Patent Description:
Modern computing and display technologies have facilitated the development of systems for so called "virtual reality" or "augmented reality" experiences, in which digitally reproduced images or portions thereof are presented to a user in a manner wherein they seem to be, or may be perceived as, real. A virtual reality, or "VR", scenario typically involves the presentation of digital or virtual image information without transparency to other actual real-world visual input; an augmented reality, or "AR", scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the user. A mixed reality, or "MR", scenario is a type of AR scenario and typically involves virtual objects that are integrated into, and responsive to, the natural world. For example, an MR scenario may include AR image content that appears to be blocked by or is otherwise perceived to interact with objects in the real world.

Referring to <FIG>, an augmented reality scene <NUM> is depicted. The user of an AR technology sees a real-world park-like setting <NUM> featuring people, trees, buildings in the background, and a concrete platform <NUM>. The user also perceives that he/she "sees" "virtual content" such as a robot statue <NUM> standing upon the real-world platform <NUM>, and a flying cartoon-like avatar character <NUM> which seems to be a personification of a bumble bee. These elements <NUM>, <NUM> are "virtual" in that they do not exist in the real world. Because the human visual perception system is complex, it is challenging to produce AR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements.

<CIT> discloses an optical system comprising an optical element comprising a metasurface, the metasurface comprising a plurality of nanostructures; and an antireflection coating for the optical element comprising the metasurface, the antireflection coating comprising a layer of an optically transparent material having a refractive index greater than <NUM> and less than a refractive index of a material comprising the metasurface, wherein the layer of optically transparent material is conformally disposed over the nanostructures of the metasurface, and the layer of optically transparent material follows contours of the nanostructures without completely filling volumes separating each of the nanostructures. <CIT> and <CIT> disclose also optical systems however without the layer of optically transparent material conformally disposed over the nanostructures.

The anti-reflective properties of the antireflection coating of these references show however limitations.

Systems and methods disclosed herein address various challenges related to AR and VR technology.

The invention is directed to an optical system according to claim <NUM>.

The invention is also directed to a method for forming an antireflection coating on a metasurface, according to claim <NUM>.

Metasurfaces, also referred to as metamaterial surfaces, provide opportunities to realize virtually flat, aberration-free optics on much smaller scales, in comparison with geometrical optics. Without being limited by theory, in some embodiments, metasurfaces include dense arrangements of surface structures, or nanostructures, that function as resonant optical antennas. The resonant nature of the light-surface structure interaction provides the ability to manipulate optical wave-fronts. In some cases, the metasurfaces may allow the replacement of bulky or difficult to manufacture optical components with thin, planar elements formed by simple patterning processes.

It will be appreciated that optical elements formed of metasurfaces may function in the reflective and/or transmissive mode. In the reflective mode, the metasurface may reflect light at desired angles. In the transmissive mode, the metasurface may transmit light through the body of the metasurface while also deflecting that light at desired angles. Undesirably, metasurfaces working in the transmissive mode may also reflect incident light, e.g., due to Fresnel reflections at interfaces with other materials. In addition, for metasurfaces working in the reflective mode, the angles at which the metasurfaces are configured to reflect light may be different from the angles at which light is reflected off of interfaces.

Undesirably, unintended reflections by metasurfaces may cause optical artifacts. For example, in display devices in which metasurfaces are used as optical elements for directing light encoded with image content (e.g., light modified by a spatial light modulator), the reflections may cause ghost images due to the reflection of some of the light back and forth along an optical path before reaching the user. For example, metasurfaces may form incoupling optical elements for incoupling light into a waveguide, which in turn is configured to output image content to a user. Where part of this light is reflected rather than incoupled into the waveguide, the reflected light may propagate back to a light projector or light source, which may then reflect the light back to the metasurface for incoupling into the waveguide, and ultimately output to a user. Due to this back-and-forth reflection, light from prior video image frames may be provided to the waveguide along with light encoding current image frames. The light encoding prior image frames may be visible to the user as a ghost image that decreases the image quality of the display device.

In some embodiments, an antireflection coating may reduce or eliminate the reflection of light from metasurfaces. The antireflection coating may be formed of an optically transmissive layer of material, such as a polymer layer, e.g., a layer of photoresist. In some embodiments, no air or other material may be present between the metasurface and the antireflection coating; the antireflection coating may directly contact the metasurface. The material forming the antireflection coating may have a refractive index lower than the refractive index of the nanostructures of the metasurface, but higher than the refractive index of the material or medium (e.g., air) forming an interface with the antireflective coating opposite the metasurface.

In some embodiments, the antireflection coating may be an interference coating and the thickness of the layer of material is selected to provide destructive interference between light reflecting off the top and bottom surfaces of the layer. Preferably, the thickness of the layer is selected to provide this interference for light of visible wavelengths. In some embodiments, the metasurfaces may be part of a color display utilizing a plurality of component colors. As a result, a particular metasurface may only be exposed to light of an associated limited range of wavelengths corresponding to a particular component color, and the antireflection coating may have a thickness selected to provide interference for light having this associated limited range of wavelengths.

In some embodiments, the antireflection coating may be a planar layer extending over and between the nanostructures forming the metasurfaces, and forming a planar surface of the nanostructures. Such a planar layer may advantageously provide antireflection properties over a wide range of angles of incident light. In some embodiments, the antireflection coating may be a conformal layer disposed on the surfaces of the nanostructures forming the metasurfaces. The conformal layer may be continuous and extend over and in between multiple nanostructures, or may be isolated on individual ones of the nanostructures.

Advantageously, the reduction in reflections may reduce or eliminate optical effects such as ghost images, thereby allowing a display device to output images with higher perceived quality. In some embodiments, an antireflection coating may reduce the amount of light reflected by a metasurface, relative to identical structure without the antireflection coating, by about <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or more. The antireflection coating may be particularly advantageously applied to metasurfaces operating in the transmissive mode, for which reflections are not part of the design of the metasurface.

Reference will now be made to the drawings, in which like reference numerals refer to like parts throughout.

In some embodiments, the metasurfaces may advantageously be applied to form optical elements in display devices, such as display devices for AR or VR display systems. These display systems may display virtual content to a user, or viewer, and AR systems may also allow the user to see the world around them by transmitting light from the surrounding environment to the user's eyes. Preferably, this virtual content is displayed on a wearable head-mounted display, e.g., as part of eyewear, that projects image information to the user's eyes. As used herein, it will be appreciated that a "head-mounted" display is a display that may be mounted on the head of a viewer.

<FIG> illustrates an example of wearable display system <NUM>. The display system <NUM> includes a head-mounted display <NUM>, and various mechanical and electronic modules and systems to support the functioning of that display <NUM>. The display <NUM> may be coupled to a frame <NUM>, which is wearable by a display system user or viewer <NUM> and which is configured to position the display <NUM> in front of the eyes of the user <NUM>. The display <NUM> may be considered eyewear in some examples. In some examples, a speaker <NUM> is coupled to the frame <NUM> and positioned adjacent the ear canal of the user <NUM> (in some examples, another speaker, not shown, is positioned adjacent the other ear canal of the user to provide for stereo/shapeable sound control). In some examples, the display system may also include one or more microphones <NUM> or other devices to detect sound. In some examples, the microphone is configured to allow the user to provide inputs or commands to the system <NUM> (e.g., the selection of voice menu commands, natural language questions, etc.), and/or may allow audio communication with other persons (e.g., with other users of similar display systems. The microphone may further be configured as a peripheral sensor to continuously collect audio data (e.g., to passively collect from the user and/or environment). Such audio data may include user sounds such as heavy breathing, or environmental sounds, such as a loud bang indicative of a nearby event. The display system may also include a peripheral sensor 30a, which may be separate from the frame <NUM> and attached to the body of the user <NUM> (e.g., on the head, torso, an extremity, etc. of the user <NUM>). The peripheral sensor 30a may be configured to acquire data characterizing the physiological state of the user <NUM> in some examples, as described further herein. For example, the sensor 30a may be an electrode.

<FIG> illustrates an example of wearable display system <NUM>. The display system <NUM> includes a display <NUM>, and various mechanical and electronic modules and systems to support the functioning of that display <NUM>. The display <NUM> may be coupled to a frame <NUM>, which is wearable by a display system user or viewer <NUM> and which is configured to position the display <NUM> in front of the eyes of the user <NUM>. The display <NUM> may be considered eyewear in some examples. In some examples, a speaker <NUM> is coupled to the frame <NUM> and configured to be positioned adjacent the ear canal of the user <NUM> (in some examples, another speaker, not shown, may optionally be positioned adjacent the other ear canal of the user to provide stereo/shapeable sound control). The display system may also include one or more microphones <NUM> or other devices to detect sound. In some examples, the microphone is configured to allow the user to provide inputs or commands to the system <NUM> (e.g., the selection of voice menu commands, natural language questions, etc.), and/or may allow audio communication with other persons (e.g., with other users of similar display systems. The microphone may further be configured as a peripheral sensor to collect audio data (e.g., sounds from the user and/or environment). In some examples, the display system may also include a peripheral sensor 120a, which may be separate from the frame <NUM> and attached to the body of the user <NUM> (e.g., on the head, torso, an extremity, etc. of the user <NUM>). The peripheral sensor 120a may be configured to acquire data characterizing a physiological state of the user <NUM> in some examples. For example, the sensor 120a may be an electrode.

With continued reference to <FIG>, the display <NUM> is operatively coupled by communications link <NUM>, such as by a wired lead or wireless connectivity, to a local data processing module <NUM> which may be mounted in a variety of configurations, such as fixedly attached to the frame <NUM>, fixedly attached to a helmet or hat worn by the user, embedded in headphones, or otherwise removably attached to the user <NUM> (e.g., in a backpack-style configuration, in a belt-coupling style configuration). Similarly, the sensor 120a may be operatively coupled by communications link 120b, e.g., a wired lead or wireless connectivity, to the local processor and data module <NUM>. The local processing and data module <NUM> may comprise a hardware processor, as well as digital memory, such as non-volatile memory (e.g., flash memory or hard disk drives), both of which may be utilized to assist in the processing, caching, and storage of data. The data include data a) captured from sensors (which may be, e.g., operatively coupled to the frame <NUM> or otherwise attached to the user <NUM>), such as image capture devices (such as cameras), microphones, inertial measurement units, accelerometers, compasses, GPS units, radio devices, gyros, and/or other sensors disclosed herein; and/or b) acquired and/or processed using remote processing module <NUM> and/or remote data repository <NUM> (including data relating to virtual content), possibly for passage to the display <NUM> after such processing or retrieval. The local processing and data module <NUM> may be operatively coupled by communication links <NUM>, <NUM>, such as via a wired or wireless communication links, to the remote processing module <NUM> and remote data repository <NUM> such that these remote modules <NUM>, <NUM> are operatively coupled to each other and available as resources to the local processing and data module <NUM>. In some examples, the local processing and data module <NUM> may include one or more of the image capture devices, microphones, inertial measurement units, accelerometers, compasses, GPS units, radio devices, and/or gyros. In some other examples, one or more of these sensors may be attached to the frame <NUM>, or may be standalone structures that communicate with the local processing and data module <NUM> by wired or wireless communication pathways.

With continued reference to <FIG>, in some examples, the remote processing module <NUM> may comprise one or more processors configured to analyze and process data and/or image information. In some examples, the remote data repository <NUM> may comprise a digital data storage facility, which may be available through the internet or other networking configuration in a "cloud" resource configuration. In some examples, the remote data repository <NUM> may include one or more remote servers, which provide information, e.g., information for generating augmented reality content, to the local processing and data module <NUM> and/or the remote processing module <NUM>. In some examples, all data is stored and all computations are performed in the local processing and data module, allowing fully autonomous use from a remote module.

With reference now to <FIG>, the perception of an image as being "three-dimensional" or "<NUM>-D" may be achieved by providing slightly different presentations of the image to each eye of the viewer. <FIG> illustrates a conventional display system for simulating three-dimensional imagery for a user. Two distinct images <NUM>, <NUM>-one for each eye <NUM>, <NUM>-are outputted to the user. The images <NUM>, <NUM> are spaced from the eyes <NUM>, <NUM> by a distance <NUM> along an optical or z-axis that is parallel to the line of sight of the viewer. The images <NUM>, <NUM> are flat and the eyes <NUM>, <NUM> may focus on the images by assuming a single accommodated state. Such <NUM>-D display systems rely on the human visual system to combine the images <NUM>, <NUM> to provide a perception of depth and/or scale for the combined image.

It will be appreciated, however, that the human visual system is more complicated and providing a realistic perception of depth is more challenging. For example, many viewers of conventional "<NUM>-D" display systems find such systems to be uncomfortable or may not perceive a sense of depth at all. Without being limited by theory, it is believed that viewers of an object may perceive the object as being "three-dimensional" due to a combination of vergence and accommodation. Vergence movements (i.e., rotation of the eyes so that the pupils move toward or away from each other to converge the lines of sight of the eyes to fixate upon an object) of the two eyes relative to each other are closely associated with focusing (or "accommodation") of the lenses and pupils of the eyes. Under normal conditions, changing the focus of the lenses of the eyes, or accommodating the eyes, to change focus from one object to another object at a different distance will automatically cause a matching change in vergence to the same distance, under a relationship known as the "accommodation-vergence reflex," as well as pupil dilation or constriction. Likewise, a change in vergence will trigger a matching change in accommodation of lens shape and pupil size, under normal conditions. As noted herein, many stereoscopic or "<NUM>-D" display systems display a scene using slightly different presentations (and, so, slightly different images) to each eye such that a three-dimensional perspective is perceived by the human visual system. Such systems are uncomfortable for many viewers, however, since they, among other things, simply provide different presentations of a scene, but with the eyes viewing all the image information at a single accommodated state, and work against the "accommodation-vergence reflex. " Display systems that provide a better match between accommodation and vergence may form more realistic and comfortable simulations of three-dimensional imagery.

<FIG> illustrates aspects of an approach for simulating three-dimensional imagery using multiple depth planes. With reference to <FIG>, objects at various distances from eyes <NUM>, <NUM> on the z-axis are accommodated by the eyes <NUM>, <NUM> so that those objects are in focus. The eyes <NUM>, <NUM> assume particular accommodated states to bring into focus objects at different distances along the z-axis. Consequently, a particular accommodated state may be said to be associated with a particular one of depth planes <NUM>, with has an associated focal distance, such that objects or parts of objects in a particular depth plane are in focus when the eye is in the accommodated state for that depth plane. In some examples, three-dimensional imagery may be simulated by providing different presentations of an image for each of the eyes <NUM>, <NUM>, and also by providing different presentations of the image corresponding to each of the depth planes. While shown as being separate for clarity of illustration, it will be appreciated that the fields of view of the eyes <NUM>, <NUM> may overlap, for example, as distance along the z-axis increases. In addition, while shown as flat for ease of illustration, it will be appreciated that the contours of a depth plane may be curved in physical space, e.g., such that all features in a depth plane are in focus with the eye in a particular accommodated state.

The distance between an object and the eye <NUM> or <NUM> may also change the amount of divergence of light from that object, as viewed by that eye. <FIG> illustrate relationships between distance and the divergence of light rays. The distance between the object and the eye <NUM> is represented by, in order of decreasing distance, R1, R2, and R3. As shown in <FIG>, the light rays become more divergent as distance to the object decreases. As distance increases, the light rays become more collimated. Stated another way, it may be said that the light field produced by a point (the object or a part of the object) has a spherical wavefront curvature, which is a function of how far away the point is from the eye of the user. The curvature increases with decreasing distance between the object and the eye <NUM>. Consequently, at different depth planes, the degree of divergence of light rays is also different, with the degree of divergence increasing with decreasing distance between depth planes and the viewer's eye <NUM>. While only a single eye <NUM> is illustrated for clarity of illustration in <FIG> and other figures herein, it will be appreciated that the discussions regarding eye <NUM> may be applied to both eyes <NUM> and <NUM> of a viewer.

Without being limited by theory, it is believed that the human eye typically can interpret a finite number of depth planes to provide depth perception. Consequently, a highly believable simulation of perceived depth may be achieved by providing, to the eye, different presentations of an image corresponding to each of these limited number of depth planes. The different presentations may be separately focused by the viewer's eyes, thereby helping to provide the user with depth cues based on the accommodation of the eye required to bring into focus different image features for the scene located on different depth plane and/or based on observing different image features on different depth planes being out of focus.

<FIG> illustrates an example of a waveguide stack for outputting image information to a user. A display system <NUM> includes a stack of waveguides, or stacked waveguide assembly, <NUM> that may be utilized to provide three-dimensional perception to the eye/brain using a plurality of waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In some examples, the display system <NUM> is the system <NUM> of <FIG>, with <FIG> schematically showing some parts of that system <NUM> in greater detail. For example, the waveguide assembly <NUM> may be part of the display <NUM> of <FIG>. It will be appreciated that the display system <NUM> may be considered a light field display in some examples.

In some examples, a single waveguide may be configured to output light with a set amount of wavefront divergence corresponding to a single or limited number of depth planes and/or the waveguide may be configured to output light of a limited range of wavelengths. Consequently, in some examples, a plurality or stack of waveguides may be utilized to provide different amounts of wavefront divergence for different depth planes and/or to output light of different ranges of wavelengths. As used herein, it will be appreciated that a depth plane may follow the contours of a flat or a curved surface. In some examples, advantageously for simplicity, the depth planes may follow the contours of flat surfaces.

With continued reference to <FIG>, the waveguide assembly <NUM> may also include a plurality of features <NUM>, <NUM>, <NUM>, <NUM> between the waveguides. In some examples, the features <NUM>, <NUM>, <NUM>, <NUM> may be one or more lenses. The waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and/or the plurality of lenses <NUM>, <NUM>, <NUM>, <NUM> may be configured to send image information to the eye with various levels of wavefront curvature or light ray divergence. Each waveguide level may be associated with a particular depth plane and may be configured to output image information corresponding to that depth plane. Image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may function as a source of light for the waveguides and may be utilized to inject image information into the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, each of which may be configured, as described herein, to distribute incoming light across each respective waveguide, for output toward the eye <NUM>. Light exits an output surface <NUM>, <NUM>, <NUM>, <NUM>, <NUM> of the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and is injected into a corresponding input surface <NUM>, <NUM>, <NUM>, <NUM>, <NUM> of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In some examples, the each of the input surfaces <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be an edge of a corresponding waveguide, or may be part of a major surface of the corresponding waveguide (that is, one of the waveguide surfaces directly facing the world <NUM> or the viewer's eye <NUM>). It will be appreciated that the major surfaces of a waveguide correspond to the relatively large area surfaces of the waveguide between which the thickness of the waveguide extends. In some examples, a single beam of light (e.g. a collimated beam) may be injected into each waveguide to output an entire field of cloned collimated beams that are directed toward the eye <NUM> at particular angles (and amounts of divergence) corresponding to the depth plane associated with a particular waveguide. In some examples, a single one of the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be associated with and inject light into a plurality (e.g., three) of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

In some examples, the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are discrete displays that each produce image information for injection into a corresponding waveguide <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, respectively. In some other examples, the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are the output ends of a single multiplexed display which may, e.g., pipe image information via one or more optical conduits (such as fiber optic cables) to each of the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. It will be appreciated that the image information provided by the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may include light of different wavelengths, or colors (e.g., different component colors, as discussed herein).

In some examples, the light injected into the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> is provided by a light projector system <NUM>, which comprises a light module <NUM>, which may include a light emitter, such as a light emitting diode (LED). The light from the light module <NUM> may be directed to and modified by a light modulator <NUM>, e.g., a spatial light modulator, via a beam splitter <NUM>. The light modulator <NUM> may be configured to change the perceived intensity of the light injected into the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> to encode the light with image information. Examples of spatial light modulators include liquid crystal displays (LCD) including a liquid crystal on silicon (LCOS) displays. It will be appreciated that the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are illustrated schematically and, in some examples, these image injection devices may represent different light paths and locations in a common projection system configured to output light into associated ones of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In some examples, the waveguides of the waveguide assembly <NUM> may function as ideal lens while relaying light injected into the waveguides out to the user's eyes. In this conception, the object may be the spatial light modulator <NUM> and the image may be the image on the depth plane.

In some examples, the display system <NUM> may be a scanning fiber display comprising one or more scanning fibers configured to project light in various patterns (e.g., raster scan, spiral scan, Lissajous patterns, etc.) into one or more waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and ultimately to the eye <NUM> of the viewer. In some examples, the illustrated image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may schematically represent a single scanning fiber or a bundle of scanning fibers configured to inject light into one or a plurality of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In some other examples, the illustrated image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may schematically represent a plurality of scanning fibers or a plurality of bundles of scanning fibers, each of which are configured to inject light into an associated one of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. It will be appreciated that one or more optical fibers may be configured to transmit light from the light module <NUM> to the one or more waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. It will be appreciated that one or more intervening optical structures may be provided between the scanning fiber, or fibers, and the one or more waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> to, e.g., redirect light exiting the scanning fiber into the one or more waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

A controller <NUM> controls the operation of one or more of the stacked waveguide assembly <NUM>, including operation of the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, the light source <NUM>, and the light modulator <NUM>. In some examples, the controller <NUM> is part of the local data processing module <NUM>. The controller <NUM> includes programming (e.g., instructions in a non-transitory medium) that regulates the timing and provision of image information to the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> according to, e.g., any of the various schemes disclosed herein. In some examples, the controller may be a single integral device, or a distributed system connected by wired or wireless communication channels. The controller <NUM> may be part of the processing modules <NUM> or <NUM> (<FIG>) in some examples.

With continued reference to <FIG>, the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be configured to propagate light within each respective waveguide by total internal reflection (TIR). The waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may each be planar or have another shape (e.g., curved), with major top and bottom surfaces and edges extending between those major top and bottom surfaces. In the illustrated configuration, the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may each include out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> that are configured to extract light out of a waveguide by redirecting the light, propagating within each respective waveguide, out of the waveguide to output image information to the eye <NUM>. Extracted light may also be referred to as out-coupled light and the out-coupling optical elements light may also be referred to light extracting optical elements. An extracted beam of light may be outputted by the waveguide at locations at which the light propagating in the waveguide strikes a light extracting optical element. The out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may, for example, be gratings, including diffractive optical features, as discussed further herein. While illustrated disposed at the bottom major surfaces of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, for ease of description and drawing clarity, in some examples, the out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be disposed at the top and/or bottom major surfaces, and/or may be disposed directly in the volume of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, as discussed further herein. In some examples, the out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be formed in a layer of material that is attached to a transparent substrate to form the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In some other examples, the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be a monolithic piece of material and the out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be formed on a surface and/or in the interior of that piece of material.

With continued reference to <FIG>, as discussed herein, each waveguide <NUM>, <NUM>, <NUM>, <NUM>, <NUM> is configured to output light to form an image corresponding to a particular depth plane. For example, the waveguide <NUM> nearest the eye may be configured to deliver collimated light (which was injected into such waveguide <NUM>), to the eye <NUM>. The collimated light may be representative of the optical infinity focal plane. The next waveguide up <NUM> may be configured to send out collimated light which passes through the first lens <NUM> (e.g., a negative lens) before it can reach the eye <NUM>; such first lens <NUM> may be configured to create a slight convex wavefront curvature so that the eye/brain interprets light coming from that next waveguide up <NUM> as coming from a first focal plane closer inward toward the eye <NUM> from optical infinity. Similarly, the third up waveguide <NUM> passes its output light through both the first <NUM> and second <NUM> lenses before reaching the eye <NUM>; the combined optical power of the first <NUM> and second <NUM> lenses may be configured to create another incremental amount of wavefront curvature so that the eye/brain interprets light coming from the third waveguide <NUM> as coming from a second focal plane that is even closer inward toward the person from optical infinity than was light from the next waveguide up <NUM>.

The other waveguide layers <NUM>, <NUM> and lenses <NUM>, <NUM> are similarly configured, with the highest waveguide <NUM> in the stack sending its output through all of the lenses between it and the eye for an aggregate focal power representative of the closest focal plane to the person. To compensate for the stack of lenses <NUM>, <NUM>, <NUM>, <NUM> when viewing/interpreting light coming from the world <NUM> on the other side of the stacked waveguide assembly <NUM>, a compensating lens layer <NUM> may be disposed at the top of the stack to compensate for the aggregate power of the lens stack <NUM>, <NUM>, <NUM>, <NUM> below. Such a configuration provides as many perceived focal planes as there are available waveguide/lens pairings. Both the out-coupling optical elements of the waveguides and the focusing aspects of the lenses may be static (i.e., not dynamic or electro-active). In some alternative examples, either or both may be dynamic using electro-active features.

In some examples, two or more of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may have the same associated depth plane. For example, multiple waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be configured to output images set to the same depth plane, or multiple subsets of the waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be configured to output images set to the same plurality of depth planes, with one set for each depth plane. This can provide advantages for forming a tiled image to provide an expanded field of view at those depth planes.

With continued reference to <FIG>, the out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be configured to both redirect light out of their respective waveguides and to output this light with the appropriate amount of divergence or collimation for a particular depth plane associated with the waveguide. As a result, waveguides having different associated depth planes may have different configurations of out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, which output light with a different amount of divergence depending on the associated depth plane. In some examples, the light extracting optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be volumetric or surface features, which may be configured to output light at specific angles. For example, the light extracting optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be volume holograms, surface holograms, and/or diffraction gratings. In some examples, the features <NUM>, <NUM>, <NUM>, <NUM> may not be lenses; rather, they may simply be spacers (e.g., cladding layers and/or structures for forming air gaps).

In some examples, the out-coupling optical elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are diffractive features that form a diffraction pattern, or "diffractive optical element" (also referred to herein as a "DOE"). Preferably, the DOE's have a sufficiently low diffraction efficiency so that only a portion of the light of the beam is deflected away toward the eye <NUM> with each intersection of the DOE, while the rest continues to move through a waveguide via TIR. The light carrying the image information is thus divided into a number of related exit beams that exit the waveguide at a multiplicity of locations and the result is a fairly uniform pattern of exit emission toward the eye <NUM> for this particular collimated beam bouncing around within a waveguide.

In some examples, one or more DOEs may be switchable between "on" states in which they actively diffract, and "off' states in which they do not significantly diffract. For instance, a switchable DOE may comprise a layer of polymer dispersed liquid crystal, in which microdroplets comprise a diffraction pattern in a host medium, and the refractive index of the microdroplets may be switched to substantially match the refractive index of the host material (in which case the pattern does not appreciably diffract incident light) or the microdroplet may be switched to an index that does not match that of the host medium (in which case the pattern actively diffracts incident light).

In some examples, a camera assembly <NUM> (e.g., a digital camera, including visible light and infrared light cameras) may be provided to capture images of the eye <NUM> and/or tissue around the eye <NUM> to, e.g., detect user inputs and/or to monitor the physiological state of the user. As used herein, a camera may be any image capture device. In some examples, the camera assembly <NUM> may include an image capture device and a light source to project light (e.g., infrared light) to the eye, which may then be reflected by the eye and detected by the image capture device. In some examples, the camera assembly <NUM> may be attached to the frame <NUM> (<FIG>) and may be in electrical communication with the processing modules <NUM> and/or <NUM>, which may process image information from the camera assembly <NUM>. In some examples, one camera assembly <NUM> may be utilized for each eye, to separately monitor each eye.

With reference now to <FIG>, an example of exit beams outputted by a waveguide is shown. One waveguide is illustrated, but it will be appreciated that other waveguides in the waveguide assembly <NUM> (<FIG>) may function similarly, where the waveguide assembly <NUM> includes multiple waveguides. Light <NUM> is injected into the waveguide <NUM> at the input surface <NUM> of the waveguide <NUM> and propagates within the waveguide <NUM> by TIR. At points where the light <NUM> impinges on the DOE <NUM>, a portion of the light exits the waveguide as exit beams <NUM>. The exit beams <NUM> are illustrated as substantially parallel but, as discussed herein, they may also be redirected to propagate to the eye <NUM> at an angle (e.g., forming divergent exit beams), depending on the depth plane associated with the waveguide <NUM>. It will be appreciated that substantially parallel exit beams may be indicative of a waveguide with out-coupling optical elements that out-couple light to form images that appear to be set on a depth plane at a large distance (e.g., optical infinity) from the eye <NUM>. Other waveguides or other sets of out-coupling optical elements may output an exit beam pattern that is more divergent, which would require the eye <NUM> to accommodate to a closer distance to bring it into focus on the retina and would be interpreted by the brain as light from a distance closer to the eye <NUM> than optical infinity.

In some examples, a full color image may be formed at each depth plane by overlaying images in each of the component colors, e.g., three or more component colors. <FIG> illustrates an example of a stacked waveguide assembly in which each depth plane includes images formed using multiple different component colors. The illustrated example shows depth planes 240a - 240f, although more or fewer depths are also contemplated. Each depth plane may have three or more component color images associated with it, including: a first image of a first color, G; a second image of a second color, R; and a third image of a third color, B. Different depth planes are indicated in the figure by different numbers for diopters (dpt) following the letters G, R, and B. Just as examples, the numbers following each of these letters indicate diopters (<NUM>/m), or inverse distance of the depth plane from a viewer, and each box in the figures represents an individual component color image. In some examples, to account for differences in the eye's focusing of light of different wavelengths, the exact placement of the depth planes for different component colors may vary. For example, different component color images for a given depth plane may be placed on depth planes corresponding to different distances from the user. Such an arrangement may increase visual acuity and user comfort and/or may decrease chromatic aberrations.

In some examples, light of each component color may be outputted by a single dedicated waveguide and, consequently, each depth plane may have multiple waveguides associated with it. In such examples, each box in the figures including the letters G, R, or B may be understood to represent an individual waveguide, and three waveguides may be provided per depth plane where three component color images are provided per depth plane. While the waveguides associated with each depth plane are shown adjacent to one another in this drawing for ease of description, it will be appreciated that, in a physical device, the waveguides may all be arranged in a stack with one waveguide per level. In some other examples, multiple component colors may be outputted by the same waveguide, such that, e.g., only a single waveguide may be provided per depth plane.

With continued reference to <FIG>, in some examples, G is the color green, R is the color red, and B is the color blue. In some other examples, other colors associated with other wavelengths of light, including magenta and cyan, may be used in addition to or may replace one or more of red, green, or blue.

It will be appreciated that references to a given color of light throughout this disclosure will be understood to encompass light of one or more wavelengths within a range of wavelengths of light that are perceived by a viewer as being of that given color. For example, red light may include light of one or more wavelengths in the range of about <NUM>-<NUM>, green light may include light of one or more wavelengths in the range of about <NUM>-<NUM>, and blue light may include light of one or more wavelengths in the range of about <NUM>-<NUM>.

In some examples, the light source <NUM> (<FIG>) may be configured to emit light of one or more wavelengths outside the visual perception range of the viewer, for example, infrared and/or ultraviolet wavelengths. In addition, the in-coupling, out-coupling, and other light redirecting structures of the waveguides of the display <NUM> may be configured to direct and emit this light out of the display towards the user's eye <NUM>, e.g., for imaging and/or user stimulation applications.

With reference now to <FIG>, in some examples, light impinging on a waveguide may need to be redirected to in-couple that light into the waveguide. An in-coupling optical element may be used to redirect and in-couple the light into its corresponding waveguide. <FIG> illustrates a cross-sectional side view of an example of a plurality or set <NUM> of stacked waveguides that each includes an in-coupling optical element. The waveguides may each be configured to output light of one or more different wavelengths, or one or more different ranges of wavelengths. It will be appreciated that the stack <NUM> may correspond to the stack <NUM> (<FIG>) and the illustrated waveguides of the stack <NUM> may correspond to part of the plurality of waveguides <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, except that light from one or more of the image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> is injected into the waveguides from a position that requires light to be redirected for in-coupling.

The illustrated set <NUM> of stacked waveguides includes waveguides <NUM>, <NUM>, and <NUM>. Each waveguide includes an associated in-coupling optical element (which may also be referred to as a light input area on the waveguide), with, e.g., in-coupling optical element <NUM> disposed on a major surface (e.g., an upper major surface) of waveguide <NUM>, in-coupling optical element <NUM> disposed on a major surface (e.g., an upper major surface) of waveguide <NUM>, and in-coupling optical element <NUM> disposed on a major surface (e.g., an upper major surface) of waveguide <NUM>. In some examples, one or more of the in-coupling optical elements <NUM>, <NUM>, <NUM> may be disposed on the bottom major surface of the respective waveguide <NUM>, <NUM>, <NUM> (particularly where the one or more in-coupling optical elements are reflective, deflecting optical elements). As illustrated, the in-coupling optical elements <NUM>, <NUM>, <NUM> may be disposed on the upper major surface of their respective waveguide <NUM>, <NUM>, <NUM> (or the top of the next lower waveguide), particularly where those in-coupling optical elements are transmissive, deflecting optical elements. In some examples, the in-coupling optical elements <NUM>, <NUM>, <NUM> may be disposed in the body of the respective waveguide <NUM>, <NUM>, <NUM>. In some examples, as discussed herein, the in-coupling optical elements <NUM>, <NUM>, <NUM> are wavelength selective, such that they selectively redirect one or more wavelengths of light, while transmitting other wavelengths of light. While illustrated on one side or corner of their respective waveguide <NUM>, <NUM>, <NUM>, it will be appreciated that the in-coupling optical elements <NUM>, <NUM>, <NUM> may be disposed in other areas of their respective waveguide <NUM>, <NUM>, <NUM> in some examples.

As illustrated, the in-coupling optical elements <NUM>, <NUM>, <NUM> may be laterally offset from one another. In some examples, each in-coupling optical element may be offset such that it receives light without that light passing through another in-coupling optical element. For example, each in-coupling optical element <NUM>, <NUM>, <NUM> may be configured to receive light from a different image injection device <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> as shown in <FIG>, and may be separated (e.g., laterally spaced apart) from other in-coupling optical elements <NUM>, <NUM>, <NUM> such that it substantially does not receive light from the other ones of the in-coupling optical elements <NUM>, <NUM>, <NUM>.

Each waveguide also includes associated light distributing elements, with, e.g., light distributing elements <NUM> disposed on a major surface (e.g., a top major surface) of waveguide <NUM>, light distributing elements <NUM> disposed on a major surface (e.g., a top major surface) of waveguide <NUM>, and light distributing elements <NUM> disposed on a major surface (e.g., a top major surface) of waveguide <NUM>. In some other examples, the light distributing elements <NUM>, <NUM>, <NUM>, may be disposed on a bottom major surface of associated waveguides <NUM>, <NUM>, <NUM>, respectively. In some other examples, the light distributing elements <NUM>, <NUM>, <NUM>, may be disposed on both top and bottom major surface of associated waveguides <NUM>, <NUM>, <NUM>, respectively; or the light distributing elements <NUM>, <NUM>, <NUM>, may be disposed on different ones of the top and bottom major surfaces in different associated waveguides <NUM>, <NUM>, <NUM>, respectively.

The waveguides <NUM>, <NUM>, <NUM> may be spaced apart and separated by, e.g., gas, liquid, and/or solid layers of material. For example, as illustrated, layer 760a may separate waveguides <NUM> and <NUM>; and layer 760b may separate waveguides <NUM> and <NUM>. In some examples, the layers 760a and 760b are formed of low refractive index materials (that is, materials having a lower refractive index than the material forming the immediately adjacent one of waveguides <NUM>, <NUM>, <NUM>). Preferably, the refractive index of the material forming the layers 760a, 760b is <NUM> or more, or <NUM> or less than the refractive index of the material forming the waveguides <NUM>, <NUM>, <NUM>. Advantageously, the lower refractive index layers 760a, 760b may function as cladding layers that facilitate total internal reflection (TIR) of light through the waveguides <NUM>, <NUM>, <NUM> (e.g., TIR between the top and bottom major surfaces of each waveguide). In some examples, the layers 760a, 760b are formed of air. While not illustrated, it will be appreciated that the top and bottom of the illustrated set <NUM> of waveguides may include immediately neighboring cladding layers.

Preferably, for ease of manufacturing and other considerations, the material forming the waveguides <NUM>, <NUM>, <NUM> are similar or the same, and the material forming the layers 760a, 760b are similar or the same. In some examples, the material forming the waveguides <NUM>, <NUM>, <NUM> may be different between one or more waveguides, and/or the material forming the layers 760a, 760b may be different, while still holding to the various refractive index relationships noted above.

With continued reference to <FIG>, light rays <NUM>, <NUM>, <NUM> are incident on the set <NUM> of waveguides. It will be appreciated that the light rays <NUM>, <NUM>, <NUM> may be injected into the waveguides <NUM>, <NUM>, <NUM> by one or more image injection devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM> (<FIG>).

In some examples, the light rays <NUM>, <NUM>, <NUM> have different properties, e.g., different wavelengths or different ranges of wavelengths, which may correspond to different colors. The in-coupling optical elements <NUM>, <NUM>, <NUM> each deflect the incident light such that the light propagates through a respective one of the waveguides <NUM>, <NUM>, <NUM> by TIR. In some examples, the incoupling optical elements <NUM>, <NUM>, <NUM> each selectively deflect one or more particular wavelengths of light, while transmitting other wavelengths to an underlying waveguide and associated incoupling optical element.

For example, in-coupling optical element <NUM> may be configured to deflect ray <NUM>, which has a first wavelength or range of wavelengths, while transmitting rays <NUM> and <NUM>, which have different second and third wavelengths or ranges of wavelengths, respectively. The transmitted ray <NUM> impinges on and is deflected by the in-coupling optical element <NUM>, which is configured to deflect light of a second wavelength or range of wavelengths. The ray <NUM> is deflected by the in-coupling optical element <NUM>, which is configured to selectively deflect light of third wavelength or range of wavelengths.

With continued reference to <FIG>, the deflected light rays <NUM>, <NUM>, <NUM> are deflected so that they propagate through a corresponding waveguide <NUM>, <NUM>, <NUM>; that is, the in-coupling optical elements <NUM>, <NUM>, <NUM> of each waveguide deflects light into that corresponding waveguide <NUM>, <NUM>, <NUM> to in-couple light into that corresponding waveguide. The light rays <NUM>, <NUM>, <NUM> are deflected at angles that cause the light to propagate through the respective waveguide <NUM>, <NUM>, <NUM> by TIR. The light rays <NUM>, <NUM>, <NUM> propagate through the respective waveguide <NUM>, <NUM>, <NUM> by TIR until impinging on the waveguide's corresponding light distributing elements <NUM>, <NUM>, <NUM>.

With reference now to <FIG>, a perspective view of an example of the plurality of stacked waveguides of <FIG> is illustrated. As noted above, the incoupled light rays <NUM>, <NUM>, <NUM>, are deflected by the in-coupling optical elements <NUM>, <NUM>, <NUM>, respectively, and then propagate by TIR within the waveguides <NUM>, <NUM>, <NUM>, respectively. The light rays <NUM>, <NUM>, <NUM> then impinge on the light distributing elements <NUM>, <NUM>, <NUM>, respectively. The light distributing elements <NUM>, <NUM>, <NUM> deflect the light rays <NUM>, <NUM>, <NUM> so that they propagate towards the out-coupling optical elements <NUM>, <NUM>, <NUM>, respectively.

In some examples, the light distributing elements <NUM>, <NUM>, <NUM> are orthogonal pupil expanders (OPE's). In some examples, the OPE's deflect or distribute light to the out-coupling optical elements <NUM>, <NUM>, <NUM> and, in some examples, may also increase the beam or spot size of this light as it propagates to the out-coupling optical elements. In some examples, the light distributing elements <NUM>, <NUM>, <NUM> may be omitted and the in-coupling optical elements <NUM>, <NUM>, <NUM> may be configured to deflect light directly to the out-coupling optical elements <NUM>, <NUM>, <NUM>. For example, with reference to <FIG>, the light distributing elements <NUM>, <NUM>, <NUM> may be replaced with out-coupling optical elements <NUM>, <NUM>, <NUM>, respectively. In some examples, the out-coupling optical elements <NUM>, <NUM>, <NUM> are exit pupils (EP's) or exit pupil expanders (EPE's) that direct light in a viewer's eye <NUM> (<FIG>). It will be appreciated that the OPE's may be configured to increase the dimensions of the eye box in at least one axis and the EPE's may be to increase the eye box in an axis crossing, e.g., orthogonal to, the axis of the OPEs. For example, each OPE may be configured to redirect a portion of the light striking the OPE to an EPE of the same waveguide, while allowing the remaining portion of the light to continue to propagate down the waveguide. Upon impinging on the OPE again, another portion of the remaining light is redirected to the EPE, and the remaining portion of that portion continues to propagate further down the waveguide, and so on. Similarly, upon striking the EPE, a portion of the impinging light is directed out of the waveguide towards the user, and a remaining portion of that light continues to propagate through the waveguide until it strikes the EP again, at which time another portion of the impinging light is directed out of the waveguide, and so on. Consequently, a single beam of incoupled light may be "replicated" each time a portion of that light is redirected by an OPE or EPE, thereby forming a field of cloned beams of light, as shown in <FIG>. In some examples, the OPE and/or EPE may be configured to modify a size of the beams of light.

Accordingly, with reference to <FIG> and <FIG>, in some examples, the set <NUM> of waveguides includes waveguides <NUM>, <NUM>, <NUM>; in-coupling optical elements <NUM>, <NUM>, <NUM>; light distributing elements (e.g., OPE's) <NUM>, <NUM>, <NUM>; and out-coupling optical elements (e.g., EP's) <NUM>, <NUM>, <NUM> for each component color. The waveguides <NUM>, <NUM>, <NUM> may be stacked with an air gap/cladding layer between each one. The in-coupling optical elements <NUM>, <NUM>, <NUM> redirect or deflect incident light (with different in-coupling optical elements receiving light of different wavelengths) into its waveguide. The light then propagates at an angle which will result in TIR within the respective waveguide <NUM>, <NUM>, <NUM>. In the example shown, light ray <NUM> (e.g., blue light) is deflected by the first in-coupling optical element <NUM>, and then continues to bounce down the waveguide, interacting with the light distributing element (e.g., OPE's) <NUM> and then the out-coupling optical element (e.g., EPs) <NUM>, in a manner described earlier. The light rays <NUM> and <NUM> (e.g., green and red light, respectively) will pass through the waveguide <NUM>, with light ray <NUM> impinging on and being deflected by in-coupling optical element <NUM>. The light ray <NUM> then bounces down the waveguide <NUM> via TIR, proceeding on to its light distributing element (e.g., OPEs) <NUM> and then the out-coupling optical element (e.g., EP's) <NUM>. Finally, light ray <NUM> (e.g., red light) passes through the waveguide <NUM> to impinge on the light in-coupling optical elements <NUM> of the waveguide <NUM>. The light in-coupling optical elements <NUM> deflect the light ray <NUM> such that the light ray propagates to light distributing element (e.g., OPEs) <NUM> by TIR, and then to the out-coupling optical element (e.g., EPs) <NUM> by TIR. The out-coupling optical element <NUM> then finally out-couples the light ray <NUM> to the viewer, who also receives the out-coupled light from the other waveguides <NUM>, <NUM>.

<FIG> illustrates a top-down plan view of an example of the plurality of stacked waveguides of <FIG> and <FIG>. As illustrated, the waveguides <NUM>, <NUM>, <NUM>, along with each waveguide's associated light distributing element <NUM>, <NUM>, <NUM> and associated out-coupling optical element <NUM>, <NUM>, <NUM>, may be vertically aligned. However, as discussed herein, the in-coupling optical elements <NUM>, <NUM>, <NUM> are not vertically aligned; rather, the in-coupling optical elements are preferably non-overlapping (e.g., laterally spaced apart as seen in the top-down view). As discussed further herein, this nonoverlapping spatial arrangement facilitates the injection of light from different resources into different waveguides on a one-to-one basis, thereby allowing a specific light source to be uniquely coupled to a specific waveguide. In some examples, arrangements including nonoverlapping spatially-separated in-coupling optical elements may be referred to as a shifted pupil system, and the in-coupling optical elements within these arrangements may correspond to sub pupils.

With reference now to <FIG>, and according to some examples, a cross-sectional side view is illustrated of an example optical structure <NUM> comprising a metasurface <NUM> formed by a plurality of nanostructures <NUM> and an antireflection coating <NUM> disposed directly over the nanostructures <NUM>. The metasurface <NUM> and antireflection coating <NUM> may be disposed on an optically transmissive substrate <NUM>. In some examples, as illustrated, the antireflection coating <NUM> fills the spaces between the nanostructures <NUM> such that no air or other material is disposed between the nanostructures <NUM> and the antireflection coating <NUM>, at least over the majority of the expanse of the metasurface <NUM>. The antireflection coating <NUM> may be optically transmissive or substantially transmissive to light.

In some examples not according to the invention, the antireflection coating <NUM> has a substantially flat top surface 1430a. The antireflection coating <NUM> may function as a planarization layer for the underlying uneven topology of the nanostructures <NUM>. In some examples, the top surface 1430a of the antireflection coating <NUM> may be substantially parallel to a generally horizontal plane defined by the top surfaces 1420a of the nanostructures <NUM>.

The thickness <NUM> of the antireflection coating <NUM> may be defined as the distance from the topmost surface 1420a of the nanostructures <NUM> to the top surface of the antireflection coating <NUM>. In some examples, the thickness <NUM> may be in a range from about <NUM> to about <NUM> microns. In some examples, the thickness <NUM> may be from about <NUM> to about <NUM> micron. In some examples, the thickness <NUM> may be from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, and from about <NUM> to about <NUM>. In some examples, the thickness <NUM> may be about <NUM>. In some examples, the thickness <NUM> may be greater than the height of the nanostructures <NUM>, where the height of the nanostructures <NUM> is the distance from the bottom of the nanostructures <NUM> to the topmost surface 1420a.

Without being bound by theory, the antireflection coating <NUM> may provide impedance matching between an overlying medium (e.g., air) and one or both of the nanostructures <NUM> and the substrate <NUM>, to reduced occurrence of reflections. It is also believed that the antireflection coating <NUM> may cause destructive interference between light reflected from the top surface of the antireflection coating 1430a and bottom surface of the antireflection coating 1430b and/or light backscattered from the surfaces of the nanostructures <NUM> and/or the surface of the substrate <NUM>. This interference is believed to lead to a reduction or elimination in the amount of light perceived to be reflected from the optical structure <NUM>. In some examples, the ability of the antireflection coating <NUM> to reduce or eliminate reflected light from the optical structure <NUM> may depend on the thickness of the antireflection coating <NUM> and the wavelength of light impinging on the antireflection coating <NUM>. Preferably, the thickness <NUM> is chosen, relative to the refractive index and dimensions of nanostructures <NUM>, and the wavelengths of light for which destructive interference is desired, to provide destructive interference as noted above.

The antireflection coating <NUM> may comprise an optically transmissive material having a refractive index lower than the refractive index of the nanostructures <NUM>, but higher than the refractive index of the medium or material directly overlying and forming an interface with the antireflective coating <NUM>. For example, the medium overlying and forming an interface with the antireflective coating <NUM> may be air. In some examples, the antireflection coating <NUM> may have a refractive index of from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>. In some examples, the antireflection coating <NUM> may have a refractive index of about <NUM>. In some examples, the refractive index of the antireflection coating <NUM> may also be lower than the refractive index of the substrate <NUM>. It will be appreciated that, in some examples, the lower refractive index of the antireflective coating <NUM> relative to the substrate <NUM> facilitates TIR of light within the substrate <NUM>, and the high refractive index of the antireflective coating <NUM> relative to the medium overlying the coating <NUM> facilitates the passage of light to the metasurface <NUM> for incoupling into the substrate <NUM>.

With continued reference to <FIG>, to reduce potential reflections caused by interfaces between additional materials, the antireflection coating <NUM> may follow the contours of the nanostructures <NUM> such that substantially no air or other material is present between the nanostructures <NUM> and the antireflection coating <NUM> for all of or substantially all of the area over which the metasurface is disposed. In some examples, as illustrated, the antireflection coating <NUM> is disposed directly on the optical structure <NUM> such that the antireflection coating <NUM> encapsulates the nanostructures <NUM> above the surface of the substrate <NUM>.

As discussed herein, the antireflection coating <NUM> preferably comprises an optically transmissive material. As an example, the optically transmissive material may be an optically transmissive organic material, such as a transparent polymer. In some examples, the antireflection coating <NUM> may comprise a resist material, such as a photoresist material. Nonlimiting examples of photoresist include positive resist and negative resist. In some examples, the antireflection coating <NUM> may comprise UV photoresist, EUV photoresist, or DUV photoresist.

It will be appreciated that the antireflective coating <NUM> are formed on the nanostructures <NUM> by various deposition processes. In some examples, the antireflection coating <NUM> may be applied to the nanostructures <NUM> as a liquid, whereby the liquid forms the antireflection coating <NUM>. For example, the antireflection coating <NUM> may be deposited on the nanostructures <NUM> as a liquid by spin-coating. In some examples, the antireflection coating <NUM> may be deposited on the nanostructures <NUM> using vapor phase precursors in a vapor deposition process, for example a chemical vapor deposition (CVD) process and atomic layer deposition (ALD).

In some examples, an antireflection coating <NUM> may reduce the amount of incident light reflected by an optical structure <NUM> operating in transmission mode by about <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or more compared to a substantially similar optical structure that does not comprise an antireflection coating <NUM>. In some examples, the antireflection coating may achieve such a reduction in reflected light over a range of incident angles from -<NUM>° to <NUM>°, -<NUM>° to <NUM>°, -<NUM>° to <NUM>°, -<NUM>° to <NUM>°, -<NUM>° to <NUM>°, or wider.

The metasurface <NUM> comprising a plurality of nanostructures <NUM> may be configured to manipulate light, such as for beam steering, wavefront shaping, separating wavelengths and/or polarizations, and combining different wavelengths and/or polarizations. Preferably, the light is visible light having a wavelength in the range of <NUM> to <NUM>. In some examples, the metasurface over which an antireflection coating is disposed may comprise nanostructures having a size and periodicity less than the wavelength of the visible light. It will be appreciated that, in some examples, the metasurface <NUM> selectively redirects some wavelengths of light, while allowing other wavelengths of light to pass without being redirected. Such properties are typically engineered with structures on micron scales (e.g., in photonics crystal fibers or distributed bragg reflectors), while various examples herein include geometries on nano-scales (e.g. <NUM>-100x smaller scales), and provide selective redirection of light in the visible part of the electromagnetic spectrum.

As an example, the metasurface <NUM> may work in a transmissive mode in which light is incident on the metasurface from a first side of the metasurface <NUM>, propagates through the body of the metasurface <NUM>, and subsequently propagates away from the metasurface <NUM> on an opposite side of the metasurface <NUM>. The light propagates away from the metasurface <NUM> in a direction different from the incident direction of the light on the first side. In some examples, an antireflection coating <NUM> may reduce or eliminate the amount of light reflected from the metasurface <NUM> as compared to a metasurface <NUM> that does not comprise an antireflection coating <NUM>. In some examples, the antireflection coating <NUM> may not substantially reduce or impact the amount of light that propagates through and away from the metasurface <NUM> as compared to a metasurface <NUM> that does not comprise an antireflection coating <NUM>.

In some examples, the substrate <NUM> supporting the metasurfaces <NUM> over which an antireflection coating <NUM> is disposed may be a waveguide and may form direct view display devices or near-eye display devices, with the waveguides configured to receive input image information and generate an output image based on the input image information in the form of light encoded with image information. These devices may be wearable and constitute eyewear in some examples, and may be the display devices described herein with respect to <FIG>. In some examples, the input image information received by the waveguides may be encoded in multiplexed light streams of different wavelengths (e.g., red, green and blue light) that are incoupled into one or more waveguides. Incoupled light may propagate through the waveguide due to total internal reflection. The incoupled light may be outcoupled (or outputted) from the waveguide by one or more outcoupling optical elements, as described above regarding <FIG>.

In some examples, the metasurfaces <NUM> over which an antireflection coating <NUM> is conformally disposed may be the incoupling optical elements, outcoupling optical elements, and/or light distributing elements of the waveguide. The compactness and planarity of the metasurface <NUM> and antireflection coating <NUM> allows for a compact waveguide, and for a compact stack of waveguides where multiple waveguides form a stack. In addition, the metasurface <NUM> may be configured to provide for a high degree of precision in incoupling and/or outcoupling light, which may provide high image quality. For example, the high selectivity may reduce channel crosstalk in configurations in which full color images are formed by outputting light of different colors or wavelengths at the same time, while the antireflection coating <NUM> may reduce ghost images.

It will be appreciated that the nanostructures <NUM> may have various sizes and be arranged in various orientations relative to one another to form the metasurface <NUM> for various applications. For example, as discussed herein, the nanostructures <NUM> may be arranged to form a diffraction grating, such as an asymmetric diffraction grating. In some examples, the metasurface <NUM> may be formed of nanostructures <NUM> that are a multilevel or tiered. For example, the nanostructures <NUM> may be relatively wide on a first level and relatively narrower on a second level. In some examples, the metasurfaces <NUM> may be formed on a single level, and have a substantially constant width on that level. Examples of metasurfaces which may be utilized as the metasurface <NUM> are described in: <CIT>. It will be appreciated that the nanostructures <NUM> disclosed herein may correspond to the protrusions, nanobeams, etc. described in these applications. In some examples, the optical structure <NUM> may be any metasurface comprising a plurality of nanostructures as is known in the art.

Examples of different configurations of the nanostructures <NUM> are described below. It will be appreciated that, for clarity of description, the nanostructures discussed below may have different reference numerals than <NUM>. Nevertheless, it will be understood that the various nanostructures (<NUM>, <NUM>) described below correspond to the nanostructures <NUM> of <FIG>.

With reference now to <FIG>, and according to some examples, a top-down view is illustrated of an example optical structure <NUM> comprising a metasurface <NUM> comprising nanostructures <NUM> forming an asymmetric Pancharatnam-Berry Phase Optical Element (PBOE), which may be advantageous for, among other things, light steering. The substrate <NUM> underlies the nanostructures <NUM>. In some examples, the substrate <NUM> may be an optically transmissive substrate, e.g., a waveguide.

With reference now to <FIG>, and according to some examples, a perspective view of an example optical element <NUM> comprising an asymmetric Pancharatnam-Berry Phase Optical Element (PBOE) and including an antireflection coating <NUM> is illustrated. As described herein, the antireflection coating <NUM> follows the contours of the nanostructures <NUM> such that substantially no air or other material is present between the nanostructures <NUM> and the antireflection coating <NUM>. Further, as described herein, the antireflection coating <NUM> may have a substantially flat top surface 1430a. The antireflection coating <NUM> may function as a planarization layer for the underlying uneven topology of the nanostructures <NUM>. In some examples, the top surface 1430a of the antireflection coating <NUM> may be substantially parallel to a generally horizontal plane defined by the top surfaces (not shown) of the nanostructures <NUM>. The thickness <NUM> of the antireflection coating <NUM> may be defined as the distance from the topmost surface of the nanostructures <NUM> to the top surface 1430a of the antireflection coating <NUM>. In some examples, the thickness <NUM> may be in a range from about <NUM> to about <NUM> microns. In some examples, the thickness <NUM> may be from about <NUM> to about <NUM> micron. In some examples, the thickness <NUM> may be from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, and from about <NUM> to about <NUM>. In some examples, the thickness <NUM> may be about <NUM>. In some examples, the thickness <NUM> may be chosen based upon the wavelength of light that the metasurface is configured to redirect and, thus, that is expected to impinge on the antireflection coating <NUM>. Preferably, the thickness <NUM> is chosen to provide destructive interference between light reflected off the top and bottom surfaces, respectively, of the antireflection coating <NUM>, where the bottom surface (not shown) is the surface of the antireflection coating forming an interface with the top surface of the nanostructures <NUM>.

<FIG> is a plot of transmission and reflection as a function of the angle of incidence of light for an optical structure having the general structure described with reference to <FIG>. Various orders of diffracted transmitted light are indicated by "T" and reflected light is indicated by "R. " In this example, the antireflection coating <NUM> is an optically transmissive photoresist having a refractive index of about <NUM>, which is lower than the refractive index of the nanostructures <NUM> and lower than the refractive index of the substrate <NUM>, which is about <NUM> and formed of polysilicon. The thickness <NUM> of the antireflection coating <NUM> is about <NUM> and air forms an interface with the topmost surface of the antireflection coating <NUM>.

As may be seen in the plot, the percentage of incident light reflected from the optical structure <NUM> remains below about <NUM>% over a wide range of incident angles, from more than -<NUM>° to more than <NUM>°. In comparison, the percentage of light reflected from a substantially similar metasurface <NUM> that does not comprise an antireflection coating was determined to be about <NUM>% (not shown) over the same range of incident angles. Thus, in this example, the antireflection coating <NUM> provides an approximately <NUM>% reduction in the amount of light reflected from the metasurface <NUM> as compared to a substantially similar metasurface <NUM> that does not comprise an antireflection coating <NUM>.

Meanwhile, the percentage of light incident on the metasurface <NUM> comprising the antireflection coating <NUM> that undergoes first order diffraction to angles suitable for TIR (T<NUM>) is about <NUM>% for an incident angle of <NUM>°, and remains at about this level for incident angles from about -<NUM>° to about <NUM>°. Advantageously, the amount of incident light diffracted at angles suitable for TIR is substantially the same as that for a substantially similar metasurface <NUM> that does not comprise an antireflection coating <NUM>. Accordingly, a metasurface <NUM> comprising an antireflection coating <NUM> may be used as an optical element <NUM> as described herein, for example an incoupling optical element, without a substantial reduction in the amount of incoupled light, while reducing the amount of reflected light, thereby reducing or eliminating potential ghost images in the display device in which the optical element is incorporated.

With reference now to <FIG>, a cross-sectional perspective view of an example optical element <NUM> comprising a metasurface <NUM> and an antireflection coating <NUM> is illustrated. The metasurface <NUM> comprises an asymmetric diffraction grating formed by nanostructures <NUM> having different widths. <FIG> illustrates a cross-sectional side view of the optical element <NUM> of <FIG>. In this example, the substrate <NUM> comprises sapphire having a refractive index of about <NUM>. The plurality of nanostructures <NUM> comprises amorphous silicon. The antireflection coating <NUM> may comprise an optically transmissive photoresist material having a refractive index of about <NUM>, and in some examples, may be conformally applied to the asymmetric diffraction grating <NUM> by spin-coating. The thickness <NUM> of the antireflection coating <NUM>, that is the distance from the topmost surface 1620a of the nanostructures <NUM> to the top surface 1430a of the antireflection coating <NUM>, is about <NUM>.

<FIG> is a plot of the transmission and reflection spectrum for an optical element having the general structure shown in <FIG>. As may be seen in the plot, the percentage of incident light reflected from the optical element <NUM> comprising the antireflection coating <NUM> remains below about <NUM>% over a wide range of incident angles, from more than -<NUM>° to more than <NUM>°. The percentage of light reflected from the optical element <NUM> comprising the antireflection coating <NUM> is about <NUM> for incident angles from about -<NUM>° to about <NUM>°.

In comparison, the percentage of light reflected from a substantially similar optical element <NUM> that does not comprise an antireflection coating <NUM> is about <NUM>% (not shown) over the same range of incident angles. Thus, in this example, the antireflection coating <NUM> provides an approximately <NUM>% reduction in the amount of light reflected from the optical element <NUM> as compared to a substantially similar optical element <NUM> that does not comprise an antireflection coating <NUM>.

Meanwhile, the percentage of light incident to the optical element <NUM> comprising the antireflection coating <NUM> that undergoes first order diffraction to TIR (T1) is greater than about <NUM>% for incident angles from about -<NUM>° to about <NUM>°. Advantageously, the amount of incident light diffracted to TIR for the optical element <NUM> comprising the antireflection coating <NUM> is substantially the same as the amount of light diffracted to TIR for a substantially similar optical element <NUM> that does not comprise an antireflection coating <NUM>. Accordingly, an optical element <NUM> comprising an antireflection coating <NUM> may be used as an optical element as described herein, for example an incoupling optical element, without a substantial reduction in the amount of incoupled light, while reducing the amount of reflected light, thereby reducing or eliminating potential ghost images, as discussed herein.

It will be appreciated that the metal surfaces and nanostructures disclosed herein may be formed by patterning, such as patterning by lithography and etching. In some examples, the metasurfaces and nanostructures may be patterned using nanoimprinting, thereby avoiding costly lithography and etch processes. Once the nanostructures are patterned, any masking materials may be removed in some examples and an antireflection coating <NUM> may be applied, deposited, or formed over the metasurface, as described herein. In some other examples, the masking materials themselves may be utilized as the antireflective coating. <FIG> and <FIG> illustrate examples of process flows for forming optical structures having antireflection coatings.

<FIG> illustrate cross-sectional views of intermediate structures 1700A-1700D, respectively, at various stages of fabrication of an optical element <NUM>, <NUM><NUM> having a metasurface <NUM>, <NUM>, <NUM> using lithography and etch, according to some examples. Referring to the intermediate structure 1700A of <FIG>, the method includes providing a substrate <NUM> having a surface <NUM> suitable for forming a metasurface <NUM>, <NUM>, <NUM> thereon. The substrate <NUM> includes an optically transmissive material having a refractive index n<NUM> and various other material attributes described above with reference to <FIG>. The method additionally includes forming on the surface <NUM> a high index layer <NUM> having an index of refraction n<NUM> bulk. The high index layer <NUM> is suitable, when patterned, for forming the one or more nanostructure <NUM>, <NUM>, <NUM>, as described above with reference to <FIG>. The high index layer <NUM> may be deposited using any suitable technique, such as chemical vapor deposition (CVD), including plasma-based CVD processes, such as plasma-enhanced chemical vapor deposition (PECVD) and thermal-based CVD processes, such as low pressure chemical vapor deposition (LPCVD), according to some examples. The high index layer <NUM> may also be deposited using physical vapor deposition (PVD), evaporation, and atomic layer deposition, among other techniques. The method additionally includes forming on the high index layer <NUM> a masking layer 1431A. The masking layer 1431A may be formed of or include one or more layers of materials that are suitable for providing a template for subsequent etching of the underlying high index layer <NUM>. In some examples, the masking layer 1431A may be a photoresist, which may be spin-coated, followed by a post-bake. In some other examples, the masking layer 1431A may include a plurality of layers, including a hard mask layer formed on the high index layer <NUM> and a photoresist layer formed on the hard mask layer. The hard mask layer may be included, for example, when a photoresist layer may not provide sufficient etch selectivity during the subsequent etch pattern transfer to the underlying high index layer <NUM>. The hard mask layer may also serve as an antireflective coating to minimize reflection during the subsequent exposure process. In some examples, the hard mask layer may be a spin-coated polymer or a film deposited by any of the deposition techniques for depositing the high index layer <NUM>. When included, the hard mask layer may provide greater etch selectivity than the overlying photoresist layer. In some examples, the photoresist may be a positive photoresist or a negative photoresist. A positive photoresist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer, whereas a negative resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer.

In some examples, the photoresist and/or the hard mask layer may be formed of a material containing silicon or silicon oxide, which may have sufficient etch selectivity against the high index layer <NUM>, such that the photoresist and/or the hard mask layer remains relatively intact through the etching of the underlying high-index layer <NUM>. In these examples, the silicon or silicon oxide-containing photoresist and/or hard mask layer may remain on top of one or more nanostructures <NUM>, <NUM>, <NUM> after patterning, as described above with reference to <FIG>.

Referring to the intermediate structure 1700B of <FIG>, after deposition and post-deposition bake, the method includes patterning the photoresist layer of the mask layer <NUM> by selectively exposing portions of the photoresist to a pattern of light. The exposure to light, e.g., coherent UV light, or an electron beam, causes a chemical change, e.g., polymeric crosslinking in the photoresist, which allows exposed portions of the photoresist to be selectively removed by a developer solution for a positive photoresist, or allows unexposed portions of the photoresist to be selectively removed by a developer solution for a negative photoresist. Upon selectively removing, the resulting patterned masking photoresist remains on the high index layer <NUM>, thereby serving as a template for the subsequent patterning the underlying hard mask layer when included by, e.g., etching. The resulting intermediate structure 1700C shows the patterned masking layer <NUM>, which includes the patterned photoresist and optionally the patterned hard mask layer when included.

Referring to the intermediate structure 1700C of <FIG>, the patterned masking layer <NUM> may be used as a template to etch the underlying high index layer <NUM> into one or more nanostructures <NUM>, <NUM>, <NUM>. It will be appreciated that the nanostructures <NUM>, <NUM>, <NUM> may be configured as desired based on the desired properties of the resulting meta-surface. In some examples, the nanostructures <NUM>, <NUM>, <NUM> may include features extending in a first lateral direction (e.g., the y-direction) and a plurality of second nanostructures <NUM>, <NUM>, <NUM> extending in a second direction (e.g., the x-direction), as described more in detail above with reference to <FIG>. In various examples, the high index layer <NUM> may be etched, e.g., anisotropically dry-etched. The etch process employed may have a suitable selectivity against the masking layer <NUM> and/or the substrate <NUM>, such that the portions of the high index layer <NUM> are removed without prematurely removing the masking layer <NUM> and/or without undesirably damaging the exposes portions of the substrate <NUM>.

Referring to the intermediate structure 1700D, in some examples, the masking layer <NUM> on the one or more nanostructures <NUM>, <NUM>, <NUM> are removed therefrom. The resist portion of the masking layer <NUM> may be removed by, e.g., using a liquid resist stripper or an oxygen-based plasma in a process referred to as ashing. If desired and when included, the underlying hard mask layer may be subsequently removed using a wet or a dry etch process which selectively removes the hard mask without substantially affecting the one or more nanostructures <NUM>, <NUM>, <NUM> or the substrate <NUM>. Subsequently, an antireflective coating may be deposited on and at the sides of the nanostructures <NUM>, <NUM>, <NUM>, e.g. by spin-coating or by chemical vapor deposition and subsequent planarization of the vapor deposited layer.

In some other examples, e.g., the examples described above with reference to <FIG>, the mask layer <NUM>, e.g., the photoresist/hard mask or the hard mask, may be left-in without being removed. In these examples the mask layer <NUM> may comprise the anti-reflection coating <NUM> as described herein with reference to <FIG>.

<FIG> illustrate cross-sectional views of intermediate structures 1800A-1800D, respectively, at various stages of fabrication of an optical element <NUM>, <NUM>, <NUM> having a metasurface <NUM>, <NUM>, <NUM> according to some examples. In some examples, the method of forming intermediate structures 1800A, 1800C and 1800D of <FIG>, respectively, is similar to the method of forming intermediate structures 1700A, 1700C and 1700D of <FIG>, respectively. However, the method of forming the intermediate structure 1800B of <FIG> is different from the method forming the intermediate structure 1700B of <FIG>, whose differences are described below.

Referring to the intermediate structure 1800B of <FIG>, unlike the method described above with reference to <FIG>, instead of patterning a photoresist layer by selectively exposing and removing portions of the photoresist using light or an electron beam, in the illustrated example, a nanoimprint template <NUM>, or a nanoimprint mold, which has predefined topological patterns in accordance with formation of the one or more nanostructure <NUM>, <NUM>, <NUM>, is brought into contact with an imprint resist of the masking layer 1431A. In some examples, the template <NUM> is pressed into an imprint resist formed of thermoplastic polymer under certain temperature, e.g., above the glass transition temperature of the imprint resist, thereby transferring the pattern of the template <NUM> into the softened imprint resist. After being cooled down, the template <NUM> is separated from the imprint resist and the patterned resist is left on the high index layer <NUM>. In some other examples, the after being pressed into the imprint resist, the imprint resist is hardened by crosslinking under UV light.

Referring to <FIG>, and according to some embodiments of the invention, after removal of masking layer <NUM> from the one or more nanostructures <NUM>, <NUM>, <NUM> as described above with respect to <FIG> and <FIG>, an antireflective coating <NUM> is conformally deposited on the nanostructures <NUM>, <NUM>, <NUM>, e.g. by a vapor deposition process such as a chemical vapor deposition process or atomic layer deposition process. Thus, the deposited antireflective coating <NUM> is a conformal layer overlying the nanostructures <NUM>, <NUM>, <NUM>, and following contours of the nanostructures <NUM>, <NUM>, <NUM>, without completely filling the volume separating those nanostructures.

In the foregoing specification, various specific embodiments have been described. It will, however, be evident that various modifications and changes may be made thereto without departing from the scope of the claims.

The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.

Indeed, it will be appreciated that the systems and methods of the disclosure each have several innovative aspects, no single one of which is solely responsible or required for the desirable attributes disclosed herein. The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure.

Certain features that are described in this specification in the context of separate embodiments also may be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment also may be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination may in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. No single feature or group of features is necessary or indispensable to each and every embodiment.

It will be appreciated that conditional language used herein, such as, among others, "can," "could," "might," "may," "e.g.," and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment. The terms "comprising," "including," "having," and the like are synonymous and are used inclusively, in an openended fashion, and do not exclude additional elements, features, acts, operations, and so forth. In addition, the articles "a," "an," and "the" as used in this application and the appended claims are to be construed to mean "one or more" or "at least one" unless specified otherwise. Similarly, while operations may be depicted in the drawings in a particular order, it is to be recognized that such operations need not be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results.

Claim 1:
An optical system comprising:
an optical element comprising a metasurface (<NUM>, <NUM>, <NUM>), the metasurface (<NUM>, <NUM>, <NUM>) comprising a plurality of nanostructures (<NUM>, <NUM>, <NUM>) forming repeating unit cells, wherein, as seen in a top-down view, each unit cell comprises:
a plurality of first nanostructures having first lengths and first widths, wherein the first lengths are elongated in a first direction, and wherein the first widths differ from one another; and
a plurality of second nanostructures having second lengths and second widths, wherein the second lengths are elongated in a second direction, and wherein the second widths differ from one another,
wherein the second direction crosses the first direction; and
an antireflection coating (<NUM>) for the optical element comprising the metasurface, the antireflection coating (<NUM>) comprising:
a layer of an optically transparent material having a refractive index greater than <NUM> and less than a refractive index of a material comprising the metasurface,
wherein the layer of optically transparent material is conformally disposed over the nanostructures (<NUM>, <NUM>, <NUM>) of the metasurface, and the layer of optically transparent material follows contours of the nanostructures (<NUM>, <NUM>, <NUM>) without completely filling volumes separating each of the nanostructures (<NUM>, <NUM>, <NUM>), wherein the unfilled volumes that separate the first nanostructures from the second nanostructures are of a different volume from the unfilled volumes that separate the first nanostructures or that separate the second nanostructures.