PATENT DOCUMENT

Publication Number: US-11775021-B2
Application Number: US-202117404783-A
Country: US
Kind Code: B2

Title: Moisture-insensitive optical touch sensors

Abstract:
An electronic device may have an optical touch sensor that is insensitive to the presence of moisture. The display may present images through a display cover layer. A light source may illuminate an external object such as a user&#39;s finger when the object contacts a surface of the display cover layer. This creates scattered light that may be detected by an array of light sensors. A metasurface grating may be used to couple light from the light source into the display cover layer at an angle such that total internal reflection within the display cover layer is sustained across the display cover layer even when the display cover layer is immersed in water or otherwise exposed to moisture. Additional metasurface gratings may be formed on the display cover layer to redirect light propagating within the display cover layer away from edges that might otherwise defeat total internal reflection.

Claims:
The invention claimed is: 
     
       1. An electronic device, comprising:
 a display; 
 a display cover layer through which the display presents images; 
 light detectors configured to detect a finger touch on the display cover layer; 
 a light source that emits light; and 
 a metasurface grating interposed between the light source and the display cover layer, wherein the metasurface grating couples a first portion of the light into the display cover layer at a first angle such that the first portion of the light is guided within a first region of the display cover layer via total internal reflection, wherein the metasurface grating couples a second portion of the light into the display cover layer at a second angle such that the second portion of the light is guided within a second region of the display cover layer via total internal reflection, and wherein the total internal reflection is locally defeated by the finger touch to scatter the light towards the light detectors. 
 
     
     
       2. The electronic device defined in  claim 1  wherein the light source comprises an array of light-emitting elements. 
     
     
       3. The electronic device defined in  claim 2  wherein the light-emitting elements comprise vertical cavity surface emitting lasers that emit the light in different directions. 
     
     
       4. The electronic device defined in  claim 3  wherein the metasurface grating comprises a first set of nanostructures optimized to receive light from a first of the light-emitting elements and a second set of nanostructures optimized to receive light from a second of the light-emitting elements. 
     
     
       5. The electronic device defined in  claim 1  wherein the metasurface grating comprises nanoposts. 
     
     
       6. The electronic device defined in  claim 5  wherein the nanoposts comprise first elliptical nanoposts having a first size and second elliptical nanoposts having a second size that is larger than the first size. 
     
     
       7. The electronic device defined in  claim 6  wherein the first elliptical nanoposts and the second elliptical nanoposts have respective major axes that are parallel to each other. 
     
     
       8. The electronic device defined in  claim 1  further comprising an additional metasurface grating on the display cover layer, wherein the light propagating within the display cover layer is incident upon the additional metasurface grating and redirected by the additional metasurface grating. 
     
     
       9. The electronic device defined in  claim 8  wherein the additional metasurface grating is located adjacent to an edge of the display cover layer and wherein the additional metasurface grating redirects the light away from the edge. 
     
     
       10. The electronic device defined in  claim 9  wherein the additional metasurface grating has at least one property that varies along a length of the edge. 
     
     
       11. The electronic device defined in  claim 1  wherein the metasurface grating comprises nanostructures formed from a first material having a first refractive index and encapsulated with a second material having a second refractive index that is lower than the first refractive index. 
     
     
       12. An electronic device, comprising:
 a display; 
 a display cover layer through which the display presents images; 
 light detectors configured to detect a finger touch on the display cover layer; 
 a light source that emits light into the display cover layer, wherein the light is guided within the display cover layer via total internal reflection; and 
 a metasurface grating on a lower surface of the display cover layer, wherein the light within the display cover layer is incident upon the metasurface grating and is redirected by the metasurface grating to sustain the total internal reflection and wherein the total internal reflection is locally defeated by the finger touch to scatter the light towards the light detectors. 
 
     
     
       13. The electronic device defined in  claim 12  wherein the metasurface grating comprises nanoposts. 
     
     
       14. The electronic device defined in  claim 13  wherein the nanoposts comprise first elliptical nanoposts having a first size and second elliptical nanoposts having a second size that is larger than the first size. 
     
     
       15. The electronic device defined in  claim 14  wherein the first elliptical nanoposts each have a first major axis and the second elliptical nanoposts each have a second major axis and wherein the first major axis is perpendicular to the second major axis. 
     
     
       16. The electronic device defined in  claim 12  wherein the display comprises pixels that serve as the light detectors. 
     
     
       17. An electronic device, comprising:
 a display; 
 a display cover layer through which the display presents images; 
 light sensors configured to detect a finger touch on the display cover layer; 
 an array of light-emitting elements that emit light in different directions towards a lower surface of the display cover layer; and 
 a metasurface grating interposed between the array of light-emitting elements and the display cover layer, wherein the metasurface grating couples the light into the display cover layer at an angle such that the light propagates within the display cover layer via total internal reflection, wherein the metasurface grating comprises first and second non-overlapping metasurface structures, wherein the first metasurface structures are optimized to receive the light from a first of the light-emitting elements and the second metasurface structures are optimized to receive the light from a second of the light-emitting elements, and wherein the total internal reflection is locally defeated by the finger touch to scatter the light towards the light sensors. 
 
     
     
       18. The electronic device defined in  claim 17  wherein the display comprises pixels that serve as the light sensors. 
     
     
       19. The electronic device defined in  claim 17  wherein the metasurface grating comprises first elliptical nanoposts having a first size and second elliptical nanoposts having a second size that is larger than the first size. 
     
     
       20. The electronic device defined in  claim 17  wherein the light-emitting elements comprise vertical cavity surface emitting lasers.

Description:
FIELD 
     This relates generally to electronic devices, and, more particularly, to electronic devices with touch sensors. 
     BACKGROUND 
     Electronic devices such as tablet computers, cellular telephones, and other equipment are sometimes provided with touch sensors. For example, displays in electronic devices are often provided with capacitive touch sensors to receive touch input. It can be challenging to operate such sensors in the presence of moisture. 
     SUMMARY 
     An electronic device may include an optical touch sensor that is insensitive to the presence of moisture. The optical touch sensor may be a two-dimensional optical touch sensor such as a total internal reflection touch sensor. The optical touch sensor may be used to gather touch input while the electronic device is immersed in water or otherwise exposed to moisture. 
     An array of pixels in the display may be used to display images. A display cover layer may overlap the array of pixels. A light source may illuminate an external object such as a finger of a user when the object contacts a surface of the display cover layer. This creates scattered light that may be detected by an array of light sensors. 
     A metasurface grating may be used to couple light from the light source into the display cover layer at an angle such that total internal reflection within the display cover layer is sustained across the display cover layer even when the display cover layer is immersed in water or otherwise exposed to moisture. Additional metasurface gratings may be formed on the display cover layer to redirect light propagating within the display cover layer away from edges that might otherwise defeat total internal reflection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an illustrative electronic device in accordance with an embodiment. 
         FIG.  2    is a perspective view of an illustrative electronic device in accordance with an embodiment. 
         FIG.  3    is a cross-sectional side view of an illustrative electronic device in accordance with an embodiment. 
         FIG.  4    is a top view of an illustrative array of pixels for an electronic device in accordance with an embodiment. 
         FIGS.  5  and  6    are cross-sectional side views of illustrative pixel arrays for electronic devices in accordance with embodiments. 
         FIG.  7    is a cross-sectional side view of an illustrative optical touch sensor arrangement in accordance with an embodiment. 
         FIG.  8    is a cross-sectional side view of an illustrative optical touch sensor arrangement based on total internal reflection in accordance with an embodiment. 
         FIG.  9    is a cross-sectional side view of an illustrative optical touch sensor arrangement in which an in-coupling metasurface grating is used to couple light into a display cover layer at a desired angle and one or more additional metasurface gratings are used to redirect light propagating within the display cover layer to sustain total internal reflection within the display cover layer in accordance with an embodiment. 
         FIG.  10    is a cross-sectional side view of an illustrative optical touch sensor arrangement in which a prism is used to couple light into a display cover layer at a desired angle and in which one or more metasurface gratings are used to redirect light propagating within the display cover layer to sustain total internal reflection within the display cover layer in accordance with an embodiment. 
         FIG.  11    is a top view of an illustrative light source for an optical touch sensor having a one-dimensional array of light-emitting elements in accordance with an embodiment. 
         FIG.  12    is a top view of an illustrative light source for an optical touch sensor having a two-dimensional array of light-emitting elements in accordance with an embodiment. 
         FIGS.  13 ,  14 ,  15 , and  16    are top views of illustrative optical touch sensors having different patterns of metasurface gratings for coupling light into a display cover layer and redirecting light within the display cover layer to ensure that total internal reflection is sustained within the display cover layer in accordance with embodiments. 
         FIG.  17    is a top view of illustrative nanostructures that may be used in a metasurface grating to couple light into a display cover layer at a desired angle in accordance with an embodiment. 
         FIG.  18    is a top view of illustrative nanostructures that may be used in a metasurface grating to redirect light propagating within a display cover layer in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A schematic diagram of an illustrative electronic device that may include an optical touch sensor is shown in  FIG.  1   . Electronic device  10  of  FIG.  1    may be a computing device such as a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wristwatch or other device worn on a user&#39;s wrist, a pendant device, a headphone or earpiece device, a head-mounted device such as eyeglasses, goggles, or other equipment worn on a user&#39;s head, or other wearable or miniature device, a television, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other electronic equipment. Illustrative configurations in which device  10  is a portable device such as a wristwatch, cellular telephone, or tablet computer and, more particularly, a portable device that is water resistant or waterproof may sometimes be described herein as an example. 
     As shown in  FIG.  1   , electronic device  10  may have control circuitry  16 . Control circuitry  16  may include storage and processing circuitry for supporting the operation of device  10 . The storage and processing circuitry may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  16  may be used to control the operation of device  10 . The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, application specific integrated circuits, etc. Control circuitry  16  may include communications circuitry for supporting wired and/or wireless communications between device  10  and external equipment. For example, control circuitry  16  may include wireless communications circuitry such as cellular telephone communications circuitry and wireless local area network communications circuitry. 
     Input-output circuitry in device  10  such as input-output devices  12  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output devices  12  may include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, haptic output devices, cameras, light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device  10  by supplying commands through input-output devices  12  and may receive status information and other output from device  10  using the output resources of input-output devices  12 . 
     Input-output devices  12  may include one or more displays such as display  14 . Display  14  may be an organic light-emitting diode display, a display formed from an array of crystalline semiconductor light-emitting diode dies, a liquid crystal display, or other display. Display  14  may be a touch screen display that includes an optical touch sensor for gathering touch input from a user. The optical touch sensor may be configured to operate even when device  10  is immersed in water or otherwise exposed to moisture. If desired, the optical touch sensor may also be configured to operate when a user is wearing gloves, which might be difficult or impossible with some capacitive touch sensors. Moreover, because the optical touch sensor operates optically, the touch sensor is not impacted by grounding effects that might impact the operation of capacitive touch sensors. 
     As shown in  FIG.  1   , input-output devices  12  may include sensors  18 . Sensors  18  may include touch sensors. Touch sensors may be provided for display  14  and/or other portions of device  10  and may be formed from an array of capacitive touch sensor electrodes, acoustic touch sensor structures, resistive touch components, force-based touch sensor structures, light-based touch sensor structures, or other suitable touch sensor arrangements. Illustrative optical touch sensor arrangements for device  10  (e.g., for display  14  of device  10 ) are sometimes described herein as an example. 
     Sensors  18  may include capacitive sensors, light-based proximity sensors, magnetic sensors, accelerometers, force sensors, touch sensors, temperature sensors, pressure sensors, inertial measurement units, accelerometers, gyroscopes, compasses, microphones, radio-frequency sensors, three-dimensional image sensors (e.g., structured light sensors with light emitters such as infrared light emitters configured to emit structured light and corresponding infrared image sensors, three-dimensional sensors based on pairs of two-dimensional image sensors, etc.), cameras (e.g., visible light cameras and/or infrared light cameras), light-based position sensors (e.g., lidar sensors), monochrome and/or color ambient light sensors, and other sensors. Sensors  18  such as ambient light sensors, image sensors, optical proximity sensors, lidar sensors, optical touch sensors, and other sensors that use light and/or components that emit light such as status indicator lights and other light-emitting components may sometimes be referred to as optical components. 
     A perspective view of an illustrative electronic device of the type that may include an optical touch sensor is shown in  FIG.  2   . In the example of  FIG.  2   , device  10  includes a display such as display  14  mounted in housing  22 . Display  14  may be a liquid crystal display, a light-emitting diode display such as an organic light-emitting diode display or a display formed from crystalline semiconductor light-emitting diode dies, or other suitable display. Display  14  may have an array of image pixels extending across some or all of front face F of device  10  and/or other external device surfaces. The array of image pixels may be rectangular or may have other suitable shapes. Display  14  may be protected using a display cover layer (e.g., a transparent front housing layer) such as a layer of transparent glass, clear plastic, sapphire, or other clear layer. The display cover layer may overlap the array of image pixels. 
     Housing  22 , which may sometimes be referred to as an enclosure or case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. As shown in the cross-sectional side view of device  10  of  FIG.  3   , housing  22  and display  14  may separate an interior region of device  10  such as interior region  30  from an exterior region surrounding device  10  such as exterior region  32 . Housing  22  may be formed using a unibody configuration in which some or all of housing  22  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). If desired, a strap may be coupled to a main portion of housing  22  (e.g., in configurations in which device  10  is a wristwatch or head-mounted device). Internal electrical components  36  (e.g., integrated circuits, discrete components, etc.) for forming control circuitry  16  and input-output devices  12  may be mounted in interior  30  of housing  22  (e.g., on one or more substrates such as printed circuit  38 ). In some configurations, components  36  may be attached to display  14  (e.g., circuitry may be mounted to the surface of display  14 ). To obtain touch input from a user&#39;s fingers or other external object (see, e.g., user finger  34 ), display  14  may include a touch sensor such as an optical touch sensor (e.g., a two-dimensional optical touch sensor that gathers information on the XY location of a user&#39;s finger or other external object when that object touches the surface of display  14 ). 
     Display  14  may include a display panel such as display panel  14 P that contains pixels P covered by display cover layer  14 CG. The pixels of display  14  may cover all of the front face of device  10  or display  14  may have pixel-free areas (e.g., notches, rectangular islands, inactive border regions, or other regions) that do not contain any pixels. Pixel-free areas may be used to accommodate an opening for a speaker and windows for optical components such as image sensors, an ambient light sensor, an optical proximity sensor, a three-dimensional image sensor such as a structured light three-dimensional image sensor, a camera flash, an illuminator for an infrared image sensor, an illuminator for a three-dimensional sensor such as a structured light sensor, a time-of-flight sensor, a lidar sensor, etc. 
       FIG.  4    is a top view of an array of illustrative pixels P in display panel (display)  14 P. As shown in  FIG.  4   , pixels P may include image pixels such as pixel P- 1  that are used in presenting images for a user of device  10 . Image pixels in display  14  may, for example, include a rectangular array of red, green, and blue light-emitting diodes or backlight red, green, and blue liquid crystal display pixels for presenting color images to a user. 
     Pixels P may also contain optical touch sensor pixels such as pixel P- 2 . Optical touch sensor pixels may include pixels that serve as light detectors and/or light emitters. Emitted light that reflects from a user&#39;s finger on the surface of display  14  may be detected using the light detectors, thereby determining the location of the user&#39;s finger. If desired, diodes or other components may be used to form pixels that can be operated both as image pixels and as touch sensor pixels. When used as touch sensor pixels, image pixels can be configured to emit optical touch sensor illumination and/or to detect optical touch sensor light. For example, a display emitter can be used to produce image light for a display while also being used to produce optical touch sensor illumination, and/or while also being used to serve as a photodetector for an optical touch sensor. 
     Image pixels such as pixels P- 1  and/or optical touch sensor pixels P- 2  may have any suitable pitch. For example, image pixels may have a density that is sufficient to display high-quality images for a user (e.g., 200-300 pixels per inch or more, as an example), whereas optical touch sensor pixels may, if desired, have a lower density (e.g., less than 200 pixel per inch, less than 50 pixels per inch, less than 20 pixels per inch, etc.). 
     Image pixels emit visible light for viewing by a user. For example, in a color display, image pixels may emit light of different colors of image light such as red, green, and blue light, thereby allowing display  14  to present color images. Optical touch sensor pixels may emit and/or detect visible light and/or infrared light (and/or, if desired, ultraviolet light). 
     In some configurations, optical touch sensor light for illuminating a user&#39;s fingers passes directly through the thickness of display cover layer  14 CG from its interior surface to its exterior surface. Optical touch sensors in which light that illuminates the user&#39;s fingers passes outwardly from light sources such as light-emitting pixels in display panel  14 P directly through the thickness of display cover layer  14 CG before being backscattered in the reverse (inward) direction to the light detectors of the optical touch sensors may sometimes be referred to herein as direct illumination optical touch sensors. 
     In other configurations, light for an optical touch sensor may be provided using edge-coupled light-emitting diodes or other light sources that emit light into the edge surface of display cover layer  14 P that is then guided within layer  14 CG in accordance with the principal of total internal reflection. For example, a light-emitting diode may emit light into the righthand edge of display cover layer  14 CG that is guided from the righthand edge of display cover layer  14 CG to the opposing lefthand edge of display cover layer  14 CG within the light guide formed by display cover layer  14 CG. In this way, light may be guided laterally across layer  14 CG in the absence of contact from a user&#39;s finger. When a user&#39;s finger touches the surface of layer  14 CG, total internal reflection can be locally defeated. This local frustration of total internal reflection scatters light inwardly toward the light detectors of the optical touch sensor. Optical touch sensors that are based on locally defeating total internal reflection may sometimes be referred to herein as total internal reflection optical touch sensors. If desired, objects other than the fingers of users (e.g., a computer stylus, a glove, and/or other external objects with appropriate optical properties) may also locally defeat total internal reflection, thereby allowing the optical touch sensors to function over a wide range of operating environments. 
     Pixels P that emit light and pixels P that detect light in display panel  14 P may be formed using shared structures and/or structures that are separate from each other. These structures may be located in the same plane (e.g., as part of a single layer of pixels on a single substrate) and/or may include components located in multiple planes (e.g., in arrangements in which some components are formed in a given layer and other components are formed in one or more additional layers above and/or below the given layer). 
     Consider, as an example, an optical touch sensor that contains an array of photodetectors formed from reverse-biased diodes. These diodes may be dedicated photodetectors or may be light-emitting didoes that serve as light detectors when reverse biased and that serve as light sources when forward biased. Light sources in the optical touch sensor may include visible light sources (e.g., visible light sources dedicated to use in the optical touch sensor or visible light sources that also serve as image pixels) and/or may include infrared light sources. Light-emitting pixels for the optical touch sensor may be formed from light-emitting diodes (e.g., dedicated light-emitting diodes or diodes that serve as light-emitting diodes when forward biased and that serve as photodetectors when reversed biased). Light-emitting pixels may also be formed from pixels P that are backlit with light from a backlight unit to form backlit pixels (e.g., backlit liquid crystal display pixels). In general, any type of photodetector signal processing circuitry may be used to detect when a photodetector has received light. For example, photodetectors may be configured to operate in a photoresistor mode in which the photodetectors change resistance upon exposure to light and corresponding photodetector signal processing circuitry may be used to measure the changes in photodetector resistance. As another example, the photodetectors may be configured to operate in a photovoltaic mode in which a voltage is produced when light is sensed and corresponding photodetector signal processing circuitry may be used to detect the voltage signals that are output from the photodetectors. Semiconductor photodetectors may be implemented using phototransistors or photodiodes. Other types of photosensitive components may be used, if desired. 
       FIG.  5    is a cross-sectional side view of an illustrative display having an array of pixels P that are not backlit. Pixels P of  FIG.  5    may include light-emitting diodes (e.g., organic light-emitting diodes such as thin-film organic light-emitting diodes and/or light-emitting diodes formed from crystalline semiconductor light-emitting diode dies). During operation, image pixels formed from the light-emitting diodes may present an image on display  14  that is visible to a user such as viewer  40  who is viewing display  14  in direction  42 . 
       FIG.  6    is a cross-sectional side view of an illustrative display having an array of pixels P that are backlit using backlight unit  44 . Backlight unit  44  may include one or more strips of light-emitting diodes that emit light into a backlight unit light guide layer (e.g., a clear optical film with light-scattering structures). As the emitted light propagates through the light guide layer, the scattered light serve as backlight illumination for pixels P (e.g., liquid crystal display pixels). In another illustrative configuration, backlight unit  44  is a direct lit backlight unit that contains an array of backlight light-emitting diodes that provide backlight (e.g., an array-type backlight unit that supports local dimming functionality). 
       FIG.  7    is a cross-sectional side view of an illustrative display with a direct illumination optical touch sensor. As shown in  FIG.  7   , visible and/or infrared light sources associated with display panel  14 P may emit illumination  46  that travels directly through display cover layer  14 CG from its inner surface to its outer surface, thereby illuminating an external object contacting the surface of display  14  such as finger  34 . This creates localized backscattered light  48  that propagates in the inward (−Z) direction and that is detected by photodetectors associated with display panel  14 P that are directly below finger  34 . In this way, the optical touch sensor can determine the lateral position (XY location) of finger  34 . 
       FIG.  8    is a cross-sectional side view of an illustrative display with a total internal reflection optical touch sensor. As shown in  FIG.  8   , display  14  may include display cover layer  14 CG and display panel  14 P. Image pixels in panel  14 P may display images that are viewable by a viewer through display cover layer  14 CG. The outermost surface of display panel  14 P may be separated from the opposing innermost surface of display cover layer  14 CG by layer  50 . Layer  50  may be formed from air, liquid, polymer (e.g., polymer adhesive such as optically clear adhesive, pressure sensitive adhesive, other polymer materials, etc.), glass, other materials, and/or combinations of these materials. Light  46  may be coupled into layer  14 CG through the sidewalls of layer  14 CG (e.g., at the righthand edge surface at the peripheral of display cover layer  14 CG in the example of  FIG.  8   ). 
     Any suitable optical coupling structures may be used to direct light  46  into display cover layer  14 CG. In the example of  FIG.  8   , light  46  is emitted by a light source such as light source  52 . Light source  52  may include one or more light-emitting diodes such as visible or infrared light-emitting diodes or one or more visible or infrared laser diode (e.g., laser diodes such as vertical cavity surface emitting lasers, edge-emitting lasers, or other laser types). Vertical cavity surface emitting diodes may have narrow spectral bandwidth (providing increased signal-to-noise ratio), may have narrow divergence, which reduces scattering from cover layer  14 CG), may allow for polarization-based sensing, and may allow for time-gating and/or temporal multiplexing). 
     Collimator  54  may be used to collimate the emitted light from light source  52  (e.g., to form a beam of light with parallel light rays). A prism such as prism  56  or other optical coupler may be coupled between collimator  54  and display cover layer  14 CG. Prism  56  may, for example, be mounted to the edge of display cover layer  14 CG to help direct light into the edge of display cover layer  14 CG. During operation, optical coupling structures such as collimator  54  and a prism or other optical coupler may be used to couple light  46  that is emitted from light source  52  into the interior of display cover layer  14 CG in a beam that is oriented at a desired angle relative to the surfaces of layer  14 CG (e.g., at an angle A with respect to surface normal n of display cover layer  14 CG). At this angle A, light  46  will propagate within layer  14 CG in accordance with the principal of total internal reflection unless total internal reflection is locally defeated by the presence of finger  34  on the outer surface of layer  14 CG. 
     Angle A is selected (and the materials used for layer  14 CG and layer  50  are selected) so that light  46  will reflect from the innermost surface of layer  14 CG in accordance with the principal of total internal reflection. Layer  14 CG may, as an example, have a refractive index n 1  (e.g., 1.5 for glass or 1.76 for sapphire as examples), whereas layer  50  may have a refractive index n 2  that is less than n 1  (e.g., less than 1.5 when layer  14 CG is glass or less than 1.76 when layer  14 CG is sapphire). The refractive index difference between n 1  and n 2  may be at least 0.05, at least 0.1, at least 0.2, or other suitable value). 
     Angle A is also selected so that light  46  will reflect from the uppermost surface of layer  14 CG in accordance with the principal of total internal reflection (in the absence of finger  34 ). In some environments, device  10  will be immersed in water  60  or otherwise exposed to moisture (rain droplets, perspiration, fresh or salt water surrounding device  10  when a user is swimming, etc.). Angle A is preferably selected to ensure that the presence of water  60  will not defeat total internal reflection while ensuring that the presence of finger  34  will locally defeat total internal reflection and thereby produce localized scattered light  48  for detection by the nearby photodetectors of the optical touch sensor. This allows the total internal reflection optical touch sensor to operate whether or not the some or all of the surface of display  14  is immersed in water or otherwise exposed to moisture. 
     Consider, as an example, a first illustrative scenario in which layer  14 CG is formed from a material with a refractive index of 1.5 (e.g., glass). Finger  34  may be characterized by a refractive index of 1.55. Water  60  may be characterized by a refractive index of 1.33. Layer  50  may have a refractive index of less than 1.5. In this first scenario, total internal reflection at the upper surface of layer  14 CG when water  60  is present is ensured by the selection of a material for layer  14 CG with a refractive index greater than water and by selecting angle A to be greater than the critical angle at the upper surface of layer  14 CG (in this example, greater than 62.46°, which is the critical angle associated with total internal reflection at the glass/water interface). To ensure total internal reflection is sustained at the lower surface of layer  14 CG, the selected value of A should be greater than the critical angle associated with the lower interface. If, as an example, layer  50  is formed from a material with a refractive index of 1.33 (the same as water) or less, the critical angle associated with the lower interface will be at least 62.46°, so A should be greater than 62.46°. If, on the other hand, layer  50  is formed from a material with a refractive index between 1.33 and 1.5, the critical angle at the lower interface will be increased accordingly and the angle A should be increased to be sufficient to ensure total internal reflection at the lower interface. Regardless of which value is selected for angle A, total internal reflection will be supported at both the lower and upper surfaces of layer  14 CG (whether layer  14 CG is in air or immersed in water), so long as finger  34  is not present. Because finger  34  has a refractive index (1.55) that is greater than that of layer  14 CG (which is 1.5 in this first scenario), whenever finger  34  is present on the upper surface of layer  14 CG, total internal reflection will be defeated at finger  34 , resulting in scattered light  48  that can be detected by the light detectors of the total internal reflection optical touch sensor associated with display  14 . 
     The refractive index of layer  14 CG need not be less than the refractive index of finger  34 . Consider, as an example, a second illustrative scenario in which layer  14 CG is formed from a crystalline material such as sapphire with a refractive index of 1.76. In this second scenario, the angle A should be selected to be both: 1) sufficiently high to ensure that total internal reflection is sustained at the upper (and lower) surfaces of layer  14 CG in the absence of finger  34  (even if water  60  is present) and 2) sufficiently low to ensure that total internal reflection at the upper surface will be locally defeated when finger  34  is touching the upper surface to provide touch input. Total internal reflection at the upper surface may be ensured by selecting a value of A that is greater than the critical angle associated with a sapphire/water interface (e.g., the value of angle A should be greater than arcsin (1.33/1.76), which is 49.08°). Total internal reflection at the lower interface is ensured by selecting a material for layer  50  that has an index of refraction of 1.33 or less (in which case A may still be greater than 49.08°) or by selecting a material for layer  50  that has a larger index (but still less than 1.55) and adjusting the value of A upwards accordingly. To ensure that total internal reflection at the upper surface can be defeated locally by finger  34 , the value of angle A should be less than the critical angle associated with a sapphire/finger interface (e.g., less than arcsin (1.55/1.76), which is 61.72°). Thus, in scenarios in which the refractive index of layer  14 CG is greater than the refractive index of finger  34 , there will be a range of acceptable values for A bounded by a lower limit (e.g., 49.08° in this example) and an upper limit (e.g., 61.72° in this example). 
     If desired, one or more gratings such as metasurface gratings may be used to couple light from light source  52  into cover layer  14 CG and/or to redirect light within cover layer  14 CG to ensure that total internal reflection is sustained within cover layer  14 CG (e.g., even in the presence of moisture, curved edges of cover layer  14 CG, and/or other elements that might otherwise defeat total internal reflection). This type of arrangement is illustrated in  FIGS.  9  and  10   . 
     In the example of  FIG.  9   , light source  52  emits light  46  vertically towards the lower surface  14 L of cover layer  14 CG (e.g., upwards along the Z-axis of  FIG.  9   ) into grating  78 A. Grating  78 A may be interposed between the lower surface  14 L of cover layer  14 CG and light source  52  and may be used to redirect light  46  into cover layer  14 CG at the desired angle to achieve total internal reflection within cover layer  14 CG (e.g., angle A of  FIG.  8   ). Using grating  78 A to couple light directly from light source  52  into cover layer  14 CG may eliminate the need for other optical coupling structures (e.g., prisms such as prism  56  of  FIG.  8   ) while also reducing the lateral footprint of the optical touch sensor (e.g., the lateral footprint in the X-Y plane of  FIG.  9   ). Gratings  78 A that are used to couple light into cover layer  14 CG may sometimes be referred to as in-coupling gratings. 
     If desired, one or more additional gratings  78 B may be formed on the lower surface  14 L of cover layer  14 CG for redirecting light within cover layer  14 CG. For example, gratings  78 B may be used to help uniformly distribute light  46  throughout cover layer  14 CG and/or to avoid portions of cover layer  14 CG that might otherwise defeat total internal reflection such as curved corners, curved edges, etc. Gratings  78 B that are used to redirect light within cover layer  14 CG may sometimes be referred to as deflecting gratings. In-coupling metasurface gratings  78 A may receive light  46  directly from light source  52 , whereas deflecting gratings  78 B may receive light  46  that is already propagating within cover layer  14 CG. 
     In the example of  FIG.  10   , optical coupling structures such as prism  56  and/or other optical coupling structures  80  may be used to couple light  46  from light source  52  into edge surface  14 E of cover layer  14 CG at the desired angle to achieve total internal reflection within cover layer  14 CG (e.g., angle A of  FIG.  8   ). One or more gratings  78 B may be formed on the lower surface  14 L of cover layer  14 CG for redirecting light within cover layer  14 CG. For example, gratings  78 B may be used to help uniformly distribute light  46  throughout cover layer  14 CG and/or to avoid portions of cover layer  14 CG that might otherwise defeat total internal reflection such as curved corners, curved edges, etc. 
     Gratings  78 A and  78 B may be surface-relief gratings, metasurface gratings, volume Bragg polarization gratings, reflective or transmissive gratings, reflective or transmissive volume holograms, etc. Arrangements in which gratings  78 A and  78 B are metasurface gratings are sometimes described herein as an illustrative example. 
     Metasurfaces for gratings  78 A and  78 B may be formed from an array of nanostructures such as silicon pillars or other structures. Metasurface polarization grating couplers may be configured to work independently under different polarizations (e.g., different linear polarization, different circular polarizations, different polarizations of infrared light, etc.). Multiwavelength operation may be supported, if desired. 
       FIGS.  11  and  12    are top views of illustrative light sources  52  that may be used to illuminate an optical touch sensor. In the example of  FIG.  11   , light source  52  includes a one-dimensional array of light-emitting elements  52 P (e.g., a one-dimensional array of laser diodes such as vertical cavity surface emitting lasers or a one-dimensional array of light-emitting diodes). Light-emitting elements  52 P may be configured to emit light  46  in different directions (e.g., to create illumination in the shape of a vertically oriented cone), and/or may be configured to emit light  46  in the same direction. 
     In the example of  FIG.  12   , light source  52  includes a two-dimensional array of light-emitting elements  52 P (e.g., a two-dimensional array of laser diodes such as vertical cavity surface emitting lasers or a two-dimensional array of light-emitting diodes). Light-emitting elements  52 P may be configured to emit light  46  in different directions and/or may be configured to emit light  46  in the same direction. 
     If desired, in-coupling gratings  78 A that receive light directly from light source  52  may be tailored to the angles at which light  46  is emitted from light sources  52 P (e.g., some metasurface structures in in-coupling grating  78 A may be optimized to receive light at a first angle from a first one of light sources  52 P while other metasurface structures in the same in-coupling grating  78 A may be optimized to receive light at a second angle from a second one of light sources  52 P). Tailoring the metasurface structures in in-coupling grating  78 A to the angle of light  46  emitted from individual light sources  52 P may ensure that light  46  is coupled into cover layer  14 CG at the desired angle to achieve total internal reflection (e.g., angle A of  FIG.  8   ). For example, surface-relief metagratings may include nanostructures that are tailored by adjusting the size, shape, spacing, periodicity, angular orientation, and/or material composition of the nanostructures. 
       FIGS.  13 ,  14 ,  15 ,  16 , and  17    are top views of cover layer  14 CG showing illustrative examples of different patterns of metasurface gratings  78 A and  78 B that may be used to couple light into cover layer  14 CG and/or to redirect light within cover layer  14 CG to sustain total internal reflection in the presence of moisture, curved edges, and/or other elements (besides finger  34 ) that might otherwise defeat total internal reflection. 
     In the example of  FIG.  13   , first and second light sources  52  may be mounted under a lower surface of cover layer  14 CG (e.g., lower surface  14 L of  FIG.  9   ) and may emit light vertically in the Z-direction of  FIG.  13   . Light sources  52  may be mounted on opposing sides of cover layer  14 CG. For example, a first light source  52  may be mounted adjacent to side  82 A of cover layer  14 CG, and a second light source  52  may be mounted adjacent to opposing side  82 D of cover layer  14 CG. Light sources  52  may each include a single light-emitting element (e.g., a single laser diode and/or a single light-emitting diode), may include a one-dimensional array of light-emitting elements (e.g., as in the example of  FIG.  11   ), or may include a two-dimensional array of light-emitting elements (e.g., as in the example of  FIG.  12   ). In-coupling metasurface gratings  78 A in regions A may be formed between light sources  52  and the lower surface of cover layer  14 CG. If desired, the in-coupling metasurface gratings  78 A in regions A may be optimized to receive different angles of light  46  from light sources  52  and to redirect light  46  into cover layer  14 CG at the desired angle to achieve total internal reflection (e.g., angle A of  FIG.  8   ). For example, gratings  78 A in regions A may receive light  46  that is initially oriented vertically in the Z-direction and may redirect light  46  so that it enters cover layer  46  at the desired angle and propagates within cover layer  14 CG along the X-Y plane. 
     Deflecting metasurface gratings  78 B may be formed on the lower surface of cover layer  14 CG in regions B and C, with a first set of gratings  78 B located along edge  82 A and a second set of gratings  78 B located along edge  82 D. Metasurface gratings  78 B in regions B and C may be configured to receive light  46  that has propagated from region A and to redirect light  46  towards the center of cover layer  14 CG. The diffraction efficiencies of gratings  78 B in regions A, B, and C may be tailored based on the desired illumination homogeneity throughout cover layer  14 CG. For example, metasurface gratings  78 B in regions B may have a higher diffraction efficiency than metasurface gratings  78 A in regions A, and metasurface gratings  78 B in regions C may have a higher diffraction efficiency than metasurface gratings  78 B in regions B. The diffraction efficiency may vary gradually along each edge  82 A and  82 D, or the diffraction efficiency may vary in blocks. Gratings  78 B in regions C may, if desired, be configured to direct light  46  parallel to and/or away from edges  82 B and  82 C (e.g., to avoid striking curved edges of cover layer  14 CG that might otherwise defeat total internal reflection). 
     In the example of  FIG.  14   , first and second light sources  52  may be mounted under a lower surface of cover layer  14 CG (e.g., lower surface  14 L of  FIG.  9   ) and may emit light vertically in the Z-direction of  FIG.  14   . Light sources  52  may be mounted on opposing sides of cover layer  14 CG. For example, a first light source  52  may be mounted adjacent to side  82 A of cover layer  14 CG, and a second light source  52  may be mounted adjacent to opposing side  82 D of cover layer  14 CG. Light sources  52  may each include a single light-emitting element (e.g., a single laser diode and/or a single light-emitting diode), may include a one-dimensional array of light-emitting elements (e.g., as in the example of  FIG.  11   ), or may include a two-dimensional array of light-emitting elements (e.g., as in the example of  FIG.  12   ). In-coupling metasurface gratings  78 A in regions A may be formed between light sources  52  and the lower surface of cover layer  14 CG. If desired, the in-coupling metasurface gratings  78 A in regions A may be optimized to receive different angles of light  46  from light sources  52  and to redirect light  46  into cover layer  14 CG at the desired angle to achieve total internal reflection (e.g., angle A of  FIG.  8   ). For example, gratings  78 A in regions A may receive light  46  that is initially oriented vertically in the Z-direction and may redirect light  46  so that it enters cover layer  46  at the desired angle and propagates within cover layer  14 CG along the X-Y plane. 
     Deflecting metasurface gratings  78 B may be formed on the lower surface of cover layer  14 CG in regions B and C, with a first set of gratings  78 B located along edge  82 A and a second set of gratings  78 B located along edge  82 D. Metasurface gratings  78 B in regions B and C may be configured to receive light  46  that has propagated from region A and to redirect light  46  towards the center of cover layer  14 CG. The diffraction efficiencies of gratings in regions A, B, and C may be tailored based on the desired illumination homogeneity throughout cover layer  14 CG. For example, metasurface gratings  78 B in regions B may have a higher diffraction efficiency than metasurface gratings  78 A in regions A, and metasurface gratings  78 B in regions C may have a higher diffraction efficiency than metasurface gratings  78 B in regions B. The diffraction efficiency may vary gradually along each edge  82 A and  82 D, or the diffraction efficiency may vary in blocks. Gratings  78 B in regions C may, if desired, be configured to direct light  46  parallel to and/or away from edges  82 B and  82 C (e.g., to avoid striking curved edges of cover layer  14 CG that might otherwise defeat total internal reflection). 
     In addition to gratings  78 A and  78 B in regions A, B, and C, cover layer  14 CG may include gratings  78 B across the center of cover layer  14 CG (e.g., extending between side  82 A and  82 D). Gratings  78 B that extend across the width of cover layer  14 CG may receive light  46  from gratings  78 A and  78 B in regions A, B, and C and may redistribute the light  46  throughout cover layer  14 CG. 
     In the example of  FIG.  15   , first and second light sources  52  may be mounted under a lower surface of cover layer  14 CG (e.g., lower surface  14 L of  FIG.  9   ) and may emit light vertically in the Z-direction of  FIG.  15   . Light sources  52  may be mounted on opposing sides of cover layer  14 CG. For example, a first light source  52  may be mounted adjacent to side  82 A of cover layer  14 CG, and a second light source  52  may be mounted adjacent to opposing side  82 D of cover layer  14 CG. Light sources  52  may each include a single light-emitting element (e.g., a single laser diode and/or a single light-emitting diode), may include a one-dimensional array of light-emitting elements (e.g., as in the example of  FIG.  11   ), or may include a two-dimensional array of light-emitting elements (e.g., as in the example of  FIG.  12   ). In-coupling metasurface gratings  78 A in regions A may be formed between light sources  52  and the lower surface of cover layer  14 CG. If desired, the in-coupling metasurface gratings  78 A in regions A may be optimized to receive different angles of light  46  from light sources  52  and to redirect light  46  into cover layer  14 CG at the desired angle to achieve total internal reflection (e.g., angle A of  FIG.  8   ). For example, gratings  78 A in regions A may receive light  46  that is initially oriented vertically in the Z-direction and may redirect light  46  so that it enters cover layer  46  at the desired angle and propagates within cover layer  14 CG along the X-Y plane. 
     Deflecting metasurface gratings  78 B may be formed on the lower surface of cover layer  14 CG in regions B and C, with a first set of gratings  78 B located along edge  82 A and a second set of gratings  78 B located along edge  82 D. Metasurface gratings  78 B in regions B and C may be configured to receive light  46  that has propagated from region A and to redirect light  46  towards the center of cover layer  14 CG. The diffraction efficiencies of gratings in regions A, B, and C may be tailored based on the desired illumination homogeneity throughout cover layer  14 CG. For example, metasurface gratings  78 B in regions B may have a higher diffraction efficiency than metasurface gratings  78 A in regions A, and metasurface gratings  78 B in regions C may have a higher diffraction efficiency than metasurface gratings  78 B in regions B. The diffraction efficiency may vary gradually along each edge  82 A and  82 D, or the diffraction efficiency may vary in blocks. Gratings  78 B in regions C may, if desired, be configured to direct light  46  parallel to and/or away from edges  82 B and  82 C (e.g., to avoid striking curved edges of cover layer  14 CG that might otherwise defeat total internal reflection). 
     In addition to gratings  78 A and  78 B in regions A, B, and C, cover layer  14 CG may include gratings  78 B in regions D, with a first grating  78 B located along edge  82 B and a second grating  78 B located along edge  82 C. Gratings  78 B in regions D may receive light  46  from gratings  78 B in regions C and may redirect the light  46  towards the center of cover layer  14 CG. Gratings  78 B in regions D may, if desired, be configured to direct light  46  away from edges  82 B and  82 C (e.g., to avoid striking curved edges of cover layer  14 CG that might otherwise defeat total internal reflection). 
     In the example of  FIG.  16   , light source  52  may be mounted under a lower surface of cover layer  14 CG (e.g., lower surface  14 L of  FIG.  9   ) and may emit light vertically in the Z-direction of  FIG.  15   . Light source  52  may be mounted adjacent to side  82 A of cover layer  14 CG. Light source  52  may include a single light-emitting element (e.g., a single laser diode and/or a single light-emitting diode), may include a one-dimensional array of light-emitting elements (e.g., as in the example of  FIG.  11   ), or may include a two-dimensional array of light-emitting elements (e.g., as in the example of  FIG.  12   ). In-coupling metasurface grating  78 A may be formed between light source  52  and the lower surface of cover layer  14 CG. If desired, in-coupling metasurface grating  78 A between light source  52  and cover layer  14 CG may extend along all or some of side  82 A and may be optimized to receive different angles of light  46  from light source  52  and to redirect light  46  into cover layer  14 CG at the desired angle to achieve total internal reflection (e.g., angle A of  FIG.  8   ). For example, grating  78 A between light source  52  and cover layer  14 CG may receive light  46  that is initially oriented vertically in the Z-direction and may redirect light  46  so that it enters cover layer  46  at the desired angle and propagates within cover layer  14 CG along the X-Y plane. 
     Deflecting metasurface gratings  78 B may be formed on the lower surface of cover layer  14 CG and may extend along one or more sides of cover layer  14 CG. In the example of  FIG.  16   , one grating  78 B is located adjacent to side  82 B of cover layer  14 CG and another grating  78 B is located adjacent to side  82 C of cover layer  14 CG. Gratings  78 B may extend along only a portion of sides  82 B and  82 C (e.g., as shown in the example of  FIG.  16   ), or may each extend along the entire side of cover layer  14 CG. Metasurface gratings  78 B along edges  82 B and  82 C may be configured to receive light  46  that has propagated from grating  78 A on side  82 A and to redirect light  46  towards the center of cover layer  14 CG. 
       FIG.  17    is a top view of an illustrative in-coupling grating  78 A formed from a metasurface structure (sometimes referred to as a metasurface grating). As shown in  FIG.  17   , metasurface grating  78 A may have nanostructures P 1  and P 2  that extend in a two-dimensional array across the surface of cover layer  14 CG (e.g., in lateral dimensions X and Y of  FIG.  17   ). Nanostructures P 1  and P 2  may be nanoposts (nanopillars) such as elliptical nanoposts, rectangular nanoposts, circular nanoposts, or nanoposts of other shapes. Nanostructures P 1  and P 2  may be formed from any suitable material (e.g., semiconductors, organic and/or inorganic dielectrics, metals, other materials, and/or combinations of materials). As an example, nanostructures P 1  and P 2  may be formed from polycrystalline silicon or amorphous silicon, which have high refractive indices. Nanostructures P 1  and P 2  may also be formed from high refractive index dielectric materials that are transparent at visible wavelengths, such as titanium dioxide (TiO2). If desired, the metagratings may be encapsulated with a lower refractive index material (e.g., polymeric materials of silicon dioxide (SiO2) and/or other materials) to protect the nanostructures from contamination and/or accidental damage. 
     Grating  78 A may include different types (e.g., different shapes, sizes, materials, spacing, etc.) of nanostructures that are organized in a random pattern without repeating elements (or other irregular and non-periodic pattern) or that are organized in a two-dimensional repeating pattern to form an array of nanostructures for grating  78 A. In the example of  FIG.  17   , in-coupling grating  78 A includes two types of nanostructures P 1  and P 2 . Nanostructure P 1  has dimensions X 1  and Y 1  along lateral dimensions X and Y, respectively. Nanostructure P 2  has dimensions X 2  and Y 2  along lateral dimensions X and Y, respectively. Dimension X 1  may be smaller than dimension X 2  and smaller than dimension Y 1 . Dimension Y 1  may be smaller than dimension Y 2 , and dimension X 2  may be smaller than dimension Y 2 . Nanostructures P 1  and P 2  have elliptical shapes in the example of  FIG.  17   , which may increase the diffraction efficiency of grating  78 A relative to other shapes. Other factors that may be changed to adjust the diffraction efficiency of grating  78 A include the spacing between nanostructures P 1  and P 2 , the height of nanostructures P 1  and P 2 , and the type of materials used to form P 1  and P 2  (e.g., a high refractive index material and an encapsulant). This is merely illustrative, however. Other shapes may be used for nanostructures P 1  and P 2 , if desired. 
     Each elliptical nanostructure P 1  and P 2  may have a major axis  84  and a minor axis  86 . The angular orientation of major axes  84  of nanostructures P 1  and P 2  may determine the direction at which light is output for a given incident angle. In some arrangements, it may be desirable to direct half of incoming light  46  in a first direction and the other half of incoming light in a second, opposite direction. When it is desired to split the light fifty-fifty in opposite directions, major axes  84  of nanostructures P 1  and P 2  may be parallel to one another, and minor axes  86  of nanostructures P 1  and P 2  may be parallel to one another. For example, grating  78 A may serve as an in-coupling grating that receives light  46  initially parallel to the Z-axis of  FIG.  17    and that redirects half of incident light  46  in the positive X direction within cover layer  14 CG (output light  46 ′) and the other half of incident light  46  in the negative X direction within cover layer  14 CG (output light  46 ′). To achieve this, major axes  84  of nanostructures P 1  and P 2  may be aligned with the Y dimension of  FIG.  17    and minor axes  86  of nanostructures P 1  and P 2  may be aligned with the X dimension of  FIG.  17   . For a given selection of nanostructure materials and the wavelength of the incoming light, the amount of in-coupled light propagating in the positive and negative X directions may be optimized by adjusting the size and ellipticity of the nanostructures. If desired, nanostructures having an arrangement of the type shown in  FIG.  17    may be used for in-coupling gratings  78 A in any of the examples shown in  FIGS.  13 ,  14 ,  15 , and  17   , and/or may be used for in-coupling gratings  78 A in other types of arrangements. 
     The example of  FIG.  17    in which there are two types of nanostructures P 1  and P 2  in grating  78 A is merely illustrative. If desired, grating  78 A may include a third type of nanostructure and/or may include more than three types of nanostructures. The third type of nanostructure may have smaller or larger dimensions than P 1  and P 2 . In one illustrative arrangement, the third type of nanostructure may be an elliptical post having smaller dimensions than P 1 . If desired, the major axis of the third type of nanostructure may be parallel to major axis  84  of P 1  and P 2 . Configurations of this type may be used to in-couple all of incident light  46  (initially parallel to the Z-dimension of  FIG.  17   ) in a single direction within cover layer  14 CG (e.g., parallel to the X-dimension of  FIG.  17   ). This is merely illustrative, however. Nanostructures within grating  78 A may have any suitable orientation relative to one another, depending on the incident angle and the desired output angle. 
       FIG.  18    is a top view of grating  78 B having metasurface structures for redirecting light that is already propagating within cover layer  14 CG. For example, grating  78 B of  FIG.  18    may be used as a deflecting grating in any of the examples shown in  FIGS.  13 ,  14 ,  15 , and  17   , and/or in other types of arrangements. 
     As shown in  FIG.  18   , metasurface grating  78 B may have nanostructures P 1  and P 2  that extend in a two-dimensional array across the surface of cover layer  14 CG (e.g., in lateral dimensions X and Y of  FIG.  18   ). Similar to the example of  FIG.  17   , grating  78 B includes two types of nanostructures P 1  and P 2 . Nanostructure P 1  has dimensions X 1  and Y 1  along lateral dimensions X and Y, respectively. Nanostructure P 2  has dimensions X 2  and Y 2  along lateral dimensions X and Y, respectively. Dimension X 1  may be smaller than dimension X 2  and smaller than dimension Y 1 . Dimension Y 1  may be smaller than dimension Y 2 , and dimension X 2  may be smaller than dimension Y 2 . Nanostructures P 1  and P 2  have elliptical shapes in the example of  FIG.  18   , which may increase the diffraction efficiency of grating  78 B relative to other shapes. This is merely illustrative, however. Other shapes may be used for nanostructures P 1  and P 2 , if desired. 
     Each elliptical nanostructure P 1  and P 2  may have a major axis  84  and a minor axis  86 . The angle between the incoming light and axes  84  and  86  of respective nanostructures P 1  and P 2  may determine the direction at which light is redirected after interacting with the metagratings. In some arrangements, it may be desirable to deflect light at 90-degree angles. To deflect the light at a right angle, major axes  84  of nanostructures P 1  and P 2  may be perpendicular to one another, minor axes  86  of nanostructures P 1  and P 2  may be perpendicular to one another, and the angle between the incoming light  46  and axes  84  and  86  of respective nanostructures P 1  and P 2  is selected to be 45 degrees. For example, grating  78 B may serve as a deflecting grating that receives incident light  46  propagating within cover layer  14 CG and that redirects the light at 90 degrees to a different region of cover layer  14 CG (see output light  46 ′ of  FIG.  18   ). To achieve this, minor axis  86  of nanostructures P 1  and major axis  84  of nanostructures P 2  may be aligned with the Y dimension of  FIG.  18   , and major axis  84  of nanostructures P 1  and minor axis  86  of nanostructures P 2  may be aligned with the X dimension of  FIG.  18   , while the azimuthal direction of the incident light  46  within the top view of the cover layer  14 CG is 45 degrees, as shown in  FIG.  18   . 
     The example of  FIG.  17    in which the major axes  84  of elliptical nanostructures P 1  and P 2  are parallel to one another and the example of  FIG.  18    in which the major axes  84  of elliptical nanostructures P 1  and P 2  are perpendicular to one another are merely illustrative. If desired, the angle between the major axes  84  of nanostructures P 1  and P 2  may be between 0 and 90 degrees, may be greater than 90 degrees, and/or may have any other suitable angle, depending on the input angle and the desired output angle. 
     If desired, other properties of gratings  78 A and  78 B may be varied to achieve the desired light distribution effect within cover layer  14 CG. Illustrative properties of grating  78 A and  78 B that may be varied to achieve the desired light distribution effect include periodicity in the X direction, periodicity in the Y direction, dimensions X 1 , Y 1 , X 2 , and Y 2 , the shape, size, material, and/or other property of nanostructures within grating  78 A (e.g., P 1  and P 2 ), the number of different types of nanostructures within grating  78 A and/or grating  78 B (e.g., grating  78 A and/or grating  78 B may include two different types of nanostructures such as P 1  and P 2 , may include only one type of nanostructure, may include more than two types of nanostructures, etc.), etc. Properties of gratings  78 A and  78 B may be varied continuously (for a gradient effect) or may be varied in sections (e.g., block-by-block portions of grating  78 A and/or grating  78 B). Properties of grating  78 A and/or grating  78 B may, for example, be varied along the length of one or more edges of cover layer  14 CG and/or may be varied across the region overlapping light source  52  (e.g., a first set of nanostructures having a first set of properties may be optimized to receive light at a first incident angle from a first light-emitting element in the light source, a second set of nanostructures having a second set of properties may be optimized to receive light at a second incident angle from a second light-emitting element in the light source, a third set of nanostructures having a third set of properties may be optimized to receive light at a third incident angle from a third light-emitting element in the light source, etc.). 
     If desired, nanostructures such as nanostructures P 1  and P 2  of  FIGS.  17  and  18    may be formed using high resolution optical projection lithography, nano-imprint lithography, and/or other suitable fabrication techniques. 
     Nanostructures such as nanostructures P 1  and P 2  of  FIGS.  17  and  18    may be designed to operate at a nominal wavelength 940 nanometers, or may be designed to operate at shorter wavelengths, including wavelengths in the visible spectrum, longer wavelengths in the infrared spectrum, and/or other suitable wavelengths. 
     One illustrative parameter that may be adjusted to optimize nanostructures in gratings  78 A and  78 B is the ellipticity of nanostructures P 1  and P 2 . Ellipticity may be defined as the ratio of the length of minor axis  86  to the length of major axis  84 . 
     For example, in arrangements where nanopillars P 1  and P 2  of  FIG.  17    are composed of amorphous silicon without an encapsulation layer, nanopillars P 1  and P 2  may have a height of 536 nm, may have a periodicity of 688 nm in the X direction, may have a periodicity of 460 nm in the Y direction, and may have ellipticities of 0.75 and 0.82, respectively. This is merely illustrative, however. If desired, nanopillars P 1  and P 2  of  FIG.  17    may be formed with different materials, heights, periodicities, and ellipticities. Ellipticity values may range from 0.3 to 1.0, for example, and may depend on the refractive indices of the nanostructures and the encapsulation material (if any), height, and periodicity in the two lateral directions. The lengths of the minor and major axes for nanostructures P 1  and P 2  in  FIG.  17    (e.g., dimensions X 1 , Y 1 , X 2 , and Y 2 ) may be 231 nm, 306 nm, 164 nm, and 199 nm, respectively (as illustrative examples). If desired, dimensions X 1 , Y 1 , X 2 , and Y 2  may range from 80 nm to 450 nm, depending on the refractive indices of the nanostructure material and the encapsulation material (if any), height, and periodicity in the two lateral directions. 
     Nanostructures such as nanopillars P 1  and P 2  of  FIG.  18    may be designed to exhibit diffraction efficiencies between 1% and 100%. In arrangements where nanostructures P 1  and P 2  of  FIG.  18    are formed from amorphous silicon without encapsulation layer and are designed to exhibit 50% diffraction efficiency and to redirect the light at an angle of 90 degrees, nanostructures P 1  and P 2  may have a height of 536 nm, a periodicity of 486 nm in the X dimension, and a periodicity of 326 nm in the Y dimension. This is merely illustrative, however. Nanostructures P 1  and P 2  may have other suitable heights and periodicity values. The lengths of the minor and major axes of nanostructures P 1  and P 2  in  FIG.  18    may be 170 nm, 224 nm, 171 nm, and 186 nm, respectively (as illustrative examples). The lengths of the minor and major axes of nanostructures P 1  and P 2  may range from 80 nm to 450 nm, depending on the refractive indices of the nanostructures and the encapsulation material (if any), height, and periodicity in the two lateral directions. The ellipticities of the nanostructures P 1  and P 2  of  FIG.  18    may be 0.76 and 0.92, for example. Different ellipticity values may be used (e.g., ranging from 0.3 to 1.0), depending on the refractive indices of nanostructure material and encapsulation material (if any), the redirection angle, the nanostructure height, and the metagrating periodicity in two lateral directions. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination. 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 Table of 
                   
                   
                   
               
               
                 Reference Numerals 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 10 
                 Electronic device 
                 12 
                 Input-output devices 
               
               
                 14 
                 Display 
                 18 
                 Sensors 
               
               
                 16 
                 Control circuitry 
                 22 
                 Housing 
               
               
                 P, P-1, P-2 
                 Pixels 
                 30 
                 Interior Region 
               
               
                 F 
                 Front face 
                 36 
                 Components 
               
               
                 32 
                 Exterior region 
                 38 
                 Substrate 
               
               
                 14P 
                 Display Panel 
                 14CG 
                 Display Cover Layer 
               
               
                 34 
                 Finger 
                 40 
                 Viewer 
               
               
                 42 
                 Direction 
                 44 
                 Backlight unit 
               
               
                 46, 46’, 48 
                 Light 
                 50 
                 Layer 
               
               
                 A 
                 Angle 
                 60 
                 Water 
               
               
                 n 
                 Surface Normal 
                 52 
                 Light Source 
               
               
                 54 
                 Collimator 
                 56 
                 Prism 
               
               
                 78A, 78B 
                 Grating 
                 80 
                 Optical Coupling Structure(s) 
               
               
                 82A, 82B, 82C, 82D 
                 Side 
                 84 
                 Major Axis 
               
               
                 86 
                 Minor Axis 
                 P1, P2 
                 Nanostructure 
               
               
                 X1, X2, Y1, Y2 
                 Dimension 
                 A, B, C, D 
                 Region

Metadata:
Filing Date: 20210817
Publication Date: 20231003
Grant Date: 20231003
Priority Date: 20210817
Inventors: SOSKIND, YAKOV G.
YEKE YAZDANDOOST, MOHAMMAD
WRIGHT, PATRICK B.
GOZZINI, GIOVANNI
Assignee: APPLE INC
CPC Classifications: [{"code": "G06F1/1656", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01L1/246", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0421", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04109", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L1/24", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F1/1656", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01L5/166", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0421", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04109", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2203/04109", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L1/246", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0421", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 85227969