WO2015021255A1

WO2015021255A1 - Light-source efficiency enhancement using metasurfaces

Info

Publication number: WO2015021255A1
Application number: PCT/US2014/050106
Authority: WO
Inventors: Vladimir M. Shalaev; Alexander V. Kildishev; Alexandra BOLTASSEVA; Nathaniel KINSEY
Original assignee: Purdue Research Foundation
Priority date: 2013-08-07
Filing date: 2014-08-07
Publication date: 2015-02-12

Abstract

A light source includes a photon generator adapted to provide light at a wavelength and a metasurface disposed over a surface of the generator, thinner than the wavelength of the emitted first light and including a plurality of nanoantennas. The surface can be for outcoupling or reflection. Each of the nanoantennas has dimensions less than the wavelength of the light and includes at least one region. The region can be a region of dielectric material arranged between two regions of conductive material so that a displacement current path is defined that crosses the region of the dielectric material, or a region of conductive material arranged between two regions of dielectric material so that a conductive current path is defined that crosses the region of the conductive material.

Description

LIGHT-SOURCE EFFICIENCY ENHANCEMENT USING METASURFACES

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application priority to and the benefit of, U.S. Provisional Patent Application Serial No. 61/862,999, filed Aug. 7, 2013 and entitled "LIGHT-SOURCE EFFICIENCY ENHANCEMENT USING METASURFACES," the entirety of which is incorporated herein by reference.

TECHNICAL FEELD

[0002] The present application relates to light sources, and specifically to improving outcoupling of light from light sources.

BACKGROUND

[0003] Waves which are incident upon an interface can experience basic phenomenon such as reflection and refraction. The direct outcome of these effects depends upon the basic properties of the materials at the interface. These properties of reflection and refraction have been used to develop waveguides and polarization filters, but in many cases these effects limit the performance of a system. For solid-state light sources formed from various semiconductor materials, there is an inherent and large mismatch between the basic material properties of the structure and the surrounding air. This impedance mismatch causes a significant reflection to occur at this interface, which limits the output efficiency of the light source. Conversely, at the reflective mirror of a laser, the light leaking through the facet increases the round-trip loss of the cavity.

[0004] Semiconductor based light sources, such as vertical-cavity surface-emitting lasers (VCSELs) using gallium arsenide (GaAs) or other materials, generally include crystalline semiconductors that have large relative permittivities. A list of permittivities for several key semiconductors used in the fabrication of these light sources is shown in Table 1. When such sources are transmitting into air (relative permittivity ~ 1), the abrupt change of permittivity at the semiconductor-air interface results in a large reflection coefficient for the incident light. Table 1 also shows, for light normally incident upon the surface, the percentage of light transmitted from the semiconductor into the air, based on the reflection coefficient (Γ) defined in Eq. (1). Some GaAs lasers have only 50-60% outcoupling. The critical angle for total internal reflection at the semiconductor to air interface is also shown. Light that is incident with only a slight angle will experience total internal reflection due to the large impedance mismatch between the semiconductor and the air.

[0005] Table 1 is a list of several exemplary semiconductors used in the fabrication of lasers along with their permittivity. The percentage of light transmitted from the device (transmitted power of electromagnetic radiation divided by incident power) is defined as 1-|Γ|², where Γ is the reflection coefficient for normal incidence in Eq. (1). The critical angle for the semiconductor into air interface is also shown and defined by Eq. (1) and is measured from the normal.

Table 1

Material Relative Percent Transmitted Critical Angle

Permittivity (1-|Γ|² ¾Γ μ_Γ=1) (Degrees)

(air ^ l)

Silicon 11.9 69.69% 16.9

Germanium 16.0 64.00% 14.5

Gallium Arsenide 13.1 67.85% 16.0

Gallium Nitride 5.7 83.22% 24.8

Indium Phosphide 12.4 68.90% 16.5

Indium Arsenide 14.6 65.76% 15.2

Aluminum Nitride 4.62 86.68% 27.7

Aluminum Arsenide 10.1 72.82% 18.3

Gallium Phosphide 11.1 71.03% 17.5

[0006] Currently, anti-reflection coatings, optical gratings, and graded index coatings are used to minimize this reflection and improve transmission. Anti-reflection coatings use destructive interference to effectively cancel reflected energy, consequently increasing transmitted energy. Anti-reflective coatings are very sensitive to the wavelength but have been shown to be quite effective. Face reflections as low as 0.01% have been achieved for AR coatings on semiconductor lasers. Gratings reduce reflections by attempting to match the impedance between the materials at the interface with structures that are similar to the size of the wavelength. They can be made more tolerant to the changes in wavelength than can AR coatings, and can be polarization dependent. In general, gratings also have a limitation in that they consider only a change in the effective permittivity of the medium since the permeability of the constituent materials in the optical regime is that of air. Gradient index layers provide more flexibility, allowing the conditions at an interface to be engineered by controlling the composition of constituent materials. Reflections are reduced by forming a smooth index transition between two materials over several wavelengths. These coatings can provide a significant suppression of reflections over a broad range. However, because solid materials have refractive indices greater than that of air, a small but abrupt change in the refractive index with air limits the total efficiency of the structure.

[0007] Also, semiconductor light sources are known have large divergence angles, causing the light to spread quickly. For many applications, a collimated or focused source is desired. Currently, an external collimating optic is used. This is bulky and is subject to misalignment. Finally, a certain percentage of the energy in the source is coupled into evanescent or non-propagating channels. This energy is not available to perform useful work.

[0008] Accordingly, there is a continuing need for structures that improve transmission of light from light sources such as semiconductor lasers, and for structures that make effective use of various modes of light.

[0009] Reference is made to U.S. Patent No. 4,536,608 to Sheng et al. BRIEF DESCRIPTION

[0010] According to an aspect, there is provided a light source comprising: a photon generator adapted to emit first light through an outcoupling surface of the photon generator into a medium, the photon generator and the medium having different indices of refraction and the emitted first light having a wavelength; and a metasurface disposed over the outcoupling surface, the metasurface having a thickness that is less than the wavelength of the emitted first light and including a plurality of nanoantennas, wherein each of the nanoantennas has dimensions less than the wavelength of the emitted first light and includes at least one region selected from the group consisting of a region of dielectric material arranged between two regions of conductive material so that a displacement current path is defined that crosses the region of the dielectric material; and a region of conductive material arranged between two regions of dielectric material so that a conductive current path is defined that crosses the region of the conductive material.

[0011] According to another aspect, there is provided a light source comprising a photon generator adapted to provide light to a surface of the photon generator, the emitted light having a selected wavelength; and a metasurface disposed over the surface, the metasurface having a thickness that is less than the wavelength of the emitted light and including an array of nanoantennas, wherein each of the nanoantennas has dimensions less than the wavelength of the emitted first light and includes at least one region selected from the group consisting of a region of dielectric material arranged between two regions of conductive material so that a displacement current path is defined that crosses the region of the dielectric material; and a region of conductive material arranged between two regions of dielectric material so that a conductive current path is defined that crosses the region of the conductive material.

[0012] Various aspects advantageously improve transmission, outcoupling, or reflectivity of light at surfaces such as interfaces between materials. Some of these aspects advantageously increase outcoupling of a laser, reducing the required input power and improving efficiency of the laser. Various aspects advantageously permit the use of lower-gain, less-expensive gain materials in lasers while maintaining performance. Various aspects advantageously direct or shape beams of light using thin layers applied or affixed to light sources such as laser cavities, advantageously mitigating misalignment problems of prior schemes.

[0013] This brief description is intended only to provide a brief overview of subject matter disclosed herein according to one or more illustrative embodiments, and does not serve as a guide to interpreting the claims or to define or limit scope, which is defined only by the appended claims. This brief description is provided to introduce an illustrative selection of concepts in a simplified form that are further described below in the Detailed Description. This brief description is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the Background.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings.

[0015] FIG. 1 depicts a wave incident upon a metasurface according to various aspects;

[0016] FIG. 2 is a perspective of an exemplary nanoantenna according to various aspects;

[0017] FIG. 3 is a plan view of an exemplary unit cell according to various aspects;

[0018] FIG. 4 is a plan view of an exemplary metasurface including a pattern of unit cells according to various aspects; [0019] FIG. 5 is an elevational section of an exemplary light source; and

[0020] FIGS. 6 and 7 are plan views of exemplary nanoantennas according to various aspects.

[0021] The attached drawings are for purposes of illustration and are not necessarily to scale.

DETAILED DESCRIPTION

[0022] Various aspects herein relate to light sources, e.g., semiconductor lasers, that include metasurfaces. Throughout this description, the term "light" refers to

electromagnetic radiation generally, and is not limited to the human- visible range of -400-700 nm. Metasurfaces are structures including arrays of small-scale mechanical features. The arrays can be regular or not, and the features can be uniform or not. The features can be, e.g., protrusions, recesses, or holes. The term "non-inverted," used in respect of a metasurface, means that the features are protrusions, e.g., metal or semiconductor islands on a dielectric support. The term "inverted," correspondingly, means that the features are recesses or holes. For example, an inverted structure can be formed from a metal or semiconductor sheet with dielectric cutouts. An example of such a mechanical feature discussed herein is a nanoantenna. The term "nanoantenna" does not require that a nanoantenna be used to transmit or receive modulated signals. The term "nanoantenna" refers to the fact that various exemplary nanoantennas described herein transfer energy between electromagnetic radiation and electrical current

(conduction or displacement), and subsequently re-radiate energy in a desired manner.

[0023] Various aspects herein relate to enhancing the efficiency of semiconductor based light sources using metasurfaces, e.g., by increasing the outcoupled light efficiency from semiconductor based light sources by means of impedance matching. This is accomplished, in various aspects, through the use a metasurface layer which is designed to eliminate reflections for the light which is incident upon the output facet. [0024] As described herein, an individual structure in a metamaterial can be tuned to have a resonant frequency for incident magnetic fields and a different resonant frequency for incident electric fields. These are preferably designed via simulation and adjustment of the structure based on simulation results. For example, software such as COMSOL Multiphysics can be used to perform such simulations.

[0025] Through the use of metasurfaces, the properties of an effective material can be designed to generate an impedance match between the high impedance light source and the low impedance surrounding, or to generate perfect boundary conditions to ensure optimal reflectivity. The application of this metasurface can greatly reduce the reflection at the interface, increasing the output efficiency of the semiconductor light source and provide tailoring of the output radiation directivity. Throughout this disclosure, the term "perfect" means "as close to ideal as can be achieved with a given laboratory or manufacturing setup and given materials."

[0026] Metasurfaces improve on previous solutions by allowing both the permittivity and permeability of an effective layer to be designed to achieve an impedance match between the materials at an interface. Metasurfaces can also be designed to provide a polarization independent design, if desired, as well as structures which are deeply sub- wavelength and require relatively simple fabrication processes such as focused ion beam milling or electron beam lithography. The application of an appropriately-designed metasurface to a semiconductor-air interface can result in substantially no light being reflected at the output facet. This can provide an output efficiency of a semiconductor light source approximately 30% higher with a metasurface than without depending upon the semiconductor materials under consideration. In addition, metasurfaces can be designed to couple or convert non-propagating evanescent modes in the cavity to propagating modes that provide far-field radiation. This increases the proportion of the radiation produced by the semiconductor light source or energy in the light source that can be used, e.g., in the far field. In some aspects, therefore, an output efficiency of a light source (e.g., light source 500, FIG. 5) is approximately 30% higher than an output efficiency of the photon generator (e.g., photon generator 56, FIG. 5) within the light source 500. In some aspects, applying a metasurface increases system efficiency of the light source from -20% to ~80%, or increases system efficiency by up to one order of magnitude. Metasurfaces can also or alternatively be used in conjunction with prior techniques such as those discussed above to further reduce the reflection for a narrow band, or to provide effective wideband antireflection surfaces.

[0027] Metasurfaces can be fabricated, e.g., by patterning metallic structures on a dielectric, or by forming voids in metallic films. Such structures or voids are referred to herein as "nanoantennas." Metamaterials are not limited to metals only. Semiconductors can be used as a base material for the nano structures. Such structures can be fabricated more easily than can some three-dimensional alternatives. For many applications these structures are useful because they do not suffer from the ohmic losses which plague metallic metasurfaces. Many types of metasurfaces have been demonstrated in the optical regime such as arrays of nanorods, spheres, bow-ties, c-shaped or v-shaped antennas, and others. These metamaterials provide an abrupt phase discontinuity.

[0028] Accordingly, for both reflective and transmissive metasurfaces as discussed below, the metasurface can include a dielectric and each of the nanoantennas can include metal or semiconductor. The metasurface can alternatively include a metallic or semiconductive film and each of the nanoantennas can include a void in the film. This is discussed below with reference to FIGS. 2—4, 6, and 7. In various examples, other electrical conductors can be used in place of metal, e.g., indium tin oxide (ITO), a transparent conductive ceramic.

[0029] FIG. 1 is a side elevational section of a wave incident upon a metasurface 24 according to various aspects. Metasurface 24 is at the interface between material 20, having permittivity ε and permeability μ_1; and material 22, having permittivity ε₂ and permeability μ₂. Metasurface 24 has a phase discontinuity and a gradient άΦ/dy of phase, as discussed below. FIG. 1 illustrates oblique incidence (incident ray 10 is not parallel to surface interface normal 26) upon a material 22 which can have a phase gradient at the interface, showing both normal and anomalous reflection and transmission.

[0030] At metasurface 24, the generalized reflection and refraction conditions are given by Eq. (2) (generalized SnelPs law) and Eq. (3) where άΦ/dy is the gradient of phase discontinuity along the interface, μ is the permeability of the material, ε is the permittivity of the material, and XQ is the free space wavelength of the incident wave.

[0031] The depiction of incident light ray 10, a wave incident upon metasurface 24, shows that there are, in general, two reflected and refracted beams, Θ_Γ, 0_ra, 0_t, and 0_ta which are the normal and anomalous reflections and the normal and anomalous refractions respectively. Normal reflections (light ray 12) and refractions (light ray 16) occur due to the periodicity of the structure. Therefore, at certain distances y, the difference in phase between the two points is άΦ/dy = 0. This is the traditional case of reflection and refraction. At all other points there is a discrete difference in the phase between two incident rays which leads to the anomalous reflection (light ray 14) and refraction (light ray 18). The gradient of phase shift in the direction of propagation of an incident ray can be, e.g., zero or constant.

[0032] Through the use of metasurfaces, an impedance match can be engineered between a high-optical-impedance semiconductor light source and low-optical- impedance air and coupling to non-propagating modes can be achieved.

[0033] Impedance matching by metasurfaces is not primarily tied to the bulk properties of permittivity ε or permeability μ. The exemplary metasurface 24 includes a very thin (<<λ₀) array of nanoantennas arranged at the interface between materials 20, 22. Each nanoantenna shifts the phase of incident light in ray 10. Differently-shaped antennas change the phase of the light differently. The shapes of the nanoantennas and their locations on the metasurface are designed so that the shift of phase varies across the interface, providing the above-noted gradient of phase. To conserve photon momentum, photons incident on each of the nanoantennas respond in ways specific to the design of those nanoantennas. This affects the reflection and refraction of those photons. The difference in refractive index n between materials 20 and 22, which index normally defines the angle of refraction, is thus no longer controlling in the presence of a metasurface. Accordingly, metasurfaces can be designed to provide matched impedance (for at least partial transmission) or a selected impedance discontinuity (for at least partial reflection) at an interface between given materials 20, 22, e.g., Si and air.

[0034] Unlike prior schemes using bulk materials or patterned structures, metasurfaces permit controlling both the permittivity and permeability of the material, e.g., at a deeply subwavelength scale. Controlling both μ and ε is similar to controlling both L and C of an electrical transmission line, and permits impedance matching. In addition to this, the design of the surface phase gradient through design of the

metasurface permits controlling the transmitted light interference. By properly designing this surface, the output radiation can be collimated by the metasurface. Prior schemes for semiconductor lasers require a separate collimating optic to achieve this. In these schemes, maintaining alignment of the laser and the optic can be difficult and, the size of optic is large compared to the light source. In contrast, metasurfaces can readily be applied to the output facet or back face of, e.g., a laser. Metasurfaces can be configured to improve the efficiency of semiconductor light sources and their outputs under deeply subwavelength dimensions in various ways. Metasurfaces can be configured to control the effective ε, μ to, e.g., match an adjacent material or provide a selected amount of reflection or transmission. Metasurfaces thus advantageously provide laser designers with an additional design parameter to adjust in configuring a laser for a particular application. [0035] FIG. 2 is a perspective of an exemplary nanoantenna 200 according to various aspects. The illustrated nanoantenna 200, or other nanoantennas described herein, can be used in a metasurface that tailors impedance to produce a desired transmission or reflection. The nanoantenna 200 affects incident light by employing resonance for both the electric and magnetic fields incident on the material. FIG.2 illustrates exemplary electric and magnetic resonances in the metasurface unit structures (nanoantennas).

[0036] A depiction of an example nanoantenna 200 with two thin metallic strips of length L=2a separated by a dielectric of thickness d is shown. In this situation, for an electromagnetic wave incident in the -z direction, the incident magnetic field 28 (H₀) generates a loop current 30, 32. Conduction current 30 flows in the metallic strips 34 in the body of the nanoantenna 200, and displacement current 32 closes the loop between the metallic strips 34. This gives rise to a magnetic resonance which varies with the geometry of the nanoantenna 200 and the base material parameters of the metal strips 34. The magnetic moment for two cylindrical nanowires is given by Eq. (4) with the resonant condition given by Eq. (5). The variables are H₀ is the applied magnetic field, a is half the length L of the nanowire 34, d is the separation of the nanowires 34, b is the radius of nanowire 34, k is the wave vector ω/c, and the factor ga is given in Eq. (5).

[0037] The incident electric field 36 (E^e ) then experiences a surface plasmon resonance 38 (induced by the electric field) with the tops of the structures 34. The induced electric dipole in the 1^th direction for an ellipsoid with axis lengths of a, b, and c is given by Eq. (6) where £ ^xt is the applied field, n are real numbers which depend on the ellipsoid geometry, and e_m, ¾ are the metal and dielectric permittivities respectively. By adjusting these resonances, the effective permittivity and permeability of the

nanoantenna 200 can be designed allowing for a perfect impedance match between the semiconductor light source and air. While there are many possible nanoantennas or other metamaterial structures which can provide both electric and magnetic resonances, they function in a similar manner. More complex geometries, such as c-shaped, v-shaped, T- shaped, and others can be designed, verified and fine-tuned through numerical simulation tools such as COMSOL.

[0038] In this and other aspects, each of the nanoantennas has dimensions less than the wavelength of the emitted first light (λ₀). Each of the nanoantennas includes at least one region. The region (or one of the regions) can be a region of dielectric material arranged between two regions of conductive material so that a displacement current path is defined that crosses the region of the dielectric material. Such configurations are useful for controlling the magnetic field response of the nanoantenna.

[0039] The region (or one of the regions) can alternatively include a region of conductive material arranged between two regions of dielectric material so that a conductive current path is defined that crosses the region of the conductive material. Such configurations are useful for controlling the electric field response of the nanoantenna.

[0040] In various aspects, nanoantennas have a geometry, e.g., a split ring resonator or "C" shaped antenna, to enhance the magnetic field response. In various aspects, the geometry enhances the electric field response. The loop displacement current results in a magnetic field response while an electrical field response may also arise, or be dominant, depending upon the geometry. For example, a standard dipole antenna like on your car is not in a loop shape, but it has a response because there is current flowing back and forth inside the conductor. This is a result mainly of the electric field rather than the magnetic field. Either geometry can be used in a nanoantenna, as each nanoantenna can be designed to apply a selected phase shift to the incident light.

[0041] Using nanoantennas, in various aspects, the metasurface 62 is configured to adjust the amplitude of the light emitted through the metasurface. In various aspects, the metasurface 62 is configured to adjust the phase of the light emitted through the metasurface, e.g., as discussed above. Moreover, in various aspects, the metasurface 62 can be configured to have a surface phase gradient. That gradient, set by the design of the metasurface 62, is configured in these aspects to shape the light passing through the metasurface into a selected beam shape. The cross-section of the shaped beam can be, e.g., a line, a spot, or a torus. Amplitude control via the metasurface 62 can also be used in beam shaping.

[0042] In an example, the beam is shaped in the far field by adjusting the phase at the interface and thus controlling the interference. This is conventionally referred to as "shaping the wavefront," which means altering the relative phase delay between portions of the beam (energy propagates perpendicular to the wavefront). For example, in a conventional convex lens, the phase of incident light is delayed more near the thick center than near the thin periphery. This spatial variation of phase delay causes the focusing effect of the convex lens. The wavefront repeats periodically, so shaping the wavefront with a fixed configuration of optical elements shapes the entire beam behind that wavefront. Beamshaping according to various aspects is performed by applying a phase profile to the light passing through the metasurface 24.

[0043] In at least one example, the metasurface 62 is configured to collimate the light passing through the metasurface. That is, the design and pattern of

nanoantennas 200 operates to provide a spatially varying phase delay (άΦ/dy) that collimates incident light. In another example, the metasurface 62 is configured to direct the light passing through the metasurface 62 at a selected angle. [0044] In another example, the metasurface 62 is configured to provide a selected percentage transmission of the emitted light from the photon generator. This is discussed below with reference to FIG. 5. These and any other functions of metasurface 62 can be combined so that, e.g., metasurface 62 can focus the extracted light 50 and direct the focused extracted light 50 at a selected angle, e.g., 45°. The surface phase gradient of the metasurface 62 can be designed to provide the desired effects or combinations of effects, e.g., with the use of COMSOL or other simulation tools.

[0045] FIG. 3 is a plan view of an exemplary unit cell 42 according to various aspects. The example metasurface unit cell 42 has several individual resonant structures 40 (nanoantennas such as nanoantenna 200, FIG. 2) used to tune the effective properties of the metasurface of which the unit cell 42 is part (see FIG. 4). The illustrated exemplary unit cell 42 can be used for both reflective and transmissive metasurfaces. The nanoantennas 40 in the unit cell 42 can have the same or different orientations, shapes, or sizes. For example, they can be the same shape, but have individual rotations (in the plane of the figure), individual sizes (as shown), or both.

[0046] Many types of nanoantennas 40 can be used, e.g., a three-rod, two-gap structure. In various aspects, each of the nanoantennas 40 has a shape. Each shape can be, e.g., a "C" shape, a "V" shape, a "T" shape, a bow-tie, or a three-rod, two-gap shape. As shown, the unit cell 42 can include nanoantennas 40 of more than one size. Each nanoantenna 40 is an individual resonant structure. The different sizes of each nanoantenna 40 correspond to respective different resonant frequencies of those nanoantennas 40. Alternatively, each nanoantenna 40 in the unit cell 42 can be the same size as the other nanoantennas 40 in the unit cell 42.

[0047] The example unit cell 42 of the metasurface 24 can be used as an impedance matched layer as discussed above. The unit cell 42 includes several individual resonant structures 40 whose geometries are slightly modified to adjust the resonant conditions. This configuration provides a άΦ/dy as described for a general metasurface 24 interface. [0048] Referring to FIG. 3 and also to FIG. 5, various light sources can use metasurfaces. For example, light source 500 (FIG. 5) can be, e.g., a laser having an outcoupling end and a reflective end. In an example, a metasurface 62 on the

outcoupling end of light source 500 includes a dielectric film with conductive

nanoantennas disposed over it. Each nanoantenna includes a region 310 of dielectric material (part of the film) arranged between two regions 320 of conductive material (parts of the conductive nanoantenna) so that a displacement current path is defined that crosses the region of the dielectric material. In an example, a metasurface 54 at the reflection end includes a metallic or other conductive film with nanoantennas forming voids in the film. Each nanoantenna includes a region 310 of conductive material (part of the film) arranged between two regions 320 of dielectric material (parts of a void or voids) so that a conductive current path is defined that crosses the region of the conductive material.

[0049] FIGS. 6 and 7 are plan views of other exemplary nanoantennas. As shown, the nanoantenna of FIG. 6 has a "K" shape and that of FIG. 7 as a more closed "C" shape than does nanoantenna 40, FIG. 3. The nanoantenna of FIG. 7 also has protrusions at the closures of the "C". In various aspects, E-shaped or L-shaped nanoantennas are also or alternatively used, or other shapes that do not have symmetry between left and right sides. In another example, V-shaped nanoantennas are used. The "V" shape can be tightly closed, widely open, or in between.

[0050] FIG. 4 is a plan view of an exemplary metasurface 400 including a pattern of unit cells 42, each having several individual nanoantennas 40 (for clarity, not all are labeled) according to various aspects. These unit cells 42 are arranged in arrays which may or may not be offset from row to row. As shown in FIG. 4, the unit cells 42 can be arranged with lateral offset 46 between unit cells 42 on the metasurface 400.

[0051] The illustrated exemplary metasurface 400 uses C-shaped resonant structures to provide a desired effective permittivity and permeability. Many other types of metasurfaces can be designed. Metasurfaces 400 such as that shown can be used in both reflective and transmissive configurations. Specifically, in this example, the nanoantennas 40 are arranged into a pattern of unit cells 42, each unit cell including a plurality of spaced apart ones of the nanoantennas 40.

[0052] According to the foregoing, an apparatus according to various aspects can include a planar material with a thickness that is less than the wavelength of the incident light in the material and with at least one other dimension greater than the thickness. The apparatus can include many metal, semiconductor, or dielectric structures that can be identical or varied in shape, size or orientation, and whose dimensions are less than the wavelength of the incident light in the material. The structures can be arranged in a periodic manner. The metasurface 400 is an example of such apparatus, when the nanoantennas 40 are deposited onto an underlying surface.

[0053] Such apparatus can provide both electric and magnetic responses to incident waves. Such apparatus can permit controlling effective permittivity and permeability according to designed physical properties and arrangements of the nanoantennas 40. Such apparatus can, e.g., advantageously provide a match of the impedance between the surrounding materials 20, 22, FIG. 1. Such apparatus can, e.g., advantageously provide far-field coupling of otherwise non-propagating, evanescent modes.

[0054] An apparatus according to various embodiments can include a planar material having a thickness that is less than the wavelength of the incident light in the material and at least one other dimension greater than the thickness. The planar material can include many voids and be, e.g., otherwise continuous. The voids can be identical or varied in shape, size or orientation, and can have dimensions less than the wavelength of the incident light in the material. The voids can be arranged in a periodic manner in the planar material. The metasurface 400 is an example of such apparatus, when the nanoantennas 40 are etched into or cut or milled out of an underlying surface.

[0055] Such apparatus can provide both electric and magnetic responses to incident waves and can provide control of effective permittivity and permeability by its design, as noted above. Such apparatus can provide a reflective surface such as an ideal reflective surface.

[0056] FIG. 5 is an elevational section of an exemplary light source 500 and illustrates, e.g., providing a metasurface 62 on an output facet of a laser light source to, e.g., increase output efficiency. In various aspects, light source 500 includes a photon generator 56 adapted to emit first light through an outcoupling surface 68 of the photon generator 56 into a medium 99. The photon generator 56 and the medium have different indices of refraction and the emitted first light has a wavelength. The photon

generator 56 can include, e.g., a lasing material, laser gain medium, or quantum-well active layer. The photon generator 56 can emit broadband light (e.g., a femtosecond laser). The photon generator 56 can also or alternatively emit pure wavelength(s) of light. The photon generator 56 can emit light in pulses or continuously. The light that exits the light source 500 is shown as extracted light 50.

[0057] A metasurface 62 is disposed over or applied to the output facet of light source 500, e.g., a semiconductor light source, and in this example is disposed over outcoupling surface 68. The term "over" does not require any particular orientation of the metasurface 62 or of the assembly of metasurface 62 and photon generator 56.

Metasurface 62 has a thickness T that is less than the wavelength of the emitted first light and includes a plurality of nanoantennas, e.g., as discussed above with reference to FIGS. 2, 3, 6, and 7. In various aspects, the photon generator 56 further emits second light having a wavelength different from the wavelength of the first light and the thickness T of the metasurface is less than the wavelength of the emitted second light. As discussed above with reference to FIG. 2, each nanoantenna includes at least one dielectric region or at least one conductive region.

[0058] The illustrated configuration is not limiting. For example, in a VCSEL, metasurface 62 (or metasurface 54, discussed below) can be patterned on the top of the cavity. Metasurfaces 62, 54 can be disposed on or over any side or side(s) of photon generator 56. [0059] In this example, metasurface 62 is positioned and configured to mitigate the impedance mismatch between photon generator 56 and medium 99, and to provide high transmissivity of light. The metasurface 62 in the illustrated configuration is disposed over a conventional layer 60 such as a graded index coating to reduce the face reflections further. Other layers 60 can include an AR coating or a grating. In another example, the metasurface is applied directly to the outcoupling surface 68 or other semiconductor-air interface to reduce reflections. Metasurface 62 can be configured to improve light extraction efficiency, to polarize the output light, or to collimate the output beam. The first light can thus be, e.g., polarized and or collimated.

[0060] Accordingly, apparatus described above according to various aspects can be placed on the output interface of a semiconductor light source opposite of the highly reflective mirror (discussed below). The apparatus can be placed either directly on the semiconductor, or on a secondary layer, or is another device of a similar kind.

[0061] In this example, the photon generator 56 is powered via top electrical contact 52. The photon generator 56 can be a semiconductor based light source, and can be mounted over a substrate 58, e.g., a semiconductor substrate.

[0062] In the example shown, a rear high reflecting metasurface 54 is disposed over the photon generator 56 at surface 70 opposite the outcoupling surface 68. A

conventional reflector can be used in place of metasurface 54. A conventional partial reflector can be used in place of the metasurface 62. Some aspects omit layer 60.

Accordingly, the illustrated light source can use a metasurface only on the outcoupling end, only on the reflection end, or on both ends.

[0063] In an example, metasurface 54 is arranged over the surface 70 and configured to reflect (e.g., using an "inverted" design with a thin film and antenna voids). In this example, metasurface 62 is not used. System performance in this example is limited by the reflection at the semiconductor/air interface (surface 68). Metasurface 54 can provide efficiency increases, focusing, collimation, and beamshaping , both inside the cavity and outside the cavity (photon generator 56). For example, metasurface 54 can reduce the required gain (increasing efficiency) by directing more light through the active region of a semiconductor laser (e.g,. by narrowing the Gaussian profile of intensity as a function of vertical position in the photon generator 56) so that the effective intensity in the active region is larger and the required gain is reduced. In this example, a smaller active region can be used than in comparative examples not using metasurface 54.

[0064] In various aspects, the light source 500 includes the photon generator 56 adapted to provide light to a surface 70 of the photon generator 56. The light has a selected wavelength. In this example, it is not required that the provided light be part of emitted light 50. This example can be used, e.g., at the fully-reflective end of a laser cavity. A metasurface 54 is disposed over the surface 70 and has a thickness that is less than the wavelength of the light. Metasurface 54 can be configured, e.g., to provide a selected percentage of reflection of the light from the photon generator.

[0065] The metasurface 54 includes an array of nanoantennas, e.g., as discussed above with reference to FIGS. 2, 3, 6, and 7. Each of the nanoantennas has dimensions less than the wavelength of the light. As discussed above with reference to FIG. 2, each nanoantenna includes at least one dielectric region or at least one conductive region.

[0066] Accordingly, apparatus such as discussed above can be placed on the interface (surface 70) of a semiconductor light source opposite of the output facet (surface 68). The apparatus can be placed either directly on the semiconductor (photon generator 56), or can be placed on a secondary layer, or can be placed on another device of a similar kind.

[0067] The metasurface 54 can be configured to adjust the amplitude of light reflected by the metasurface 54; to adjust the phase of light reflected by the

metasurface 54; to have a surface phase gradient configured to shape the light reflected by the metasurface 54 into a selected beam shape; to collimate the light reflected by the metasurface 54; or to direct the light reflected by the metasurface 54 at a selected angle. Any of these can be combined so that, e.g., the metasurface 54 can focus the reflected light and direct the focused reflected light at a selected angle, e.g., 45°. The surface phase gradient of the metasurface 54 can be designed to provide the desired effects or combinations of effects, e.g., with the use of COMSOL or other simulation tools.

[0068] In various aspects, metasurfaces 62, 54 are used on one or both of surfaces 68, 70 to provide desired reflectivities of the ends of a laser cavity. In an example, metasurface 54 is designed to provide set as close to 100% reflectivity as possible for given materials and fabrication techniques. Metasurface 62 (in conjunction with layer 60, in some examples) is designed to provide a selected reflectivity <100%. The light transmitted through metasurface 62 is the output of the laser (light source 500). The light reflected by metasurface 62 remains in the cavity and contributes to the gain of the laser. Accordingly, the reflectance of metasurface 62 can be adjusted to provide a desired output power or to meet other specifications of the laser.

[0069] In various aspects, the metasurface 62 is designed to provide a gradient of phase that controls interference of the transmitted radiation. The metasurface 62 can be designed, e.g., to collimate the output beam (extracted light 50) of the light source 500. The metasurface 62 can also be made polarization selective such that only a single polarization is transmitted. In addition, metasurface 62 can be designed to couple non- propagating or evanescent modes within the cavity into the far-field. Using prior schemes, these modes would not enter the far field due to their evanescent decay, but by converting them into radiating modes some or all of the modes within the light source can be made use of in the far-field.

[0070] Accordingly, in various aspects, the emitted light from the photon

generator 56 includes light of a first polarization and light of a second polarization.

Metasurface 62 is configured to transmit a higher percentage of the light of the first polarization than of the light of the second polarization. In some of these aspects, metasurface 62 is configured to substantially block the light of the second polarization. [0071] In an example using semiconductor lasers, a metasurface 54 designed to provide perfect boundary conditions (e.g., the normal component of the complex

Poynting vector is zero at the surface) can be placed opposite the output facet of the laser (over surface 70, opposite surface 68). By reducing the leakage at surface 70, more feedback will enter the laser, advantageously reducing the round-trip loss of the cavity and increasing efficiency of the laser. Metasurface 54 can also be made polarization selective such that only a single polarization is efficiently reflected. The single polarization will therefore have a lower lasing threshold than other, lossy polarization(s), effectively polarizing the output radiation. Metasurface 54 can also introduce a phase gradient to control the angle of the reflected light. This will allow for tailoring of the mode shape inside the cavity.

[0072] Accordingly, in various aspects above, the gradient of phase shift discussed above can control the interference of transmitted waves. The transmitted radiation of the apparatus can be controlled by this interference so as to collimate the output radiation. The apparatus can be configured to provide a polarization selective response so as to provide a substantially polarized output. In various aspects, the gradient of phase shift discussed above can control the interference of transmitted waves. The transmitted radiation of the apparatus may be controlled by this interference so as to collimate the output radiation.

[0073] Accordingly, a light source can include a photon generator adapted to emit light through an outcoupling surface of the photon generator into a medium, the photon generator and the medium having different indices of refraction; and a metasurface disposed over the outcoupling surface, the metasurface configured to match the optical impedances of the photon generator and the medium.

[0074] The invention is inclusive of combinations of the aspects described herein. References to "a particular aspect" and the like refer to features that are present in at least one aspect of the invention. Separate references to "an aspect" or "particular aspects" or the like do not necessarily refer to the same aspect or aspects; however, such aspects are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to "method" or "methods" and the like is not limiting. The word "or" is used in this disclosure in a non-exclusive sense, unless otherwise explicitly noted.

[0075] The invention has been described in detail with particular reference to certain preferred aspects thereof, but it will be understood that variations, combinations, and modifications can be effected by a person of ordinary skill in the art within the spirit and scope of the invention.

Claims

CLAIMS:

1. A light source comprising:

a) a photon generator adapted to emit first light through an outcoupling surface of the photon generator into a medium, the photon generator and the medium having different indices of refraction and the emitted first light having a wavelength; and

b) a metasurface disposed over the outcoupling surface, the metasurface having a thickness that is less than the wavelength of the emitted first light and including a plurality of nanoantennas,

c) wherein each of the nanoantennas has dimensions less than the wavelength of the emitted first light and includes at least one region selected from the group consisting of:

i) a region of dielectric material arranged between two regions of conductive material so that a displacement current path is defined that crosses the region of the dielectric material; and

ii) a region of conductive material arranged between two regions of dielectric material so that a conductive current path is defined that crosses the region of the conductive material.

2. The light source according to claim 1, wherein the photon generator further emits second light having a wavelength different from the wavelength of the first light and the thickness of the metasurface is less than the wavelength of the emitted second light.

3. The light source according to claim 1 , wherein each of the nanoantennas has a shape, each shape selected from the group consisting of a "C" shape, a "V" shape, a "T" shape, a bow-tie, or a three-rod, two-gap shape.

4. The light source according to claim 1, wherein the nanoantennas are arranged into a pattern of unit cells, each unit cell including a plurality of spaced apart ones of the nanoantennas.

5. The light source according to claim 1, wherein the metasurface is configured to adjust the amplitude of the light emitted through the metasurface.

6. The light source according to claim 1, wherein the metasurface is configured to adjust the phase of the light emitted through the metasurface.

7. The light source according to claim 6, wherein the metasurface is configured to have a surface phase gradient configured to shape the light passing through the metasurface into a selected beam shape.

8. The light source according to claim 6, wherein the metasurface is configured to collimate the light passing through the metasurface.

9. The light source according to claim 6, wherein the metasurface is configured to direct the light passing through the metasurface at a selected angle.

10. The light source according to claim 1, wherein the metasurface is configured to provide a selected percentage transmission of the emitted light from the photon generator.

11. The light source according to claim 1 , wherein the emitted light includes light of a first polarization and light of a second polarization and the metasurface is configured to transmit a higher percentage of the light of the first polarization than of the light of the second polarization.

12. The light source according to claim 11, wherein the metasurface is configured to substantially block the light of the second polarization.

13. The light source according to claim 1, wherein an output efficiency of the light source is approximately 30% higher than an output efficiency of the photon generator.

14. The light source according to claim 1, wherein the metasurface includes a dielectric and each of the nanoantennas includes metal or semiconductor.

15. The light source according to claim 1, wherein the metasurface includes a metallic or semiconductive film and each of the nanoantennas includes a void in the film.

16. A light source comprising:

a) a photon generator adapted to provide light to a surface of the photon generator, the light having a selected wavelength; and

b) a metasurface disposed over the surface, the metasurface having a thickness that is less than the wavelength of the light and including an array of nanoantennas,

c) wherein each of the nanoantennas has dimensions less than the wavelength of the light and includes at least one region selected from the group consisting of:

17. The light source according to claim 16, wherein the metasurface is configured to provide a selected percentage of reflection of the light from the photon generator.

18. The light source according to claim 16, wherein each of the nanoantennas has a shape, each shape selected from the group consisting of a "C" shape, a "V" shape, a "T" shape, a bow-tie, or a three-rod, two-gap shape.

19. The light source according to claim 16, wherein the nanoantennas are arranged into a pattern of unit cells, each unit cell including a plurality of spaced- apart ones of the nanoantennas.

20. The light source according to claim 16, wherein the metasurface is configured to adjust the amplitude of light reflected by the metasurface.

21. The light source according to claim 16, wherein the metasurface is configured to adjust the phase of light reflected by the metasurface.

22. The light source according to claim 21, wherein the metasurface is configured to have a surface phase gradient configured to shape the light reflected by the metasurface into a selected beam shape.

23. The light source according to claim 21 , wherein the metasurface is configured to collimate the light reflected by the metasurface.

24. The light source according to claim 21, wherein the metasurface is configured to direct the light reflected by the metasurface at a selected angle.

25. The light source according to claim 16, wherein the metasurface includes a dielectric and each of the nanoantennas includes metal.

26. The light source according to claim 16, wherein the metasurface includes a metallic film and each of the nanoantennas includes a void in the metallic film.