Abstract:
To maximize the critical angle, θ c , and the reflectance, R, in total internal reflection reflective image displays, the difference in the refractive indices between the surface of the transparent front sheet and the liquid medium comprising of electrophoretically mobile particles must be maximized. High index optical glasses may be used to fabricate the front sheet but are costly and difficult to manufacture with fine structural features. Polymers may be used to fabricate the transparent front sheet as they are cheaper and simpler to process into desired structures but typically have low indices of refraction. Polymers comprising of dispersed high refractive index particles may be used to increase the refractive index of the transparent front sheet. The polymers may be formed from UV-curable liquid monomers.

Description:
[0001]    This application claims priority to the filing date of Provisional Application No. 62/098,333, filed Dec. 31, 2014, the specification of which is incorporated herein in its entirety. 
     
    
     BACKGROUND 
     Field 
       [0002]    The disclosure generally relates to reflective image displays. Specifically, the disclosure relates to total internal reflection (TIR) image displays comprising high refractive index composite front sheets. 
       Background 
       [0003]    Conventional TIR-based reflective image displays comprise a transparent high refractive index front sheet with a plurality of convex protrusions in contact with a low refractive index fluid containing electrophoretically mobile particles.  FIG. 1  depicts a cross-section of a portion of a prior art TIR-based reflective image display  100 . Display  100  comprises a high refractive index transparent front sheet  102  with outward surface  104  facing viewer  106 . Front sheet  102  further comprises a plurality of convex protrusions  108  on the inward side. The protrusions may be in the shape of a hemisphere  110  as shown in  FIG. 1  or may be other shapes. The protrusions  110  may be embedded beads or may be part of a continuous front sheet. 
         [0004]    Display  100  further comprises a transparent front electrode  112  on the inward surface of sheet  102 , rear support sheet  114  with rear electrode layer  116 . Within the cavity or containment reservoir formed by front sheet  102  and rear sheet  114  contains electrophoretically mobile particles  118  dispersed in low refractive index medium  120 . Display  100  further comprises voltage bias source  122 . Display  100  may further comprise at least one optional dielectric layer located on one or both of the electrodes  112 ,  116 . 
         [0005]    Application of a bias may move at least one particle  118  near the surface of the front sheet and into the evanescent wave region. At this location, TIR is frustrated and incident light rays may be absorbed creating a dark state. When particles are moved away from the front sheet  102  and out of the evanescent wave region light may be totally internally reflected. This creates a bright or white state of the display. Combinations of dark and bright states formed by movement of the particles  118  in and out of the evanescent wave region by the electrodes creates images. The images may convey information to viewer  106 . 
         [0006]    As is well known, the TIR interface between two media having different refractive indices is characterized by a critical angle θ c . The critical angle characterizes the interface between the surface of the transparent front sheet (with refractive index η 1 )  102 , and the low refractive index fluid (with refractive index η 3 )  120 . Light rays incident upon the interface at angles less than θ c  may be transmitted through the interface. Light rays incident upon the interface at angles greater than θ c  may undergo TIR at the interface. A small critical angle is preferred at the TIR interface since this affords a large range of angles over which TIR may occur. The critical angle, θ c , is calculated by the following equation (Eq. 1): 
         [0000]    
       
         
           
             
               
                 
                   
                     θ 
                     c 
                   
                   = 
                   
                     
                       sin 
                       
                         - 
                         1 
                       
                     
                      
                     
                       ( 
                       
                         
                           η 
                           3 
                         
                         
                           η 
                           1 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0007]    It is important to minimize the critical angle, θ c , to allow for a large range of angles of incident light rays over which TIR may occur and thus maximize the reflectance of the display. It may be prudent to have a fluid medium  120  with preferably as small a refractive index (η 3 ) as possible and to have a transparent front sheet  102  composed of a material having a refractive index (η 1 ) preferably as large as possible. The reflectance, R, may be calculated for each individual protrusion  110  of the transparent front sheet  102  as follows in equation 2 (Eq. 2): 
         [0000]    
       
         
           
             
               
                 
                   R 
                   = 
                   
                     1 
                     - 
                     
                       
                         ( 
                         
                           
                             η 
                             3 
                           
                           
                             η 
                             1 
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0008]    It should be noted that in order to calculate the reflectance, R, of the entire front sheet  102  comprising of the plurality of convex protrusions  108  a multiplier must be used to account for the fill factor of the individual protrusions  110 . The calculations described herein are for an individual protrusion  110  and are for illustrative purposes only. 
         [0009]    The effect of the refractive index on θ c  and R is illustrated by comparing two different hypothetical systems, A and B. It is assumed that each system uses the same liquid medium with a refractive index η 3 =1.27. In system A, the refractive index (η 1 ) of the protrusions  110  is assumed to be 1.5 while the protrusions  110  of system B have a higher refractive index (η 1 ) of 1.8. As a result, system A with the lower refractive index (η 1 ) of each protrusion  110  has a higher critical angle (θ c ) of about 58° and lower reflectance (R) of about 28%. System B with a higher refractive index of the protrusions ( 72   1 )  110  has a lower critical angle of about 45° and higher reflectance of about 50%. See the data listed in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Calculation of critical angle (θ c ) and reflectance (R) as function 
               
               
                 of the refractive index (η 1 ) of the protrusions. 
               
             
          
           
               
                   
                 η 3   
                   
                 θ c  (critical 
                 R 
               
               
                 System 
                 (medium) 
                 η 1  (bead) 
                 angle) 
                 (reflectance) 
               
               
                   
               
               
                 A 
                 1.27 
                 1.5 
                 58° 
                 28% 
               
               
                 B 
                 1.27 
                 1.8 
                 45° 
                 50% 
               
               
                   
               
             
          
         
       
     
         [0010]    In order to maximize reflectance and minimize the critical angle it is important to maximize the difference between the refractive indices of the protrusions  110  and the liquid medium  120  (which may include electrophoretic particles  118 ). 
         [0011]    For a large number of applications in optical devices, such as TIR-based reflective image displays, materials having a high index of refraction are required or would be advantageous with respect to traditional materials such as polymers or standard glasses (e.g. soda-lime glasses and borosilicate glasses). Both polymers and standard glasses have indices of refraction in the range of about 1.4-1.6. For many optical applications, it is necessary to structure the material to achieve the required optical functionality. Optical glasses are known to have refractive indices up to about 2.0 but the possibilities to structure such glasses are limited, often time-consuming and costly. Polymers, on the other hand, are limited in their range of refractive indices but can be easily structured by a variety of methods such as molding, casting, embossing and extrusion. Although polymers are known with a refractive index of greater than 1.6, their optical properties are often insufficient for many applications. 
         [0012]    It is known that polymer composite materials may be prepared with higher refractive indices by doping the polymer with high-index inorganic nanoparticles of a size range where optical scattering effects do not occur. However, the doping process itself is difficult with polymers which are normally solid at room temperature or which have a very high viscosity. Alternatively, one may use ultra-violet (UV) light curing (curing may also be referred to as polymerizing) monomers as the basis for preparing doped, high-index polymers that may easily be molded using standard processes to produce structured optical devices or subcomponents for such devices. UV-cured polymers have the advantage that most are low-viscosity liquid monomers at room temperature in the uncured state. Such liquids may easily be doped with the above-mentioned high-index nanoparticles. They may be structured using a variety of known processes and cured with UV light to form a solid, structured, high-index layer or body. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]    These and other embodiments of the disclosure will be discussed with reference to the following exemplary and non-limiting illustrations, in which like elements are numbered similarly, and where: 
           [0014]      FIG. 1  depicts a cross-section of a portion of a prior art TIR-based reflective image display; 
           [0015]      FIG. 2  depicts a cross-section of a portion of a continuous high refractive index composite front sheet of a TIR-based reflective image display; 
           [0016]      FIG. 3  depicts a cross-section of a portion of a TIR-based reflective image display comprising a high refractive index composite front sheet; and 
           [0017]      FIG. 4  schematically illustrates an exemplary system for implementing an embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The exemplary embodiments provided herein improve reflectance of TIR displays. In an exemplary embodiment, the disclosure provides a composite high refractive index transparent front sheet. The composite high refractive index transparent front sheet comprises high refractive index particles dispersed in a polymer matrix. The composite high refractive index transparent front sheet increases the difference of the refractive indices of the front sheet and low refractive index medium containing electrophoretically mobile particles. As a result the reflectance properties of the display increases. 
         [0019]      FIG. 2  depicts a cross-section of a portion of a continuous high refractive index composite front sheet of a TIR-based reflective image display. This is a close-up view of an optically transparent composite front sheet  200  comprising a plurality of convex protrusions  202  on the inward surface. In an exemplary embodiment the plurality of convex protrusions  202  comprises at least one protrusion  204  in the shape of a hemisphere as illustrated in  FIG. 2 . In other embodiments, front sheet  200  may comprise beads embedded on the inward surface. 
         [0020]    In an exemplary embodiment, composite front sheet  200  comprises high index particles  206  dispersed in an optically transparent polymer matrix  208  such that the refractive index of the composite is higher than in the absence of particles  206 . In some embodiments, the diameter of the particles  206  may be less than about 400 nanometers. In other embodiments, the size of the particles  206  may be less than about 250 nanometers. In an exemplary embodiment, the particles  206  may be have an average size of about 10-20 nanometers or less. In certain embodiments, the particles may have a refractive index of about 1.65 or higher. In some embodiments, the particles may have a refractive index of about 1.8 or higher. In other embodiments, the particles may have a refractive index of about 2.0 or higher. The particles  206  may be comprised of TiO 2 , diamond, cubic zirconia, ZnS, ZnSe, germanium or other similar high refractive index optical glass materials or a combination thereof. 
         [0021]    In an exemplary embodiment, composite front sheet  200  may comprise high index particles  206  of at least about 5% by volume. In other embodiments, front sheet  200  may comprise high index particles  206  of at least about 5% to about 90% by volume. As the volume of particles  206  increases in the polymer matrix  208 , the resulting index of refraction of the hemispheres  204  may also increase. It may be advantageous to maximize the volume % of the high index particles  206  in the polymer matrix  208  to maximize the refractive index. Many factors may need to be considered when determining the volume fraction of particles  206  in the polymer matrix  208  such as processability, brittleness, tensile strength and optical properties. In an exemplary embodiment the composite front sheet  200  may have a refractive index of about 1.65 or higher. In other embodiments, the composite front sheet  200  may have a refractive index of about 1.85 or higher. 
         [0022]    In an exemplary embodiment, polymer matrix  208  may be formed from a UV-curable monomer. Polymer matrix  208  may comprise polystyrene, polyacrylate, polymethacrylate, polylactone, polylactam, polycyclic ether, polycyclic acetal, polyvinyl ether, poly-N-vinyl carbazole or polycyclic siloxane-based polymers or a combination thereof. In an exemplary embodiment, poly-1,6-hexane-diol diacrylate may be used as the polymer matrix  208 . 
         [0023]    In an exemplary method to create a composite front sheet  200 , high index particles  206  may be suspended and substantially uniformly dispersed in a liquid medium comprising of a monomer and photo-initiator. The suspension may be poured into a mold or over a structured surface comprising a negative image of the desired structure. The suspension may then be irradiated by UV-light in order to cure or polymerize the monomer and freeze the high index particles  206  in place in a substantially uniform manner throughout the polymer matrix  208 . 
         [0024]    In other embodiments, polymer matrix  208  may be a melt processable polymer. High index particles  206  may be dispersed in a high temperature liquid state of polymer  208  then cooled to room temperature in a mold to create composite front sheet  200 . In other embodiments composite front sheet  200  may be formed by embossing or stamping. 
         [0025]      FIG. 3  depicts a cross-section of a portion of a TIR-based reflective image display comprising a high refractive index composite front sheet. Display  300  embodiment comprises an optically transparent composite front sheet  302  with an outward surface  304  facing viewer  306  and a plurality of convex protrusions  308  on the inward side. Sheet  302  is similar to sheet  200  in  FIG. 2 . In an exemplary embodiment, display  300  comprises at least one protrusion  310  in the shape of a hemisphere. In an exemplary embodiment the composite front sheet  302  may have a refractive index of about 1.65 or higher. In other embodiments, the composite front sheet  302  may have a refractive index of about 1.85 or higher. 
         [0026]    Composite sheet  302  may further comprise high refractive index particles  312  dispersed in an optically transparent polymer matrix  314 . In some embodiments the diameter of the particles  312  may be less than about 400 nanometers. In other embodiments, particles  312  may be less than about 250 nanometers. In an exemplary embodiment the particles  312  may be about 10-20 nanometers in average diameter. In some embodiments, the particles may have a refractive index of about 1.8 or higher. In other embodiments the particles may have a refractive index of about 2.0 or higher. The particles  312  may be comprised of TiO 2 , diamond, cubic zirconia, ZnS, ZnSe, germanium or other similar high refractive index optical glass materials or a combination thereof. 
         [0027]    In an exemplary embodiment, polymer matrix  314  may be formed from a UV-curable monomer. Polymer matrix  314  may comprise polystyrene, polyacrylate, polymethacrylate, polylactone, polylactam, polycyclic ether, polycyclic acetal, polyvinyl ether, poly-N-vinyl carbazole or polycyclic siloxane-based polymers or a combination thereof. In an exemplary embodiment, poly-1,6-hexane-diol diacrylate may be used as the polymer matrix  314 . 
         [0028]    In other embodiments, polymer matrix  314  may be a melt-processable polymer. High index particles  312  may be dispersed in a high temperature liquid state of polymer  314  then cooled to room temperature in a mold to create composite front sheet  302 . In still other embodiments, composite front sheet  302  may be formed by embossing or stamping. 
         [0029]    Display  300  may further comprise a transparent front electrode layer  316  on the inward surface of sheet  302 . Layer  316  may comprise at least one of indium tin oxide (ITO), electrically conducting polymer or conductive metal nanoparticles dispersed in a clear polymer matrix. 
         [0030]    Display  300  comprises rear support sheet  318  and rear electrode layer  320 . Rear electrode layer  320  may be located on the inward surface of sheet  318 . Rear electrode layer  320  may comprise a thin film transistor (TFT) array, direct drive patterned array or a passive matrix array of electrodes. 
         [0031]    Display  300  may further comprise at least one dielectric layer (not shown) on the surface of one or both the front  316  and rear  320  electrode layers. A dielectric layer may protect the electrode layers. The dielectric layer may comprise at least one of an organic polymer or inorganic material. In an exemplary embodiment, the dielectric layer may comprise parylene. In other embodiments the dielectric layer may comprise a halogenated parylene. In other embodiments the dielectric layer may comprise polyimide or SiO 2 . 
         [0032]    Display  300  comprises a low refractive index medium  322  within the cavity or containment reservoir formed by the composite front sheet  302  and rear support sheet  318 . Medium  322  may be air or a liquid. In an exemplary embodiment, medium  322  may be an inert, fluorinated liquid such as a fluorinated hydrocarbon. In an exemplary embodiment, medium  322  may be Fluorinert™ perfluorinated hydrocarbon liquid available from 3M, St. Paul, Minn. 
         [0033]    Display  300  further comprises a plurality of light absorbing, electrophoretically mobile particles  324  dispersed in medium  322 . Particles  324  may be a dye or a pigment or a combination thereof. Particles  324  may be at least one of carbon black, a metal or metal oxide. Particles  324  may comprise a positive polarity or a negative polarity or both a positive and negative polarity. 
         [0034]    Display  300  in  FIG. 3  may further comprise an optional voltage bias source  326 . Bias source  326  may apply a negative or positive bias across medium  322  comprising electrophoretically mobile particles  324 . The applied bias may move at least one particle  324  through medium  322  towards the front electrode  316  or rear electrode  320  layers. 
         [0035]    Display  300  may be operated as follows. A bias of opposite polarity to certain particles  324  may be applied by voltage source  326  at the rear electrode layer  320 . At least one of the electrophoretically mobile particles  324  may move near and collect at the rear electrode  320  as shown on the left side of dotted line  328 . Incident light rays may pass through the composite front sheet  302  and may be totally internally reflected at the surface of the plurality of hemispherical protrusions  308 . This is represented by incident light ray  330  in  FIG. 3  that is totally internally reflected and exits the display as reflected light ray  332  towards viewer  306 . This may create a bright or light state of the display as observed by a viewer. 
         [0036]    A bias may be applied by source  326  of opposite polarity of the electrophoretically mobile particles  324  at the front electrode layer  316  as shown to the right of dotted line  328 . Particles  324  may move towards and collect at the front electrode  316 . Particle  324  may enter the evanescent wave region and frustrate TIR. Incident light rays may pass through the composite front sheet  302  and may be absorbed by particles  324  that have collected at the front electrode  316 . This is illustrated by incident light rays  334  and  336  in  FIG. 3 . This may create a dark state of the display. 
         [0037]    In other embodiments, any of the image displays comprising a transparent composite front sheet containing high refractive index particles may further include at least one spacer structure. Spacer structures may be used in order to control the gap between the front and rear electrodes. Spacer structures may be used to support the various layers in the displays. The spacer structures may be in the shape of circular or oval beads, blocks, cylinders or other geometrical shapes or combinations thereof. The spacer structures may comprise glass, metal, plastic or other resin. 
         [0038]    In other embodiments, any of the image displays comprising a transparent composite front sheet containing high refractive index particles may further include at least one edge seal. An edge seal may be a thermally or photo-chemically cured material. The edge seal may comprise one or more of an epoxy, silicone or other polymer based material. 
         [0039]    In other embodiments, the image displays comprising a transparent composite front sheet containing high refractive index particles may further include at least one sidewall (may also be referred to as cross-walls). Sidewalls limit particle settling, drift and diffusion to improve display performance and bistability. Sidewalls may be located within the light modulation layer. Sidewalls may completely or partially extend from the front electrode, rear electrode or both the front and rear electrodes. Sidewalls may comprise plastic or glass. 
         [0040]    In an exemplary embodiment, a directional front light may be employed with the display embodiments comprising a transparent composite front sheet containing high refractive index particles. The light source may be a light emitting diode (LED), a cold-cathode fluorescent lamp (CCFL) or a surface mount technology (SMT) incandescent lamp. 
         [0041]    In some embodiments a light diffusive layer may be used with the display embodiments comprising a transparent composite front sheet containing high refractive index particles to “soften” the reflected light observed by the viewer. In other embodiments a light diffusive layer may be used in combination with a front light. 
         [0042]    Various control mechanism for the invention may be implemented fully or partially in software and/or firmware. This software and/or firmware may take the form of instructions contained in or on a non-transitory computer-readable storage medium. Those instructions may then be read and executed by one or more processors to enable performance of the operations described herein. The instructions may be in any suitable form, such as but not limited to source code, compiled code, interpreted code, executable code, static code, dynamic code, and the like. Such a computer-readable medium may include any tangible non-transitory medium for storing information in a form readable by one or more computers, such as but not limited to read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; a flash memory, etc. 
         [0043]    In some embodiments, a tangible machine-readable non-transitory storage medium that contains instructions may be used in combination with the reflective displays comprising a transparent composite front sheet containing high refractive index particles. In other embodiments the tangible machine-readable non-transitory storage medium may be further used in combination with one or more processors. 
         [0044]      FIG. 4  shows an exemplary system for controlling a display according to one embodiment of the disclosure. In  FIG. 4 , display  400  is controlled by controller  440  having processor  430  and memory  420 . Other control mechanisms and/or devices may be included in controller  440  without departing from the disclosed principles. Controller  440  may define hardware, software or a combination of hardware and software. For example, controller  440  may define a processor programmed with instructions (e.g., firmware). Processor  430  may be an actual processor or a virtual processor or a combination thereof. Similarly, memory  420  may be an actual memory (i.e., hardware) or virtual memory (i.e., software) or a combination thereof. 
         [0045]    Memory  420  may store instructions to be executed by processor  430  for driving display  400 . The instructions may be configured to operate display  400 . In one embodiment, the instructions may include biasing electrodes associated with display  400  (not shown) through power supply  450 . When biased, the electrodes may cause movement of electrophoretic particles to a region to thereby absorb or reflect light that passes through the transparent composite front sheet containing high refractive index particles. By appropriately biasing the electrodes (not shown), mobile light absorbing particles (e.g., particles  324 ,  FIG. 3 ) may be attracted to a location at or near the transparent composite front sheet (e.g., front sheet  302  or  314 ,  FIG. 3 ) containing high refractive index particles in order to absorb or reflect the incoming light. Absorbing the incoming light creates a dark state. Reflecting the incoming light creates a light state. 
         [0046]    In some embodiments, a porous reflective layer may be used in combination with the reflective displays comprising a transparent composite front sheet containing high refractive index particles. The porous reflective layer may be interposed between the front and rear electrode layers. In other embodiments the rear electrode may be located on the surface of the porous electrode layer. 
         [0047]    In the display embodiments described herein, they may be used in such applications including electronic book readers, portable computers, tablet computers, cellular telephones, smart cards, signs, watches, wearables, shelf labels, flash drives and outdoor billboards or outdoor signs comprising a display. 
         [0048]    The following exemplary and non-limiting embodiments provide various implementations of the disclosure. 
         [0049]    Example 1 is directed to an image display, comprising: a front sheet with a refractive index of about 1.65 or higher, the front sheet having an outward surface and an inward surface; a plurality of protrusions formed on the inward surface of the front sheet, at least one of the plurality of the protrusions further comprising a plurality of high refractive index nanoparticles in a polymer matrix, wherein the plurality of high refractive index nanoparticles have a refractive index of about 1.8 or higher; and a backplane electrode layer, wherein the backplane electrode and the inward surface of the front sheet forms a cavity. 
         [0050]    Example 2 is directed to the image display of example 1, wherein the front sheet comprises an optically transparent sheet. 
         [0051]    Example 3 is directed to the image display of examples 1 or 2, wherein the plurality of protrusions define a plurality of beads formed on an inward surface of the front sheet. 
         [0052]    Example 4 is directed to the image display of any preceding example, wherein the plurality of protrusions define a plurality of hemispherical protrusions comprising the polymer matrix. 
         [0053]    Example 5 is directed to the image display of any preceding example, wherein the cavity is configured to receive an electrophoresis medium with a plurality of electrophoretically mobile particles suspended in the medium. 
         [0054]    Example 6 is directed to the image display of any preceding example, further comprising a voltage source for applying a voltage across the cavity to move the plurality of electrophoretically mobile particles within the medium. 
         [0055]    Example 7 is directed to the image display of any preceding example, wherein the plurality of high refractive index nanoparticles in a polymer matrix have a diameter of about 400 nm or less. 
         [0056]    Example 8 is directed to the image display of any preceding example, wherein the plurality of high refractive index nanoparticles in a polymer matrix have a diameter of about 250 nm or less. 
         [0057]    Example 9 is directed to the image display of any preceding example, wherein the polymer matrix comprises polystyrene, polyacrylate, polymethacrylate, polylactone, polylactam, polycyclic ether, polycyclic acetal, polyvinyl ether, poly-N-vinyl carbazole, poly-1,6-hexane-diol diacrylate or a polycyclic siloxane or a combination thereof. 
         [0058]    Example 10 is directed to the image display of any preceding example, wherein the polymer matrix is formed by UV-curing a monomer. 
         [0059]    Example 11 is directed to a method to form an image display, the method comprising: providing a front sheet with a refractive index of about 1.65 or higher, the front sheet having an outward surface and an inward surface; forming a plurality of protrusions on the inward surface of the front sheet, at least one of the plurality of the protrusions further comprising a plurality of high refractive index nanoparticles in a polymer matrix, wherein the plurality of high refractive index nanoparticles have a refractive index of about 1.8 or higher; and forming a backplane electrode layer facing the plurality of protrusions to form a cavity between the backplane electrode and the plurality of protrusions. 
         [0060]    Example 12 is directed to the method of example 11, wherein forming the plurality of protrusions further comprises forming a plurality of beads over the inward surface of the front sheet. 
         [0061]    Example 13 is directed to the method of examples 11 or 12, wherein forming the plurality of protrusions further comprises forming a plurality of hemispherical protrusions including the polymer matrix. 
         [0062]    Example 14 is directed to the method of any preceding example, wherein the cavity is configured to receive an electrophoresis medium with a plurality of electrophoretically mobile particles suspended in the medium. 
         [0063]    Example 15 is directed to the method of any preceding example, further comprising applying a voltage across the cavity to move the plurality of electrophoretically mobile particles within the medium. 
         [0064]    Example 16 is directed to the method of any preceding example, wherein the plurality of high refractive index nanoparticles in a polymer matrix have a diameter of about 400 nm or less. 
         [0065]    Example 17 is directed to the method of any preceding example, wherein the plurality of high refractive index nanoparticles in a polymer matrix have a diameter of about 250 nm or less. 
         [0066]    Example 18 is directed to the method of any preceding example, wherein the polymer matrix comprises polystyrene, polyacrylate, polymethacrylate, polylactone, polylactam, polycyclic ether, polycyclic acetal, polyvinyl ether, poly-N-vinyl carbazole, poly-1,6-hexane-diol diacrylate or a polycyclic siloxane or a combination thereof. 
         [0067]    Example 19 is directed to the method of any preceding example, wherein the polymer matrix is formed by UV-curing a monomer. 
         [0068]    While the principles of the disclosure have been illustrated in relation to the exemplary embodiments shown herein, the principles of the disclosure are not limited thereto and include any modification, variation or permutation thereof.