Abstract:
Light-emitting devices that include a scattering element of arbitrary shape and processes for fabrication thereof are described. In one aspect a method for forming a light-emitting device includes providing a light-emitting element (LEE) on a base surface of a base substrate; coupling a first optical element with the base surface of the base substrate, where the first optical element has a first surface facing the LEE and a second surface opposing the first surface; disposing the first optical element in a curable or settable fluid so that the fluid conforms to the second surface; and curing or setting the fluid to form a second optical element including a molded transparent layer adjacent the second surface of the first optical element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e)(1) of U.S. Provisional Application No. 61/780,537, filed on Mar. 13, 2013, which is incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The described technology relates to fabrication of light-emitting devices, for example fabrication of light-emitting devices that include a scattering element of arbitrary shape. 
       BACKGROUND 
       [0003]    The present technology relates generally to light-emitting devices and, in particular, to light-emitting devices that feature a solid state light-emitting element and a scattering element and an extractor element remote from a light-emitting element. 
         [0004]    Light-emitting elements are ubiquitous in the modern world, being used in applications ranging from general illumination (e.g., light bulbs) to lighting electronic information displays (e.g., backlights and front-lights for LCDs) to medical devices and therapeutics. Solid state light emitting devices, which include light emitting diodes (LEDs), are increasingly being adopted in a variety of fields, promising low power consumption, high luminous efficacy and longevity, particularly in comparison to incandescent and other conventional light sources. 
         [0005]    One example of a SSL device increasingly being used for in luminaires is a so-called “white LED.” Conventional white LEDs typically include an LED that emits blue or ultraviolet light and a phosphor or other luminescent material. The device generates white light via down-conversion of blue or UV light from the LED (referred to as “pump light”) by the phosphor. Such devices are also referred to as phosphor-based LEDs (PLEDs). Although subject to losses due to light-conversion, various aspects of PLEDs promise reduced complexity, better cost efficiency and durability of PLED-based luminaires in comparison to other types of luminaires. 
         [0006]    While new types of phosphors are being actively investigated and developed, configuration of PLED-based light-emitting devices, however, provides further challenges due to the properties of available luminescent materials. Challenges include light-energy losses from photon conversion, phosphor self-heating from Stokes loss, dependence of photon conversion properties on operating temperature, degradation due to permanent changes of the chemical and physical composition of phosphors in effect of overheating or other damage, dependence of the conversion properties on intensity of light, propagation of light in undesired directions in effect of the random emission of converted light that is emitted from the phosphor, undesired chemical properties of phosphors, and controlled deposition of phosphors in light-emitting devices, for example. 
       SUMMARY 
       [0007]    The described technology relates to fabrication of light-emitting devices, for example fabrication of light-emitting devices that include a scattering element of arbitrary shape. 
         [0008]    In one aspect, a light-emitting device includes a base substrate that has a base surface; a light-emitting element (LEE) that is configured to emit light, where the LEE is disposed on the base surface; a first optical element having a first surface that is spaced apart from the LEE and positioned to receive light from the LEE, where the first optical element includes scattering centers that are arranged to scatter light from the LEE, and where the first optical element has a non-hemispherical shape; a second optical element that has an exit surface, where the second optical element is transparent and in contact with the first optical element, there being an optical interface between the first and second optical elements at the place of contact, where the optical interface is opposite the first surface of the first optical element, and where the second optical element is arranged to receive at least a portion of the light through the optical interface; and a transparent shell that has an inner surface and an opposing outer surface, where the inner surface is in contact with the exit surface of the second optical element, where: a medium adjacent to the first surface of the first optical element has a refractive index n 0 , the first optical element includes a material that has a refractive index n 1 , where n 0 &lt;n 1 , the second optical element includes a material that has a refractive index n 2 , where n 0 &lt;n 2 , and the transparent shell includes a material that has a refractive index n 3 , where n 3 ≧n 2 . 
         [0009]    The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. In some embodiments, the first optical element includes phosphor. In some embodiments, a hardness of the transparent shell can be larger than a hardness of the second optical element. In some embodiments, the material of the transparent shell can be solid and the material of the second optical element can be liquid or gel. In some embodiments, the light-emitting device can further include a reflective layer disposed on the base surface of the base substrate. In some embodiments, the outer surface of the transparent shell can be a transparent surface that is shaped such that an angle of incidence on the outer surface of the light provided by the first optical element that directly impinges on the outer surface is less than a critical angle for total internal reflection. In some embodiments, the outer surface of the transparent shell can be a transparent surface that is shaped such that an angle of incidence on the outer surface of the light provided by the first optical element that directly impinges on the outer surface is less than a Brewster angle. 
         [0010]    In another aspect, a method for forming a light-emitting device includes providing a light-emitting element (LEE) on a base surface of a base substrate; coupling a first optical element with the base surface of the base substrate, where the first optical element has a first surface facing the LEE and a second surface opposing the first surface, the first surface is spaced apart from the LEE, the first optical element includes a material that has a first refractive index n 1 , and where a medium adjacent to the first surface of the first optical element has a refractive index n 0 &lt;n 1 ; disposing the first optical element in a curable or settable fluid so that the fluid conforms to the second surface; and curing or setting the fluid to form a second optical element adjacent the second surface of the first optical element, where the second optical element has a refractive index n 2 &gt;n 0 . 
         [0011]    The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. In some embodiments the second optical element includes a molded transparent layer. In some embodiments, the second surface of the first optical element has a non-hemispherical shape. In some embodiments, the fluid is provided in a transparent shell. In some embodiments, the transparent shell has an outer surface that corresponds to the exit surface of the second optical element. In some embodiments, the transparent shell remains with the light-emitting device and an outer surface of the transparent shell can be a transparent surface that is shaped such that an angle of incidence on the outer surface of the light provided by the second optical element that directly impinges on the outer surface is less than a critical angle for total internal reflection. 
         [0012]    In some embodiments, the curing or setting includes exposing the fluid to UV radiation. In some embodiments, the curing or setting includes heating the fluid. In some embodiments, the method further includes providing a reflective layer on the base surface, where the reflective layer can be configured to diffusely or specularly reflect light. In some embodiments, the exit surface of the second optical element can be a transparent surface that is shaped such that an angle of incidence on the exit surface of the light provided by the first optical element that directly impinges on the exit surface is less than a critical angle for total internal reflection. In some embodiments, the exit surface of the second optical element can be a transparent surface that is shaped such that an angle of incidence on the exit surface of the light provided by the first optical element that directly impinges on the exit surface is less than a Brewster angle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows a cross-sectional view of an example of a light-emitting device. 
           [0014]      FIGS. 2A-2E  show aspects of an example of a fabrication process of light-emitting devices. 
       
    
    
       [0015]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0016]      FIG. 1  shows a cross-sectional side view of a light-emitting device  100 . The light-emitting device  100  can include a base substrate  105 , one or more light-emitting elements, such as light-emitting element  110  (e.g., a blue pump LED), a scattering element  120 , and an extractor element  130 . The scattering element  120  is arranged on the base substrate  105  and is shaped to at least partially surround the light-emitting element  110  in an enclosure  140 . An index of refraction n 0  of a medium inside the enclosure  140  is smaller than an index of refraction n p  of the scattering element  120 . An index of refraction of the scattering element  120  can be smaller than or equal to an index of refraction of the extractor element  130 . Such a choice of relative values for the indexes of refraction n 0 &lt;n p ≦n 1  enables asymmetric propagation of light emitted by the light-emitting element  110  through the light-emitting device  100 . In the example illustrated in  FIG. 1 , the scattering element  120  is located on the inside of the extractor element  130  adjacent the enclosure  140  of the extractor element  130  to form an optical interface  125 . 
         [0017]    The shape of the optical interface  125  between the scattering element  120  and the extractor element  130  can affect the distribution of light output by the light-emitting device  100 . For example, an optical interface  125  shaped as an oblate dome (having a dome height shorter than a base diameter) provides an intensity distribution biased along the optical axis of the light-emitting device  100 , e.g., the +z axis. As another example, an optical interface  125  shaped as an oblong dome (having a dome height longer than a base diameter) provides a laterally-biased intensity distribution (biased away from the optical axis of the light-emitting device  100 .) In general, when fabricating light-emitting devices, the shape of the optical interface between the scattering element and extractor element can be limited by available machining processes. The fabrication processes described in detail below provide the capability to produce light-emitting devices having a variety of shapes of the optical interfaces between the scattering element and extractor element not readily producible by conventional fabrication processes. 
         [0018]    In some implementations, the scattering element  120  can have an irregular shape (as illustrated). Furthermore, the scattering element  120  can have a uniform or non-uniform geometrical or effective thickness, generally referred to as thickness and as the case may be referring to a geometrical or effective thickness as appropriate. The effective thickness refers to a combination of geometrical thickness and scattering/conversion properties of the scattering element  120 . Depending on the embodiment, a regular or irregular shaped scattering element  120  may have a regular or irregular thickness. The scattering element  120  includes a plurality of scattering centers configured to scatter light. Depending on the embodiment, the scattering centers can be configured to elastically, inelastically, or elastically and inelastically scatter light. 
         [0019]    In some implementations, the light-emitting elements can be pre-packaged LEDs, for example LED dies encapsulated in silicone. A size of encapsulated LEDs can be 1, 3 or 5 mm in diameter. The base substrate  105  has a surface  108  on which the light-emitting element  110  can be disposed. The surface  108  of the base substrate  105  can be reflective (e.g., a mirror). In some implementations, a reflective layer  145  can be deposited on the surface  108  of the base substrate  105 , as described below in connection with  FIG. 2A . The reflective layer  145  can be a metal mirror (e.g., Ag, Al), a dielectric mirror, a non-absorbing diffuser, or any combination thereof. 
         [0020]    At least a portion of light emitted by the light-emitting element  110 , or back-scattered by the scattering element  120 , can be reflected by the surface  108  (or the reflective layer  145 ). The scattering element  120  can include active scattering centers, e.g., phosphors configured to inelastically scatter pump (e.g., blue) light emitted by the light-emitting element  110  to inelastically scattered light (green, yellow, etc.). The scattering element  120  can include passive scattering centers configured to elastically scatter the pump light, without changing its color. The scattering element  120  has a first surface  115  spaced apart from the light-emitting element  110  and positioned to receive the light from the light-emitting element  110 . In some implementations, the scattering element  120  has uniform thickness. The thickness of the scattering element  120  can be 0.02, 0.20, 0.50, or 1 mm, for example. In general, the first surface  115  of the scattering element  120  can have a desired shape, e.g., spherical, parabolic, elliptical, or an arbitrary, mostly concave (with respect to the enclosure  140 ) shape, as illustrated in  FIG. 1 . 
         [0021]    The light-emitting element  110  is disposed on the surface  108  of the base substrate  105 , in an opening/enclosure  140  that is, at least in part, defined by the first surface  115 . The enclosure  140  is referred to as a recovery enclosure, as described below. In some implementations, the reflective layer  145  disposed on the surface  108  of the base substrate  105  extends to at least the first surface  115  of the scattering element  120 . In other implementations, the reflective layer  145  extends to at least an exit surface  135  of the extractor element  130 . In some implementations, the reflective layer  145  extends beyond the exit surface  135  of the extractor element  130 . The enclosure  140  can be filled with a medium (e.g., gas or air) and encloses the light-emitting element  110 , and at least a portion of the reflective layer  145 . 
         [0022]    In general, the shape of exit surface  135  can vary as desired, e.g., spherical or cylindrical (as shown in  FIG. 1 ), parabolic, elliptical, etc., with or without facets/steps. In the example illustrated in  FIG. 1 , the exit surface  135  of the extractor element  130  has a radius R 1  that is concentric with a notional surface of radius R o  located within the extractor element  130 , such that the notional surface contains the optical interface  125  between the scattering element  120  and the extractor element  130 . In some implementations, the extractor element  130  satisfies a Weierstrass configuration R 1 ≧R 1W , such that an angle of incidence on the exit surface  135  of the scattered light that directly impinges on the exit surface  135  is less than a critical angle. In this manner, the scattered light that directly impinges on the exit surface  135  experiences little or no total internal reflection thereon. The Weierstrass radius is given by R 1W =R O ·n 1 , where R O  is the radius of the notional surface that encloses at least the scattering element  120  of the light-emitting device  100 , and n 1  denotes the index of refraction of the material of the extractor element  130 . In some implementations, the extractor element  130  satisfies a Brewster configuration R 1 ≧R 1B , such that an angle of incidence on the exit surface  135  of the scattered light that directly impinges on the exit surface  135  is less than the Brewster angle. In this case, the scattered light that directly impinges on the exit surface  135  experiences even less total internal reflection thereon than in the Weierstrass configuration. The Brewster radius is given by R 1B =R O (1+n1 2 ) +1/2 , where R O  is the radius of the notional surface that encloses at least the scattering element  120  of the light-emitting device  100 , and n 1  denotes the index of refraction of the material of the extractor element  130 . The Brewster radius R 1B  is larger than the Weierstrass radius R 1W , R 1B &gt;R 1W . Moreover, further increasing the radius R 1  of the extractor element  130  beyond the Brewster radius R 1B  renders reduction in Fresnel reflections that plateaus off. Therefore, the Brewster radius R 1B  can be used as an upper bound for the radius R 1  of the extractor element  130 . 
         [0023]    In this example, light propagation asymmetry arises from the relative values of the indexes of refraction of materials in the enclosure  140  (index n 0 ), in the scattering element  120  (index n p ), and in the extractor element  130  (index n 1 ). For instance, if n p =1.5 and n 0 =1.0, that is n 0 &lt;n p , a large fraction (˜75%) of the isotropically distributed photons impinging on the first surface  115  will be reflected by total internal reflection (TIR) back into the scattering element  120  and only a smaller fraction (˜25%) will be transmitted backwards into the recovery enclosure  140  from where only few may reach the light-emitting element  110 . At the optical interface  125 , the condition n p ≦n 1  will guarantee that substantially all photons reaching the optical interface  125  will transition into the extractor element  130 , and the Brewster condition will further guarantee that practically all these photons will transmit into air without TIR through the exit surface  135 . Only a small fraction (down to about ˜4% depending on incidence angle) will be returned by Fresnel reflection at the exit surface  135 . For the above examples, when the radius R O  of the notional surface enclosing the scattering element  120  is 3 mm, the Weierstrass radius R 1W =4.50 mm, and the Brewster radius R 1B =5.41 mm. 
         [0024]    In some implementations, the scattering element  120  can be pre-formed in a desired shape. For example, scattering elements can be fabricated (e.g., molded) separately and procured/provided as pre-formed components. As noted above, the shape of an optical interface  125  between the scattering element  120  and the extractor element  130  can influence the illumination pattern output by the light-emitting device  100 . Accordingly, it is desirable to form (e.g., mold) the extractor element  130  to correspond to a desired shape of the outer surface of the scattering element  120 . 
         [0025]      FIGS. 2A-E  show an example of a fabrication process of a light-emitting device, such as light-emitting device  100  described herein. Cartesian coordinates are provided for reference. 
         [0026]    At  210 , a base substrate  105  having a surface  108  is provided, as shown in  FIG. 2A . Substrate  105  extends in the x-y plane and has a relatively short z-dimension. A reflective layer  145  (e.g., a layer formed from a specular and/or diffuse reflective material, such as aluminum or silver) is disposed on surface  108 . The reflective layer  145  includes a gap  147  to accommodate a light-emitting element  110 . 
         [0027]    At  220 , the light-emitting element  110  is disposed on the base substrate  105  in gap  147 , as shown in  FIG. 2B . Generally, a variety of light-emitting elements can be used, such as, for example, packaged light-emitting elements (e.g., an LED die encapsulated in silicone). 
         [0028]    At  230 , a scattering element  120  (e.g., a phosphor-containing composite) is secured to base substrate  105 , as shown in  FIG. 2C . In some implementations, the scattering element  120  can be secured to the base substrate  105  using an adhesive, such as a silicone adhesive. As described above in connection with  FIG. 1 , the scattering element  120  can be pre-formed in a variety of shapes. The scattering element  120  encloses the light-emitting element  110 , such that a first surface  115  is spaced apart from the light-emitting element  110 , creating an enclosure  140 . The refractive index n 0  of the medium within the enclosure  140  (e.g., air) is smaller than the refractive index n p  of the scattering element  120 . 
         [0029]    At  240 , an extractor element is formed to accommodate a shape of the outer surface  127  of the scattering element  120 , as shown in  FIG. 2D . The base substrate  105  with the light-emitting element  110  and the scattering element  120  is positioned over a mold  242  having a cavity  246  having a size and shape corresponding to the desired shape of the extractor element&#39;s exit surface. Cavity  246  is filled with a curable or settable fluid  130 - u  (e.g., uncured silicone, liquid high index glass, epoxy, gel, etc.). The scattering element  120  is placed in a desired position with respect to the cavity  246 , for example, an optical axis of the light-emitting element  110  can be aligned with the center axis of the cavity  246 . 
         [0030]    In some implementations, the cavity  246  (i) is sized to contain a notional surface of radius R O  (not shown in  FIG. 2D ), such that the notional surface inscribes the scattering element  120 , and (ii) has a radius of curvature R 1  that satisfies the Weierstrass condition, R 1 ≧R O /n 1c , as described above in connection with  FIG. 1 . In some implementations, the radius of curvature R 1  of the notional surface that inscribes the scattering element  120  satisfies the Brewster condition, R 1 ≧R O (1+n 1c   2 ) 1/2 , as described above in connection with  FIG. 1 . 
         [0031]    The fill level of the fluid  130 - u  in the cavity  246  can be configured such that an unfilled volume enclosed by the cavity  246  is slightly less (e.g., 2%, 5%, 10% less) than a volume enclosed by an outer surface  127  of the scattering element  120 . 
         [0032]    In some implementations, a hard optical-quality surface  244  (e.g., having a surface polished to a 0.01, 0.1, 1 of a wavelength) is disposed in the cavity  246  before it is filled with the fluid  130 - u.  In this case, the hard optical-quality surface remains integrated with the extractor element  130  after the curing operation performed at  250 . The mold  242  can include one or more channels (not shown) to dispose of excess fluid displaced at  240  when forming the extractor. 
         [0033]    In some implementations, the hard optical-quality surface can be configured to provide a shell for the uncured fluid  130 - u.  Such a shell can be configured to be held in place by the mold  242  as illustrated in  FIG. 2D  or other support (not illustrated). For example, such other support can be configured to support the shell in one or more point-like locations. 
         [0034]    The base substrate  105  is brought against the mold  242  such that the scattering element  120  is immersed in the fluid  130 - u.  The base substrate  105  and the mold  242  can be brought together in a motion in the Z-direction as indicated by the arrows shown in  FIG. 2D . In some implementations, the base substrate  105  and the mold  242  can be brought together in a manner that provides an opening to displace air from the cavity  246  between a top surface of the fluid  130 - u  and the scattering element  120  (e.g., due to the shape of region  128  of the scattering element  120 ) to avoid formation of voids, for example via trapping air bubbles between surface  127  or  145  and fluid  130 - u.  For example, the base substrate  105  can be rocked back-and-forth about an axis parallel to the y-axis, while the scattering element  120  advances into the fluid  130 - u  in a direction antiparallel to the z-axis. The mold  242  and the base substrate  105  are pressed together to force the fluid  130 - u  to fill all voids. In some implementations, a vacuum source can be used to extract excess air. 
         [0035]    At  250 , the fluid  130 - u  that forms the extractor element is cured to a solid state, as indicated in  FIG. 2E . For example, the curing can be performed by applying (e.g., through the mold  242 ) heat or UV light  255  or both (successively or concurrently). The curing of the extractor element molded at  240  results in a cured extractor element  130 , to form a light-emitting device. 
         [0036]    The cured extractor element  130  has an index of refraction n 1c  that is substantially the same or larger than the index of refraction n p  of the scattering element  120 . If a hard optical-quality surface  244  was disposed in the cavity  246  at  240 , the hard optical-quality surface  244  can form a part of the cured extractor element  130 , for example, to form a protective shell around the exit surface  135  of the cured extractor element  130 . In this case, the hard optical-quality surface  244  has an index of refraction that is substantially the same or larger than the index of refraction n 1c  of the bulk of the cured extractor element  130 . In some implementations, a hardness of the hard optical-quality surface  244  can be larger than a hardness of the bulk of the extractor element  130 , e.g., 1.05×, 1.10×, 1.50×, 2×, 10× harder. 
         [0037]    The mold  242  can be decoupled from the light-emitting device prior to, during or after curing, at  250 , the extractor element  130 . In some implementations, post-curing can be performed to further harden the extractor element  130 . In some embodiments, a release agent is used to facilitate removal of the extractor element from the mold. In some implementations, it is desirable to fabricate extractor elements that have shapes extending beyond hemispheres, such that a pattern of light output from the light-emitting devices subtends more than 180°. In such cases, the mold  242  can include two or more pieces that are separated after cure. 
         [0038]    In some implementations, the extractor element  130  can be formed using an injection molding process. The base substrate  105  with the scattering element  120  can be brought together with the mold  242  and the fluid  130 - u  can be pressure injected into the mold  242  through inlet channels (not shown in  FIG. 2D .) A vacuum can be created to extract the air from the cavity  246 . The pressure injected fluid  130 - u  can be cured as described herein at  250  and the mold  242  is then separated from the light-emitting device. 
         [0039]    In some implementations, the hard optical-quality surface  244  can be coupled to the base substrate (e.g., by using silicone as adhesive) to form an enclosure for the fluid  130 - u,  which can remain in a liquid or gel state. In such configurations, the fluid  130 - u  has an index of refraction n 1u  that is substantially the same as or smaller than the index of refraction of the hard optical-quality surface  244 , but larger than the index of refraction n p  of the scattering element  120 . In this case, a hardness of the hard optical-quality surface  244  can be larger than a hardness of the bulk of the extractor element  130 , as the latter is in gel state. 
         [0040]    Though the example fabrication process has been described with respect to a light-emitting device including a single light-emitting element, the process described herein can also be implemented for an array of light-emitting elements that can be separated into individual light-emitting devices after completing the fabrication process, for example. In some implementations, a light-emitting device  100  can include multiple light-emitting elements. In some implementations, a light-emitting device  100  can include multiple scattering elements.