Patent Publication Number: US-9890912-B2

Title: Light-emitting apparatus including photoluminescent layer

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a light-emitting apparatus including a photoluminescent layer. 
     2. Description of the Related Art 
     Optical devices, such as lighting fixtures, displays, and projectors, that output light in the necessary direction are required for many applications. Photoluminescent materials, such as those used for fluorescent lamps and white light-emitting diodes (LEDs), emit light in all directions. Thus, those materials are used in combination with optical elements such as reflectors and lenses to output light only in a particular direction. For example, Japanese Unexamined Patent Application Publication No. 2010-231941 discloses an illumination system including a light distributor and an auxiliary reflector to provide sufficient directionality. 
     SUMMARY 
     In one general aspect, the techniques disclosed here feature a light-emitting apparatus that includes a light-emitting device and an excitation light source. The excitation light source is integrally provided with the light-emitting device and emits excitation light. The light-emitting device includes: a photoluminescent layer having a first surface perpendicular to a thickness direction thereof, an area of the first surface being larger than a sectional area of the photoluminescent layer perpendicular to the first surface; and a light-transmissive layer located on the photoluminescent layer. At least one of the photoluminescent layer and the light-transmissive layer has a submicron structure having at least projections or recesses arranged perpendicular to the thickness direction of the photoluminescent layer. The photoluminescent layer receives the excitation light and emits light including first light having a wavelength λ a  in air. At least one of the photoluminescent layer and the light-transmissive layer has a light emitting surface perpendicular to the thickness direction of the photoluminescent layer, the first light being emitted from the light emitting surface. A distance D int  between adjacent projections or recesses and a refractive index n wav-a  of the photoluminescent layer for the first light satisfy λ a /n wav-a &lt;D int &lt;λ a . A thickness of the photoluminescent layer, the refractive index n wav-a , and the distance D int  are set to limit a directional angle of the first light emitted from the light emitting surface. 
     It should be noted that general or specific embodiments may be implemented as a device, an apparatus, a system, a method, or any elective combination thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of the structure of a light-emitting device according to an embodiment; 
         FIG. 1B  is a fragmentary cross-sectional view of the light-emitting device illustrated in  FIG. 1A ; 
         FIG. 1C  is a perspective view of the structure of a light-emitting device according to another embodiment; 
         FIG. 1D  is a fragmentary cross-sectional view of the light-emitting device illustrated in  FIG. 1C ; 
         FIG. 2  is a graph showing the calculation results of the enhancement of light output in the front direction with varying emission wavelengths and varying a period of a periodic structure; 
         FIG. 3  is a graph illustrating the conditions for m=1 and m=3 in the inequality (10); 
         FIG. 4  is a graph showing the calculation results of the enhancement of light output in the front direction with varying emission wavelengths and varying thicknesses t of a photoluminescent layer; 
         FIG. 5A  is a graph showing the calculation results of the electric field distribution of a mode to guide light in the x direction for a thickness t of 238 nm; 
         FIG. 5B  is a graph showing the calculation results of the electric field distribution of a mode to guide light in the x direction for a thickness t of 539 nm; 
         FIG. 5C  is a graph showing the calculation results of the electric field distribution of a mode to guide light in the x direction for a thickness t of 300 nm; 
         FIG. 6  is a graph showing the calculation results of the enhancement of light performed under the same conditions as in  FIG. 2  except that the polarization of the light was assumed to be the TE mode, which has an electric field component perpendicular to the y direction; 
         FIG. 7A  is a plan view of a two-dimensional periodic structure; 
         FIG. 7B  is a graph showing the results of calculations performed as in  FIG. 2  for the two-dimensional periodic structure; 
         FIG. 8  is a graph showing the calculation results of the enhancement of light output in the front direction with varying emission wavelengths and varying refractive indices of the periodic structure; 
         FIG. 9  is a graph showing the results obtained under the same conditions as in  FIG. 8  except that the photoluminescent layer was assumed to have a thickness of 1,000 nm; 
         FIG. 10  is a graph showing the calculation results of the enhancement of light output in the front direction with varying emission wavelengths and varying heights of the periodic structure; 
         FIG. 11  is a graph showing the results of calculations performed under the same conditions as in  FIG. 10  except that the periodic structure was assumed to have a refractive index n p  of 2.0; 
         FIG. 12  is a graph showing the results of calculations performed under the same conditions as in  FIG. 9  except that the polarization of the light was assumed to be the TE mode, which has an electric field component perpendicular to the y direction; 
         FIG. 13  is a graph showing the results of calculations performed under the same conditions as in  FIG. 9  except that the photoluminescent layer was assumed to have a refractive index n wav  of 1.5; 
         FIG. 14  is a graph showing the results of calculations performed under the same conditions as in  FIG. 2  except that the photoluminescent layer and the periodic structure were assumed to be located on a transparent substrate having a refractive index of 1.5; 
         FIG. 15  is a graph illustrating the condition represented by the inequality (15); 
         FIG. 16  is a schematic view of a light-emitting apparatus including a light-emitting device illustrated in  FIGS. 1A and 1B  and a light source that directs excitation light into a photoluminescent layer; 
         FIGS. 17A to 17D  illustrate structures in which excitation light is coupled into a quasi-guided mode to efficiently output light:  FIG. 17A  illustrates a one-dimensional periodic structure having a period p x  in the x direction,  FIG. 17B  illustrates a two-dimensional periodic structure having a period p x  in the x direction and a period p y  in the y direction,  FIG. 17C  illustrates the wavelength dependence of light absorptivity in the structure in  FIG. 17A , and  FIG. 17D  illustrates the wavelength dependence of light absorptivity in the structure in  FIG. 17B ; 
         FIG. 18A  is a schematic view of a two-dimensional periodic structure; 
         FIG. 18B  is a schematic view of another two-dimensional periodic structure; 
         FIG. 19A  is a schematic view of a modified example in which a periodic structure is formed on a transparent substrate; 
         FIG. 19B  is a schematic view of another modified example in which a periodic structure is formed on a transparent substrate; 
         FIG. 19C  is a graph showing the calculation results of the enhancement of light output from the structure illustrated in  FIG. 19A  in the front direction with varying emission wavelengths and varying periods of the periodic structure; 
         FIG. 20  is a schematic view of a mixture of light-emitting devices in powder form; 
         FIG. 21  is a plan view of a two-dimensional array of periodic structures having different periods on the photoluminescent layer; 
         FIG. 22  is a schematic view of a light-emitting device including photoluminescent layers each having a textured surface; 
         FIG. 23  is a cross-sectional view of a structure including a protective layer between a photoluminescent layer and a periodic structure; 
         FIG. 24  is a cross-sectional view of a structure including a periodic structure formed by processing only a portion of a photoluminescent layer; 
         FIG. 25  is a cross-sectional transmission electron microscopy (TEM) image of a photoluminescent layer formed on a glass substrate having a periodic structure; 
         FIG. 26  is a graph showing the results of measurements of the spectrum of light output from a sample light-emitting device in the front direction; 
         FIG. 27A  is a schematic view of a light-emitting device that can emit linearly polarized light of the TM mode, rotated about an axis parallel to the line direction of the one-dimensional periodic structure; 
         FIG. 27B  is a graph showing the results of measurements of the angular dependence of light output from the sample light-emitting device rotated as illustrated in  FIG. 27A ; 
         FIG. 27C  is a graph showing the results of calculations of the angular dependence of light output from the sample light-emitting device rotated as illustrated in FIG.  27 A; 
         FIG. 27D  is a schematic view of a light-emitting device that can emit linearly polarized light of the TE mode, rotated about an axis parallel to the line direction of the one-dimensional periodic structure; 
         FIG. 27E  is a graph showing the results of measurements of the angular dependence of light output from the sample light-emitting device rotated as illustrated in  FIG. 27D ; 
         FIG. 27F  is a graph showing the results of calculations of the angular dependence of light output from the sample light-emitting device rotated as illustrated in  FIG. 27D ; 
         FIG. 28A  is a schematic view of a light-emitting device that can emit linearly polarized light of the TE mode, rotated about an axis perpendicular to the line direction of the one-dimensional periodic structure; 
         FIG. 28B  is a graph showing the results of measurements of the angular dependence of light output from the sample light-emitting device rotated as illustrated in  FIG. 28A ; 
         FIG. 28C  is a graph showing the results of calculations of the angular dependence of light output from the sample light-emitting device rotated as illustrated in  FIG. 28A ; 
         FIG. 28D  is a schematic view of a light-emitting device that can emit linearly polarized light of the TM mode, rotated about an axis perpendicular to the line direction of the one-dimensional periodic structure; 
         FIG. 28E  is a graph showing the results of measurements of the angular dependence of light output from the sample light-emitting device rotated as illustrated in  FIG. 28D ; 
         FIG. 28F  is a graph showing the results of calculations of the angular dependence of light output from the sample light-emitting device rotated as illustrated in  FIG. 28D ; 
         FIG. 29  is a graph showing the results of measurements of the angular dependence of light (wavelength: 610 nm) output from the sample light-emitting device; 
         FIG. 30  is a schematic perspective view of a slab waveguide; 
         FIG. 31A  is a schematic cross-sectional view of a light-emitting apparatus,  FIG. 31B  is a schematic cross-sectional view of a light-emitting apparatus, and  FIG. 31C  is a schematic cross-sectional view of a light-emitting apparatus; 
         FIG. 32A  is a schematic cross-sectional view of the structure of a bottom emission LED, and  FIG. 32B  is a schematic cross-sectional view of the structure of a top emission LED; 
         FIG. 33A  is a schematic cross-sectional view of a light-emitting apparatus,  FIG. 33B  is a schematic cross-sectional view of a light-emitting apparatus, and  FIG. 33C  is a schematic cross-sectional view of a light-emitting apparatus; 
         FIG. 34A  is a schematic cross-sectional view of a light-emitting apparatus,  FIG. 34B  is a schematic cross-sectional view of a light-emitting apparatus, and  FIG. 34C  is a schematic cross-sectional view of a light-emitting apparatus; 
         FIG. 35  is a schematic cross-sectional view of a light-emitting apparatus; 
         FIG. 36A  is an explanatory schematic view of the structure of an edge-emitting laser, and  FIG. 36B  is an explanatory schematic view of the structure of a surface-emitting laser; 
         FIG. 37A  is a schematic cross-sectional view of a light-emitting apparatus, and  FIG. 37B  is a schematic cross-sectional view of a light-emitting apparatus; and 
         FIG. 38  is a schematic cross-sectional view of a light-emitting apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure includes the following light-emitting devices and light-emitting apparatuses:
     [Item 1] A light-emitting device including   

     a photoluminescent layer, 
     a light-transmissive layer located on or near the photoluminescent layer, and 
     a submicron structure that is formed on at least one of the photoluminescent layer and the light-transmissive layer and that extends in a plane of the photoluminescent layer or the light-transmissive layer, 
     wherein the submicron structure has projections or recesses, 
     light emitted from the photoluminescent layer includes first light having a wavelength λ a  in air, and 
     the distance D int  between adjacent projections or recesses and the refractive index n wav-a  of the photoluminescent layer for the first light satisfy λ a /n wav-a &lt;D int &lt;λ a .
     [Item 2] The light-emitting device according to Item 1, wherein the submicron structure includes at least one periodic structure comprising the projections or recesses, and the at least one periodic structure includes a first periodic structure having a period p a  that satisfies λ a /n wav-a &lt;p a &lt;λ a .   [Item 3] The light-emitting device according to Item 1 or 2, wherein the refractive index n t-a  of the light-transmissive layer for the first light is lower than the refractive index n wav-a  of the photoluminescent layer for the first light.   [Item 4] The light-emitting device according to any one of Items 1 to 3, wherein the first light has the maximum intensity in a first direction determined in advance by the submicron structure.   [Item 5] The light-emitting device according to Item 4, wherein the first direction is normal to the photoluminescent layer.   [Item 6] The light-emitting device according to Item 4 or 5, wherein the first light emitted in the first direction is linearly polarized light.   [Item 7] The light-emitting device according to any one of Items 4 to 6, wherein the directional angle of the first light with respect to the first direction is less than 15 degrees.   [Item 8] The light-emitting device according to any one of Items 4 to 7, wherein second light having a wavelength λ b  different from the wavelength λ a  of the first light has the maximum intensity in a second direction different from the first direction.   [Item 9] The light-emitting device according to any one of Items 1 to 8, wherein the light-transmissive layer has the submicron structure.   [Item 10] The light-emitting device according to any one of Items 1 to 9, wherein the photoluminescent layer has the submicron structure.   [Item 11] The light-emitting device according to any one of Items 1 to 8, wherein   

     the photoluminescent layer has a flat main surface, and 
     the light-transmissive layer is located on the flat main surface of the photoluminescent layer and has the submicron structure.
     [Item 12] The light-emitting device according to Item 11, wherein the photoluminescent layer is supported by a transparent substrate.   [Item 13] The light-emitting device according to any one of Items 1 to 8, wherein   

     the light-transmissive layer is a transparent substrate having the submicron structure on a main surface thereof, and 
     the photoluminescent layer is located on the submicron structure.
     [Item 14] The light-emitting device according to Item 1 or 2, wherein the refractive index n t-a  of the light-transmissive layer for the first light is higher than or equal to the refractive index n wav-a  of the photoluminescent layer for the first light, and each of the projections or recesses in the submicron structure has a height or depth of 150 nm or less.   [Item 15] The light-emitting device according to any one of Items 1 and 3 to 14, wherein   

     the submicron structure includes at least one periodic structure comprising the projections or recesses, and the at least one periodic structure includes a first periodic structure having a period p a  that satisfies λ a /n wav-a &lt;p a &lt;λ a , and 
     the first periodic structure is a one-dimensional periodic structure.
     [Item 16] The light-emitting device according to Item 15, wherein   

     light emitted from the photoluminescent layer includes second light having a wavelength λ b  different from the wavelength λ a  in air, 
     the at least one periodic structure further includes a second periodic structure having a period p b  that satisfies λ b /n wav-b &lt;p b &lt;λ b , wherein n wav-b  denotes a refractive index of the photoluminescent layer for the second light, and 
     the second periodic structure is a one-dimensional periodic structure.
     [Item 17] The light-emitting device according to any one of Items 1 and 3 to 14, wherein the submicron structure includes at least two periodic structures comprising the projections or recesses, and the at least two periodic structures include a two-dimensional periodic structure having periodicity in different directions.   [Item 18] The light-emitting device according to any one of Items 1 and 3 to 14, wherein   

     the submicron structure includes periodic structures comprising the projections or recesses, and 
     the periodic structures include periodic structures arranged in a matrix.
     [Item 19] The light-emitting device according to any one of Items 1 and 3 to 14, wherein   

     the submicron structure includes periodic structures comprising the projections or recesses, and 
     the periodic structures include a periodic structure having a period p ex  that satisfies λ ex /n wav-ex &lt;p ex &lt;λ ex , wherein λ ex  denotes the wavelength of excitation light in air for a photoluminescent material contained in the photoluminescent layer, and n wav-ex  denotes the refractive index of the photoluminescent layer for the excitation light.
     [Item 20] A light-emitting device including   

     photoluminescent layers and light-transmissive layers, 
     wherein at least two of the photoluminescent layers are independently the photoluminescent layer according to any one of Items 1 to 19, and at least two of the light-transmissive layers are independently the light-transmissive layer according to any one of Items 1 to 19.
     [Item 21] The light-emitting device according to Item 20, wherein the photoluminescent layers and the light-transmissive layers are stacked on top of each other.   [Item 22] A light-emitting device including   

     a photoluminescent layer, 
     a light-transmissive layer located on or near the photoluminescent layer, and 
     a submicron structure that is formed on at least one of the photoluminescent layer and the light-transmissive layer and that extends in a plane of the photoluminescent layer or the light-transmissive layer, 
     wherein light for forming a quasi-guided mode in the photoluminescent layer and the light-transmissive layer is emitted.
     [Item 23] A light-emitting device including   

     a waveguide layer capable of guiding light, and 
     a periodic structure located on or near the waveguide layer, 
     wherein the waveguide layer contains a photoluminescent material, and 
     the waveguide layer includes a quasi-guided mode in which light emitted from the photoluminescent material is guided while interacting with the periodic structure.
     [Item 24] A light-emitting device including   

     a photoluminescent layer, 
     a light-transmissive layer located on or near the photoluminescent layer, and 
     a submicron structure that is formed on at least one of the photoluminescent layer and the light-transmissive layer and that extends in a plane of the photoluminescent layer or the light-transmissive layer, 
     wherein the submicron structure has projections or recesses, and 
     the distance D int  between adjacent projections or recesses, the wavelength λ ex  of excitation light in air for a photoluminescent material contained in the photoluminescent layer, and the refractive index n wav-ex  of a medium having the highest refractive index for the excitation light out of media present in an optical path to the photoluminescent layer or the light-transmissive layer satisfy λ ex /n wav-ex &lt;D int &lt;λ ex .
     [Item 25] The light-emitting device according to Item 24, wherein the submicron structure includes at least one periodic structure comprising the projections or recesses, and the at least one periodic structure includes a first periodic structure having a period p ex  that satisfies λ ex /n wav-ex &lt;p ex &lt;λ ex .   [Item 26] A light-emitting device including   

     a light-transmissive layer, 
     a submicron structure that is formed in the light-transmissive layer and extends in a plane of the light-transmissive layer, and 
     a photoluminescent layer located on or near the submicron structure, 
     wherein the submicron structure has projections or recesses, 
     light emitted from the photoluminescent layer includes first light having a wavelength λ a  in air, 
     the submicron structure includes at least one periodic structure comprising the projections or recesses, and 
     the refractive index n wav-a  of the photoluminescent layer for the first light and the period p a  of the at least one periodic structure satisfy λ a /n wav-a &lt;p a &lt;λ a .
     [Item 27] A light-emitting device including   

     a photoluminescent layer, 
     a light-transmissive layer having a higher refractive index than the photoluminescent layer, and 
     a submicron structure that is formed in the light-transmissive layer and extends in a plane of the light-transmissive layer, 
     wherein the submicron structure has projections or recesses, 
     light emitted from the photoluminescent layer includes first light having a wavelength λ a  in air, 
     the submicron structure includes at least one periodic structure comprising the projections or recesses, and 
     the refractive index n wav-a  of the photoluminescent layer for the first light and the period p a  of the at least one periodic structure satisfy λ a /n wav-a &lt;p a &lt;λ a .
     [Item 28] A light-emitting device including   

     a photoluminescent layer, and 
     a submicron structure that is formed in the photoluminescent layer and extends in a plane of the photoluminescent layer, 
     wherein the submicron structure has projections or recesses, 
     light emitted from the photoluminescent layer includes first light having a wavelength λ a  in air, 
     the submicron structure includes at least one periodic structure comprising the projections or recesses, and 
     the refractive index n wav-a  of the photoluminescent layer for the first light and the period p a  of the at least one periodic structure satisfy λ a /n wav-a &lt;p a &lt;λ a .
     [Item 29] The light-emitting device according to any one of Items 1 to 21 and 24 to 28, wherein the submicron structure has both the projections and the recesses.   [Item 30] The light-emitting device according to any one of Items 1 to 22 and 24 to 27, wherein the photoluminescent layer is in contact with the light-transmissive layer.   [Item 31] The light-emitting device according to Item 23, wherein the waveguide layer is in contact with the periodic structure.   [Item 32] A light-emitting apparatus including   

     the light-emitting device according to any one of Items 1 to 31, and 
     an excitation light source for irradiating the photoluminescent layer with excitation light.
     [Item 33] A light-emitting apparatus including a light-emitting device and an excitation light source, wherein   

     the excitation light source is integrally provided with the light-emitting device and emits excitation light, and 
     the light-emitting device includes:
         a photoluminescent layer having a first surface perpendicular to a thickness direction thereof, an area of the first surface being larger than a sectional area of the photoluminescent layer perpendicular to the first surface; and   a light-transmissive layer located on the photoluminescent layer, wherein   at least one of the photoluminescent layer and the light-transmissive layer has a submicron structure having at least projections or recesses arranged perpendicular to the thickness direction of the photoluminescent layer,   the photoluminescent layer receives the excitation light and emits light including first light having a wavelength λ a  in air,   at least one of the photoluminescent layer and the light-transmissive layer has a light emitting surface perpendicular to the thickness direction of the photoluminescent layer, the first light being emitted from the light emitting surface,   a distance D int  between adjacent projections or recesses and a refractive index n wav-a  of the photoluminescent layer for the first light satisfy λ a /n wav-a &lt;D int &lt;λ a , and       a thickness of the photoluminescent layer, the refractive index n wav-a , and the distance D int  are set to limit a directional angle of the first light emitted from the light emitting surface.   [Item 34] The light-emitting apparatus according to Item 33, wherein the submicron structure includes at least one periodic structure comprising the projections or recesses, and the at least one periodic structure includes a first periodic structure having a period p a  that satisfies λ a /n wav-a &lt;p a &lt;λ a .   [Item 35] The light-emitting apparatus according to Item 33 or 34, wherein   

     the light-emitting device further includes a low-refractive-index layer between the photoluminescent layer and the excitation light source and between the light-transmissive layer and the excitation light source, and 
     the low-refractive-index layer has a lower refractive index for the first light than the photoluminescent layer.
     [Item 36] The light-emitting apparatus according to Item 35, wherein the low-refractive-index layer has second projections facing the excitation light source.   [Item 37] The light-emitting apparatus according to Item 33 or 34, wherein a light output side of the excitation light source is in direct contact with the photoluminescent layer or the light-transmissive layer.   [Item 38] The light-emitting apparatus according to Item 35 or 36, wherein a light output side of the excitation light source is in direct contact with the low-refractive-index layer.   [Item 39] The light-emitting apparatus according to any one of Items 33 to 38, wherein the light-emitting device further includes a selective reflective layer that selectively reflects light passing through the photoluminescent layer.   [Item 40] A light-emitting apparatus including a light-emitting device and an excitation light source, wherein   

     the excitation light source is integrally provided with the light-emitting device and emits excitation light, and 
     the light-emitting device includes:
         a light-transmissive layer having a submicron structure; and   a photoluminescent layer that is located on the submicron structure and emits light in response to the excitation light, wherein   the photoluminescent layer has a first surface perpendicular to a thickness direction thereof, an area of the first surface being larger than a sectional area of the photoluminescent layer perpendicular to the first surface,   the light emitted from the photoluminescent layer includes first light having a wavelength λ a  in air,   at least one of the photoluminescent layer and the light-transmissive layer has a light emitting surface perpendicular to the thickness direction of the photoluminescent layer, the first light being emitted from the light emitting surface,   the submicron structure includes at least one periodic structure comprising at least projections or recesses arranged perpendicular to the thickness direction of the photoluminescent layer,   a refractive index n wav-a  of the photoluminescent layer for the first light and a period p a  of the at least one periodic structure satisfy λ a /n wav-a &lt;p a &lt;λ a , and       a thickness of the photoluminescent layer, the refractive index n wav-a  and the period p a  are set to limit a directional angle of the first light emitted from the light emitting surface.   [Item 41] A light-emitting apparatus including a light-emitting device and an excitation light source, wherein   

     the excitation light source is integrally provided with the light-emitting device and emits excitation light, and 
     the light-emitting device includes:
         a photoluminescent layer emitting light in response to the excitation light; and   a light-transmissive layer that has a higher refractive index than the photoluminescent layer, and has a submicron structure, wherein   the light emitted from the photoluminescent layer includes first light having a wavelength λ a  in air,   the photoluminescent layer has a first surface perpendicular to a thickness direction thereof, an area of the first surface being larger than a sectional area of the photoluminescent layer perpendicular to the first surface,   at least one of the photoluminescent layer and the light-transmissive layer has a light emitting surface perpendicular to the thickness direction of the photoluminescent layer, the first light being emitted from the light emitting surface,   the submicron structure includes at least one periodic structure comprising at least projections or recesses arranged perpendicular to the thickness direction of the photoluminescent layer,   a refractive index n wav-a  of the photoluminescent layer for the first light and a period p a  of the at least one periodic structure satisfy λ a /n wav-a &lt;p a &lt;λ a , and       a thickness of the photoluminescent layer, the refractive index n wav-a , and the period p a  are set to limit a directional angle of the first light emitted from the light emitting surface.   [Item 42] The light-emitting apparatus according to any one of Items 33 to 41, wherein the photoluminescent layer is in contact with the light-transmissive layer.   [Item 43] A light-emitting apparatus comprising a light-emitting device and an excitation light source, wherein   

     the excitation light source is integrally provided with the light-emitting device and emits excitation light, 
     the light-emitting device includes a photoluminescent layer emitting light in response to the excitation light, and has a submicron structure, 
     the photoluminescent layer has a first surface perpendicular to a thickness direction thereof, an area of the first surface being larger than a sectional area of the photoluminescent layer perpendicular to the first surface, 
     the submicron structure includes at least one periodic structure comprising at least projections or recesses arranged perpendicular to the thickness direction of the photoluminescent layer, 
     the light emitted from the photoluminescent layer includes first light having a wavelength λ a  in air, 
     the photoluminescent layer has a light emitting surface perpendicular to the thickness direction of the photoluminescent layer, the first light being emitted from the light emitting surface, 
     a refractive index n wav-a  of the photoluminescent layer for the first light and a period p a  of the at least one periodic structure satisfy λ a /n wav-a &lt;p a &lt;λ a , and 
     a thickness of the photoluminescent layer, the refractive index n wav-a , and the period p a  are set to limit a directional angle of the first light emitted from the light emitting surface.
     [Item 44] The light-emitting apparatus according to any one of Items 33 to 43, wherein the submicron structure has both the projections and the recesses.   [Item 45] The light-emitting apparatus according to any one of Items 33 to 44, wherein the photoluminescent layer includes a phosphor.   [Item 46] The light-emitting apparatus according to any one of Items 33 to 45, wherein 380 nm≦λ a ≦780 nm is satisfied.   [Item 47] The light-emitting apparatus according to any one of Items 33 to 46, wherein the thickness of the photoluminescent layer, the refractive index n wav-a , and the distance D int  are set to allow an electric field to be formed in the photoluminescent layer, in which antinodes of the electric field are located in areas, the areas each corresponding to respective one of the projections and/or recesses.   [Item 48] The light-emitting apparatus according to any one of Items 33 to 47, wherein the light-transmissive layer is located indirectly on the photoluminescent layer.   [Item 49] The light-emitting apparatus according to any one of Items 33 to 48, wherein the thickness of the photoluminescent layer, the refractive index n wav-a , and the distance D int  are set to allow an electric field to be formed in the photoluminescent layer, in which antinodes of the electric field are located at, or adjacent to, at least the projections or recesses.   [Item 50] The light-emitting apparatus according to any one of Items 33 to 49, wherein the light-emitting device further includes a substrate that has a refractive index n s-a  for the first light and is located on the photoluminescent layer, wherein λ a /n wav-a &lt;D int &lt;λ a /n s-a  is satisfied.   

     A light-emitting device according to an embodiment of the present disclosure includes a photoluminescent layer, a light-transmissive layer located on or near the photoluminescent layer, and a submicron structure that is formed on at least one of the photoluminescent layer and the light-transmissive layer and that extends in a plane of the photoluminescent layer or the light-transmissive layer. The submicron structure has projections or recesses, light emitted from the photoluminescent layer includes first light having a wavelength λ a  in air, and the distance D int  between adjacent projections or recesses and the refractive index n wav-a  of the photoluminescent layer for the first light satisfy λ a /n wav-a &lt;D int &lt;λ a . The wavelength λ a  is, for example, within the visible wavelength range (for example, 380 to 780 nm). 
     The photoluminescent layer contains a photoluminescent material. The term “photoluminescent material” refers to a material that emits light in response to excitation light. The term “photoluminescent material” encompasses fluorescent materials and phosphorescent materials in a narrow sense, encompasses inorganic materials and organic materials (for example, dyes), and encompasses quantum dots (that is, tiny semiconductor particles). The photoluminescent layer may contain a matrix material (host material) in addition to the photoluminescent material. Examples of matrix materials include resins and inorganic materials such as glasses and oxides. 
     The light-transmissive layer located on or near the photoluminescent layer is made of a material with high transmittance to the light emitted from the photoluminescent layer, for example, inorganic materials or resins. For example, the light-transmissive layer is desirably formed of a dielectric material (particularly, an insulator having low light absorptivity). For example, the light-transmissive layer may be a substrate that supports the photoluminescent layer. If the surface of the photoluminescent layer facing air has the submicron structure, the air layer can serve as the light-transmissive layer. 
     In a light-emitting device according to an embodiment of the present disclosure, a submicron structure (for example, a periodic structure) on at least one of the photoluminescent layer and the light-transmissive layer forms a unique electric field distribution inside the photoluminescent layer and the light-transmissive layer, as described in detail later with reference to the results of calculations and experiments. This electric field distribution is formed by an interaction between guided light and the submicron structure and may also be referred to as a “quasi-guided mode”. 
     The quasi-guided mode can be utilized to improve the luminous efficiency, directionality, and polarization selectivity of photoluminescence, as described later. The term “quasi-guided mode” may be used in the following description to describe novel structures and/or mechanisms contemplated by the inventors. However, such a description is for illustrative purposes only and is not intended to limit the present disclosure in any way. 
     For example, the submicron structure has projections, and the distance (the center-to-center distance) D int  between adjacent projections satisfies λ a /n wav-a &lt;D int &lt;λ a . Instead of the projections, the submicron structure may have recesses. For simplicity, the following description will be directed to a submicron structure having projections. The symbol λ denotes the wavelength of light, and the symbol λ a  denotes the wavelength of light in air. The symbol n wav  denotes the refractive index of the photoluminescent layer. If the photoluminescent layer is a medium containing materials, the refractive index n wav  denotes the average refractive index of the materials weighted by their respective volume fractions. 
     Although it is desirable to use the symbol n wav-a  to refer to the refractive index for light having a wavelength λ a  because the refractive index n generally depends on the wavelength, it may be abbreviated for simplicity. The symbol n wav  basically denotes the refractive index of the photoluminescent layer; however, if a layer having a higher refractive index than the photoluminescent layer is adjacent to the photoluminescent layer, the refractive index n wav  denotes the average refractive index of the layer having a higher refractive index and the photoluminescent layer weighted by their respective volume fractions. This is optically equivalent to a photoluminescent layer composed of layers of different materials. 
     The effective refractive index n eff  of the medium for light in the quasi-guided mode satisfies n a &lt;n eff &lt;n wav , wherein n a  denotes the refractive index of air. If light in the quasi-guided mode is assumed to be light propagating through the photoluminescent layer while being totally reflected at an angle of incidence θ, the effective refractive index n eff  can be written as n eff =n wav  sin θ. The effective refractive index n eff  is determined by the refractive index of the medium present in the region where the electric field of the quasi-guided mode is distributed. 
     For example, if the submicron structure is formed in the light-transmissive layer, the effective refractive index n eff  depends not only on the refractive index of the photoluminescent layer but also on the refractive index of the light-transmissive layer. Because the electric field distribution also varies depending on the polarization direction of the quasi-guided mode (that is, the TE mode or the TM mode), the effective refractive index n eff  can differ between the TE mode and the TM mode. 
     The submicron structure is formed on at least one of the photoluminescent layer and the light-transmissive layer. If the photoluminescent layer and the light-transmissive layer are in contact with each other, the submicron structure may be formed on the interface between the photoluminescent layer and the light-transmissive layer. In such a case, the photoluminescent layer and the light-transmissive layer have the submicron structure. The photoluminescent layer may have no submicron structure. In such a case, a light-transmissive layer having a submicron structure is located on or near the photoluminescent layer. 
     A phrase like “a light-transmissive layer (or its submicron structure) located on or near the photoluminescent layer”, as used herein, typically means that the distance between these layers is less than half the wavelength λ a . This allows the electric field of a guided mode to reach the submicron structure, thus forming a quasi-guided mode. However, the distance between the submicron structure of the light-transmissive layer and the photoluminescent layer may exceed half the wavelength λ a  if the light-transmissive layer has a higher refractive index than the photoluminescent layer. If the light-transmissive layer has a higher refractive index than the photoluminescent layer, light reaches the light-transmissive layer even if the above relationship is not satisfied. In the present specification, if the photoluminescent layer and the light-transmissive layer have a positional relationship that allows the electric field of a guided mode to reach the submicron structure and form a quasi-guided mode, they may be associated with each other. 
     The submicron structure, which satisfies λ a /n wav-a &lt;D int &lt;λ a , as described above, is characterized by a submicron size. The submicron structure includes at least one periodic structure, as in the light-emitting devices according to the embodiments described in detail later. The at least one periodic structure has a period p a  that satisfies λ a /n wav-a &lt;p a &lt;λ a . Thus, the submicron structure includes a periodic structure in which the distance D int  between adjacent projections is constant at p a . If the submicron structure includes a periodic structure, light in the quasi-guided mode propagates while repeatedly interacting with the periodic structure so that the light is diffracted by the submicron structure. Unlike the phenomenon in which light propagating through free space is diffracted by a periodic structure, this is the phenomenon in which light is guided (that is, repeatedly totally reflected) while interacting with the periodic structure. This can efficiently diffract light even if the periodic structure causes a small phase shift (that is, even if the periodic structure has a small height). 
     The above mechanism can be utilized to improve the luminous efficiency of photoluminescence by the enhancement of the electric field due to the quasi-guided mode and also to couple the emitted light into the quasi-guided mode. The angle of travel of the light in the quasi-guided mode is varied by the angle of diffraction determined by the periodic structure. This can be utilized to output light of a particular wavelength in a particular direction (that is, significantly improve the directionality). Furthermore, high polarization selectivity can be simultaneously achieved because the effective refractive index n eff  (=n wav  sin θ) differs between the TE mode and the TM mode. For example, as demonstrated by the experimental examples below, a light-emitting device can be provided that outputs intense linearly polarized light (for example, the TM mode) of a particular wavelength (for example, 610 nm) in the front direction. The directional angle of the light output in the front direction is, for example, less than 15 degrees. The term “directional angle” refers to the angle of one side with respect to the front direction, which is assumed to be 0 degrees. 
     Conversely, a submicron structure having a lower periodicity results in a lower directionality, luminous efficiency, polarization, and wavelength selectivity. The periodicity of the submicron structure may be adjusted depending on the need. The periodic structure may be a one-dimensional periodic structure, which has a higher polarization selectivity, or a two-dimensional periodic structure, which allows for a lower polarization. 
     The submicron structure may include periodic structures. For example, these periodic structures may have different periods or different periodic directions (axes). The periodic structures may be formed on the same plane or may be stacked on top of each other. The light-emitting device may include photoluminescent layers and light-transmissive layers, and each of the layers may have submicron structures. 
     The submicron structure can be used not only to control the light emitted from the photoluminescent layer but also to efficiently guide excitation light into the photoluminescent layer. That is, the excitation light can be diffracted and coupled into the quasi-guided mode to guide light in the photoluminescent layer and the light-transmissive layer by the submicron structure to efficiently excite the photoluminescent layer. A submicron structure may be used that satisfies λ ex /n wav-ex &lt;D int &lt;λ ex , wherein λ ex  denotes the wavelength in air of the light that excites the photoluminescent material, and n wav-ex  denotes the refractive index of the photoluminescent layer for the excitation light. The symbol n wav-ex  denotes the refractive index of the photoluminescent layer for the emission wavelength of the photoluminescent material. Alternatively, a submicron structure may be used that includes a periodic structure having a period p ex  that satisfies λ ex /n wav-ex &lt;p ex &lt;λ ex . The excitation light has a wavelength λ ex  of 450 nm, for example, but may have a shorter wavelength than visible light. If the excitation light has a wavelength within the visible range, it may be output together with the light emitted from the photoluminescent layer. 
     1. UNDERLYING KNOWLEDGE FORMING BASIS OF THE PRESENT DISCLOSURE 
     The underlying knowledge forming the basis for the present disclosure will be described before describing specific embodiments of the present disclosure. As described above, photoluminescent materials such as those used for fluorescent lamps and white LEDs emit light in all directions and thus require optical elements such as reflectors and lenses to emit light in a particular direction. These optical elements, however, can be eliminated (or the size thereof can be reduced) if the photoluminescent layer itself emits directional light. This results in a significant reduction in the size of optical devices and equipment. With this idea in mind, the inventors have conducted a detailed study on the photoluminescent layer to achieve directional light emission. 
     The inventors have investigated the possibility of inducing light emission with particular directionality so that the light emitted from the photoluminescent layer is localized in a particular direction. Based on Fermi&#39;s golden rule, the emission rate Γ, which is a measure characterizing light emission, is represented by the equation (1): 
     
       
         
           
             
               
                 
                   
                     Γ 
                     ⁡ 
                     
                       ( 
                       r 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         2 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         π 
                       
                       ℏ 
                     
                     ⁢ 
                     
                       
                         〈 
                         
                           ( 
                           
                             d 
                             · 
                             
                               E 
                               ⁡ 
                               
                                 ( 
                                 r 
                                 ) 
                               
                             
                           
                           ) 
                         
                         〉 
                       
                       2 
                     
                     ⁢ 
                     
                       ρ 
                       ⁡ 
                       
                         ( 
                         λ 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In the equation (1), r is the vector indicating the position, λ is the wavelength of light, d is the dipole vector, E is the electric field vector, and ρ is the density of states. For many substances other than some crystalline substances, the dipole vector d is randomly oriented. The magnitude of the electric field E is substantially constant irrespective of the direction if the size and thickness of the photoluminescent layer are sufficiently larger than the wavelength of light. Hence, in most cases, the value of &lt;(d·E(r))&gt; 2  does not depend on the direction. Accordingly, the emission rate Γ is constant irrespective of the direction. Thus, in most cases, the photoluminescent layer emits light in all directions. 
     As can be seen from the equation (1), to achieve anisotropic light emission, it is necessary to align the dipole vector d in a particular direction or to enhance the component of the electric field vector in a particular direction. One of these approaches can be employed to achieve directional light emission. In the present disclosure, the results of a detailed study and analysis on structures for utilizing a quasi-guided mode in which the electric field component in a particular direction is enhanced by the confinement of light in the photoluminescent layer will be described below. 
     2. STRUCTURE FOR ENHANCING ELECTRIC FIELD ONLY IN PARTICULAR DIRECTION 
     The inventors have investigated the possibility of controlling light emission using a guided mode with an intense electric field. Light can be coupled into a guided mode using a waveguide structure that itself contains a photoluminescent material. However, a waveguide structure simply formed using a photoluminescent material outputs little or no light in the front direction because the emitted light is coupled into a guided mode. Accordingly, the inventors have investigated the possibility of combining a waveguide containing a photoluminescent material with a periodic structure (including projections or recesses or both). When the electric field of light is guided in a waveguide while overlapping with a periodic structure located on or near the waveguide, a quasi-guided mode is formed by the effect of the periodic structure. That is, the quasi-guided mode is a guided mode restricted by the periodic structure and is characterized in that the antinodes of the amplitude of the electric field have the same period as the periodic structure. Light in this mode is confined in the waveguide structure to enhance the electric field in a particular direction. This mode also interacts with the periodic structure to undergo diffraction so that the light in this mode is converted into light propagating in a particular direction and can thus be output from the waveguide. The electric field of light other than the quasi-guided mode is not enhanced because little or no such light is confined in the waveguide. Thus, most light is coupled into a quasi-guided mode with a large electric field component. 
     That is, the inventors have investigated the possibility of using a photoluminescent layer containing a photoluminescent material as a waveguide (or a waveguide layer including a photoluminescent layer) in combination with a periodic structure located on or near the waveguide to couple light into a quasi-guided mode in which the light is converted into light propagating in a particular direction, thereby providing a directional light source. 
     As a simple waveguide structure, the inventors have studied slab waveguides. A slab waveguide has a planar structure in which light is guided.  FIG. 30  is a schematic perspective view of a slab waveguide  110 S. There is a mode of light propagating through the waveguide  110 S if the waveguide  110 S has a higher refractive index than a transparent substrate  140  that supports the waveguide  110 S. If such a slab waveguide includes a photoluminescent layer, the electric field of light emitted from an emission point overlaps largely with the electric field of a guided mode. This allows most of the light emitted from the photoluminescent layer to be coupled into the guided mode. If the photoluminescent layer has a thickness close to the wavelength of the light, a situation can be created where there is only a guided mode with a large electric field amplitude. 
     If a periodic structure is located on or near the photoluminescent layer, the electric field of the guided mode interacts with the periodic structure to form a quasi-guided mode. Even if the photoluminescent layer is composed of a plurality of layers, a quasi-guided mode is formed as long as the electric field of the guided mode reaches the periodic structure. Not all parts of the photoluminescent layer needs to be formed of a photoluminescent material, provided that at least a portion of the photoluminescent layer functions to emit light. 
     If the periodic structure is made of a metal, a mode due to the guided mode and plasmon resonance is formed. This mode has different properties from the quasi-guided mode. This mode is less effective in enhancing emission because a large loss occurs due to high absorption by the metal. Thus, it is desirable to form the periodic structure using a dielectric material having low absorptivity. 
     The inventors have studied the coupling of light into a quasi-guided mode that can be output as light propagating in a particular angular direction using a periodic structure formed on a waveguide (for example, a photoluminescent layer).  FIG. 1A  is a schematic perspective view of a light-emitting device  100  including a waveguide (for example, a photoluminescent layer)  110  and a periodic structure (for example, a light-transmissive layer)  120 . The light-transmissive layer  120  is hereinafter also referred to as a periodic structure  120  if the light-transmissive layer  120  forms a periodic structure (that is, if a periodic submicron structure is formed on the light-transmissive layer  120 ). In this example, the periodic structure  120  is a one-dimensional periodic structure in which stripe-shaped projections extending in the y direction are arranged at regular intervals in the x direction.  FIG. 1B  is a cross-sectional view of the light-emitting device  100  taken along a plane parallel to the xz plane. If a periodic structure  120  having a period p is provided in contact with the waveguide  110 , a quasi-guided mode having a wave number k wav  in the in-plane direction is converted into light propagating outside the waveguide  110 . The wave number k out  of the light can be represented by the equation (2): 
                     k   out     =       k   wav     -     m   ⁢       2   ⁢           ⁢   π     p                 (   2   )               
wherein m is an integer indicating the diffraction order.
 
     For simplicity, the light guided in the waveguide  110  is assumed to be a ray of light propagating at an angle θ wav . This approximation gives the equations (3) and (4): 
     
       
         
           
             
               
                 
                   
                     
                       
                         k 
                         wav 
                       
                       ⁢ 
                       
                         λ 
                         0 
                       
                     
                     
                       2 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       π 
                     
                   
                   = 
                   
                     
                       n 
                       wav 
                     
                     ⁢ 
                     sin 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       θ 
                       wav 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         k 
                         out 
                       
                       ⁢ 
                       
                         λ 
                         0 
                       
                     
                     
                       2 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       π 
                     
                   
                   = 
                   
                     
                       n 
                       out 
                     
                     ⁢ 
                     sin 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       θ 
                       out 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In these equations, λ 0  denotes the wavelength of the light in air, n wav  denotes the refractive index of the waveguide  110 , n out  denotes the refractive index of the medium on the light output side, and θ out  denotes the angle at which the light is output from the waveguide  110  to a substrate or air. From the equations (2) to (4), the output angle θ out  can be represented by the equation (5):
 
 n   out  sin θ out   =n   wav  sin θ wav   −mλ   0   /p   (5)
 
     If n wav  sin θ wav =mλ 0 /p in the equation (5), this results in θ out =0, meaning that the light can be emitted in the direction perpendicular to the plane of the waveguide  110  (that is, in the front direction). 
     Based on this principle, light can be coupled into a particular quasi-guided mode and be converted into light having a particular output angle using the periodic structure to output intense light in that direction. 
     There are some constraints to achieving the above situation. To form a quasi-guided mode, the light propagating through the waveguide  110  has to be totally reflected. The conditions therefor are represented by the inequality (6):
 
 n   out   &lt;n   wav  sin θ wav   (6)
 
     To diffract the quasi-guided mode using the periodic structure and thereby output the light from the waveguide  110 , −1&lt;sin θ out &lt;1 has to be satisfied in the equation (5). Hence, the inequality (7) has to be satisfied: 
     
       
         
           
             
               
                 
                   
                     - 
                     1 
                   
                   &lt; 
                   
                     
                       
                         
                           n 
                           wav 
                         
                         
                           n 
                           out 
                         
                       
                       ⁢ 
                       sin 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         wav 
                       
                     
                     - 
                     
                       
                         m 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           λ 
                           0 
                         
                       
                       
                         
                           n 
                           out 
                         
                         ⁢ 
                         p 
                       
                     
                   
                   &lt; 
                   1 
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Taking into account the inequality (6), the inequality (8) may be satisfied: 
     
       
         
           
             
               
                 
                   
                     
                       m 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         λ 
                         0 
                       
                     
                     
                       2 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         out 
                       
                     
                   
                   &lt; 
                   p 
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     To output the light from the waveguide  110  in the front direction (θ out =0), as can be seen from the equation (5), the equation (9) has to be satisfied:
 
 p=mλ   0 /( n   wav  sin θ wav )  (9)
 
     As can be seen from the equation (9) and the inequality (6), the required conditions are represented by the inequality (10): 
     
       
         
           
             
               
                 
                   
                     
                       m 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         λ 
                         0 
                       
                     
                     
                       n 
                       wav 
                     
                   
                   &lt; 
                   p 
                   &lt; 
                   
                     
                       m 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         λ 
                         0 
                       
                     
                     
                       n 
                       out 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     If the periodic structure  120  as illustrated in  FIGS. 1A and 1B  is provided, it may be designed based on first-order diffracted light (that is, m=1) because higher-order diffracted light having m of 2 or more has low diffraction efficiency. In this embodiment, the period p of the periodic structure  120  is determined so as to satisfy the inequality (11), which is given by substituting m=1 into the inequality (10): 
     
       
         
           
             
               
                 
                   
                     
                       λ 
                       0 
                     
                     
                       n 
                       wav 
                     
                   
                   &lt; 
                   p 
                   &lt; 
                   
                     
                       λ 
                       0 
                     
                     
                       n 
                       out 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     If the waveguide (photoluminescent layer)  110  is not in contact with a transparent substrate, as illustrated in  FIGS. 1A and 1B , n out  is equal to the refractive index of air (approximately 1.0). Thus, the period p may be determined so as to satisfy the inequality (12): 
     
       
         
           
             
               
                 
                   
                     
                       λ 
                       0 
                     
                     
                       n 
                       wav 
                     
                   
                   &lt; 
                   p 
                   &lt; 
                   
                     λ 
                     0 
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     Alternatively, a structure as illustrated in  FIGS. 1C and 1D  may be employed in which the photoluminescent layer  110  and the periodic structure  120  are formed on a transparent substrate  140 . The refractive index n s  of the transparent substrate  140  is higher than the refractive index of air. Thus, the period p may be determined so as to satisfy the inequality (13), which is given by substituting n out =n s  into the inequality (11): 
     
       
         
           
             
               
                 
                   
                     
                       λ 
                       0 
                     
                     
                       n 
                       wav 
                     
                   
                   &lt; 
                   p 
                   &lt; 
                   
                     
                       λ 
                       0 
                     
                     
                       n 
                       s 
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     Although m=1 is assumed in the inequality (10) to give the inequalities (12) and (13), m≧2 may be assumed. That is, if both surfaces of the light-emitting device  100  are in contact with air layers, as shown in  FIGS. 1A and 1B , the period p may be determined so as to satisfy the inequality (14): 
                       m   ⁢           ⁢     λ   0         n   wav       &lt;   p   &lt;     m   ⁢           ⁢     λ   0               (   14   )               
wherein m is an integer of 1 or more.
 
     Similarly, if the photoluminescent layer  110  is formed on the transparent substrate  140 , as in the light-emitting device  100   a  illustrated in  FIGS. 1C and 1D , the period p may be determined so as to satisfy the inequality (15): 
     
       
         
           
             
               
                 
                   
                     
                       m 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         λ 
                         0 
                       
                     
                     
                       n 
                       wav 
                     
                   
                   &lt; 
                   p 
                   &lt; 
                   
                     
                       m 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         λ 
                         0 
                       
                     
                     
                       n 
                       s 
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     By determining the period p of the periodic structure so as to satisfy the above inequalities, the light emitted from the photoluminescent layer  110  can be output in the front direction, thus providing a directional light-emitting device. 
     3. VERIFICATION BY CALCULATIONS 
     3-1. Period and Wavelength Dependence 
     The inventors verified, by optical analysis, whether the output of light in a particular direction as described above is actually possible. The optical analysis was performed by calculations using DiffractMOD available from Cybernet Systems Co., Ltd. In these calculations, the change in the absorption of external light incident perpendicular to a light-emitting device by a photoluminescent layer was calculated to determine the enhancement of light output perpendicular to the light-emitting device. The calculation of the process by which external incident light is coupled into a quasi-guided mode and is absorbed by the photoluminescent layer corresponds to the calculation of a process opposite to the process by which light emitted from the photoluminescent layer is coupled into a quasi-guided mode and is converted into propagating light output perpendicular to the light-emitting device. Similarly, the electric field distribution of a quasi-guided mode was calculated from the electric field of external incident light. 
       FIG. 2  shows the calculation results of the enhancement of light output in the front direction with varying emission wavelengths and varying periods of the periodic structure, where the photoluminescent layer was assumed to have a thickness of 1 μm and a refractive index n wav  of 1.8, and the periodic structure was assumed to have a height of 50 nm and a refractive index of 1.5. In these calculations, the periodic structure was assumed to be a one-dimensional periodic structure uniform in the y direction, as shown in  FIG. 1A , and the polarization of the light was assumed to be the TM mode, which has an electric field component parallel to the y direction. The results in  FIG. 2  show that there are enhancement peaks at certain combinations of wavelength and period. In  FIG. 2 , the magnitude of the enhancement is expressed by different shades of color; a darker color (black) indicates a higher enhancement, whereas a lighter color (white) indicates a lower enhancement. 
     In the above calculations, the periodic structure was assumed to have a rectangular cross section as shown in  FIG. 1B .  FIG. 3  is a graph illustrating the conditions for m=1 and m=3 in the inequality (10). A comparison between  FIGS. 2 and 3  shows that the peaks in  FIG. 2  are located within the regions corresponding to m=1 and m=3. The intensity is higher for m=1 because first-order diffracted light has a higher diffraction efficiency than third- or higher-order diffracted light. There is no peak for m=2 because of low diffraction efficiency in the periodic structure. 
     In  FIG. 2 , a plurality of lines are observed in each of the regions corresponding to m=1 and m=3 in  FIG. 3 . This indicates the presence of a plurality of quasi-guided modes. 
     3-2. Thickness Dependence 
       FIG. 4  is a graph showing the calculation results of the enhancement of light output in the front direction with varying emission wavelengths and varying thicknesses t of the photoluminescent layer, where the photoluminescent layer was assumed to have a refractive index n wav  of 1.8, and the periodic structure was assumed to have a period of 400 nm, a height of 50 nm, and a refractive index of 1.5.  FIG. 4  shows that the enhancement of the light peaks at a particular thickness t of the photoluminescent layer. 
       FIGS. 5A and 5B  show the calculation results of the electric field distributions of a mode to guide light in the x direction for a wavelength of 600 nm and thicknesses t of 238 nm and 539 nm, respectively, at which there are peaks in  FIG. 4 . For comparison,  FIG. 5C  shows the results of similar calculations for a thickness t of 300 nm, at which there is no peak. In these calculations, as in the above calculations, the periodic structure was a one-dimensional periodic structure uniform in the y direction. In each figure, a black region indicates a higher electric field intensity, whereas a white region indicates a lower electric field intensity. Whereas the results for t=238 nm and t=539 nm show high electric field intensity, the results for t=300 nm shows low electric field intensity as a whole. This is because there are guided modes for t=238 nm and t=539 nm so that light is strongly confined. Furthermore, regions with the highest electric field intensity (that is, antinodes) are necessarily present in or directly below the projections, indicating the correlation between the electric field and the periodic structure  120 . Thus, the resulting guided mode depends on the arrangement of the periodic structure  120 . A comparison between the results for t=238 nm and t=539 nm shows that these modes differ in the number of nodes (white regions) of the electric field in the z direction by one. 
     3-3. Polarization Dependence 
     To examine the polarization dependence, the enhancement of light was calculated under the same conditions as in  FIG. 2  except that the polarization of the light was assumed to be the TE mode, which has an electric field component perpendicular to the y direction.  FIG. 6  shows the results of these calculations. Although the peaks in  FIG. 6  differ slightly in position from the peaks for the TM mode ( FIG. 2 ), they are located within the regions shown in  FIG. 3 . This demonstrates that the structure according to this embodiment is effective for both of the TM mode and the TE mode. 
     3-4. Two-Dimensional Periodic Structure 
     The effect of a two-dimensional periodic structure was also studied.  FIG. 7A  is a partial plan view of a two-dimensional periodic structure  120 ′ including recesses and projections arranged in both of the x direction and the y direction. In  FIG. 7A , the black regions indicate the projections, and the white regions indicate the recesses. For a two-dimensional periodic structure, both of the diffraction in the x direction and the diffraction in the y direction have to be taken into account. Although the diffraction in only the x direction or the y direction is similar to that in a one-dimensional periodic structure, a two-dimensional periodic structure can be expected to give different results from a one-dimensional periodic structure because diffraction also occurs in a direction containing both of an x component and a y component (for example, a direction inclined at 45 degrees).  FIG. 7B  shows the calculation results of the enhancement of light for the two-dimensional periodic structure. The calculations were performed under the same conditions as in  FIG. 2  except for the type of periodic structure. As shown in  FIG. 7B , peaks matching the peaks for the TE mode in  FIG. 6  were observed in addition to peaks matching the peaks for the TM mode in  FIG. 2 . These results demonstrate that the two-dimensional periodic structure also converts and outputs the TE mode by diffraction. For a two-dimensional periodic structure, the diffraction that simultaneously satisfies the first-order diffraction conditions in both of the x direction and the y direction also has to be taken into account. Such diffracted light is output in the direction at the angle corresponding to √2 times (that is, 2 1/2  times) the period p. Thus, peaks will occur at √2 times the period p in addition to peaks that occur in a one-dimensional periodic structure. Such peaks are observed in  FIG. 7B . 
     The two-dimensional periodic structure does not have to be a square grid structure having equal periods in the x direction and the y direction, as illustrated in  FIG. 7A , but may be a hexagonal grid structure, as illustrated in  FIG. 18A , or a triangular grid structure, as illustrated in  FIG. 18B . The two-dimensional periodic structure may have different periods in different directions (for example, in the x direction and the y direction for a square grid structure). 
     In this embodiment, as demonstrated above, light in a characteristic quasi-guided mode formed by the periodic structure and the photoluminescent layer can be selectively output only in the front direction through diffraction by the periodic structure. With this structure, the photoluminescent layer can be excited with excitation light such as ultraviolet light or blue light to output directional light. 
     4. STUDY ON CONSTRUCTIONS OF PERIODIC STRUCTURE AND PHOTOLUMINESCENT LAYER 
     The effects of changes in various conditions such as the constructions and refractive indices of the periodic structure and the photoluminescent layer will now be described. 
     4-1. Refractive Index of Periodic Structure 
     The refractive index of the periodic structure was studied. In the calculations performed herein, the photoluminescent layer was assumed to have a thickness of 200 nm and a refractive index n wav  of 1.8, the periodic structure was assumed to be a one-dimensional periodic structure uniform in the y direction, as shown in  FIG. 1A , having a height of 50 nm and a period of 400 nm, and the polarization of the light was assumed to be the TM mode, which has an electric field component parallel to the y direction.  FIG. 8  shows the calculation results of the enhancement of light output in the front direction with varying emission wavelengths and varying refractive indices of the periodic structure.  FIG. 9  shows the results obtained under the same conditions except that the photoluminescent layer was assumed to have a thickness of 1,000 nm. 
     The results show that a photoluminescent layer having a thickness of 1,000 nm ( FIG. 9 ) results in a smaller shift in the wavelength at which the light intensity peaks (referred to as a peak wavelength) with the change in the refractive index of the periodic structure than a photoluminescent layer having a thickness of 200 nm ( FIG. 8 ). This is because the quasi-guided mode is more affected by the refractive index of the periodic structure as the photoluminescent layer is thinner. Specifically, a periodic structure having a higher refractive index increases the effective refractive index and thus shifts the peak wavelength toward longer wavelengths, and this effect is more noticeable as the photoluminescent layer is thinner. The effective refractive index is determined by the refractive index of the medium present in the region where the electric field of the quasi-guided mode is distributed. 
     The results also show that a periodic structure having a higher refractive index results in a broader peak and a lower intensity. This is because a periodic structure having a higher refractive index outputs light in the quasi-guided mode at a higher rate and is therefore less effective in confining the light, that is, has a lower Q value. To maintain a high peak intensity, a structure may be employed in which light is moderately output using a quasi-guided mode that is effective in confining the light (that is, has a high Q value). This means that it is undesirable to use a periodic structure made of a material having a much higher refractive index than the photoluminescent layer. Thus, in order to increase the peak intensity and Q value, the refractive index of a dielectric material constituting the periodic structure (that is, the light-transmissive layer) can be lower than or similar to the refractive index of the photoluminescent layer. This is also true if the photoluminescent layer contains materials other than photoluminescent materials. 
     4-2. Height of Periodic Structure 
     The height of the periodic structure was then studied. In the calculations performed herein, the photoluminescent layer was assumed to have a thickness of 1,000 nm and a refractive index n wav  of 1.8, the periodic structure was assumed to be a one-dimensional periodic structure uniform in the y direction, as shown in  FIG. 1A , having a refractive index n p  of 1.5 and a period of 400 nm, and the polarization of the light was assumed to be the TM mode, which has an electric field component parallel to the y direction.  FIG. 10  shows the calculation results of the enhancement of light output in the front direction with varying emission wavelengths and varying heights of the periodic structure.  FIG. 11  shows the results of calculations performed under the same conditions except that the periodic structure was assumed to have a refractive index n p  of 2.0. Whereas the results in  FIG. 10  show that the peak intensity and the Q value (that is, the peak line width) do not change above a certain height of the periodic structure, the results in  FIG. 11  show that the peak intensity and the Q value decrease with increasing height of the periodic structure. If the refractive index n wav  of the photoluminescent layer is higher than the refractive index n p  of the periodic structure ( FIG. 10 ), the light is totally reflected, and only a leaking (that is, evanescent) portion of the electric field of the quasi-guided mode interacts with the periodic structure. If the periodic structure has a sufficiently large height, the influence of the interaction between the evanescent portion of the electric field and the periodic structure remains constant irrespective of the height. In contrast, if the refractive index n wav  of the photoluminescent layer is lower than the refractive index n p  of the periodic structure ( FIG. 11 ), the light reaches the surface of the periodic structure without being totally reflected and is therefore more influenced by a periodic structure with a larger height. As shown in  FIG. 11 , a height of approximately 100 nm is sufficient, and the peak intensity and the Q value decrease above a height of 150 nm. Thus, if the refractive index n wav  of the photoluminescent layer is lower than the refractive index n p  of the periodic structure, the periodic structure may have a height of 150 nm or less to achieve a high peak intensity and Q value. 
     4-3. Polarization Direction 
     The polarization direction was then studied.  FIG. 12  shows the results of calculations performed under the same conditions as in  FIG. 9  except that the polarization of the light was assumed to be the TE mode, which has an electric field component perpendicular to the y direction. The TE mode is more influenced by the periodic structure than the TM mode because the electric field of the quasi-guided mode leaks more largely for the TE mode than for the TM mode. Thus, the peak intensity and the Q value decrease more significantly for the TE mode than for the TM mode if the refractive index n p  of the periodic structure is higher than the refractive index n wav  of the photoluminescent layer. 
     4-4. Refractive Index of Photoluminescent Layer 
     The refractive index of the photoluminescent layer was then studied.  FIG. 13  shows the results of calculations performed under the same conditions as in  FIG. 9  except that the photoluminescent layer was assumed to have a refractive index n wav  of 1.5. The results for the photoluminescent layer having a refractive index n wav  of 1.5 are similar to the results in  FIG. 9 . However, light having a wavelength of 600 nm or more was not output in the front direction. This is because, from the inequality (10), λ 0 &lt;n wav ×p/m=1.5×400 nm/1=600 nm. 
     The above analysis demonstrates that a high peak intensity and Q value can be achieved if the periodic structure has a refractive index lower than or similar to the refractive index of the photoluminescent layer or if the periodic structure has a higher refractive index than the photoluminescent layer and a height of 150 nm or less. 
     5. MODIFIED EXAMPLES 
     Modified Examples of the present embodiment will be described below. 
     5-1. Structure Including Substrate 
     As described above, the light-emitting device may have a structure in which the photoluminescent layer  110  and the periodic structure  120  are formed on the transparent substrate  140 , as illustrated in  FIGS. 1C and 1D . Such a light-emitting device  100   a  may be produced by forming a thin film of the photoluminescent material for the photoluminescent layer  110  (optionally containing a matrix material; the same applies hereinafter) on the transparent substrate  140  and then forming the periodic structure  120  thereon. In this structure, the refractive index n s  of the transparent substrate  140  has to be lower than or equal to the refractive index n wav  of the photoluminescent layer  110  so that the photoluminescent layer  110  and the periodic structure  120  function to output light in a particular direction. If the transparent substrate  140  is provided in contact with the photoluminescent layer  110 , the period p has to be set so as to satisfy the inequality (15), which is given by replacing the refractive index n out  of the output medium in the inequality (10) by n s . 
     To demonstrate this, calculations were performed under the same conditions as in  FIG. 2  except that the photoluminescent layer  110  and the periodic structure  120  were assumed to be located on a transparent substrate  140  having a refractive index of 1.5.  FIG. 14  shows the results of these calculations. As in the results in  FIG. 2 , light intensity peaks are observed at particular periods for each wavelength, although the ranges of periods where peaks appear differ from those in  FIG. 2 .  FIG. 15  is a graph illustrating the condition represented by the inequality (15), which is given by substituting n out =n s  into the inequality (10). In  FIG. 14 , light intensity peaks are observed in the regions corresponding to the ranges shown in  FIG. 15 . 
     Thus, for the light-emitting device  100   a , in which the photoluminescent layer  110  and the periodic structure  120  are located on the transparent substrate  140 , a period p that satisfies the inequality (15) is effective, and a period p that satisfies the inequality (13) is significantly effective. 
     5-2. Light-Emitting Apparatus Including Excitation Light Source 
       FIG. 16  is a schematic view of a light-emitting apparatus  200  including the light-emitting device  100  illustrated in  FIGS. 1A and 1B  and a light source  180  that emits excitation light toward the photoluminescent layer  110 . In this embodiment, as described above, the photoluminescent layer can be excited with excitation light such as ultraviolet light or blue light to output directional light. The light source  180  can be configured to emit such excitation light to provide a directional light-emitting apparatus  200 . Although the wavelength of the excitation light emitted from the light source  180  is typically within the ultraviolet or blue range, it is not necessarily within these ranges, but may be determined depending on the photoluminescent material for the photoluminescent layer  110 . Although the light source  180  illustrated in  FIG. 16  is configured to direct excitation light into the bottom surface of the photoluminescent layer  110 , it may be configured otherwise, for example, to direct excitation light into the top surface of the photoluminescent layer  110 . 
     The excitation light may be coupled into a quasi-guided mode to efficiently output light.  FIGS. 17A to 17D  illustrate this method. In this example, as in the structure illustrated in  FIGS. 1C and 1D , the photoluminescent layer  110  and the periodic structure  120  are formed on the transparent substrate  140 . As illustrated in  FIG. 17A , the period p x  in the x direction is first determined so as to enhance light emission. As illustrated in  FIG. 17B , the period p y  in the y direction is then determined so as to couple the excitation light into a quasi-guided mode. The period p x  is determined so as to satisfy the condition given by replacing p in the inequality (10) by p x . The period p y  is determined so as to satisfy the inequality (16): 
                       m   ⁢           ⁢     λ   ex         n   wav       &lt;     p   y     &lt;       m   ⁢           ⁢     λ   ex         n   out               (   16   )               
wherein m is an integer of 1 or more, λ ex  is the wavelength of the excitation light, and n out  is the refractive index of the medium having the highest refractive index of the media in contact with the photoluminescent layer  110  except the periodic structure  120 .
 
     In the example in  FIGS. 17A to 17D , n out  is the refractive index n s  of the transparent substrate  140 . For a structure including no transparent substrate  140 , as illustrated in  FIG. 16 , n out  denotes the refractive index of air (approximately 1.0). 
     In particular, the excitation light can be more effectively converted into a quasi-guided mode if m=1, that is, if the period p y  is determined so as to satisfy the inequality (17): 
     
       
         
           
             
               
                 
                   
                     
                       λ 
                       ex 
                     
                     
                       n 
                       wav 
                     
                   
                   &lt; 
                   
                     p 
                     y 
                   
                   &lt; 
                   
                     
                       λ 
                       ex 
                     
                     
                       n 
                       out 
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
     Thus, the excitation light can be converted into a quasi-guided mode if the period p y  is set so as to satisfy the condition represented by the inequality (16) (particularly, the condition represented by the inequality (17)). As a result, the photoluminescent layer  110  can efficiently absorb the excitation light of the wavelength λ ex . 
       FIGS. 17C and 17D  are the calculation results of the proportion of absorbed light to light incident on the structures illustrated in  FIGS. 17A and 17B , respectively, for each wavelength. In these calculations, p x =365 nm, p y =265 nm, the photoluminescent layer  110  was assumed to have an emission wavelength λ of about 600 nm, the excitation light was assumed to have a wavelength λ ex  of about 450 nm, and the photoluminescent layer  110  was assumed to have an extinction coefficient of 0.003. As shown in  FIG. 17D , the photoluminescent layer  110  has high absorptivity not only for the light emitted from the photoluminescent layer  110  but also for the excitation light, that is, light having a wavelength of approximately 450 nm. This indicates that the incident light is effectively converted into a quasi-guided mode to increase the proportion of the light absorbed into the photoluminescent layer  110 . The photoluminescent layer  110  also has high absorptivity for the emission wavelength, that is, approximately 600 nm. This indicates that light having a wavelength of approximately 600 nm incident on this structure is similarly effectively converted into a quasi-guided mode. The periodic structure  120  shown in  FIG. 17B  is a two-dimensional periodic structure including structures having different periods (that is, different periodic components) in the x direction and the y direction. Such a two-dimensional periodic structure including periodic components allows for high excitation efficiency and high output intensity. Although the excitation light is incident on the transparent substrate  140  in  FIGS. 17A and 17B , the same effect can be achieved even if the excitation light is incident on the periodic structure  120 . 
     Also available are two-dimensional periodic structures including periodic components as shown in  FIGS. 18A and 18B . The structure illustrated in  FIG. 18A  includes periodically arranged projections or recesses having a hexagonal planar shape. The structure illustrated in  FIG. 18B  includes periodically arranged projections or recesses having a triangular planar shape. These structures have major axes (axes  1  to  3  in the examples in  FIGS. 18A and 18B ) that can be assumed to be periodic. Thus, the structures can have different periods in different axial directions. These periods may be set so as to increase the directionality of light beams of different wavelengths or to efficiently absorb the excitation light. In any case, each period is set so as to satisfy the condition corresponding to the inequality (10). 
     5-3. Periodic Structure on Transparent Substrate 
     As illustrated in  FIGS. 19A and 19B , a periodic structure  120   a  may be formed on the transparent substrate  140 , and the photoluminescent layer  110  may be located thereon. In the example in  FIG. 19A , the photoluminescent layer  110  is formed along the texture of the periodic structure  120   a  on the transparent substrate  140 . As a result, a periodic structure  120   b  with the same period is formed in the surface of the photoluminescent layer  110 . In the example in  FIG. 19B , the surface of the photoluminescent layer  110  is flattened. In these examples, directional light emission can be achieved by setting the period p of the periodic structure  120   a  so as to satisfy the inequality (15). 
     To verify the effect of these structures, the enhancement of light output from the structure in  FIG. 19A  in the front direction was calculated with varying emission wavelengths and varying periods of the periodic structure. In these calculations, the photoluminescent layer  110  was assumed to have a thickness of 1,000 nm and a refractive index n wav  of 1.8, the periodic structure  120   a  was assumed to be a one-dimensional periodic structure uniform in the y direction having a height of 50 nm, a refractive index n p  of 1.5, and a period of 400 nm, and the polarization of the light was assumed to be the TM mode, which has an electric field component parallel to the y direction.  FIG. 19C  shows the results of these calculations. In these calculations, light intensity peaks were observed at the periods that satisfy the condition represented by the inequality (15). 
     5-4. Powder 
     According to the above embodiment, light of any wavelength can be enhanced by adjusting the period of the periodic structure and the thickness of the photoluminescent layer. For example, if the structure illustrated in  FIGS. 1A and 1B  is formed using a photoluminescent material that emits light over a wide wavelength range, only light of a certain wavelength can be enhanced. Accordingly, the structure of the light-emitting device  100  as illustrated in  FIGS. 1A and 1B  may be provided in powder form for use as a fluorescent material. Alternatively, the light-emitting device  100  as illustrated in  FIGS. 1A and 1B  may be embedded in resin or glass. 
     The single structure as illustrated in  FIGS. 1A and 1B  can output only light of a certain wavelength in a particular direction and is therefore not suitable for outputting, for example, white light, which has a wide wavelength spectrum. Accordingly, as shown in  FIG. 20 , light-emitting devices  100  that differ in the conditions such as the period of the periodic structure and the thickness of the photoluminescent layer may be mixed in powder form to provide a light-emitting apparatus with a wide wavelength spectrum. In such a case, the individual light-emitting devices  100  have sizes of, for example, several micrometers to several millimeters in one direction and can include, for example, one- or two-dimensional periodic structures with several periods to several hundreds of periods. 
     5-5. Array of Structures with Different Periods 
       FIG. 21  is a plan view of a two-dimensional array of periodic structures having different periods on the photoluminescent layer. In this example, three types of periodic structures  120   a ,  120   b , and  120   c  are arranged without any space therebetween. The periods of the periodic structures  120   a ,  120   b , and  120   c  are set so as to output, for example, light in the red, green, and blue wavelength ranges, respectively, in the front direction. Thus, structures having different periods can be arranged on the photoluminescent layer to output directional light with a wide wavelength spectrum. The periodic structures are not necessarily configured as described above, but may be configured in any manner. 
     5-6. Layered Structure 
       FIG. 22  illustrates a light-emitting device including photoluminescent layers  110  each having a textured surface. A transparent substrate  140  is located between the photoluminescent layers  110 . The texture on each of the photoluminescent layers  110  corresponds to the periodic structure or the submicron structure. The example in  FIG. 22  includes three periodic structures having different periods. The periods of these periodic structures are set so as to output light in the red, green, and blue wavelength ranges in the front direction. The photoluminescent layer  110  in each layer is made of a material that emits light of the color corresponding to the period of the periodic structure in that layer. Thus, periodic structures having different periods can be stacked on top of each other to output directional light with a wide wavelength spectrum. 
     The number of layers and the constructions of the photoluminescent layer  110  and the periodic structure in each layer are not limited to those described above, but may be selected as appropriate. For example, for a structure including two layers, first and second photoluminescent layers are formed opposite each other with a light-transmissive substrate therebetween, and first and second periodic structures are formed on the surfaces of the first and second photoluminescent layers, respectively. In such a case, the first photoluminescent layer and the first periodic structure may together satisfy the condition corresponding to the inequality (15), whereas the second photoluminescent layer and the second periodic structure may together satisfy the condition corresponding to the inequality (15). For a structure including three or more layers, the photoluminescent layer and the periodic structure in each layer may satisfy the condition corresponding to the inequality (15). The positional relationship between the photoluminescent layers and the periodic structures in  FIG. 22  may be reversed. Although the layers illustrated by the example in  FIG. 22  have different periods, they may all have the same period. In such a case, although the spectrum cannot be broadened, the emission intensity can be increased. 
     5-7. Structure Including Protective Layer 
       FIG. 23  is a cross-sectional view of a structure including a protective layer  150  between the photoluminescent layer  110  and the periodic structure  120 . The protective layer  150  may be provided to protect the photoluminescent layer  110 . However, if the protective layer  150  has a lower refractive index than the photoluminescent layer  110 , the electric field of the light leaks into the protective layer  150  only by about half the wavelength. Thus, if the protective layer  150  is thicker than the wavelength, no light reaches the periodic structure  120 . As a result, there is no quasi-guided mode, and the function of outputting light in a particular direction cannot be achieved. If the protective layer  150  has a refractive index higher than or similar to that of the photoluminescent layer  110 , the light reaches the interior of the protective layer  150 ; therefore, there is no limitation on the thickness of the protective layer  150 . Nevertheless, a thinner protective layer  150  is desirable because more light is output if most of the portion in which light is guided (this portion is hereinafter referred to as “waveguide layer”) is made of a photoluminescent material. The protective layer  150  may be made of the same material as the periodic structure (light-transmissive layer)  120 . In such a case, the light-transmissive layer  120  having the periodic structure functions as a protective layer. The light-transmissive layer  120  desirably has a lower refractive index than the photoluminescent layer  110 . 
     6. MATERIALS AND PRODUCTION METHODS 
     Directional light emission can be achieved if the photoluminescent layer (or waveguide layer) and the periodic structure are made of materials that satisfy the above conditions. The periodic structure may be made of any material. However, a photoluminescent layer (or waveguide layer) or a periodic structure made of a medium with high light absorption is less effective in confining light and therefore results in a lower peak intensity and Q value. Thus, the photoluminescent layer (or waveguide layer) and the periodic structure may be made of media with relatively low light absorption. 
     For example, the periodic structure may be formed of a dielectric material having low light absorptivity. Examples of candidate materials for the periodic structure include magnesium fluoride (MgF 2 ), lithium fluoride (LiF), calcium fluoride (CaF 2 ), quartz (SiO 2 ), glasses, resins, magnesium oxide (MgO), indium tin oxide (ITO), titanium oxide (TiO 2 ), silicon nitride (SiN), tantalum pentoxide (Ta 2 O 5 ), zirconia (ZrO 2 ), zinc selenide (ZnSe), and zinc sulfide (ZnS). To form a periodic structure having a lower refractive index than the photoluminescent layer, as described above, MgF 2 , LiF, CaF 2 , SiO 2 , glasses, and resins can be used, which have refractive indices of approximately 1.3 to 1.5. 
     The term “photoluminescent material” encompasses fluorescent materials and phosphorescent materials in a narrow sense, encompasses inorganic materials and organic materials (for example, dyes), and encompasses quantum dots (that is, tiny semiconductor particles). In general, a fluorescent material containing an inorganic host material tends to have a higher refractive index. Examples of fluorescent materials that emit blue light include M 10 (PO 4 ) 6 Cl 2 :Eu 2+  (wherein M is at least one element selected from Ba, Sr, and Ca), BaMgAl 10 O 17 :Eu 2+ , M 3 MgSi 2 O 8 :Eu 2+  (wherein M is at least one element selected from Ba, Sr, and Ca), and M 5 SiO 4 Cl 6 :Eu 2+  (wherein M is at least one element selected from Ba, Sr, and Ca). Examples of fluorescent materials that emit green light include M 2 MgSi 2 O 7 :Eu 2+  (wherein M is at least one element selected from Ba, Sr, and Ca), SrSi 5 AlO 2 N 7 :Eu 2+ , SrSi 2 O 2 N 2 :Eu 2+ , BaAl 2 O 4 :Eu 2+ , BaZrSi 3 O 9 :Eu 2+ , M 2 SiO 4 :Eu 2+  (wherein M is at least one element selected from Ba, Sr, and Ca), BaSi 3 O 4 N 2 :Eu 2+ , Ca 8 Mg(SiO 4 ) 4 Cl 2 :Eu 2+ , Ca 3 SiO 4 Cl 2 :Eu 2+ , CaSi 12-(m+n) Al (m+n) O n N 16-n :Ce 3+ , and β-SiAlON:Eu 2+ . Examples of fluorescent materials that emit red light include CaAlSiN 3 :Eu 2+ , SrAlSi 4 O 7 :Eu 2+ , M 2 Si 5 N 8 :Eu 2+  (wherein M is at least one element selected from Ba, Sr, and Ca), MSiN 2 :Eu 2+  (wherein M is at least one element selected from Ba, Sr, and Ca), MSi 2 O 2 N 2 :Yb 2+  (wherein M is at least one element selected from Sr and Ca), Y 2 O 2 S:Eu 3+ , Sm 3+ , La 2 O 2 S:Eu 3+ , Sm 3+ , CaWO 4 :Li 1+ , Eu 3+ , Sm 3+ , M 2 SiS 4 :Eu 2+  (wherein M is at least one element selected from Ba, Sr, and Ca), and M 3 SiO 5 :Eu 2+  (wherein M is at least one element selected from Ba, Sr, and Ca). Examples of fluorescent materials that emit yellow light include Y 3 Al 5 O 12 :Ce 3+ , CaSi 2 O 2 N 2 :Eu 2+ , Ca 3 Sc 2 Si 3 O 12 :Ce 3+ , CaSc 2 O 4 :Ce 3+ , α-SiAlON:Eu 2+ , MSi 2 O 2 N 2 :Eu 2+  (wherein M is at least one element selected from Ba, Sr, and Ca), and M 7 (SiO 3 ) 6 Cl 2 :Eu 2+  (wherein M is at least one element selected from Ba, Sr, and Ca). 
     Examples of quantum dots include materials such as CdS, CdSe, core-shell CdSe/ZnS, and alloy CdSSe/ZnS. Light of various wavelengths can be emitted depending on the material. Examples of matrices for quantum dots include glasses and resins. 
     The transparent substrate  140 , as shown in, for example,  FIGS. 1C and 1D , is made of a light-transmissive material having a lower refractive index than the photoluminescent layer  110 . Examples of such materials include magnesium fluoride (MgF 2 ), lithium fluoride (LiF), calcium fluoride (CaF 2 ), quartz (SiO 2 ), glasses, and resins. 
     Exemplary production methods will be described below. 
     A method for forming the structure illustrated in  FIGS. 1C and 1D  includes forming a thin film of the photoluminescent layer  110  on the transparent substrate  140 , for example, by evaporation, sputtering, or coating of a fluorescent material, forming a dielectric film, and then patterning the dielectric film, for example, by photolithography to form the periodic structure  120 . Alternatively, the periodic structure  120  may be formed by nanoimprinting. As shown in  FIG. 24 , the periodic structure  120  may also be formed by partially processing the photoluminescent layer  110 . In such a case, the periodic structure  120  is made of the same material as the photoluminescent layer  110 . 
     The light-emitting device  100  illustrated in  FIGS. 1A and 1B  can be manufactured, for example, by fabricating the light-emitting device  100   a  illustrated in  FIGS. 1C and 1D  and then stripping the photoluminescent layer  110  and the periodic structure  120  from the substrate  140 . 
     The structure shown in  FIG. 19A  can be manufactured, for example, by forming the periodic structure  120   a  on the transparent substrate  140  by a process such as a semiconductor manufacturing processes or nanoimprinting and then depositing thereon the material for the photoluminescent layer  110  by a process such as evaporation or sputtering. The structure shown in  FIG. 19B  can be manufactured by filling the recesses in the periodic structure  120   a  with the photoluminescent layer  110  by a process such as coating. 
     The above methods of manufacture are for illustrative purposes only, and the light-emitting devices according to the embodiments of the present disclosure may be manufactured by other methods. 
     EXPERIMENTAL EXAMPLES 
     Light-emitting devices according to embodiments of the present disclosure are illustrated by the following examples. 
     A sample light-emitting device having the structure as illustrated in  FIG. 19A  was prepared and evaluated for its properties. The light-emitting device was prepared as described below. 
     A one-dimensional periodic structure (stripe-shaped projections) having a period of 400 nm and a height of 40 nm was formed on a glass substrate, and a photoluminescent material, that is, YAG:Ce, was deposited thereon to a thickness of 210 nm.  FIG. 25  shows a cross-sectional transmission electron microscopy (TEM) image of the resulting light-emitting device.  FIG. 26  shows the results of measurements of the spectrum of light emitted from the light-emitting device in the front direction when YAG:Ce was excited with an LED having an emission wavelength of 450 nm.  FIG. 26  shows the results (ref) for a light-emitting device including no periodic structure, the results for the TM mode, and the results for the TE mode. The TM mode has a polarization component parallel to the one-dimensional periodic structure. The TE mode has a polarization component perpendicular to the one-dimensional periodic structure. The results show that the intensity of light of a particular wavelength in the case with the periodic structure is significantly higher than without a periodic structure. The results also show that the light enhancement effect is greater for the TM mode, which has a polarization component parallel to the one-dimensional periodic structure. 
       FIGS. 27A to 27F  and  FIGS. 28A to 28F  show the results of measurements and calculations of the angular dependence of the intensity of light output from the same sample.  FIGS. 27B and 27E  show the results of measurements and  FIGS. 27C and 27F  show the results of calculations for rotation about an axis parallel to the line direction of the one-dimensional periodic structure (that is, the periodic structure  120 ).  FIGS. 28B and 28E  show the results of measurements and  FIGS. 28C and 28F  show the results of calculations for rotation about an axis perpendicular to the line direction of the one-dimensional periodic structure (that is, the periodic structure  120 ).  FIGS. 27A to 27F  and  FIGS. 28A to 28F  show the results for linearly polarized light in the TM mode and the TE mode.  FIG. 27A  shows the results for linearly polarized light in the TM mode.  FIGS. 27D to 27F  show the results for linearly polarized light in the TE mode.  FIGS. 28A to 28C  show the results for linearly polarized light in the TE mode.  FIGS. 28D to 28F  show the results for linearly polarized light in the TM mode. As can be seen from  FIGS. 27A to 27F  and  FIGS. 28A to 28F , the enhancement effect is greater for the TM mode, and the enhanced wavelength shifts with angle. For example, light having a wavelength of 610 nm is observed only in the TM mode and in the front direction, indicating that the light is directional and polarized. In addition, the top and bottom parts of each figure match each other. Thus, the validity of the above calculations was experimentally demonstrated. 
     Among the above results of measurements, for example,  FIG. 29  shows the angular dependence of the intensity of light having a wavelength of 610 nm for rotation about an axis perpendicular to the line direction. As shown in  FIG. 29 , the light was significantly enhanced in the front direction and was little enhanced at other angles. The directional angle of the light output in the front direction is less than 15 degrees. The directional angle is the angle at which the intensity is 50% of the maximum intensity and is expressed as the angle of one side with respect to the direction with the maximum intensity. This demonstrates that directional light emission was achieved. In addition, all the light was in the TM mode, which demonstrates that polarized light emission was simultaneously achieved. 
     Although YAG:Ce, which emits light in a wide wavelength range, was used in the above experiment, directional and polarized light emission can also be achieved using a similar structure including a photoluminescent material that emits light in a narrow wavelength range. Such a photoluminescent material does not emit light of other wavelengths and can therefore be used to provide a light source that does not emit light in other directions or in other polarized states. 
     7. LIGHT-EMITTING APPARATUS INCLUDING LIGHT-EMITTING DEVICE AND EXCITATION LIGHT SOURCE INTEGRALLY FORMED 
     A light-emitting apparatus according to an embodiment of the present disclosure includes a light-emitting device and an excitation light source. The light-emitting device and the excitation light source are integrally formed. Like components having substantially the same function are denoted by like reference numerals throughout the drawings and may not be further described. 
     A light-emitting apparatus according to an embodiment will be described below with reference to  FIG. 31A .  FIG. 31A  is a schematic cross-sectional view of a light-emitting apparatus  300 . 
     The light-emitting apparatus  300  includes a light-emitting device  100  and an excitation light source  180 . The light-emitting device  100  includes a photoluminescent layer  110 , a light-transmissive layer  120 , and a submicron structure. The light-transmissive layer  120  is located on or near the photoluminescent layer  110 . The submicron structure is formed on at least one of the photoluminescent layer  110  and the light-transmissive layer  120  and extends in a plane of the photoluminescent layer  110  or the light-transmissive layer  120 . The submicron structure has projections or recesses. The distance between adjacent projections or recesses is denoted by D int . Light emitted from the photoluminescent layer  110  includes first light having a wavelength λ a  in air. The photoluminescent layer  110  has a refractive index n wav-a  for the first light. These satisfy λ a /n wav-a &lt;D int &lt;λ a . The excitation light source  180  emits excitation light to be directed into the photoluminescent layer  110 . The light-emitting device  100  and the excitation light source  180  are integrally formed. For the sake of simplicity, the excitation light source  180  is denoted by the same reference numeral as the light source  180 . 
     The light-emitting apparatus  300 , which includes the light-emitting device  100  and the excitation light source  180  integrally formed, can realize a compact light-emitting apparatus that includes a light-emitting device having high directionality and luminous efficiency and an excitation light source. 
     As illustrated in  FIG. 31A , the excitation light source  180  may be located on the bottom surface of the light-emitting device  100 . The light-emitting device  100  may be supported by the excitation light source  180 . 
     Excitation light from the excitation light source  180  typically has a wavelength in an ultraviolet or blue region. Excitation light from the excitation light source  180  may have another wavelength depending on the photoluminescent material of the photoluminescent layer  110 . The excitation light source  180  may be a light-emitting diode (LED). The LED may be of a bottom emission type or a top emission type. 
     The structure of LEDs will be described below with reference to  FIGS. 32A and 32B .  FIG. 32A  is a schematic cross-sectional view of a bottom emission LED.  FIG. 32B  is a schematic cross-sectional view of a top emission LED. The structure of LEDs is not limited to those illustrated in the figures. The excitation light source  180  may be a known LED. The structure and principle of LEDs are well known and are not described in detail here. 
     As illustrated in  FIG. 32A , the bottom emission LED may include a first electrode  181 , a second electrode  182 , a semiconductor multilayer structure  183  located between these electrodes, and a substrate  184 , which supports these components. If the substrate  184  is formed of a semiconductor, the second electrode  182  may be located on the bottom surface of the substrate  184 . The first electrode  181  may reflect light emitted from the semiconductor multilayer structure  183 . The first electrode  181  may be formed of a metal having high reflectance. The second electrode  182  and the substrate  184  may transmit light emitted from the semiconductor multilayer structure  183 . The second electrode  182  may be formed of a semiconductor doped with an impurity. Light from the semiconductor multilayer structure  183  is emitted from the bottom surface (of the substrate  184 ) of the LED. 
     In the case that the excitation light source  180  of the light-emitting apparatus  300  is a bottom emission LED, the light-emitting device  100  may be located on the substrate  184 . Thus, the light output side of the excitation light source  180  may be in contact with the light-emitting device  100 . The light output side of the excitation light source  180  may be in direct contact with the photoluminescent layer  110  or the light-transmissive layer  120 . The light output side of excitation light sources refers to a side from which excitation light is mainly emitted. The light output side is typically perpendicular to the output direction of excitation light. For example, the light-emitting apparatus  300  that includes the light-emitting device  100  supported by the excitation light source  180  can be produced by depositing a photoluminescent material on a substrate of a bottom emission LED. 
     The direct contact between the light output side of the excitation light source  180  and the photoluminescent layer  110  or the light-transmissive layer  120  can decrease excitation light loss and allows excitation light to be efficiently directed to the photoluminescent layer  110 . The excitation light loss refers to light that is not directed to the photoluminescent layer  110 , for example, light leaking from the light-emitting apparatus. 
     In the top emission LED illustrated in  FIG. 32B , light from the semiconductor multilayer structure  183  is emitted from the top surface of the LED. The top emission LED differs from the bottom emission LED in the following points. The points of difference are that the first electrode  181  transmits light emitted from the semiconductor multilayer structure  183  and that a reflective film  189  that reflects light emitted from the semiconductor multilayer structure  183  is located on the underside of the semiconductor multilayer structure  183  (opposite the first electrode  181 ). As illustrated in the figure, the reflective film  189  may be located on the underside of the substrate  184  (opposite the semiconductor multilayer structure  183 ). The reflective film  189  may be located between the second electrode  182  and the substrate  184 . If the second electrode  182  reflects light emitted from the semiconductor multilayer structure  183 , the reflective film  189  may be omitted. In the case that the excitation light source  180  is a top emission LED, the light-emitting device  100  may be located on the first electrode  181 . The light output side of the excitation light source  180  may be in contact with the light-emitting device  100 . The light output side of the excitation light source  180  may be in direct contact with the photoluminescent layer  110  or the light-transmissive layer  120 . 
     In the light-emitting apparatus  300 , the refractive index of a layer of the excitation light source  180  in contact with the light-emitting device  100  may be lower than the refractive index of the photoluminescent layer  110 . This relationship of the refractive indices holds for first light having a wavelength λ a  in air out of light emitted from the photoluminescent layer  110 . Thus, the excitation light source  180  probably has a small effect on the quasi-guided mode in the light-emitting device  100 . The light-emitting apparatus  300  can realize a compact light-emitting apparatus that includes a light-emitting device and an excitation light source without reducing the directionality and luminous efficiency of the light-emitting device  100 . In the case that the excitation light source  180  is a bottom emission LED, a layer of the excitation light source  180  in contact with the light-emitting device  100  may be the substrate  184 . The substrate  184  may be formed of sapphire (refractive index: 1.76) or gallium nitride (GaN) (refractive index: 2.4). In the case that the excitation light source  180  is a top emission LED, a layer of the excitation light source  180  in contact with the light-emitting device  100  may be the first electrode  181 . The first electrode  181  that transmits light emitted from the semiconductor multilayer structure  183  may be formed of ITO. 
     The light-emitting apparatus may have another structure. The refractive index of a layer of the excitation light source  180  in contact with the light-emitting device  100  may be higher than the refractive index of the photoluminescent layer  110 . For example, as described later with reference to  FIGS. 33A to 33C , the light-emitting device may include a low-refractive-index layer between the photoluminescent layer and the excitation light source and between the light-transmissive layer and the excitation light source. 
     A light-emitting apparatus  301  and a light-emitting apparatus  302  according to other embodiments will be described below with reference to  FIGS. 31B and 31C .  FIG. 31B  is a schematic cross-sectional view of the light-emitting apparatus  301 .  FIG. 31C  is a schematic cross-sectional view of the light-emitting apparatus  302 . The light-emitting apparatus  301  and the light-emitting apparatus  302  are different from the light-emitting apparatus  300  in the structure of the light-emitting device  100 . The light-emitting apparatus  301  and the light-emitting apparatus  302  may be the same as the light-emitting apparatus  300  except the structure of the light-emitting device  100 . 
     In the light-emitting apparatus  300  illustrated in  FIG. 31A , the light-transmissive layer  120  is supported by the photoluminescent layer  110 . The light-emitting device  100  may have another structure. For example, as in the light-emitting device  100  of the light-emitting apparatus  301  illustrated in  FIG. 31B , the photoluminescent layer  110  may be located on the light-transmissive layer  120 . For example, as in the light-emitting device  100  of the light-emitting apparatus  302  illustrated in  FIG. 31C , the light-transmissive layer  120  may be located on the top and bottom surfaces of the photoluminescent layer  110 . Not limited to the embodiments illustrated in the figures, the light-emitting device  100  may be any of the light-emitting devices according to the embodiments of the present disclosure. The light-emitting device  100  may be a combination of two or more of the light-emitting devices according to the embodiments of the present disclosure. 
     The excitation light source  180  in the light-emitting apparatus  301  and the light-emitting apparatus  302  may be a bottom emission LED. In the light-emitting apparatus  301  and the light-emitting apparatus  302 , the refractive index of a layer of the excitation light source  180  in contact with the light-emitting device  100  may be lower than the refractive indices of the photoluminescent layer  110  and the light-transmissive layer  120 . 
     In the production of the light-emitting apparatus  301  and the light-emitting apparatus  302 , for example, a bottom emission LED is prepared, and a predetermined texture (pattern) depending on the submicron structure is formed on the substrate of the LED. The texture may be formed by a known semiconductor manufacturing processes. A material for the light-transmissive layer  120  and a material for the photoluminescent layer  110  are then successively deposited on the substrate of the LED to produce the light-emitting apparatus  301  and the light-emitting apparatus  302 . 
     The light-emitting device  100  of the light-emitting apparatus  302  have a plurality of submicron structures. The submicron structures may include periodic structures formed of projections or recesses. The periodic structures may include a periodic structure having a period p ex  that satisfies λ ex /n wav-ex &lt;p ex &lt;λ ex . λ ex  denotes the wavelength of excitation light in air emitted from the excitation light source  180 . n wav-ex  denotes the refractive index of the photoluminescent layer  110  for excitation light. 
     A light-emitting apparatus according to still another embodiment will be described below with reference to  FIG. 33A .  FIG. 33A  is a schematic cross-sectional view of a light-emitting apparatus  310 . 
     The light-emitting device  100  of the light-emitting apparatus  310  is different from the light-emitting device  100  of the light-emitting apparatus  300  in that the light-emitting device  100  of the light-emitting apparatus  310  further includes a low-refractive-index layer  130  between the photoluminescent layer  110  and the excitation light source  180  and between the light-transmissive layer  120  and the excitation light source  180 . The low-refractive-index layer  130  has a lower refractive index than the photoluminescent layer  110 . The light-emitting apparatus  310  may be the same as the light-emitting apparatus  300  except the low-refractive-index layer  130 . 
     The light-emitting apparatus  310 , which includes the light-emitting device  100  and the excitation light source  180  integrally formed, can realize a compact light-emitting apparatus that includes a light-emitting device having high directionality and luminous efficiency and an excitation light source. 
     The low-refractive-index layer  130  has a lower refractive index for the first light than the photoluminescent layer  110 . Thus, the excitation light source  180  probably has a small effect on the quasi-guided mode in the light-emitting device  100 . The light-emitting apparatus  310  can realize a compact light-emitting apparatus that includes a light-emitting device and an excitation light source without reducing the directionality and luminous efficiency of the light-emitting device  100 . In the light-emitting apparatus  310  including the low-refractive-index layer  130 , the refractive index of a layer of the excitation light source  180  in contact with the light-emitting device  100  may be higher than the refractive index of the photoluminescent layer  110 . As a matter of course, the refractive index of a layer of the excitation light source  180  in contact with the light-emitting device  100  may be lower than the refractive index of the photoluminescent layer  110 . In the light-emitting apparatus  310  including the low-refractive-index layer  130 , the material of a layer of the excitation light source  180  in contact with the light-emitting device  100  can be selected from a larger number of materials. The low-refractive-index layer  130  may be formed of silicon dioxide (SiO 2 ), MgF 2 , CaF 2 , BaF 2 , SrF 2 , or room-temperature curing glass (HSQ·SOG). It is desirable that the low-refractive-index layer  130  have a thickness greater than the wavelength λ a  of the first light in air, for example. The thickness of the low-refractive-index layer  130  is a thickness in the direction normal to the photoluminescent layer  110 . 
     In the light-emitting apparatus  310 , the light output side of the excitation light source  180  may be in direct contact with the low-refractive-index layer  130 . If the light output side of the excitation light source  180  is in direct contact with the low-refractive-index layer  130 , excitation light can efficiently reach the low-refractive-index layer  130 . This can improve the introduction efficiency of excitation light into the photoluminescent layer  110 . 
     The method for producing the light-emitting apparatus  310  may be the same as the method for producing the light-emitting apparatus  300  except the formation of the low-refractive-index layer  130 . The low-refractive-index layer  130  is formed by depositing a material for the low-refractive-index layer  130  on the excitation light source  180 . 
     A light-emitting apparatus  311  and a light-emitting apparatus  312  according to still other embodiments will be described below with reference to  FIGS. 33B and 33C .  FIG. 33B  is a schematic cross-sectional view of the light-emitting apparatus  311 .  FIG. 33C  is a schematic cross-sectional view of the light-emitting apparatus  312 . The light-emitting apparatus  311  and the light-emitting apparatus  312  are different from the light-emitting apparatus  310  in the structure of the light-emitting device  100 . The light-emitting apparatus  311  and the light-emitting apparatus  312  may be the same as the light-emitting apparatus  310  except the structure of the light-emitting device  100 . The light-emitting apparatus  311  is different from the light-emitting apparatus  301  in that the light-emitting apparatus  311  further includes the low-refractive-index layer  130 . The light-emitting apparatus  311  may be the same as the light-emitting apparatus  301  except the low-refractive-index layer  130 . The light-emitting apparatus  312  is different from the light-emitting apparatus  302  in that the light-emitting apparatus  312  further includes the low-refractive-index layer  130 . The light-emitting apparatus  312  may be the same as the light-emitting apparatus  302  except the low-refractive-index layer  130 . 
     In the light-emitting apparatus  311  and the light-emitting apparatus  312 , the low-refractive-index layer  130  has a greater thickness than the light-transmissive layer  120 . It is desirable that the low-refractive-index layer  130  have a thickness greater than the sum of the thickness of the light-transmissive layer  120  and the wavelength λ a  of the first light in air, for example. This is because if the distance between the light-transmissive layer  120  and the excitation light source  180  is shorter than the wavelength λ a  of the first light in air, evanescent light between the light-emitting device  100  and the excitation light source  180  may leak into the excitation light source  180 . The thicknesses of the low-refractive-index layer  130  and the light-transmissive layer  120  are thicknesses in the direction normal to the photoluminescent layer  110 . In the embodiments illustrated in the figures, the thickness of the low-refractive-index layer  130  refers to the length from the interface between the low-refractive-index layer  130  and the excitation light source  180  to the interface between the low-refractive-index layer  130  and the photoluminescent layer  110  in the direction normal to the photoluminescent layer  110 . 
     In the production process of the light-emitting apparatus  311  and the light-emitting apparatus  312 , a material for the low-refractive-index layer  130  is deposited on the excitation light source  180  to form a film, and the low-refractive-index layer  130  is then patterned. The patterning may be performed by a known semiconductor manufacturing processes. The patterning forms the light-transmissive layer  120  (and the submicron structure) under the photoluminescent layer  110 . 
     The excitation light source  180  in the light-emitting apparatus  311  and the light-emitting apparatus  312  may be a bottom emission LED. The excitation light source  180  in the light-emitting apparatus  311  and the light-emitting apparatus  312  may be a top emission LED. 
     A light-emitting apparatus  313  according to still another embodiment will be described below with reference to  FIG. 34A .  FIG. 34A  is a schematic cross-sectional view of the light-emitting apparatus  313 . 
     The low-refractive-index layer  130  of the light-emitting apparatus  313  has second projections  131   a  adjacent to the excitation light source  180  and is different from the low-refractive-index layer  130  of the light-emitting apparatus  310  in this point. The distance between adjacent second projections  131   a  may be shorter or longer than D int . The light-emitting apparatus  313  may be the same as the light-emitting apparatus  310  except the second projections  131   a.    
     The light-emitting apparatus  313 , which includes the light-emitting device  100  and the excitation light source  180  integrally formed, can realize a compact light-emitting apparatus that includes a light-emitting device having high directionality and luminous efficiency and an excitation light source. 
     The second projections  131   a  can reduce the reflection of excitation light at the interface between the excitation light source  180  and the low-refractive-index layer  130 . In the light-emitting apparatus  313  including the second projections  131   a , excitation light from the excitation light source  180  is efficiently directed to the photoluminescent layer  110  through the low-refractive-index layer  130 . The reflection of excitation light at the interface includes reflection of excitation light at the interface and total reflection of excitation light at the interface. The second projections  131   a  reduce reflection loss and/or total reflection loss and allow excitation light to be efficiently directed to the photoluminescent layer  110 . 
     If the distance between adjacent second projections  131   a  is shorter than D int , the second projections  131   a  may constitute a moth-eye structure. A layer of the excitation light source  180  in contact with the light-emitting device  100  may have a refractive index higher than the refractive index of the low-refractive-index layer  130 . The second projections  131   a  may be formed of the material of a layer of the excitation light source  180  in contact with the light-emitting device  100 . The second projections  131   a  continuously change the effective refractive index for excitation light emitted from the excitation light source  180  in the direction normal to the photoluminescent layer  110  from the refractive index of the excitation light source  180  to the refractive index of the low-refractive-index layer  130 . This allows excitation light emitted from the excitation light source  180  to be efficiently directed to the photoluminescent layer  110 . The light-emitting device of the light-emitting apparatus  313  has further improved directionality and luminous efficiency. 
     If the second projections  131   a  are formed of a material different from the material of a layer of the excitation light source  180  in contact with the light-emitting device  100 , the second projections  131   a  may be formed of a material having a higher refractive index than the low-refractive-index layer  130 . 
     The second projections  131   a  may be generally conical or generally pyramidal. The second projections  131   a  may be a cone or pyramid with a rounded top (apex). The second projections  131   a  may be truncated conical or truncated pyramidal. The second projections  131   a  may be generally hemispherical. The second projections  131   a  may be generally cylindrical or generally prismatic. The second projections  131   a  may be arranged at regular or irregular intervals. 
     The second projections  131   a  are not limited to the moth-eye structure. The distance between adjacent second projections  131   a  may be longer than D int . The second projections  131   a  may have a periodic texture. The second projections  131   a  may constitute a diffraction grating. The second projections  131   a  may be arranged at irregular intervals. The shape, height, size, and period of the second projections  131   a  may be appropriately designed to cause diffraction and/or scattering of excitation light. The second projections  131   a  can effectively reduce total reflection at the interface between the excitation light source  180  and the low-refractive-index layer  130 . The second projections  131   a  can cause diffraction and/or scattering of excitation light and allows excitation light emitted from the excitation light source  180  to be efficiently directed to the photoluminescent layer  110  through the low-refractive-index layer  130 . 
     The low-refractive-index layer  130  has a thickness greater than the height of the second projections  131   a . The second projections  131   a  may have a height in the range of 50 nm to 50 μm. It is desirable that the low-refractive-index layer  130  have a thickness greater than the sum of the height of the second projections  131   a  and the wavelength λ a  of the first light in air, for example. The thickness of the low-refractive-index layer  130  and the height of the second projections  131   a  are lengths in the direction normal to the photoluminescent layer  110 . 
     The second projections  131   a  may be arranged at regular intervals. The second projections  131   a  may have a period (pitch) in the range of 200 nm to 50 μm. The period of the second projections  131   a  refers to the distance between adjacent second projections  131   a  in a plane parallel to the photoluminescent layer  110 . The size of the second projections  131   a  may be the same as the period of the second projections  131   a . The size of the second projections  131   a  may be smaller than the period of the second projections  131   a . The size of the second projections  131   a  is the size of the second projections  131   a  in a plane parallel to the photoluminescent layer  110  (for example, the diameter of the second projections  131   a  having a generally circular bottom, or the length of a side of the second projections  131   a  having a rectangular bottom). 
     The light-emitting apparatus  313  can be produced as described below. First, the second projections  131   a  are formed on the excitation light source  180 , for example, by a semiconductor manufacturing processes or a transfer process (such as nanoimprinting). After the second projections  131   a  are formed, a material for the low-refractive-index layer  130 , a material for the photoluminescent layer  110 , and a material for the light-transmissive layer  120  are successively deposited on the excitation light source  180 . The material for the light-transmissive layer  120  is then patterned to form the submicron structure. 
     A light-emitting apparatus  314  and a light-emitting apparatus  315  according to still other embodiments will be described below with reference to  FIGS. 34B and 34C .  FIG. 34B  is a schematic cross-sectional view of the light-emitting apparatus  314 .  FIG. 34C  is a schematic cross-sectional view of the light-emitting apparatus  315 . The light-emitting apparatus  314  and the light-emitting apparatus  315  are different from the light-emitting apparatus  313  in the structure of the light-emitting device  100 . The light-emitting apparatus  314  and the light-emitting apparatus  315  may be the same as the light-emitting apparatus  313  except the structure of the light-emitting device  100 . The light-emitting apparatus  314  is different from the light-emitting apparatus  311  in that the light-emitting apparatus  314  further includes the second projections  131   a . The light-emitting apparatus  314  may be the same as the light-emitting apparatus  311  except the second projections  131   a . The light-emitting apparatus  315  is different from the light-emitting apparatus  312  in that the light-emitting apparatus  315  further includes the second projections  131   a . The light-emitting apparatus  315  may be the same as the light-emitting apparatus  312  except the second projections  131   a.    
     In the light-emitting apparatus  314  and the light-emitting apparatus  315 , the low-refractive-index layer  130  has a thickness greater than the sum of the height of the second projections  131   a  and the thickness of the light-transmissive layer  120 . The second projections  131   a  may have a height in the range of 50 nm to 50 μm. It is desirable that the low-refractive-index layer  130  have a thickness greater than the sum of the height of the second projections  131   a  and the wavelength λ a  of the first light in air, for example. The thickness of the low-refractive-index layer  130 , the thickness of the light-transmissive layer  120 , and the height of the second projections  131   a  are lengths in the direction normal to the photoluminescent layer  110 . In the embodiments illustrated in the figures, the thickness of the low-refractive-index layer  130  refers to the length from the bottom of the second projections  131   a  to the interface between the low-refractive-index layer  130  and the photoluminescent layer  110  in the direction normal to the photoluminescent layer  110 . 
       FIG. 35  is a schematic cross-sectional view of a light-emitting apparatus  316  according to still another embodiment. As illustrated in  FIG. 35 , the excitation light source  180  may have a reflective film  188  on its side surfaces. Leakage of excitation light from a side surface of the excitation light source  180  may cause excitation light loss (light not directed to the photoluminescent layer  110 ). The reflective film  188  on the side surfaces can efficiently direct excitation light to the photoluminescent layer  110 . The side surfaces of the excitation light source  180  may be surfaces perpendicular to the light output side. The reflective film  188  may be formed of a dielectric multilayer film or a metal. The reflective film  188  may also be formed of TiO 2  or BaSO 4 . 
     Although the excitation light source  180  is an LED in these embodiments, the excitation light source  180  may be another device. The excitation light source  180  of the light-emitting apparatuses according to these embodiments may be a laser. In general, laser beams are superior in monochromaticity to LED light and can therefore improve the introduction efficiency of excitation light into the photoluminescent layer  110 . 
     The structure of a laser that can be used as the excitation light source  180  will be described below with reference to  FIGS. 36A and 36B .  FIG. 36A  is an explanatory schematic view of the structure of an edge-emitting laser, and  FIG. 36B  is an explanatory schematic view of the structure of a surface-emitting laser. The structure of the laser is not limited to those illustrated in the figures. The excitation light source  180  may be a known laser. The structure and principle of lasers are well known and are not described in detail here. 
     As illustrated in  FIG. 36A , for example, the edge-emitting laser includes a resonator  185  on the substrate  184 . The resonator  185  may include a reflector  185   a  at each end thereof in a plane parallel to the substrate  184 . A cleavage end surface of the substrate  184  may function as a reflector and constitute the resonator  185 . In the edge-emitting laser, a laser beam is emitted in a direction parallel to the substrate  184 . As illustrated in  FIG. 36B , the surface-emitting laser may be used as the excitation light source  180 . In the surface-emitting laser, a reflector  185   a  of a resonator  185  may be parallel to the substrate  184 . The laser beam is emitted in a direction perpendicular to the substrate  184 . 
     In a light-emitting apparatus including a laser as the excitation light source  180 , the light output side of the excitation light source  180  may be in contact with the light-emitting device  100 . The light output side of the excitation light source  180  may be in direct contact with the photoluminescent layer  110  or the light-transmissive layer  120 . The excitation light source  180  may be a semiconductor laser. In semiconductor lasers, laser oscillation occurs by the application of a voltage to a pn junction. For example, a semiconductor laser containing a gallium nitride semiconductor material may be used. 
       FIGS. 37A and 37B  are schematic cross-sectional views of a light-emitting apparatus  317  and a light-emitting apparatus  318 , respectively, each including an edge-emitting laser as the excitation light source  180 . As illustrated in  FIG. 37A , the photoluminescent layer  110  may be perpendicular to the direction of excitation light emitted from the excitation light source  180 . As illustrated in  FIG. 37B , the photoluminescent layer  110  may be parallel to the direction of excitation light emitted from the excitation light source  180 . 
     The light-emitting apparatuses according to the embodiments may further include a selective reflective layer  170  on the top surface of the photoluminescent layer  110 . A light-emitting apparatus  320  will be described below with reference to  FIG. 38 .  FIG. 38  is a schematic cross-sectional view of the light-emitting apparatus  320 . 
     The light-emitting device  100  in the light-emitting apparatus  320  further includes the selective reflective layer  170  on the top surface of the photoluminescent layer  110 . In the embodiment illustrated in  FIG. 38 , the light-emitting apparatus  320  is the same as the light-emitting apparatus  314  except the selective reflective layer  170 . The light-emitting apparatus  320  is not limited to this. Except for further including the selective reflective layer  170 , the light-emitting apparatus  320  may be any of the light-emitting apparatuses according to the embodiments of the present disclosure or a combination of two or more of the light-emitting apparatuses. Except for further including the selective reflective layer  170 , the light-emitting device  100  in the light-emitting apparatus  320  may be any of the light-emitting devices according to the embodiments of the present disclosure or a combination of two or more of the light-emitting devices. 
     The light-emitting apparatus  320 , which includes the light-emitting device  100  and the excitation light source  180  integrally formed, can realize a compact light-emitting apparatus that includes a light-emitting device having high directionality and luminous efficiency and an excitation light source. 
     The selective reflective layer  170  is an optical filter that can transmit and reflect only light in a particular wavelength region. For example, excitation light emitted from the excitation light source  180  and passing through the photoluminescent layer  110  is reflected from the selective reflective layer  170 . This can improve the introduction efficiency of excitation light into the photoluminescent layer  110 . The light-emitting device of the light-emitting apparatus  320  can achieve high directionality and luminous efficiency. The selective reflective layer  170  may be configured to transmit light emitted from the photoluminescent layer  110  rather than reflecting the light. This can further improve the directionality and luminous efficiency of the light-emitting device of the light-emitting apparatus  320 . The wavelength of light emitted from the light-emitting apparatus  320  can be selected by the selective reflective layer  170 . Light from the light-emitting apparatus  320  can have high monochromaticity. 
     The selective reflective layer  170  is not limited to the embodiments described above and may be configured in any way. For example, the selective reflective layer  170  may be configured to transmit both excitation light passing through the photoluminescent layer  110  and light emitted from the photoluminescent layer  110 . The selective reflective layer  170  may be configured such that both excitation light passing through the photoluminescent layer  110  and light emitted from the photoluminescent layer  110  are emitted in the direction normal to the photoluminescent layer  110 . In such a case, light beams emitted from the light-emitting apparatus  320  are (directional) light beams emitted in the direction normal to the photoluminescent layer  110  and may have different wavelengths. Thus, the light-emitting apparatus  320  can emit polychromatic light and white light. 
     The selective reflective layer  170  may have a dielectric multilayer film structure. As is well known, a dielectric multilayer film is formed by alternately stacking dielectric materials having different refractive indices on top of one another. The material (for example, the refractive index) and thickness of each dielectric film depend on the reflectivity and/or optical transparency of the selective reflective layer  170 . The selective reflective layer  170  may be a band-pass filter or a dichroic mirror. 
     Light-emitting devices according to the present disclosure can be used to provide directional light-emitting apparatuses and can be applied to optical devices, such as lighting fixtures, displays, and projectors.