Collimating metalens and illumination device

Abstract

A collimating metalens (15) comprises a transparent substrate (100) having a first main surface (101) and a second main surface (102). The first and the second main surfaces (101, 102) are parallel to a first direction and a second direction, the first direction being perpendicular to the second direction. The collimating metalens (15) further comprises a first metasurface (108) formed on the first main surface (101), the first metasurface (108) comprising a first region (111). The collimating metalens (15) further comprises a second metasurface (109) formed on the second main surface (102), the second metasurface (109) comprising a second region (112). The first region (111) acts as a cylindrical concave lens, and the second region (112) acts as a convex lens.

Description

[0001] COLLIMATING METALENS AND ILLUMINATION DEVICE Projection devices, e.g. near-to-eye displays comprising AR (augmented reality) / VR (virtual reality) devices, head-up dis- plays and other low-power and compact projection devices often use edge-emitting laser devices. Edge-emitting laser devices usually emit beams of electromagnetic radiation having a strong ellipticity. Accordingly, there is a need to provide an improved optical device that may collimate and circularize light beams. It is an objective of the present disclosure to provide an im- proved collimating metalens and an improved illumination de- vice. Embodiments of the present disclosure relate to the subject- matter of the independent claims. According to embodiments, a collimating metalens comprises a transparent substrate having a first main surface and a second main surface. The first and the second main surfaces are par- allel to a first direction and a second direction, the first direction being perpendicular to the second direction. The collimating metalens further comprises a first metasurface formed on the first main surface, the first metasurface com- prising a first region, and a second metasurface formed on the second main surface, the second metasurface comprising a sec- ond region. The first region acts as a cylindrical concave lens, and the second region acts as a convex lens. For example, the first region may be non-diffracting in the first direction and the first region may be diffracting in the second direction. According to embodiments, a thickness of the substrate meas- ured in a third direction may be less than a focal length of the convex lens. Further, a first focal length of the convex lens implemented by the second region in the first direction may be different from a second focal length of the convex lens implemented by the second region in the second direction. According to embodiments, any of the first and second regions comprises an arrangement of pillars or ridges of a dielectric material selected from the group of materials consisting of a high refractive index low-loss dielectric oxide, a high re- fractive index low-loss dielectric carbide, a high refractive index low-loss dielectric diamond, a high refractive index low-loss dielectric sulfide, a high refractive index low-loss dielectric nitride, a high-index silicone polymer, and a high- index acrylic polymer over a substrate. For example, the pillars may have a diameter in a range of 40 to 200 nm. Further, a height of the pillars may be less than 1000 nm. For example, a first pattern of the first region may comprise ridges extending in a pattern direction. According to embodiments, a first pattern of the pillars of the first region changes along a first pattern direction and is unchanged along a second pattern direction perpendicular to the first pattern direction. For example, the first pattern direction may correspond to the first direction. Further, the second pattern direction may correspond to the second direc- tion. For example, the pillars of the second region may be arranged along a second pattern comprising concentric features with re- spect to an optical axis of the convex lens. According to embodiments, the collimating metalens further comprises a cover layer over the pillars or ridges. For exam- ple, the cover layer may cover sidewalls of the pillars or ridges. According to embodiments, the cover layer may be absent from sidewalls of the pillars or ridges. According to embodiments, an illumination device comprises a laser device and the collimating metalens as described above, wherein the laser device is configured to generate a radiation pattern having a first extension length in the second direc- tion larger than in the first direction. The collimating metalens is arranged so that the first metasurface is facing the laser device. For example, the laser device may be a transverse single mode semiconductor laser. According to further embodiments, the laser device is a semi- conductor laser having a ridge having a width so that a trans- verse multi-mode laser is implemented. According to further embodiments, an illumination device com- prises a multi-ridge laser device and the collimating metalens as described above. The collimating metalens is configured to receive electromagnetic radiation from all the ridges of the multi-ridge laser. The illumination device may further comprise a control device which is configured to control a light emission from each of the ridges. For example, the control device may be operable to simultane- ously address different ridges. According to embodiments, each of the ridges may be operable to emit electromagnetic radiation, wherein a wavelength of electromagnetic radiation emitted by at least two of the ridges is different. According to further embodiments, an illumination device com- prises an edge emitting semiconductor laser and the collimat- ing metalens as described above. The collimating metalens is configured to receive electromagnetic radiation emitted by the edge emitting semiconductor laser. The illumination device further comprises an adjustable grating. The adjustable grat- ing is configured to receive electromagnetic radiation trans- mitted by the collimating metalens, to reflect a first portion of the electromagnetic radiation to the edge emitting semicon- ductor laser and to outcouple a second portion of the electro- magnetic radiation. Further embodiments relate to an electronic device comprising the illumination device as described above. For example, the electronic device may be implemented as an AR / VR lighting de- vice or a projector. The accompanying drawings are included to provide a further understanding of embodiments of the invention and are incorpo- rated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles. Other embodiments of the invention and many of the intended advantages will be readily appreciated, as they be- come better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numbers desig- nate corresponding similar parts. Fig. 1A shows a cross-sectional view of an illumination device in a first direction (fast axis direction). Fig. 1B shows a cross-sectional view of an illumination device in a second direction (slow axis direction). Fig. 2A shows an example of a metalens. Fig. 2B shows a further example of a portion of a metalens. Fig. 2C shows pillar structures that may form a metalens. Fig. 2D shows a perspective view of an arrangement of ridges that may form a component of a metalens. Figs. 3A to 3B schematically illustrate a method of manufac- turing a metalens. Figs. 4A to 4D schematically illustrate a method of forming a metalens. Fig. 5A shows a cross-sectional view of a portion of a metalens according to embodiments. Fig. 5B shows a cross-sectional view of a portion of a metalens according to further embodiments. Fig. 5C shows a cross-sectional view of a portion of a metalens according to further embodiments. Fig. 6A shows an illumination device according to further em- bodiments. Fig. 6B shows a top view of an illumination device according to further embodiments. Fig. 6C shows a top view of an illumination device according to further embodiments. Fig. 7 shows a cross-sectional view of an illumination device according to embodiments. Fig. 8 is a schematic view of an electronic device according to embodiments. In the following detailed description reference is made to the accompanying drawings, which form a part hereof and in which are illustrated by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional ter- minology such as "top", "bottom", "front", "back", "over", "on", "above", "leading", "trailing" etc. is used with reference to the orientation of the Figures being described. Since components of embodiments of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope defined by the claims. The description of the embodiments is not limiting. In particu- lar, elements of the embodiments described hereinafter may be combined with elements of different embodiments. The present disclosure relates to compounds such as silicon oxide or silicon nitride without further specifying a stoichometric ratio. Where such unspecific compounds are described, it is in- tended that this description relates to any known stoichometric ratio including mixtures thereof. Accordingly, the term “silicon oxide” may refer to SiO, SiO2and mixtures of SiO and SiO2. The terms “lateral” and “horizontal” as used in this specifi- cation intends to describe an orientation parallel to a first surface of a substrate or semiconductor body. This can be for instance the surface of a wafer or a die. The term “vertical” as used in this specification intends to describe an orientation which is arranged perpendicular to the first surface of a substrate or semiconductor body. The present disclosure relates to tunable metalenses. Generally, the term “metalens” or “metaoptic structure” refers to a lens that bends light using an array of nanostructures that are formed on a (ideally flat) surface of a substrate, instead of via re- fraction. More specifically, a metalens includes a metasurface that includes an array of nanostructures, wherein the nanostruc- ture array is configured to bend light incident thereon by al- tering its phase. As will be described, the phase change imparted by the metasurface can create a new wave front in a region down field of the lens. For example, a metalens consistent with the present disclosure can include an array of nanostructures that can impart a phase change to incident light having a spherical or hemispherical wave front up field of a lens, such that the light in a region down field of the lens has a planar wave front (i.e., a plane wave). Stated more generally, metalenses are designed to shape light to a dedicated radiation pattern. This may be accomplished due to phase shifts and interference effects of geometrical structures having sizes below the wavelength of the electromagnetic radia- tion to be shaped by the metalenses. According to an approxima- tion, the phase shifts may be interpreted as waveguide modes propagating in the pillars, generating a phase shift dependent on the height of the pillar of a given geometry and size. Furthermore, the propagation in materials may also change the polarization direction of light. After passing areas having a structured pattern of polarization rotation material, interfer- ences may be generated, the interferences redirecting the light. For example, nanostructures that may be a component of metalenses are generally configured to function as resonators or waveguides that impart a phase change to light incident thereon. As will be described below, the metalenses may comprise pillars or ridges of a suitable material. In this context, the pillars / ridges may constitute the nanostructures mentioned above. Examples of a suitable material forming the pillars or ridges comprise a dielectric material which may be selected from the group of materials consisting of a high refractive index, low- loss dielectric oxide, a high refractive index low-loss dielec- tric carbide, a high refractive index low-loss dielectric dia- mond, a high refractive index low-loss dielectric sulfide, a high refractive index low-loss dielectric nitride, a high-index silicone polymer and a high-index acrylic polymer which is ar- ranged over a substrate. For example, the material may comprise TiO2. Fig. 1A shows a cross-sectional view of an illumination device 10 comprising a collimating metalens 15 according to embodi- ments. The collimating metalens 15 comprises a transparent substrate 100 having a first main surface 101 and a second main surface 102. The first main surface 101 is parallel to a first direction, e.g. the y-direction, and a second direction, e.g. the z-direction. Further, the second main surface 102 is parallel to the first direction and the second direction. The first direction is perpendicular to the second direction. The collimating metalens 15 further comprises a first metasurface 108 that is formed on the first main surface 101. The first metasurface 108 comprises a first region 111. The collimating metalens 15 further comprises a second metasurface 109 which is formed on the second main surface 102. The second metasur- face 109 comprises a second region 112. The first region 111 acts as a cylindrical concave lens. Further, the second region 112 acts as a convex lens. The illumination device shown in Fig. 1A further comprises a laser device 107. The laser device 107 may be an edge-emitting semiconductor laser. For example, the laser device 107 may be a transverse single mode semicon- ductor laser. The first metasurface 108 may be arranged on a side of the collimating metalens 15 facing the laser device 107. The second metasurface 109 may be arranged on a side of the collimating metalens 15 which is remote from the laser de- vice 107. The cross-sectional view of Fig. 1A is taken along the so- called fast axis of the edge-emitting laser device 107. In particular, the cross-sectional view of Fig. 1A is taken par- allel to the x-z plane, wherein the z-direction corresponds to a vertical direction, e.g. a direction in which the semicon- ductor layers for forming the laser device 107 have been grown. A thickness d of the substrate 100 may be less than a focal length of the convex lens implemented by the second re- gion 112. According to embodiments, the convex lens implemented by the second region 112 may comprise two different focal lengths that may be measured in two different directions, e.g. a first direction (fast axis) and the second direction (second axis). In this case, the thickness d of the substrate 100 may be less than any of the focal lengths of the convex lens implemented by the second region 112. As a result, the concave lens imple- mented by the first region 111 is arranged within the focal length of the convex lens. Accordingly, the focal points of the convex lens and the concave lens may be arranged on a side of the laser device 107. The configuration implements an in- verted Galilean telescope. The substrate 100 may comprise a transparent material and may be selected, e.g. from silicon oxide or another transparent material, e.g. a polymer. For example, the concave lens implemented by the first region 111 may be non-diffracting in the z-direction. Accordingly, light beams are simply refracted at the interface between air and the substrate 100 without any lens action due to diffrac- tion. The beams are then collimated by the convex metalens im- plemented by the second region 112. The first virtual source 106 is determined by the intersection of the fast axis edge rays 103 at angle ^v’. The first virtual source 106 lies before the emission surface of the laser device 107. Fig. 1B shows a cross-sectional view of the illumination de- vice 10 parallel to the x-y plane, e.g. along the slow axis of the edge-emitting laser device 107. The concave lens imple- mented by the first region 111 is diffracting. As is shown, slow axis edge rays 104 are deflected by the first region 111 at a higher angle in comparison to specular refraction. Ac- cordingly, when the slow axis edge ray 104 reaches the second region 112, the spot size of the slow axis rays match those formed by the fast axis rays. As a consequence, the outgoing beam will be collimated and circular. In Fig. 1A, D1vdenotes a distance between the fast axis edge rays 103 when entering the substrate 100, i.e. after passing the concave lens formed by the first region 111. The distance D2vdenotes the distance of the fast axis edge rays 103 after passing the convex lens formed by the second region 112. In Fig. 1B the distance D1hdenotes the distance of the slow axis edge rays 104 after passing the concave lens formed by the first region 111. The distance D2hdenotes the distance be- tween the slow axis edge rays 104 after passing the convex lens formed by the second region 112. The laser device 107 is arranged at a distance a to the first main surface 101 of the substrate 100. The beam leaving the collimating metalens 15 is considered to be circular, when D2vis equal to D2h. With a given divergence angle ^h’, a second virtual source 126 is formed by the concave lens implemented by the first region 111 at an intermediate distance difrom the first main surface 101. In the geometric optics limit, the size of the second virtual source wLD’is smaller than the hor- izontal emission region size wLDof the laser device 107. The size of the second virtual source wLD’determines the geometric optics limits of the divergence ϑhof the collimated beam. How- ever, a prediction of the virtual source sizes may be made more accurate using waveoptics. On the basis of the divergence along the slow axis and the fast axis, the design of the two lenses may be determined. If the laser device itself shows astigmatism, this may be addi- tionally taken into account for the design of the lenses. Considering the cross-sectional view of Fig. 1A and fixing the thickness d of the substrate and the desired beam diameter D2v, the required spacing a to the laser device 107 may be deter- mined. From these values, the optics parameters for the slow axis may be formed. The required divergence angle ^h’implies that the virtual image, as would appear in the substrate mate- rial, must be at a distance difrom the first main surface 101. Accordingly, the concave lens implemented by region 111 is a one-dimensional diverging lens which images the emission re- gion of the laser device 107 along the slow axis at this loca- tion, when the distance a to the laser device 107 is given. In order to assure that rays along the slow axis and the fast axis are collimated, the convex lens implemented by the second region 112 is designed to be astigmatic. In more detail, the focal length f2valong the fast axis is different from the fo- cal length f2halong the slow axis. As a consequence, the locations of the virtual source and the position of the virtual image are different for the respective axes, respectively. As is illustrated in Figs. 1A and 1B, a collimating metalens 15 may comprise two metasurfaces 108, 109 that are arranged on opposing surfaces 101, 102 of a single flat substrate 100. As a consequence, the collimating metalens 15 is configured to shape wavefronts at very high curvatures (or ray angles). Hence, the collimating metalens 15 is configured to provide almost unlimited wavefront shaping functionality. The metasur- faces 108, 109 may also have a low shadowing impact, much more precise wavefront control and a high accuracy fabrication in comparison with traditional optical elements. As a result, the collimating metalens 15 may provide high quality collimation and circularization, with low wavefront distortion. The design of the respective metalenses forming the concave lens and the convex lens may be determined to first order on the basis of geometric optics considerations. For example, the conditions for the edge rays 103, 104 may be determined by first defining distance ratios: The second equality results from the view of Fig. 1B. For the following equations, spot diameters or sizes and ray angles are taken to represent the half-maximum (HM) intensity values. For example, the horizontal beam width D2hmay be given by D2h= 2d*tanθh(β-1+α) The condition for a circular beam, D2h= D2v= D2implies that ^^^^^^^ − ^^^^^ ^ =^^ ^^^^^− ^^^^^Moreover, since along the fast axis, the concave lens imple- mented by the first region 111 is not refracting, the follow- ing equation holds The above equations uniquely determine the ratios ^ and ^ for given desired full-width hm (FWHM) beam diameter D2, and the provided divergences ^vand ^hfor the laser device 107. ^^^^ 0 < ^ <^^^^^^^From Figs. 1A and 1B, the focal lengths of the convex lens in the substrate material implemented by the second region 112 are as follows: ^ ^^^^= ^^^^^^^ ^ ^^^^= 2^^^^^ ^ wherein the above equations can be used to determine the focal lengths in terms of the divergences of the laser device 107. The front focal length f1hin air may be determined by the par- axial length equation for the concave lens implemented by the first region 111: The focal length is negative, representing a diverging lens. Based on these results and taking experimental divergence measurements of ^v= 22.0° and ^h= 8° for a blue TEM00edge emitting laser diode, the resulting geometric optics design parameters may be obtained: Parameter Description Final values D2FA beam diameter (2ndlens) 0.499 mm (FHWM) D1vFA beam diameter (1stlens) 0.237 mm (FWHM) D1hSA beam diameter (1st lens) 0.0852 mm (FWHM) M SA magnification (1st lens) 0.235 f1h1st lens SA paraxial front -0.182 mm focal length (in air) f2v2ndlens FA front focal 1.902 mm length f2h2ndlens SA front focal 1.206 mm length n1Air refractive index 1.00 n2Substrate refractive index 1.466 a Distance between emission 0.609 mm surface of laser device to the first main surface diSA distance of the virtual 0.206 mm source (in substrate) ϑhHorizontal divergence ½ angle 0.014° ϑvVertical divergence ½ angle Diffraction limit θvFWHM vertical divergence of 23.3° laser device θhFWHM horizontal divergence of 7.1° laser device wherein “FA” denotes “fast axis“ and „SA” denotes „slow axis. The concave lens implemented by the first region 111 is effec- tively forming a (virtual) image of the emission region of the laser device 107. For example, an expanding spherical wave and forming a virtual spherical contracting wave, a hyperbolic phase delay will be applied for determining parameters of the concave lens. For example, it may be assumed that the phase delay of the concave lens is one-dimensional and may be ex- pressed as follows: The parameters may be determined based on geometric optics de- sign. The wavevectors kj= njk0, where k0= 2 ^ / ^0, and j = 1, 2 refers to the respective media. For example, for a wavelength of 450 nm, the parameters may be determined as is shown in the table below. The parameters may be further modified to compen- sate for astigmatism of the laser device 107. For example, an astigmatic Gauss model may be applied and may further be re- placed by a near-field model at the emitting facet of the la- ser device 107. Parameter Value λ0450 nm z1f0.6092 mm z1b-0.2057 mm ^02π Using an astigmatic Gaussian model for the laser device 107 and a vector propagation simulation, a phase delay function may be determined for the convex lens. For example, to create a collimated output beam, the parameters in the equation below can be adjusted such that the convex lens phase conjugates the calculated incident field, following propagation from the con- cave metalens. In this case the phase in the equation below is the negative of the phase of the incident field on the convex metasurface. A thickness d of the substrate may be assumed to be 1.00 mm. The resulting phase delay function may be two-di- mensional and astigmatic and may comprise a symmetric hyperbo- loid term and polynomial correction or astigmatic terms: The table below shows parameters for a phase function for the convex lens implemented by the second region 112. Note that k and f are chosen to give a convergent fit, not necessarily the physical values. They can also be fit parameters. Parameter value λ0 450 nm k n1k0f 1.206 mm ^02πa100.23302 a01-0.03197 a20-0.052496 a020.040312 a110.086851 Fig. 2A shows a portion of the concave lens implemented by the first region which may be designed using a periodic method us- ing the following parameters: Period P: 252 nm, square lattice. Height H: 600 nm at an assumed emission wavelength of 450 nm. As is shown in Fig. 2A, the pattern changes along a first di- rection, e.g. the y-direction, and does not change in a second direction, e.g. the z-direction. Accordingly, the resulting metasurface is diffracting in a first direction, e.g. the y- direction, and is not diffracting in a second direction, e.g. the z-direction. Fig. 2B shows an example of a view of the convex lens imple- mented by the second region 112. As is shown, the pattern changes in the first and in the second direction. Fig. 2C shows an example of a metasurface comprising pillars 113 which may be made of a dielectric material 110 and which are arranged over the substrate 100. According to design meth- ods or simulations, for visible light, based on a high-refrac- tive oxide such as TiO2, the diameter of the pillars may be larger than 40 nm. For example, a diameter may be less than 200 nm. Further, the pillars may have a height less than 1000 nm. According to further embodiments, the laser device 107 may be operated at larger or lower wavelengths. For example, for a wavelength larger than 800 nm, a diameter of the pillars may be in the range of 100 nm – 500 nm with a height from 1000 nm and 1500 nm for a wavelength of 1000 nm, assuming common metal oxide dielectrics are used. Often crystalline or amorphous Si is used for IR applications which has much higher refractive index than metal oxides and which has low optical absorption at IR wavelengths. In this case, pillar diameters from 100 nm to 300 nm with a height of less than 500 nm may be used to achieve a high efficiency. For near-UV wavelengths in the range of 300 nm to 375 nm, the diameter of the pillars 113 may range from 40 nm to 200 nm. A pillar height may be less than 500 nm. In general, one can roughly scale pillar dimensions and lattice periods according to: L_new = (lambda_new / lambda_ref)*L_ref, where L_new and L_ref represent a new scaled and current reference pillar di- mension or period (such as height or diameter) and lambda_new and lambda_ref represent wavelength for the new and reference wavelengths. This scaling only strictly holds if the pillar material (and surrounding media) have the same refractive in- dex at the different wavelengths. However, it can provide a useful range estimate, even if pillar refractive indices are modestly dispersive. Fig. 2D shows an example of ridges 114 which are made of die- lectric material 110 and which may be arranged over a sub- strate 100. As is shown, the ridges extend in a pattern direc- tion, e.g. the z-direction. Accordingly, the resulting surface is non-diffractive in the z-direction and is diffractive in a direction perpendicular to the z-direction, e.g. in the x di- rection. As is clearly to be understood, the metalenses may be designed using further design methods which are generally known. It is to be noted that due to high phase gradients that can occur in phase delay functions, the metalens design may benefit from design methods that do not use a strict periodic lattice for placement of the pillars. For example, a local grating method, in which the lattice period is varied with the distance be- tween Fresnel zones can often provide higher efficiency than a metalens based on a pure periodic lattice pillar arrangement. Figs. 3A and 3B illustrate a method of manufacturing the col- limating metalens 15 which has been described above. According to embodiments, the metalens 15 comprises an arrangement of pillars 113 or ridges 114 of a dielectric material 110. The dielectric material 110 may be selected from the group of ma- terials consisting of a high refractive index, low-loss die- lectric oxide, a high refractive index low-loss dielectric carbide, a high refractive index low-loss dielectric diamond, a high refractive index low-loss dielectric sulfide, a high refractive index low-loss dielectric nitride, a high-index silicone polymer and a high-index acrylic polymer which is ar- ranged over a substrate. For example, the material may com- prise TiO2. As is shown in Fig. 3A, the first metasurface 108 may be formed over a first carrier 116. For example, the carrier 116 may comprise a transparent material. The first metasurface may be formed e.g. by depositing a layer of a dielectric material 110, followed by a patterning method and etching. Patterning methods may include e-beam, deep UV or extreme UV photolithog- raphy, and nanoimprinting. As a result, pillars 113 or ridges 114 of a dielectric material 110 may be formed over the first carrier 116. The second metasurface 109 may be formed over a second carrier 117 in a corresponding manner. The first and the second carriers 116, 117 may be attached to each other us- ing a glue 115. The total thickness of the first and the second carriers 116, 117 and a few microns of a transparent UV-curing glue 115 may correspond to the desired total thickness. For example, the glue may have a refractive index which is different from the refractive index of the first and second carriers 116, 117. For example, an alignment of the two carriers 116, 117 may be accomplished via scattering-based or metallization-based fidu- cials which may be generated as part of the lithographic pat- terns. These fiducials are observable with laser illumination or incoherent light and optically magnified cameras. Moreover, viewing far-field and near-field patterns upon laser transmis- sion, the first and the second metasurfaces 108, 109 may be actively aligned. After aligning the first and the second metasurfaces 108, 109, the glue may be cured, e.g. by UV radi- ation. Fig. 3B shows an example of a resulting structure. According to further implementations, as is shown in Fig. 4A, the metasurfaces may be manufactured over a single substrate 100. For example, one of the metasurfaces 109 may be protected during fabrication of a further one of the metasurfaces 108 using a temporary organic encapsulation material 118. In more detail, as is illustrated in Fig. 4A, e.g. a second metasur- face 109 may be formed over the substrate 100. As is clearly to be understood, instead, the first metasurface 108 may be formed. As is further shown in Fig. 4A, the second metasurface 109 may be protected using a cover element 118 that may com- prise an organic material. Fig. 4B shows a schematic view of the second metasurface 109 comprising the cover element 118. Thereafter, the first metasurface 108 is formed over the first main surface 101 of the substrate. Fig. 4C shows an example of a resulting structure. Thereafter, as is shown in Fig. 4D, the cover element 118 is removed from the second metasurface 109. Fig. 4D shows an example of a re- sulting collimating metalens 15. According to the processes described, alignment of patterns on each main surface 101, 102 of the substrate may be accom- plished by forming, e.g. metallization-based fiducials, gener- ally using photolithography or e-beam. As is shown in Fig. 4C, after protecting the second metasurface 109 with a cover ele- ment 118, the substrate is flipped. A second set of fiducials is aligned with the first e.g. in a photomask aligner and then the resist is exposed. The second set which may also comprise metallization fiducials, may be made using a lift-off process. Once, the metal fiducials are fabricated, one can use these to precisely define and align local coordinates for e-beam or other high resolution lithography methods on both sides of the substrate. The optical axis of the metasurface to be formed in the process shown in Fig. 4C can be aligned to the optical axis of the already formed metasurface to within the required sub-micron position. The methods shown in Figs. 3 and 4 may be used for wafer level fabrication. Fig. 5A shows a cross-sectional view of a portion of a metasurface 108, 109 comprising pillars 113 or ridges 114 ac- cording to embodiments. The metasurface illustrated in Fig. 5A may be a first or a second metasurface 108, 109. Fig. 5B shows a cross-sectional view of a portion of a metasurface according to further embodiments. As is illus- trated, a cover layer 120 may be formed over the surface of the individual pillars 113 or ridges 114. Moreover, a portion of the cover layer 120 may be arranged between adjacent pil- lars 113 or ridges 114. For example, the cover layer may com- prise a dielectric material, e.g. SiO2. According to further embodiments, further dielectric materials comprise organic ma- terial, low-index fluorides, low-index oxides or low-index silicates. Further examples comprise inorganic materials such as Al2O3, MgF, BeO, BeO3, Be2SiO4, Mg2SiO4. For example, a thick- ness of the cover layer 120 may be less than 150 nm. Moreover, a thickness of the cover layer 120 may be larger than 100 nm. Accordingly, the metasurfaces may be encapsulated to protect the pillars or the ridges from physical damage or environmen- tal interactions. In the case of the cover layer as shown in Fig. 5B having a small thickness, a negative optical impact may be reduced, while the pillars or ridges are protected against humidity, sulfur and other compounds in the air. According to embodi- ments that are shown in Fig. 5C, the cover layer 120 may be formed so as to cover the sidewalls of the ridges 114 or the pillars 113. For example, the cover layer 120 is formed as a conformal layer so that the resulting surface is not planar and comprises voids. Moreover, a thickness of the cover layer 120 over the sidewalls is less than 150 nm. In embodiments described hereinabove, for example, the laser device 107 may be a transverse single-mode semiconductor la- ser. According to further embodiments, the laser device 107 may be a multi-mode or higher order edge-emitting laser de- vice. For example, if the laser device 107 is a laser device which is operated at a higher power, a wider horizontal wave- guide structure may be used in order to reduce the current density. Further, multi-ridge edge-emitting lasers may be used in AR / VR applications to provide both wavelength diversity and for providing multiple scanned spots in laser-beam scanning configurations. For example, Fig. 6A shows a illumination device 10 comprising a laser device 107 which may be implemented as a multi-ridge laser. In more detail, the laser device 107 may comprise a plurality of laser ridges 1221, 1222, 1223. Each of the ridges may be individually addressed. For example, Fig. 6A shows a control device 128 which is configured to separately control each of the ridges 1221… 1223. Accordingly, simultaneous pix- els may be generated for laser-beam scanning AR / VR projectors. Further, each of the ridges 1221, … 1223may be configured to emit a slightly different wavelength. As a result, coherent artifacts e.g. speckles may be reduced or avoided. Furthermore, the illumination device 10 of Fig. 6A comprises the collimating metalens 15 which has been described with ref- erence to Figs. 1A and 1B, for example. As is shown, there may be one collimating metalens 15 for a plurality of ridges 1221… 1223of the laser device 107. Accordingly, one metalens 105 may be used to collimate light emitted by any of the ridges. The collimating metalens 105 provides a high degree of off-axis collimation which is beneficial in this illumination device 10. According to further implementations, the ridges 1221… 1223may interact, so that they are coherently phase-locked or op- erating as phased-array lasers. In this case, the phase on each ridge may be continuously controlled so that the coherent sum of radiation emitted by the ridges 1221… 1223may be used along with the collimating metalens 105 for beam-scanning or beam-shaping applications in the far field. For example, when the laser device implements a fixed phased-array of ridges, the resulting “super-mode” may also be used for special beam- shaping or collimation applications. Fig. 6B shows an illumination device according to further em- bodiments. As is shown, the laser device 107 comprises one single wide laser ridge 123. For example, the laser device 107 may be a multi-mode laser. For example, the laser device 107 may be partially coherent due to different emission wave- lengths of each lateral mode when view along the slow axis. The illumination device 10 further comprises the collimating metalens 15 which has been explained above with reference to Figs. 1A and 1B. Fig. 6C shows an example of an illumination device that may implement an external cavity laser 11. In more detail, the de- vice shown in Fig. 6C comprises a laser device 107 which may be a single-mode semiconductor laser. The external cavity la- ser 11 further comprises a collimating metalens 15 which has been explained above with reference to Figs. 1A and 1B. The external cavity laser 11 further comprises an adjustable grat- ing 125. By adjusting a tilt angle of the adjustable grating 125, a wavelength of radiation which is reflected back to the laser device 107 may be adjusted. Further wavelengths of radi- ation which is reflected towards outside may be adjusted. In more detail, due to the grating 125, a partial reflection from one of the grating orders may be obtained. This partial reflection corresponds to a direct counter-propagating reflec- tion of the collimated beam at the desired laser wavelength. This feedback provides a higher net gain at a specific laser wavelength which is determined by the grating. A feedback am- plitude and output coupling efficiency may be determined by the grating order efficiency. According to further implementations, the grating 125 may be implemented as a reflective metaoptic device which may provide further beam-shaping and efficiency control. For example, this functionality may take advantage of a polarization state of the edge-emitting laser device 107. Fig. 7 shows a vertical cross-sectional view of an illumina- tion device 10 according to embodiments. The illumination de- vice comprises a laser device 107 which may be mounted to a carrier 105. Moreover, the collimating metalens 15 which has been explained above with reference to Figs. 1A and 1B, for example, may be arranged over the carrier 105. As has been de- scribed before, the collimating metalens 15 comprises a first metasurface 108 which faces the laser device 107 and a second metasurface 109 on a side remote from the laser device 107. The first metasurface 108 and the second metasurface 109 are attached to the substrate 100. In more detail, the first metasurface 108 is attached to the first main surface 101 of the substrate. Moreover, the second metasurface 109 is at- tached to the second main surface 102 of the substrate 100. One can also make a TO-can based device, where the collimating metalens 15 is attached to the laser die mounting finger or bonded separately within the TO-can. Fig. 8 shows a schematic view of an electronic device 20 ac- cording to embodiments. The electronic device 20 comprises the illumination device 10 as has been explained above. For exam- ple, the electronic device 20 may be implemented as an AR / VR lighting device or a projector. The collimating metalens described above is configured to cir- cularize and collimate an elliptical beam emitted by an edge emitting laser device. The collimating metalens described above has a greatly reduced size in comparison to refractive optics designs. In particular, due to the special inverse Gal- ilean design, the size may be considerably reduced. In compar- ison to refractive designs having a similar size, it is easier to achieve an appropriate wavefront quality both from the de- sign and fabrication precision standpoints. Further, nearly all non-ideal wavefront characteristics of an edge emitting laser may be compensated in a single optical component, including ellipticity, astigmatism and mode imper- fections. As has been described, the collimating metalens may be manufactured using semiconductor processes. As a result, cost may be reduced. The multi-functionality of the described metalens may be used to optimize collimation from a multi-mode or a multi-ridge edge-emitting laser. While embodiments of the invention have been described above, it is obvious that further embodiments may be implemented. For example, further embodiments may comprise any subcombination of features recited in the claims or any subcombination of el- ements described in the examples given above. Accordingly, this spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein.

[0002] LIST OF REFERENCES 10 illumination device 11 external cavity laser 15 collimating metalens 20 electronic device 100 substrate 101 first main surface 102 second main surface 103 fast axis edge ray 104 slow axis edge ray 105 carrier 107 laser device 108 first metasurface 109 second metasurface 110 dielectric material 111 first region 112 second region 113 pillar 114 ridge 115 glue 116 first carrier 117 second carrier 118 cover element 120 cover layer 1221, 1222,…1223 laser ridge 123 wide laser ridge 125 adjustable grating 127 virtual laser device 128 control device

Claims

CLAIMS 1. A collimating metalens (15), comprising: a transparent substrate (100) having a first main sur- face (101) and a second main surface (102), the first and the second main surfaces (101, 102) being parallel to a first di- rection and a second direction, the first direction being per- pendicular to the second direction; a first metasurface (108) formed on the first main sur- face (101), the first metasurface (108) comprising a first re- gion (111); and a second metasurface (109) formed on the second main surface (102), the second metasurface (109) comprising a sec- ond region (112), wherein the first region (111) acts as a cylindrical concave lens, and the second region (112) acts as a convex lens.

2. The collimating metalens (15) according to claim 1, wherein the first region (111) is non-diffracting in the first direction and the first region (111) is diffracting in the second direction.

3. The collimating metalens (15) according to any of the preceding claims wherein a thickness of the substrate (100) measured in a third direction is less than a focal length of the convex lens.

4. The collimating metalens (15) according to claim 2 or 3, wherein a first focal length of convex lens implemented by the second region (112) in the first direction is different from a second focal length of the convex lens implemented by the second region (112) in the second direction.

5. The collimating metalens (15) according to any of the preceding claims, wherein any of the first and second regions (111, 112) comprises an arrangement of pillars (113) or ridges (114) of a dielectric material (110) selected from the group of materials consisting of a high refractive index low-loss dielectric oxide, a high refractive index low-loss dielectric carbide, a high refractive index low-loss dielectric diamond, a high refractive index low-loss dielectric sulfide, a high refractive index low-loss dielectric nitride, a high-index silicone polymer, and a high-index acrylic polymer over a sub- strate.

6. The collimating metalens (15) according to claim 4, wherein the pillars (13) have a diameter in a range of 40 to 200 nm.

7. The collimating metalens (15) according to claim 4 or 5, wherein the pillars (13) have a height less than 1000 nm.

8. The collimating metalens (15) according to claim 5, wherein a first pattern of the first region (111) comprises ridges (114) extending in a pattern direction.

9. The collimating metalens (15) according to any of claims 5 to 7, wherein a first pattern of the pillars (113) of the first region (111) changes along a first pattern direction and is unchanged along a second pattern direction perpendicu- lar to the first pattern direction.

10. The collimating metalens (15) according to any of claims 4 to 7 or 8, wherein the pillars (113) of the second region (112) are arranged along a second pattern comprisingconcentric features with respect to an optical axis of the convex lens.

11. The collimating metalens (15) according to any of claims 5 to 9, further comprising a cover layer (120) over the pillars (113) or ridges (114).

12. The collimating metalens (15) according to claim 11, wherein the cover layer (120) covers sidewalls of the pillars (113) or ridges (114).

13. The collimating metalens (15) according to claim 11, wherein the cover layer (120) is absent from sidewalls of the pillars (113) or ridges (114).

14. An illumination device (10) comprising a laser device (107) and the collimating metalens (15) according to any of the preceding claims, wherein the laser device (107) is con- figured to generate a radiation pattern having a first exten- sion length in the second direction larger than in the first direction, wherein the collimating metalens (15) is arranged so that the first metasurface (108) is facing the laser device (107).

15. The illumination device (10) according to claim 14, wherein the laser device (107) is a transverse single mode semiconductor laser.

16. The illumination device (10) according to claim 14, wherein the laser device (107) is a semiconductor laser having a ridge (114) having a width so that a transverse multi-mode laser is implemented.

17. An illumination device (10) comprising a multi-ridge laser device and the collimating metalens (15) according to any of claims 1 to 13, wherein the collimating metalens (15) is configured to receive electromagnetic radiation from all ridges (1221, 1222, 1223) of the multi-ridge laser.

18. The illumination device (10) according to claim 17, further comprising a control device (128) configured to con- trol a light emission from each of the ridges (1221, 1222, 1223).

19. The illumination device (10) according to claim 18, wherein the control device (128) is operable to simultaneously address different ridges (1221, 1222, 1223).

20. The illumination device (10) according to any of claims 16 to 19, wherein each of the ridges (1221, 1222, 1223) is op- erable to emit electromagnetic radiation, wherein a wavelength of electromagnetic radiation emitted by at least two of the ridges is different.

21. An illumination device (10) comprising an edge emitting semiconductor laser (107); the collimating metalens (15) according to any of claims 1 to 13, the collimating metalens (15) being configured to receive electromagnetic radiation emitted by the edge emit- ting semiconductor laser (107), and an adjustable grating (125), the adjustable grating (125) being configured to receive electromagnetic radiation transmitted by the collimating metalens (15), to reflect a first portion of the electromagnetic radiation to the edge emitting semiconductor laser (107) and to outcouple a second portion of the electromagnetic radiation.

22. An electronic device (20) comprising the illumination device (10) according to any of claims 14 to 21.

23. The electronic device (20) according to claim 22, being implemented as an AR / VR lighting device or a projector.

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