Abstract:
The disclosure relates to a light emitting device. The light emitting device includes an insulative transparent substrate, a light emitting material layer, and a metal metamaterial layer. The metal metamaterial layer is located between the insulative transparent substrate and the light emitting material layer. The metal metamaterial layer includes a number of periodically aligned metamaterial units. Because the plasmon of the metamaterial can control electromagnetic properties in nanoscale, light from the light emitting device can be polarized in nanoscale. Thus, the light emitting device can emit polarized light. The display device using the light emitting device is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201410423957.7, filed on Aug. 26, 2014, in the China Intellectual Property Office, disclosure of which is incorporated herein by reference. 
       FIELD 
       [0002]    The subject matter herein generally relates to light emitting devices and display devices, in particular, to light emitting devices and display devices based on metamaterial. 
       BACKGROUND 
       [0003]    Currently, liquid crystal displays (LCDs) are widely used. 
         [0004]    Polarizer is used in the LCD to polarize the inputting light. The polarizer is a usually a film polarizer and will waste half of the incident light intensity. Thus, it not only reduces the brightness of the LCD but also waste the electric energy. Although an additional liquid crystal layer is used to replace the polarizer to polarize the incident light. However, the polarizations of the incident light are all based on far field operation. 
         [0005]    What is needed, therefore, is to provide a light emitting device and a display device for solving the problem discussed above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Implementations of the present technology will now be described, by way of example only, with reference to the attached figures, wherein: 
           [0007]      FIG. 1  is a schematic view of one embodiment of a light emitting device. 
           [0008]      FIG. 2  is a cross-sectional view along line II-II of  FIG. 1 . 
           [0009]      FIG. 3  shows a plurality of metamaterial units in different shapes. 
           [0010]      FIG. 4  is a Scanning Electron Microscope (SEM) image of one embodiment of a metamaterial unit. 
           [0011]      FIG. 5  shows how the light emitting device of  FIG. 1  works by irradiating from the front surface and outputting light from the back surface. 
           [0012]      FIG. 6  is a polarization testing result of the light emitting device of  FIG. 1  on the work mode of  FIG. 5 . 
           [0013]      FIG. 7  shows how the light emitting device of  FIG. 1  works by irradiating from the back surface and outputting light from the front surface. 
           [0014]      FIG. 8  is a polarization testing result of the light emitting device of  FIG. 1  on the work mode of  FIG. 7 . 
           [0015]      FIG. 9  shows how a light emitting device of a compare embodiment works. 
           [0016]      FIG. 10  is a polarization testing result of the light emitting device of  FIG. 9 . 
           [0017]      FIG. 11  shows testing results of transmission, reflection and absorption of a metamaterial layer in a far field of another compare embodiment. 
           [0018]      FIG. 12  is a schematic view of one embodiment of a display device. 
           [0019]      FIG. 13  is a schematic view of another one embodiment of a light emitting device. 
           [0020]      FIG. 14  is a schematic view of another one embodiment of a light emitting device. 
           [0021]      FIG. 15  is an SEM image of one embodiment of a metamaterial unit. 
           [0022]      FIG. 16  is a schematic view of another one embodiment of a light emitting device. 
           [0023]      FIG. 17  is a schematic view of another one embodiment of a light emitting device. 
           [0024]      FIG. 18  is a schematic view of another one embodiment of a light emitting device. 
           [0025]      FIG. 19  is a schematic view of another one embodiment of a light emitting device. 
           [0026]      FIG. 20  is a schematic view of another one embodiment of a light emitting device. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures and components have not been described in detail so as not to obscure the related relevant feature being described. The drawings are not necessarily to scale and the proportions of certain parts may be exaggerated to better illustrate details and features. The description is not to be considered as limiting the scope of the embodiments described herein. 
         [0028]    Several definitions that apply throughout this disclosure will now be presented. 
         [0029]    The term “coupled” is defined as connected, whether directly or indirectly through intervening components, and is not necessarily limited to physical connections. The connection can be such that the objects are permanently connected or releasably connected. The term “outside” refers to a region that is beyond the outermost confines of a physical object. The term “inside” indicates that at least a portion of a region is partially contained within a boundary formed by the object. The term “substantially” is defined to be essentially conforming to the particular dimension, shape or other word that substantially modifies, such that the component need not be exact. For example, substantially cylindrical means that the object resembles a cylinder, but can have one or more deviations from a true cylinder. The term “comprising” means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in a so-described combination, group, series and the like. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
         [0030]    References will now be made to the drawings to describe, in detail, various embodiments of the present light emitting devices and display devices based on metamaterial. 
         [0031]    Referring to  FIGS. 1-2 , a light emitting device  100  of one embodiment includes an insulative transparent substrate  110 , a metamaterial layer  120  and a light emitting layer  130 . The insulative transparent substrate  110 , the metamaterial layer  120  and the light emitting layer  130  are stacked with each other. 
         [0032]    The metamaterial layer  120  is located on a surface of the insulative transparent substrate  110 . The light emitting layer  130  is located on a surface of the metamaterial layer  120  so that the metamaterial layer  120  is sandwiched between the insulative transparent substrate  110  and the light emitting layer  130 . The light emitting layer  130  covers the metamaterial layer  120 . Furthermore, an optional transparent protective layer (not shown) can be located on a surface of the light emitting layer  130  that is spaced from the metamaterial layer  120 . 
         [0033]    The insulative transparent substrate  110  can be flat or curved and configured to support other elements. The insulative transparent substrate  110  can be made of rigid materials such as silicon oxide, silicon nitride, ceramic, glass, quartz, diamond, plastic or any other suitable material. The insulative transparent substrate  110  can also be made of flexible materials such as polycarbonate (PC), polymethyl methacrylate acrylic (PMMA), polyimide (PI), polyethylene terephthalate (PET), polyethylene (PE), polyether polysulfones (PES), polyvinyl polychloride (PVC), benzocyclobutenes (BCB), polyesters, or acrylic resin. The size and shape of the insulative transparent substrate  110  can be selected according to need. For example, the thickness of the insulative transparent substrate  110  is in a range from about 100 micrometers to about 500 micrometers. In one embodiment, the insulative transparent substrate  110  is a silicon dioxide layer with a thickness of 200 micrometers. If the metamaterial layer  120  and the light emitting layer  130  is free standing, the insulative transparent substrate  110  is optional. 
         [0034]    The metamaterial layer  120  includes metamaterial which is artificial material engineered to have properties that have not yet been found in nature, such as negative refractive index. The metamaterial layer  120  includes a plurality of metamaterial units  122  arranged to form a periodic array. The plurality of metamaterial units  122  can be a plurality of bulges protruded from a surface of the insulative transparent substrate  110  or a plurality of apertures/holes defined by and extending through the insulative transparent substrate  110 . The plurality of bulges are spaced from each other so that the metamaterial layer  120  allows light to pass through. The shapes of the plurality of metamaterial units  122  can be the patterns as shown in  FIG. 3 , or mirror image of the patterns of  FIG. 3 , or the patterns of  FIG. 3  being rotated. The patterns of the metamaterial units  122  of  FIG. 3  can be          ,          ,          ,          ,          , and          . 
         [0035]    The thickness h of the metamaterial units  122  can be in a range from about 30 nanometers to about 100 nanometers, the period of the metamaterial units  122  can be in a range from about 300 nanometers to about 500 nanometers, and the line width of the metamaterial unit  122  can be in a range from about 30 nanometers to about 40 nanometers. The size of the metamaterial unit  122  can be less than or equal to wavelength of the light emitted from the light emitting layer  130 . In one embodiment, the size of the metamaterial unit  122  in each direction is less than 100 nanometers. The material of the metamaterial layer  120  is metal which can generate surface plasmons (SPS). The metal can be gold, silver, copper, iron, aluminum, nickel or alloys thereof. The metamaterial layer  120  can be fabricated by treating a metal layer by focusing ion beam etching or electron beam lithography. In one embodiment, the metamaterial layer  120  is made by depositing a gold film on the surface of the silicon dioxide layer and focusing ion beam etching the gold film to obtain a plurality of strip-shaped apertures arranged to form a periodic array. The plurality of strip-shaped aperture are used as the metamaterial units  122 . The thickness of the gold film is 50 nanometers. The period of the strip-shaped apertures is 250 nanometers. As shown in  FIG. 4 , the length of the strip-shaped aperture is 90.38 nanometers. The width of the strip-shaped aperture is 26.53 nanometers. As shown in Table 1 below, the metamaterial unit  122  can be classified into four categories according to the properties of chirality symmetry, isotropy and polarized light. The metamaterial units  122  of strip-shaped apertures are belong to category 4 of the Table 1. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Chirality 
                   
                   
               
               
                 Categories 
                 Symmetry 
                 Isotropy 
                 Classification of polarized light 
               
               
                   
               
             
             
               
                 1 
                 Yes 
                 Yes 
                 Circularly polarized light 
               
               
                 2 
                 Yes 
                 No 
                 Elliptically polarized light 
               
               
                 3 
                 No 
                 Yes 
                 Non-polarized light 
               
               
                 4 
                 No 
                 No 
                 Linearly polarized light 
               
               
                   
               
             
          
         
       
     
         [0036]    The light emitting layer  130  includes photoluminescent material, such as semiconductor quantum dots, dye molecules or fluorescent powder. The semiconductor quantum dots can be PbS quantum dots, CdSe quantum dots or GaAs quantum dots. The diameter of the semiconductor quantum dot can be in a range from about 10 nanometers to about 200 nanometers. The dye molecules can be rhodamine 6G. The light emitting layer  130  is located on a surface of the metamaterial layer  120  and extends through the metamaterial layer  120  to be in direct contact with the insulative transparent substrate  110 . The surface of the light emitting layer  130  that is spaced from the metamaterial layer  120  can be flat or curved. The thickness H of the light emitting layer  130  can be in a range from about 50 nanometers to about 500 nanometers, such as from about 100 nanometers to about 200 nanometers. The light emitting layer  130  can be fabricated by spinning coating, spraying, printing, or depositing. In one embodiment, the light emitting layer  130  includes a polymer matrix  132  and a plurality of CdSe quantum dots  134  dispersed in the polymer matrix  132 . The thickness of the light emitting layer  130  is 100 nanometers. The light emitting layer  130  is made by dispersing the CdSe quantum dots  134  in photoresist to form a mixture solution, and then spinning coating the mixture solution on the metamaterial layer  120 . 
         [0037]    The surface of the light emitting layer  130  that is spaced from the insulative transparent substrate  110  is defined as a front surface  102 . The surface of the insulative transparent substrate  110  that is spaced from the light emitting layer  130  is defined as back surface  104 . As shown in  FIG. 5 , when the incident light  140  irradiate the light emitting device  100  from the front surface  102 , light emitted from the light emitting layer  130  will pass through the metamaterial layer  120  to output from the back surface  104  to form the emitted light  150 . Usually, the incident light  140  is laser light. As shown in  FIG. 6 , the degree of linear polarization of the emitted light  150  from the back surface  104  is 95%. As shown in  FIG. 7 , when the incident light  140  irradiate the light emitting device  100  from the back surface  104 , light emitted from the light emitting layer  130  will output from the front surface  102  directly to form the emitted light  150 . As shown in  FIG. 8 , the linear polarization of the emitted light  150  from the front surface  102  is 10%. The linear polarization of the emitted light  150  of  FIG. 5  is much greater than the linear polarization of the emitted light  150  of  FIG. 7 . When the emitted light  150  pass through the metamaterial layer  120 , the linear polarization of the emitted light  150  is enhanced. 
         [0038]    Usually, a light source with a distance far than a wavelength can be seen as a far field light source, and a light source with a distance close to 1/10 wavelength can be seen as a near field light source. The wavelength of visible light is in a range from about 390 nanometers to about 770 nanometers. Usually, the electromagnetic field is localized in the subwavelength scale near the surface of the metamaterials. Therefore, the light emitting layer  130  of visible light with a thickness less than 100 nanometers is within the near field domain of the metamaterial layer  120 , which guarantees the strong interaction between the metamaterial layer  120  and the light emitting layer  130 . 
         [0039]    The metamaterial layer  120  can be regarded as a nano antenna array for the electromagnetic waves and will cause scattering to the electromagnetic waves nearby. According to the classical electromagnetic theory the electromagnetic waves that were previously emitted by the dipole sources undergo a series of scattering events on the antenna elements, which would rebound back and, in turn, work as the driving field for the dipole moments. Secondary emission would be induced, which influences the total emission fields through superimposing on the previously emitted fields. Notably, the secondary emitted field is polarized identically to the scattering driving fields. In one embodiment, the metamaterial layer  120  of  FIG. 1  show different scattering strengths for orthogonally polarizations, the scattering along Y-direction could be enhanced, whereas the scattering fields along the X-direction is overwhelmed. As a result, the linearly Y-polarized emission in the far-field happens. According to the Fresnel rule and the boundary conditions of electromagnetic fields, the emitted light  150  on the back surface  104  has higher polarization. 
         [0040]    Furthermore, as shown in  FIGS. 9-10 , in one compare embodiment, the light emitting layer  130  is directly located on a surface of the insulative transparent substrate  110  without any metamaterial layer therebetween. When the incident light  140  irradiate the light emitting layer  130  from the front surface  102 , the emitted light  150  from the back surface  104  is non-linearly polarized light. Thus, the polarization property of the light emitting device  100  is caused by the metamaterial layer  120 . 
         [0041]    In another compare embodiment, the transmission, reflection and absorption of the metamaterial layer  120  are tested when a far field plane wave light source is used to irradiate the light emitting device  100 . The far field plane wave light source emits white light, which would not activate the light emitting layer  130  to emit light, to irradiate the light emitting device  100  from front side  102 . As shown in  FIG. 11 , Ty/Tx is about 5, where the Ty represents the transmission of the Y polarized light of the emitted light  150  and Tx represents the transmission of the X polarized light of the emitted light  150 . The linear polarization of the transmission light can be calculated by (I max −I min )/(I max +I min )=(5−1)/(5+1)˜67%. Therefore, the polarization of the metamaterial layer  120  for far field light source is about 67%, but for near field light source is about 95%. Therefore, the polarization of the emitted light  150  of the light emitting device  100  is not caused simply by the transmission of the metamaterial layer  120 , but caused by that the metamaterial layer  120  adjust the radiation rate of the light emitting layer  130  which is a near field light source. 
         [0042]    The light emitting device  100  has following advantages. First, the brightness of the emitted light  150  can be enhanced because the plasmon resonance of the metamaterial layer  120 . Second, the light emitted from the light emitting layer  130  are polarized in nano-scale by the polarization of plasmon resonance of the metamaterial layer  120  so that the light emitting device  100  can emit polarized light directly. 
         [0043]    Referring to  FIG. 12 , a display device  10  is provided. The display device  10  includes the light emitting device  100 , a light guide plate  160  and a liquid crystal panel  170 . The light emitting device  100 , the light guide plate  160  and the liquid crystal panel  170  are stacked with each other in that order. The light guide plate  160  is located on the back surface  104  of the insulative transparent substrate  110  and sandwiched between the light emitting device  100  and the liquid crystal panel  170 . The light emitting device  100  is used as a light source of the display device  10 . Because the light emitting device  100  can emit polarized light directly, the display device  10  is simple and does not need other polarizer. The display device  10  can also include the light emitting devices  200 ,  300 ,  400  of embodiments below. 
         [0044]    Referring to  FIG. 13 , a light emitting device  200  of one embodiment includes the insulative transparent substrate  110 , the metamaterial layer  120 , the light emitting layer  130  and a reflection layer  180 . The insulative transparent substrate  110 , the metamaterial layer  120 , the light emitting layer  130  and the reflection layer  180  are stacked with each other. 
         [0045]    The light emitting device  200  is similar with the light emitting device  100  except that the reflection layer  180  is located on and covers the light emitting layer  130  so that the light emitting layer  130  is sandwiched between the insulative transparent substrate  110  and the reflection layer  180 . The reflection layer  180  can be a metal film such as a gold film. Because part of the light that is emitted from the light emitting layer  130  and travel to the reflection layer  180  will be reflected by the reflection layer  180  to pass through the metamaterial layer  120  to output from the back surface  104 , the light emitting efficiency of the light emitting device  200  is enhanced. 
         [0046]    In work of the light emitting device  200 , the incident light  140  can irradiate the light emitting device  200  from the back surface  104  or side surface  106 . The emitted light  150  output from the back surface  104 . In one embodiment, the incident light  140  irradiate the light emitting device  200  from entire side surface  106  so that more emitted light  150  can output from the back surface  104 . 
         [0047]    Referring to  FIG. 14 , a light emitting device  300  of one embodiment includes the insulative transparent substrate  110 , the metamaterial layer  120 , and the light emitting layer  130 . The insulative transparent substrate  110 , the metamaterial layer  120 , and the light emitting layer  130  are stacked with each other. 
         [0048]    The light emitting device  300  is similar with the light emitting device  100  except that the metamaterial layer  120  includes a plurality of strip-shaped bulges arranged to form a periodic array and used as a plurality of metamaterial units  122 . The metamaterial layer  120  defined a plurality of spaces  124  between adjacent metamaterial units  122 . The light emitting layer  130  is wave-shaped and has a uniform thickness. The light emitting layer  130  has a plurality of first surfaces and a plurality of second surface depressed from the plurality of first surfaces. As shown in  FIG. 15 , the plurality of metamaterial units  122  are arranged to form a two dimensional array. In one embodiment, the thickness of the strip-shaped bulges is 50 nanometers, the period of the strip-shaped bulges is 300 nanometers, the length of the strip-shaped bulge is 152 nanometers, and the width of the strip-shaped bulge is 116 nanometers. 
         [0049]    Referring to  FIG. 16 , a light emitting device  400  of one embodiment includes the insulative transparent substrate  110 , the metamaterial layer  120 , and the light emitting layer  130 . The insulative transparent substrate  110 , the metamaterial layer  120 , and the light emitting layer  130  are stacked with each other. 
         [0050]    The light emitting device  400  is similar with the light emitting device  100  except that the plurality of metamaterial units  122  are a plurality of           shaped apertures arranged to form a periodic two dimensional array. The plurality of           shaped apertures is fabricated by etching a gold film. In one embodiment, the thickness of the gold film is 50 nanometers, the period of the           shaped apertures is 400 nanometers, and the line width of the           shaped aperture is 40 nanometers. 
         [0051]    The light emitting devices  100 ,  200 ,  300 ,  400  are all optical pumping light emitting devices and work by light irradiating. The light emitting devices  500 ,  600 ,  700 ,  800  below are electric pumping light emitting devices and work by supplying a voltage or current. 
         [0052]    Referring to  FIG. 17 , a light emitting device  500  of one embodiment is a vertical structure light emitting diode (LED) and includes a first electrode  510 , a first semiconductor layer  520 , an active layer  530 , a second semiconductor layer  540  and a second electrode  550 . 
         [0053]    The first electrode  510 , the first semiconductor layer  520 , the active layer  530 , the second semiconductor layer  540  and the second electrode  550  are stacked with each other in that order. The first electrode  510  is electrically connected to the first semiconductor layer  520 . The second electrode  550  is electrically connected to the second semiconductor layer  540 . At least one of the first electrode  510  and the second electrode  550  is a metal metamaterial layer, and the distance between the metal metamaterial layer and the active layer  530  is less than or equal to 100 nanometers. In one embodiment, the distance between the metal metamaterial layer and the active layer  530  is less than or equal to 50 nanometers. The active layer  530  can be seen as a near field light source of the metal metamaterial layer. 
         [0054]    If the first semiconductor layer  520  is an N-type semiconductor, the second semiconductor layer  540  is a P-type semiconductor, and vice versa. The N-type semiconductor layer provides negative electrons, and the P-type semiconductor layer provides positive holes. The N-type semiconductor layer can be made of N-type gallium nitride, N-type gallium arsenide, or N-type copper phosphate. The P-type semiconductor layer can be made of P-type gallium nitride, P-type gallium arsenide, or P-type copper phosphate. The first semiconductor layer  520  can have a thickness of about 50 nanometers to about 3 micrometers. The second semiconductor layer  540  can have a thickness of about 50 nanometers to about 3 micrometers. If the first electrode  510  is a metal metamaterial layer, the thickness of the first semiconductor layer  520  should be less than 50 nanometers so that the distance between the first electrode  510  and the active layer  530  is less than 50 nanometers. If the second electrode  550  is a metal metamaterial layer, the thickness of the second semiconductor layer  540  should be less than 50 nanometers so that the distance between the second electrode  550  and the active layer  530  is less than 50 nanometers. 
         [0055]    The active layer  530  is sandwiched between the first semiconductor layer  520  and the second semiconductor layer  540 . The active layer  530  is a photon exciting layer and can be one of a single quantum well layer or multilayer quantum well films. The active layer  530  can be made of gallium indium nitride (GaInN), aluminum indium gallium nitride (AlGaInN), gallium arsenide (GaSn), aluminum gallium arsenide (AlGaSn), gallium indium phosphide (GaInP), or aluminum gallium arsenide (GaInSn). The active layer  530 , in which the electrons fill the holes, can have a thickness of about 0.01 micrometers to about 0.6 micrometers. 
         [0056]    The first electrode  510  may be a P-type or an N-type electrode and is the same type as the first semiconductor layer  520 . The second electrode  550  may be a P-type or an N-type electrode and is the same type as the second semiconductor layer  540 . The thickness of the first electrode  510  can range from about 0.01 micrometers to about 2 micrometers. The thickness of the second electrode  550  can range from about 0.01 micrometers to about 2 micrometers. The material of the first electrode  510  and the second electrode  550  is metal such as gold, silver, copper, iron, aluminum, nickel, titanium, or alloys thereof. 
         [0057]    In one embodiment, the first semiconductor layer  520  is an N-type gallium nitride layer with a thickness of 0.3 micrometers, and the second semiconductor layer  540  is a P-type gallium nitride layer with a thickness of 100 nanometers, and the active layer  530  includes a GaInN layer and a GaN layer stacked with each other and has a thickness of about 0.03 micrometers. The first electrode  510  is N-type electrode and includes a nickel layer and a gold layer. The thickness of the nickel layer is about 15 nanometers. The thickness of the gold layer is about 200 nanometers. The second electrode  550  is P-type electrode and includes a metal metamaterial layer having the same pattern as the metamaterial layer  120  of  FIG. 16  and a thickness of 100 nanometers. 
         [0058]    In work, a voltage is supplied to the light emitting device  500  through the first electrode  510  and the second electrode  550 . The active layer  530  is activated to produce photons. The photons output from the second electrode  550 . Because the second electrode  550  is a metal metamaterial layer spaced from the active layer  530  with a distance less than 100 nanometers, the light emitting rate of the active layer  530  can be enhanced by the plasmons of the metal metamaterial layer. Furthermore, the light emitting device  500  can emit polarized light directly because of the polarization of the metal metamaterial layer. 
         [0059]    In anther embodiment, both the first electrode  510  and the second electrode  550  is metal metamaterial layer, and both the first semiconductor layer  520  and the second semiconductor layer  540  has thickness less than 100 nanometers. The photons produced from the active layer  530  can output from both the first electrode  510  and the second electrode  550 . 
         [0060]    Referring to  FIG. 18 , a light emitting device  600  of one embodiment is a vertical structure LED and includes a reflection layer  580 , a first electrode  510 , a first semiconductor layer  520 , an active layer  530 , a second semiconductor layer  540  and a second electrode  550 . 
         [0061]    The light emitting device  600  is similar with the light emitting device  500  except that a reflection layer  580  is located on a surface of the first electrode  510  that is spaced from the first semiconductor layer  520 . The reflection layer  580  covers the first electrode  510 . Because part of the light that is emitted from the active layer  530  and travel to the reflection layer  580  will be reflected by the reflection layer  580  to pass through, be polarized and enhanced by the metal metamaterial layer of the second electrode  550 , the light emitting efficiency of the light emitting device  600  is enhanced. 
         [0062]    Referring to  FIG. 19 , a light emitting device  700  of one embodiment is a horizontal structure LED and includes a substrate  560 , a first electrode  510 , a first semiconductor layer  520 , an active layer  530 , a second semiconductor layer  540  and a second electrode  550 . 
         [0063]    The light emitting device  700  is similar with the light emitting device  500  except that part of the first semiconductor layer  520  is exposed to form an exposed part, and the first electrode  510  is located on the exposed part of the first semiconductor layer  520 . In one embodiment, the substrate  560 , the first semiconductor layer  520 , the active layer  530 , the second semiconductor layer  540  and the second electrode  550  are stacked with each other in that order. The area of the active layer  530 , the second semiconductor layer  540  and the second electrode  550  are the same and smaller than that of the first semiconductor layer  520  so that part of the first semiconductor layer  520  is exposed. The second electrode  550  is a metal metamaterial layer spaced from the active layer  530  with a distance less than 100 nanometers. 
         [0064]    Referring to  FIG. 20 , a light emitting device  800  of one embodiment is a LED and includes a first electrode  510 , a first semiconductor layer  520 , an active layer  530 , a second semiconductor layer  540 , a second electrode  550  and a metal metamaterial layer  570 . 
         [0065]    The light emitting device  800  is similar with the light emitting device  500  except the metal metamaterial layer  570 . The metal metamaterial layer  570  is located on a side surface of the light emitting device  800  that is perpendicular with each of the first electrode  510 , the first semiconductor layer  520 , the active layer  530 , the second semiconductor layer  540 , and the second electrode  550 . In one embodiment, the light emitting device  800  is a cuboid and has four side surfaces. The metal metamaterial layer  570  is located on only one of the four side surfaces. The other three side surfaces can also be coated with metal reflection films. The first electrode  510  and the second electrode  550  can also be a metal metamaterial layer. The metal metamaterial layer  570  has a pattern same as that of the metamaterial layer  120  of  FIG. 1 . Because the metal metamaterial layer  570  is in direct contact with the active layer  530 , the photons produced from the active layer  530  and output from the metal metamaterial layer  570  on the side surface can be enhanced and polarized by the metal metamaterial layer  570 . Thus, the light emitting device  800  has an enhanced brightness and can emit polarized light directly. 
         [0066]    The embodiments shown and described above are only examples. Even though numerous characteristics and advantages of the present technology have been set forth in the foregoing description, together with details of the structure and function of the present disclosure, the disclosure is illustrative only, and changes may be made in the detail, including in matters of shape, size and arrangement of the parts within the principles of the present disclosure up to, and including, the full extent established by the broad general meaning of the terms used in the claims. 
         [0067]    Depending on the embodiment, certain of the steps of methods described may be removed, others may be added, and the sequence of steps may be altered. The description and the claims drawn to a method may include some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps.