Patent Description:
The combiner <NUM> should be highly transmissive to light coming through the windshield, except within a small but finite angle at three selective wavelengths near the red, green and blue wavelengths emitted by the projector. Low-cost and mass producible materials for HUD display combiners are needed that can provide both narrow band reflection for polarized input signals at oblique incident elevation angles, and high transmission of unpolarized ambient light. <CIT> discloses an optical combiner for a head-mounted display device, wherein the optical combiner is based on a metasurface which is optimised in terms of its angular and spectral response.

Traditional diffraction gratings are angle dependent, meaning that as the incident angle of an input signal changes, a resonant wavelength of the output signal also changes. To reduce the angular variation for a reflection resonance from a diffraction grating, the spectral line width of the resonance can be controlled, resulting in a wider wavelength band with broader peaks. In some examples, this poor angular tolerance can lower the broadband transmission of the diffraction grating, and alter the hue of the output signal.

In one aspect, the present disclosure is directed to optical devices using perturbed arrangements of periodic structures to flatten the angular response of the resonant effect, while simultaneously providing narrow-wavelength reflective resonances. In some examples, the perturbed structures can produce a flat band output signal at a desired incident angle with narrower reflection bands compared to a conventional diffraction grating.

In some examples, the optical device included an optical combiner with a first structured layer of a material with a first refractive index. The first structured layer includes a structured surface with a periodic two-dimensional arrangement of structures such as, for example, an array of depressions. A second layer of a material with a second refractive index overlies the structured surface and occupies at least a substantial portion of the volume of each of the structures. The difference between the first refractive index and the second refractive index, measured at <NUM>, is less than about <NUM>. For polarized or unpolarized light incident on the structured surface at an oblique elevation angle greater than about <NUM>° (± <NUM>°), the optical combiner can provide an output signal with one or more distinct narrowband reflection peaks each having an average reflection of greater than about <NUM>%, and the average reflection peaks in the output signal can be provided within a range of ± <NUM>°, or ± <NUM>°, or ± <NUM>° of the elevation angle.

The optical combiners of the present disclosure utilize inexpensive and widely available materials, and can be manufactured on a larger scale at relatively low costs using, for example, a roll-to-roll manufacturing process.

In one aspect, the present disclosure is directed to an optical combiner that includes a first layer with a periodic two-dimensional arrangement of structures arranged to support resonance for an input signal of a target wavelength, wherein the structures have a first refractive index. A second layer overlies the structures on the first layer, wherein the second layer includes a second material with a second refractive index, and wherein a difference between the first refractive index and the second refractive index, measured at <NUM>, is less than about <NUM>. The periodic arrangement of structures is configured such that the optical combiner produces, for the input signal incident on the first layer from air at an oblique elevation angle of greater than about <NUM>°, an output signal with a reflection peak with an average reflection of greater than about <NUM>% within a ± <NUM>° range of the elevation angle.

In another aspect, the present disclosure is directed to a windshield for a vehicle, which includes an exterior glass layer, an interior glass layer, and an optical combiner film between the exterior glass layer and the interior glass layer. The optical combiner film includes first layer with a periodic two-dimensional arrangement of structures arranged to support resonance for an input signal of a target wavelength, wherein the structures have a first refractive index; and a second layer that overlies the structures on the first layer, wherein the second layer includes a second material with a second refractive index, and wherein a difference between the first refractive index and the second refractive index, measured at <NUM>, is less than about <NUM>. The periodic arrangement of structures is configured such that the optical combiner produces, for the input signal incident on the first layer from air at an oblique elevation angle of greater than about <NUM>°, an output signal with a reflection peak with an average reflection of greater than about <NUM>% within a ± <NUM>° range of the elevation angle.

In another aspect, the present disclosure is directed to a heads-up display (HUD) system, which includes a computer with a processor that generates an output including HUD display data, and a projector unit interfaced with the computer, wherein the projector unit includes a laser that emits narrow band red/green/blue (RGB) input signal onto a windshield for display by a viewer. The windshield includes an exterior glass layer, an interior glass layer, and an optical combiner film between the exterior glass layer and the interior glass layer. The optical combiner film includes a first layer with a periodic two-dimensional arrangement of structures arranged to support resonance for an input signal of a target wavelength, wherein the structures have a first refractive index; and a second layer that overlies the structures on the first layer, wherein the second layer includes a second material with a second refractive index, and wherein a difference between the first refractive index and the second refractive index, measured at <NUM>, is less than about <NUM>. The periodic arrangement of structures is configured such that the optical combiner produces, for the RGB input signal incident on the first layer from air at an oblique elevation angle of greater than about <NUM>°, an output signal having a reflection peak with an average reflection of greater than about <NUM>% within a ± <NUM>° range of the elevation angle.

In another aspect, the present disclosure is directed to an optical combiner film that includes a structured layer overlain by a cover layer. The structured layer includes a periodic lattice of regularly repeating depressions, wherein the lattice of depressions has a perturbed hexagonal unit cell, and wherein a difference between a refractive index of the structures and a refractive index of the cover layer, measured at <NUM>, is less than about <NUM>. The periodic lattice is configured such that the optical combiner film produces, for an input signal incident on the structured layer from air at an oblique elevation angle of greater than about <NUM>°, an output signal having a reflection peak with an average reflection of greater than about <NUM>% within a ± <NUM>° range of the elevation angle.

In another aspect, the present disclosure is directed to a method for making an optical combiner film. The method includes forming a first layer on a polymeric support film, wherein the first layer includes a periodic arrangement of depressions and having a perturbed hexagonal unit cell arrangement, and wherein the first layer includes a material with a first refractive index; and applying a cover layer on the first layer, wherein the cover layer includes a material with a second refractive index, and wherein a difference between the first refractive index and the second refractive index, measured at <NUM>, is less than about <NUM>. The structures in the first layer are configured such that the optical combiner film produces, for an input signal incident on the first layer from air at an oblique elevation angle of greater than about <NUM>°, an output signal having a reflection peak with an average reflection of greater than about <NUM>% within a ± <NUM>° range of the elevation angle.

In another aspect, the present disclosure is directed to an optical combiner, including: a first structured layer of a first material with a first refractive index, wherein the first structured layer includes a first periodic two-dimensional arrangement of structures arranged to support resonance for an input signal of a first target wavelength and a second target wavelength; and a second structured layer of a second material with second refractive index, wherein the second structured layer includes a second periodic two-dimensional arrangement of structures arranged to support resonance for an input signal of a third target wavelength different from the first and the second target wavelengths, wherein the first structured layer and the second structured layer are stacked on each other such that light incident on the first structured layer is diffracted, in succession, by the first structured layer and the second structured layer. The first structured layer and the second structured layer are encapsulated in a third material with a third refractive index such that a refractive index difference, measured at <NUM>, between each of the first and the second refractive indices and the third refractive index is less than about <NUM>. The structures in the first and the second structured layers are configured such that the optical combiner produces, for an input signal incident on first periodic arrangement of structures from air at an oblique elevation angle of greater than about <NUM>°, an output signal having three reflection peaks with an average reflection of greater than about <NUM>% within a ± <NUM>° range of the elevation angle.

Like symbols in the drawings indicate like elements.

Referring again to <FIG>, if the input signal incident on the optical combiner <NUM> includes a discrete set of narrow frequency channels (e.g., the three wavelengths λred ± Δλ , λgreen ± Δλ , and λblue ± Δλ with narrow linewidths Δλ « λ), the response of the beam splitter can be engineered to be spectrally selective so that ηart(λred,green,blue) ≈ <NUM>, but <NUM> at all other wavelengths. In this way, the overall efficiency of external information may also be approximately unity, ηext = ∫ dληext(λ) ≈ <NUM>. For example, guided mode resonance (GMR) filters can be configured to produce robust command over the bandwidth Δλ of the resonances via the depth of corrugation (deeper corrugations increase Δλ ). However, in conventional GMR filters, the dispersion ωres(k) of the guided modes confers strong angular selectivity in addition to the spectral selectivity, so that for a certain frequency ω the reflectance satisfies R(ω,k) ≈ <NUM> only at the single wavevector k that satisfies ω = ωres(k).

Referring to <FIG>, for non-normal angles of incidence, the dispersion of a conventional diffraction grating has the approximate form ωres(k) = ω<NUM> + vgk , where ω<NUM> is the resonant frequency at normal incidence and vg ≈ c / neff is the group velocity of the guided mode ( c is the speed of light in vacuum and neff is the effective index of the guided mode, of order unity). Hence, for a resonant linewidth Δω = ω<NUM> / Q (where Q is the Q-factor, characterizing the spectral selectivity), the angular tolerance (defined herein as the width of the angular range where reflection is at least <NUM>% of the max reflection) of a GMR is Δθ ≈ neff / Q.

In a conventional diffraction grating structure such as a GMR filter one needs a broad spectral bandwidth to have a large field of view (FoV), in direct conflict with the narrow bandwidths desired for high ηext. Alternatively, the FoV could be extended by aperiodic patterning such that the resonant frequency shifts spatially to counteract the angular dependence. However, both for manufacturing practicality and installation tolerances, periodic structures are highly preferable for compatibility with scalable approaches such as roll-to-roll manufacturing and nano-imprint lithography.

Referring now to <FIG>, the optical combiners of the present disclosure utilize a first structured layer with a periodic two-dimensional arrangement of structures arranged to support resonance for an input signal of a target wavelength. The first structured layer is overlain by a second layer, and the difference between the refractive indices of the first and the second layers is less than about <NUM>, as measured at <NUM>. The periodic arrangement of structures is configured such that the optical combiner produces, for the input signal incident on the first layer from air at an oblique angle of greater than about <NUM>°, an output signal with a reflection peak with an average reflection of greater than about <NUM>% within a ± <NUM>° range of the target wavelength. In this application, average reflection measurements do not include Fresnel reflections from adjoining surfaces or layers such as, for example, layers of glass.

In some examples, the structures in the first layer are quasi-bound states in the continuum (QBIC) structures, which produce resonances that can be precisely controlled through a symmetry-lowering perturbation instead of the depth of corrugation utilized in GMR devices. In addition to the symmetry-controlled Q-factor (following Q ∝ <NUM> / δ<NUM> for a magnitude of perturbation δ), QBICs have symmetry-controlled polarization dependence capable of reflecting any desired polarization, and the polarization control of flat optical filters even extends to manipulation of circularly polarized states. In some examples, QBICs are also compatible with high index contrast systems, in which case large in-plane Bragg scattering may produce flat band structures with increased angular tolerance (reduced angular selectivity).

In particular, QBICs are often well-approximated by having parabolic band structures, following a second order Taylor expansion ωres(k) ≈ ω<NUM> + b(k - kc)<NUM> / <NUM>, where kc is the momentum at which the first derivative vanishes and b is the Taylor expansion coefficient with units m<NUM> / s. In this case, when operating near θop = sin-<NUM>(kcc / ω<NUM>) the angular tolerance of the resonant reflectance has the form <MAT>. Hence not only does the angular tolerance scale much more favorably with Q-factor (Q-<NUM>/<NUM> instead of Q-<NUM> ), the parameter b offers an independent degree of freedom governing the trade-off between angular and spectral selectivity, and can vary over several orders of magnitude (in contrast to neff ).

Unfortunately, by symmetry the momenta kc at which the first derivative vanishes (usually corresponding to the band edge mode) are conventionally limited to normal incidence, kc = <NUM>, or at the edge of the first Brillouin zone (FBZ), kc = ±π / a (where a is the lattice constant). The former would require operating at normal incidence, which is prohibitive for an optical combiner, and the latter exist either under the light line (in which case the modes are bound and cannot resonantly reflect light) or over the diffraction line (in which case parasitic diffraction orders will greatly reduce ηart and potentially introduce unwanted diffractive rainbow effects distorting the external information). 'Accidental' mode mixing between two unrelated q-BICs can open up band gaps at arbitrary kc , but (i) the resulting bands does not rely on Bragg scattering but on the strength of mode-coupling, and hence b may not be arbitrarily small, (ii) the phenomenon relies on 'accidental' alignment between two modes, reducing the robustness from both a design and practical (fabrication) point of view, (iii) it inherently requires the presence of an additional resonance, thus reducing ηext, and (iv) it increases the complexity of the control over the linewidth and polarization properties of the QBICs. In contrast, a solution controlling kc directly from symmetry considerations would solve these limitations, simultaneously bringing to bear the advantages of spectrally isolated QBICs in terms of both lifetime and polarization control.

The optical combiners of the present disclosure utilize a zone-folding method based on lattice perturbations, which offers robust control over the Q-factor, polarization state, and operating angle of the band edge mode. In the arrangement of structures in the structured layer of the present disclosure, a non-rectangular lattice is transformed into a rectangular lattice, and Bragg-localized modes at the edge of the FBZ are folded to a non-zero momentum within the FBZ whose value is fixed by the discrete translation symmetry of the lattice. By stretching or compressing the lattice in a direction orthogonal to the plane of incidence, the momentum may be tuned, and thus the angle of incidence, of this band edge mode.

Since the angle of incidence breaks the inversion symmetry of the system compared to normal incidence excitation, the band-edge QBICs are compatible with extrinsic chirality, and thus offer selectability of the resonant polarization state to be any elliptical polarization (including full circular dichroism). Since zone-folded QBICs are periodic, subwavelength in dimension, and have broadband transmissive behavior, these structures offer a scalable platform for projective optical combiners and other AR displays with minimal compromises regarding fidelity of the external information and the efficiency and FoV of the input signal.

As shown in the schematic diagram of <FIG>, when used in an optical combiner component of an optical device such as, for example, a heads-up display (HUD) system, the structured surfaces of the present disclosure, or set of cascaded metasurfaces, specularly reflect up to three selected wavelengths (for example, red, green, and blue). In some examples, in a HUD the structured surfaces reject external information (represented by broadband light) only in these bands while overlaying the information projected by projector in HUD, the confluence of which arrives at the user. <FIG> shows a characteristic reflection spectrum at the operating angle θop for the selected wavelengths.

<FIG> schematically depicts the angular response of an optical combiner utilizing the structured surfaces of the present disclosure, in an example system wherein the information by the HUD is viewed by the user centered at an elevation angle θop but having extremal values subtended by the angle Δθ. The performance of the optical combiner utilizing the structures of the present disclosure for the full FoV of the HUD image is correspondingly depicted in <FIG>, wherein the spectral shift of the resonant peaks is smaller than their linewidths, maintaining enhanced reflectance over all of the angles of operation. Not depicted in <FIG> is the same requirement in the orthogonal direction.

In some examples, to avoid double images coming from direct reflection off the air-glass interfaces, the external interfaces of the HUD system should have an anti-reflection coating. In some examples, the operating elevation angle θop should be at or near Brewster's angle (approximately <NUM>°), requiring p-polarized light to be sent by the HUD and resonantly reflected by the metasurface.

<FIG> illustrates an example of how a perturbation applied to a hexagonal photonic crystal can yield a rectangular lattice. The dashed hexagon in FG. 5A represents the unit cell of the unperturbed lattice, while the solid rectangle represents the unit cell of the perturbed structure. The example perturbation shown in <FIG>, which is not intended to be limiting, shifts every other row of the lattice vertically by a distance δ , and perturbed structures arranged in this type of array can be used to excite a targeted TE (transverse electric), or s-polarized lattice of structures with TM (transverse magnetic) (p-polarized) light. In this application, TE polarized light is characterized by its electric field being perpendicular to the plane of incidence. For TE polarized light, the magnetic field - always perpendicular to the electric field in isotropic materials - thus lies in the plane of incidence.

The result of the perturbation in reciprocal space is depicted in <FIG>, where the zones of the unperturbed FBZ (dashed hexagon) are folded into the perturbed FBZ (solid rectangle) by translation of reciprocal lattice vector of the perturbed structure. In <FIG>, the modes at the K points in the unperturbed FBZ (where Bragg scattering is maximal in a hexagonal lattice) have folded to a point kc away from the Γ point and k f away from the edge of the perturbed FBZ. In the present application, k-points refer to sampling points in the FBZ of the material, i.e. the specific region of reciprocal-space which is closest to the origin (<NUM>,<NUM>,<NUM>) (the Γ point).

As a result, as shown in the example band diagram of <FIG> of such a perturbed device, a flat band is centered at kc away from the Γ point. In this case, the operating angle is such that the light line (shown as dotted line in <FIG> centered at ω<NUM> ) for incident light, <MAT> where k<NUM> = <NUM>π / λ , intersects the flat band at ω<NUM> (the resonant frequency at kc ). For a given lattice, this operating angle evidently is fixed by the resonant frequency. However, the ratio f = ay / ax of the lattice constant in the y direction, ay , to that in the x direction ax , can be used as a separate degree of freedom to tune k f. We note that the hexagonal case corresponds to <MAT>. In particular, we find that the distance k f in this more general case satisfies <MAT> <MAT>.

<FIG> shows the band structure along the Γ - M'x direction for various choices of f , showing broad tunability of kf , and hence, kc. Since k f becomes smaller as f grows, kc = π / ax - kf = π / ax grows. In other words, by stretching the lattice in the y direction by a distance δ, wherein δ is greater than <NUM> and less than ay/<NUM>, (increasing a dimension in real space), the angle of the flat band mode grows (increasing the dimension in reciprocal space).

As shown in <FIG>, the optical combiner <NUM> of the present disclosure includes a first structured layer <NUM> of a material with a first refractive index. The structured layer <NUM> includes a surface <NUM> with a periodic arrangement <NUM> of generally cylindrical depressions <NUM>. A second layer <NUM> of a material with a second refractive index overlies the surface <NUM> and at least substantially occupies a volume of each of the depressions <NUM>. The difference between the first refractive index and the second refractive index, measured at <NUM>, is less than about <NUM>.

The light incident on the structures <NUM> can be polarized or unpolarized. For example, the input signal <NUM> incident on the structures <NUM> can be transverse magnetic (TM) polarized (p-polarized) light, or transverse electric (TE) polarized (s-polarized) light. The input signal <NUM> is incident on the structures <NUM> of the first structured layer <NUM> at an oblique elevation angle θ of greater than about <NUM>° (± <NUM>°), greater than about <NUM>°, greater than about <NUM>°, or greater than about <NUM>°. In some examples, the oblique incident elevation angle θ is about <NUM>° to about <NUM>°, about <NUM>° to about <NUM>°, about <NUM>° to about <NUM>°, about <NUM>° to about <NUM>°, or about <NUM>°.

In some examples, the input signal <NUM> can be polarized narrow band signals such as, for example, signals with red, green, and blue (RGB) visible wavelengths. In some examples, the polarized RGB input signal includes a red band with a wavelength of about <NUM> to about <NUM>, a green band with a wavelength of about <NUM> to about <NUM>, and a blue band with a wavelength of about <NUM> to about <NUM>.

In some examples, the distinct narrowband reflection peaks of the output signal of the optical combiner <NUM> of <FIG> can be configured to provide an average reflection of greater than about <NUM>%, or greater than about <NUM>%, or greater than about <NUM>%, or greater than about <NUM>%. The optical combiner <NUM> provides the average reflection peaks in the output signal within a range of ± <NUM>°, or ± <NUM>°, or ± <NUM>° of a of a selected elevation signal θ. In some examples, the optical combiner <NUM> can produce the reflection peaks over a wavelength range of about <NUM> to about <NUM>, or about <NUM> to about <NUM> microns (µm).

In some examples, the optical combiner <NUM> can provide the reflection peaks in the output signal over a range of azimuthal angles φ (<FIG>) of about -<NUM>° to about <NUM>°, or about -<NUM>° to about <NUM>°.

To provide good optical performance in an application such as a HUD, a suitable optical combiner <NUM> should provide desired output signal reflection peaks within a desired range of both elevation angles θ and azimuthal angles φ. However, since the azimuthal angle φ centers around <NUM>°, in some examples the elevational angle θ can be more sensitive to angular shifts, and as such the discussion of the present disclosure focuses on the elevation angle θ. In the present disclosure, unless a reference to an angle specifically notes that the angle is an azimuthal angle, or an angle φ, the angle referred to is the elevation angle θ.

In some examples, the optical combiner <NUM> has a haze of less than about <NUM>% for transmitted light over a wavelength range for unpolarized light over a wavelength range of about <NUM> to about <NUM> incident on the structured surface at any angle incident angle.

In another example, the optical combiner <NUM> has a reflection of less than about <NUM>% for unpolarized light over a wavelength range of about <NUM> to about <NUM> incident on the structured surface at any incident angle.

In some examples, the first structured layer <NUM> is a material with a refractive index of less than about <NUM>. In one example, the first structured layer <NUM> includes, but is not limited to, materials such as titanium dioxide (TiO<NUM>), which has a refractive index n = <NUM>. Other suitable materials for the first structured layer <NUM> include zirconia or titania-filled acrylate resins which may be deposited via coating, for example; and metal oxides, nitrides, and oxynitrides including oxides, nitrides, and oxynitrides of Si, Ti, Zr, Hf, Nb, Ta, or Ce, which may be vapor deposited. Since silicon is a metalloid, silicon oxides, silicon nitrides, and silicon oxynitrides are considered to be metal oxides, metal nitrides, and metal oxynitrides, respectively. In some cases, titania (TiO<NUM>) may be preferred for optical applications involving visible light.

In some examples, the first structured layer <NUM> has a thickness of less than about <NUM>, or less than about <NUM>, or less than about <NUM>, or less than about <NUM>.

In some examples, the second layer <NUM> may be a polymeric material with a refractive index selected to provide a refractive index difference of less than about <NUM>. In some examples, the second layer <NUM> may be a polymeric material with a refractive index selected to provide a refractive index difference of less than about <NUM>. Suitable examples include, but are not limited to, poly(methylmethacrylate)(PMMA), polycarbonate (PC), polypropylene (PP), polyethylene (PE), polystyrene (PS), polyester, polyimides, and mixtures and combinations thereof.

In some examples, the second layer <NUM> overlying the structures <NUM> on the first structured layer <NUM> may be a material with a refractive index of less than <NUM> such as, for example, TiO<NUM>, or any of the other materials listed above, and the first structured layer <NUM> may be a polymeric material with a refractive index selected to provide a refractive index difference of less than about <NUM>.

In some embodiments, the first structured layer <NUM>, the second layer <NUM>, or both, may reside on or between one or more optional support layers <NUM>. In some examples, which are not intended to be limiting, the optional support layer <NUM> may be made of any suitable optical material including glass, polymeric materials such as acrylates, polyethylene terephthalate (PET), polyethylene napthalate (PEN), polymethylmethacrylate (PMMA), cyclic olefin copolymers (COP), polycarbonate (PC), as well as multilayered polymeric optical films. The support layer <NUM> may include single or multiple layers of the same or dissimilar materials.

The periodic arrangement of structures <NUM> in the first structured layer <NUM> may vary widely depending on the intended application of the optical combiner, and the structures <NUM> may have any shape, size, and spacing capable of separating the input light <NUM> into selected component wavelengths. In the example of <FIG>, the periodic arrangement in the first structured layer <NUM> includes a lattice <NUM> of regularly repeating cylindrical depressions <NUM>.

As shown in detail in <FIG>, the lattice <NUM> of depressions <NUM> includes a hexagonal unit cell arrangement that is perturbed to form a rectangular unit cell. The perturbed lattice of depressions in the surface <NUM> has a ratio of a lattice constant ay in a second (y) direction in the plane of an unperturbed lattice of depressions to a lattice constant ax in the first (x) direction in the plane of the unperturbed lattice of depressions of r(<NUM>) <NUM>/<NUM>, wherein the dimensional factor r is from about <NUM> to about <NUM>. In some examples, ax and ay are less than about <NUM>, or ax is less than about <NUM>, and ay is less than about <NUM>. The perturbed lattice <NUM> includes cylindrical depressions <NUM> arranged in rows along the first (x) direction in a plane of the surface <NUM>, and wherein every other row of the cylindrical depressions is shifted laterally by a distance δ of about <NUM> to about <NUM>, or about <NUM> to about <NUM>, in a second (y) direction in the plane of the surface <NUM>.

In some example embodiments, the lateral distance δ is about <NUM> to about <NUM>, and ax and ay are less than about <NUM>, or δ is about <NUM> to about <NUM>, wherein ax is less than about <NUM>, and wherein ay is less than about <NUM>.

In various example embodiments, the cylindrical depressions <NUM> have a diameter D of less than about <NUM>, or less than <NUM>.

In another embodiment shown in <FIG>, the periodic arrangement in the zone-folded metasurface <NUM> of the first structured layer <NUM> of <FIG> includes a lattice <NUM> of regularly repeating rectangular depressions <NUM>. The lattice <NUM> of depressions <NUM> includes a hexagonal unit cell arrangement that is perturbed to form a rectangular unit cell, and the perturbed lattice of rectangular depressions in the surface <NUM> has a lattice constant ay in a second (y) direction in the plane of an unperturbed lattice of depressions and a lattice constant ax in the first (x) direction in the plane of the unperturbed lattice of depressions. In some examples, ay and ax are each less than about <NUM>, or ay = <NUM> and ax = <NUM>.

In some examples, the rectangular depressions <NUM> have a length of about <NUM> to <NUM> and a width of about <NUM> to <NUM>, or a length of about <NUM> to <NUM> and a width of about <NUM> to about <NUM>.

As shown in <FIG>, in the lattice <NUM> every other row is shifted by a distance δ = L-W, wherein L = a length of the rectangular depressions and W = a width of the rectangular depressions, wherein δ is about <NUM> to about <NUM>, or about <NUM> to about <NUM>. The lattice <NUM> is now doubled in the x direction compared to the device in <FIG>, and now incorporates two sets of rectangular depressions <NUM>. In one row, the rectangular depressions <NUM> are oriented at angles α<NUM> and α<NUM> + <NUM>° , alternating every other depression. In an adjacent row, the depressions <NUM> slits are oriented at angles α<NUM> and α<NUM> + <NUM>° , wherein α<NUM> ≠ α<NUM>. In some examples, α<NUM> = <NUM>° and α<NUM> = <NUM>°.

The corresponding zone folding is shown in <FIG>, showing again that the states are folded from the edge of the unperturbed FBZ to a non-zero quasi-momentum kc away from the Γ point.

In another example, the structured layer includes a cascaded arrangement with a plurality of periodic structures stacked on each other, such that incident light successively interacts with the periodic structures. For example, a first periodic structure in the stack can be configured to support a dual resonance, and a second periodic structure in the stack can be configured to support a single resonance. For example, a first structured surface including a first arrangement of depressions configured to support a dual resonance may be stacked a predetermined distance over a second arrangement of depressions configured to support a single resonance.

As shown schematically in <FIG>, an optical combiner <NUM> includes a first structured layer <NUM> with a first surface <NUM> including an arrangement of periodic structures <NUM> configured to support a dual resonance such as, for example, green-blue (GB). A second structured layer <NUM> includes a second surface <NUM> with a second arrangement of periodic structures <NUM> configured to support a single resonance such as, for example, red (R).

The structures in the first structured layer <NUM> and the second structured layer <NUM> are regularly repeating lattices of cylindrical depressions <NUM>, <NUM>, similar to those shown in <FIG>. The lattices <NUM>, <NUM> include a hexagonal unit cell arrangement that is perturbed to form a rectangular unit cell. The perturbed lattice of depressions in the surfaces <NUM>, <NUM> have a ratio of a lattice constant ay in a second (y) direction in the plane of an unperturbed lattice of depressions to a lattice constant ax in the first (x) direction in the plane of the unperturbed lattice of depressions of r(<NUM>)<NUM>/<NUM>, wherein r is from about <NUM> to about <NUM>. In some examples, ax and ay are less than about <NUM>, or ax is less than about <NUM>, and ay is less than about <NUM>.

The perturbed lattices <NUM>, <NUM> include cylindrical depressions <NUM> arranged in rows along the first (x) direction in a plane of the surfaces <NUM>, <NUM>, wherein every other row of the cylindrical depressions is shifted laterally by a distance δ of about <NUM> to about <NUM>, or about <NUM> to about <NUM>, in a second (y) direction in the plane of the surface <NUM>.

In one example, which is not intended to be limiting, to form a dual resonance BG filter, the first structured layer <NUM> included cylindrical depressions <NUM> arranged in rows such that ax = <NUM>, ay = <NUM>, Dx = <NUM>, Dy = <NUM>, δ = <NUM>, and a layer thickness of <NUM>. To form a single resonance R filter, he second structured layer <NUM> also included cylindrical depressions <NUM> arranged in rows such that ax = <NUM>, ay = <NUM>, Dx = Dy = <NUM>, δ = <NUM>, and a layer thickness of <NUM>. The layers <NUM>, <NUM> were positioned a distance ds of <NUM> apart in an encapsulating layer (not shown in <FIG>) such that the encapsulating material substantially occupied the volume of the cylindrical depressions <NUM>, <NUM>. The refractive indices of the first and second structured layers <NUM>, <NUM> and the encapsulating layer had a refractive index difference of less than about <NUM>.

As discussed above with respect to the embodiment of <FIG>, the layers <NUM>, <NUM> may be made of a material with a refractive index of less than about <NUM> such as, for example, TiO<NUM>. The encapsulating layer may be a polymeric material with a refractive index selected to provide a refractive index difference of less than about <NUM> with respect to the refractive index of the layers <NUM>, <NUM>. In some examples, the encapsulating layer <NUM> may be a dielectric material such as TiO<NUM>, and the layers <NUM>, <NUM> may be made of a polymeric material with a refractive index selected to provide a refractive index difference of less than about <NUM> between the dielectric material and the polymeric material.

Referring now to the plot of <FIG>, the <NUM>th order reflection spectrum over a range of oblique incidence elevation angles θ = <NUM>-<NUM>° is plotted for the first structured layer <NUM> of <FIG>, and shows distinct peaks at <NUM> and <NUM>. <FIG> shows the 0th order reflection spectrum over a range of oblique incidence elevation angles θ = <NUM>-<NUM>° plotted for the second structured layer <NUM> of <FIG>, and shows a peak at <NUM>.

As shown in the plot of the reflectance for TM polarized light at oblique incidence in <FIG>, the optical combiner <NUM> of <FIG> including both the structured layers <NUM>, <NUM> provided an output signal with three resonance peaks. The <NUM>th order reflection and transmission spectra for the optical combiner <NUM> are shown in <FIG> and <FIG>, respectively.

By varying basic parameters of the structures forming the metasurface, such as height and duty cycle, the resonant frequency of the reflective output signal can be tuned. By varying the magnitude of the symmetry-lowering perturbation δ , the linewidth of the resulting resonant state follows Q ∝ <NUM>/ δ<NUM>. For instance, in the device of <FIG>, δ is implemented as a lateral shift of a circular hole, while in the device in <FIG> it is δ = L-W. The operating angle of the device is then set by the folded quasi-momentum kc controlled by the ratio f = ay/ax of the unperturbed lattice dimensions. By adding additional degrees of freedom the polarization state may be varied at will, including exhibiting full circular dichroism.

Leveraging the zero first-order dispersion of a band-edge mode is useful in particular for applications requiring narrowband reflection features over a substantial range of incident angles. Since a band-edge mode is born of mode mixing (opening up a band gap) due to the periodicity of a device (breaking continuous translational symmetry), it has well-defined momentum properties determined by the symmetries of the periodic device (discrete translational symmetries). Perturbing a high-symmetry lattice can be used to select the momentum of free-space that couples to this mode. While band-edge modes may exist at arbitrary angles in conventional approaches by engineering the mode mixing of several modes, in the devices of the present disclosure this functionality is achieved in a symmetry-protected manner. That is, for a wide range of parameters there will exist a flat band mode near the desired operating angle, without requiring any precise alignment of several unrelated modes. A maximally Bragg-localized mode (e.g., the K points of a hexagonal lattice) may be accessed at any operating angle for a given refractive index contrast system.

The symmetry-based perturbative approach employed in the present disclosure is also useful with respect to the broadband features: only narrowband features are present in the visible spectral region, leaving the response to the majority of visible light transparent and undistorted. In some examples, in addition to HUD, the optical combiners of the present disclosure may be useful for applications such as AR displays, where high clarity and visibility of the external information is required. The metasurfaces of the present disclosure provide narrow spectral features yielding enhanced reflection over a large angular range, which can be desirable for superimposing the artificial information in a projected AR display. While in most cases transverse magnetic (TM, or p-polarized) light is of particular interest (so that the Brewster's angle may be used to eliminate double images born of reflection off the air-glass interfaces), the nonlocal metasurfaces of the present disclosure may control the polarization state of these zone-folded QBIC structures, offering additional flexibility in optical design.

The enhanced light-matter interactions of these long-lived states make the optical combiners of the present disclosure useful for a wide array of applications. For instance, thermal emission engineering using band-edge QBICs (existing at normal incidence) can be used to produce compact optical sources, and the present approach may augment these concepts by enabling direct engineering of off-normal band-edge modes with distinct polarization states depending on the direction of light. The enhanced light-matter interactions afforded by q-BICs can also be used to produce active devices tuned by electro-optic or thermal methods, wherein the localization of a band-edge mode can make possible more compact devices. Similarly, applications such as biological sensing and nonlinear optics benefit from long-lived states and localization. The optical combiners of the present disclosure may be extended to any linewidth, off-normal operating angle and polarization of choice.

The zone-folded metasurfaces of the present disclosure have symmetry-controlled scalar and vectoral properties. The perturbative approach used to form the metasurfaces introduces narrowband features with on- demand symmetry-controlled linewidths, operating angles, and resonant polarization states. In some examples, the resulting devices offer enhanced reflection of narrowband information with increased angular tolerance over conventional designs, while leaving the majority of the broadband response undistorted and highly transmissive. This combination of features in a periodic structure is uniquely suited as a scalable solution (compatible with roll-to-roll manufacturing) for AR applications.

Referring now to the schematic diagram of <FIG>, which is not to scale, in another aspect the present disclosure is directed to a method <NUM> for making the optical combiners shown above. In the method <NUM>, in step <NUM> a layer of glass <NUM> with a layer <NUM> of TiO<NUM> thereon has applied thereon a layer <NUM> of a metal such as, for example, chromium, by e-beam evaporation. A layer <NUM> of e-beam resist is spin-coated on the layer of Cr <NUM>.

In step <NUM>, the e-beam resist layer <NUM> is exposed to e-beam in selected areas to form a pattern <NUM> suitable for creation of a zone folded metasurface. In step <NUM>, after development, the pattern <NUM> is transferred to the Cr layer <NUM> via a dry etching process. In step <NUM>, the pattern <NUM> is transferred to the TiO<NUM> layer <NUM> via dry etching to form depressions <NUM>, and the residual portions of the Cr layer <NUM> are removed via a wet etching process.

As shown in step <NUM>, a polymeric layer <NUM> is then spin coated onto the TiO<NUM> layer <NUM>, and fills the depressions <NUM> therein.

In another embodiment shown in <FIG>, a method <NUM> for making an optical combiner includes a step <NUM> of providing a structured polymeric film <NUM> including a pattern <NUM> of depressions <NUM> suitable for creation of a zone folded metasurface. The polymeric film <NUM> may be structured by a wide variety of techniques to form the depressions <NUM>, including etching, laser drilling, microreplication with a metal tool, and combinations thereof.

As shown in step <NUM>, the structured polymeric film <NUM> may be coated with an encapsulating layer <NUM> of a dielectric material such as, for example, TiO<NUM>, such that the dielectric material occupies the depressions <NUM>.

One or more optional support layers (not shown in <FIG>) may be added on the dielectric layer <NUM> or on the polymeric film <NUM>.

As shown in <FIG>, in another aspect, the present disclosure is directed to an optical combiner film <NUM> that includes one or more optional support layers <NUM>, a structured layer <NUM> with a pattern <NUM> of depressions <NUM>, and a cover or encapsulating layer <NUM>. In some examples, the combiner film <NUM> may be manufactured at relatively low cost using a roll-to-roll process, and may easily be made or cut into large formats for use in optical displays, vehicular windshields, and the like.

Referring again to <FIG>, in some examples, the optical combiner film <NUM> can be laminated between glass layers 350A, 350B to form a windshield construction <NUM> for use in a vehicle, aircraft, and the like. In some examples, the optical combiner film <NUM> can be laminated to either of glass layers 350A, 350B, or may even be laminated to an interior surface of the windshield proximal the vehicle or aircraft operator.

As shown schematically in <FIG>, the windshield construction <NUM> may be incorporated into a heads up display (HUD) system for use in vehicles, aircraft, and the like, or may be used as a component of an optical system in an AR device.

The devices of the present disclosure will now be further described in the following nonlimiting examples.

The gratings were fabricated with a standard top-down lithographic process shown schematically in <FIG>.

A layer of TiO<NUM> was deposited on a <NUM>-mm-thick quartz substrate and etched down to desired thickness. A <NUM>-nm-thick layer of chromium was deposited via e-beam evaporation, and a <NUM>-nm-thick layer of e-beam resist (ZEP <NUM>-A, available from Zeon Corp. , Marunouchi, JP) was spin-coated on top of the samples. The photonic crystal pattern is written with an electron beam tool (Elionix <NUM> keV, available from Elionix, Inc. , Tokyo, JP). After development, the pattern was transferred to the chromium layer via a Cl<NUM>-O<NUM> dry etching process performed in an ICP machine (Oxford PlasmaPro System <NUM> Cobra, available from Oxford Instruments, Bristol, UK). After removing the ZEP mask, the pattern was further transferred to the titania layer with a CF4-Ar-O<NUM> dry etching process performed in the same ICP machine. The residual chromium mask was then removed via wet etching. To embed the titania metasurface into a glass-like (n=<NUM>) dielectric environment, a thin layer of polymethylmethacrylate (PMMA) <NUM> (MicroChem, Newton, MA) was spin-coated on top of the sample. A denser version of the same polymer (PMMA A11, MicroChem) was then drop-cast on top of the sample and used as an adhesion layer to glue a <NUM>-mm-thick microscope coverslip. The fabricated metasurfaces had an in-plane dimensions of about <NUM> microns to about <NUM> microns.

The angle-dependent transmission and reflection spectra were acquired by placing the sample on a motor-controlled rotation stage as shown in <FIG>, which allowed accurate control of the angle θ. For transmission measurements (<FIG>), a broadband white light was linearly polarized and weakly focused on the sample (focal length f=<NUM>) to obtain an excitation spot with a diameter of about <NUM>. This excitation configuration was chosen as a compromise to ensure that the beam spot was smaller than the grating cross section even for large angles θ, while simultaneously minimizing the excitation angular spread.

The beam was collected from the other side of the sample with an identical lens and directed either to a CCD camera (for alignment purposes) or to a fiber coupled spectrometer. For each angle the lamp spectra transmitted through the grating, Sgrat(λ, θ) was acquired, and through the bare glass-like substrate, S<NUM>(λ, θ), and the transmission spectra <MAT> was calculated. This procedure properly considers the lateral beam shifts introduced by the thick glass substrate at large angles, which can alter the collection efficiency. Due to the normalization used, T(λ, θ) does not include the effect of the air/glass and glass/air interfaces. To correct for this, the angle-dependent incoherent transmission spectrum was calculated through a thick glass slab, Tglass(λ, θ), which was used to calculate the absolute transmission of the sample, Tabs(λ, θ) ≡ T(λ, θ) × Tglass(λ, θ).

For reflection measurements (<FIG>), a linearly polarized tunable laser (SuperK Fianium, available from NKT Photonics, Boston, MA) was used as a source. Before being focused on the sample, a portion of the laser was extracted with a beam splitter and directed to a power meter (P1) for power calibration. The beam reflected by the grating was collimated by a lens and measured by a second power meter (P2). Both collecting lens and power meter P2 were placed on a second rotation stage, whose angle is set to <NUM>° - <NUM> to measure the specular reflection. The absolute reflection spectra were then obtained by sweeping the excitation wavelength and recording the powers measured by P1 and P2.

The device of <FIG> above, which included ax = <NUM>, ay = <NUM>, D = <NUM>, and δ = <NUM>), the p-polarized transmission spectra (<FIG> for the simulated response and <FIG> for the measured response) showed the expected feature: the upper band (i.e., the longestwavelength mode) was flattened at an incident angle θ of about <NUM>°, due to the avoided crossing between this and a lower band (see also right inset of <FIG> for a zoom-in). As a result, the transmission dip associated with this band (at λ of about <NUM>) was almost dispersion-less in the angular range of interest (θ = <NUM>° ± <NUM>°), as also shown in <FIG>.

The s-polarized transmission spectra (<FIG> for the simulated response and <FIG> for the measured response) showed similar features, with different bands (typically narrower than the p-polarization case) and the presence of an avoided crossing, although at a different angle. While not wishing to be bound by any theory, the difference in the absolute values of the transmission between simulations and measurements, especially visible for s-polarization (<FIG>, <FIG>) at large angles, was likely due to the fact that in simulations the presence of the two air/glass interfaces is neglected. As expected, for s-polarized excitation the measured transmission decreased at large angles because of the large reflection at the two glass/air interfaces. For p-polarized excitation this effect was less important because at the Brewster angle (θ = about <NUM>°) the reflection at the air/glass interfaces is zero.

As mentioned above, an important figure of merit to ensure that these devices can be used for augmented reality (AR) applications is that, apart from the wavelengths of operation, they are mainly transmissive for an unpolarized broadband signal in the visible range. To quantify this, the average between the two datasets in <FIG> (to emulate an unpolarized beam) was calculated, and further averaged across the wavelengths in the visible range. The obtained curve (<FIG>, curve B) shows that the average unpolarized transmission was almost <NUM>% at normal incidence, while it dropped to about <NUM>% at the elevation angle of operation θ = <NUM>°. The slow decrease of the average transmission as θ increases was mainly due to the reduced transmission of the TE component. For comparison, the curve R in <FIG> shows the average unpolarized transmission of a thick glass slab.

To confirm that the transmission dips are due to large reflection, and to quantify the loss, reflection spectra were measured at select angles using the techniques described above in the discussion of <FIG>. In <FIG> the reflection spectra at θ = <NUM>° (line B) are compared with the transmission spectra (line R) at the same angle. The three transmission dips in the <NUM> - <NUM> range are accompanied by corresponding reflection peaks, with almost equal magnitude. Again, while not wishing to be bound by any theory, the small discrepancy between the central wavelengths of the transmission dips and reflection peaks is believed to be due to a small spectral detuning between the different tools used for reflection and transmission measurements. For the peak of interest (λ = about <NUM>) a transmission dip of <NUM>% is accompanied by a reflection peak of <NUM>%, indicating the presence of on-resonance ~<NUM>% loss. In some examples, this loss can likely be attributed to fabrication imperfections (i.e. roughness), and to residuals of the metallic mask used for fabrication.

In another example, the device of <FIG> was prepared, with four rectangular apertures of dimension <NUM>×<NUM>nm rotated at angles α<NUM>,α<NUM> + <NUM>°, α<NUM>, α<NUM> +<NUM>°. The slab had height H = <NUM> and the lattice constants were ax = <NUM> and ay = <NUM>.

The expected eigenpolarization was as follows: <MAT>
where c<NUM>, c<NUM> depend on how the dimensions of the rectangular, W × L, differ from a square (i.e., c<NUM> ∝ L<NUM> - W<NUM> and c<NUM> ∝ L<NUM> - W<NUM>). The factor i = (-<NUM>)<NUM>/<NUM> comes from the quarter period shift of the two rows in the x direction, and kx is the in-plane momentum and hence is positive when light is incident with lateral momentum pointing in the +x direction and - when pointing in the -x direction. A linear eigenpolarization ϕeig ≈ α<NUM> was expected when α<NUM> = α<NUM>, similar to the response of the device in <FIG>. However, in this case elliptical dichroism may be observed when α<NUM> ≠ α<NUM>.

Claim 1:
An optical combiner (<NUM>; <NUM>), comprising:
a first layer (<NUM>; <NUM>) comprising a periodic two-dimensional arrangement (<NUM>; <NUM>; <NUM>) of structures arranged to support resonance for an input signal (<NUM>) of a target wavelength, wherein the structures have a first refractive index;
a second layer (<NUM>) that overlies the structures on the first layer, wherein the second layer comprises a second material with a second refractive index, and wherein a difference between the first refractive index and the second refractive index, measured at <NUM>, is less than <NUM>; and
wherein the periodic arrangement of structures is configured such that the optical combiner produces, for the input signal incident on the first layer from air at an oblique elevation angle of greater than <NUM>°, an output signal comprising a reflection peak with an average reflection of greater than <NUM>% within a ± <NUM>° range of the elevation angle.
wherein the periodic two-dimensional arrangement of structures comprises a lattice of regularly repeating depressions (<NUM>; <NUM>; <NUM>), wherein the lattice of depressions has a perturbed hexagonal unit cell, said perturbed hexagonal unit cell being a hexagonal unit cell arrangement that is perturbed so that the perturbed lattice of depressions has a rectangular unit cell, and
wherein the second material occupies at least a portion of a volume of each depression.