Actively deformable meta-mirror

Abstract

The optical assembly (10) includes a plurality of metamirrors (12), each of which includes a substrate (20), a reflective layer (22) formed on the substrate (20), an array of optical meta-elements (24) extending from the reflective layer (22), and an array of microactuators (36) coupled to the substrate (20) opposite the reflective layer (22). The combination of the microactuators (36) is controlled to control the orientation and curvature of the metamirror (12) and to set how the meta-elements (24) focus a light beam reflected from the reflective layer (22).

Description

[Technical Field]

[0001]

[0001] The present disclosure relates generally to actively deformable metamirrors, and more particularly to optical assemblies having an array of actively deformable metamirrors, each of which includes an array of metaelements formed on an array of microactuators. [Background technology]

[0002]

[0002] Refractive and reflective optical elements have many applications in various optical systems for focusing light and other electromagnetic waves. Some of these optical systems often require large apertures to collect enough light for their intended purpose, and therefore improvements in the size and weight of optical elements are desirable. This is particularly true for large aperture ground-based mirrors, and even more so for airborne and space-based optical systems that need to collect more light. However, the use of traditional refractive optical materials, such as glass, and materials for large mirrors, such as SiC, are limited by weight, size, and thickness, making it difficult to provide large apertures using these components.

[0003]

[0003] Metamaterials are artificially engineered composite materials that derive their properties primarily from their internal microstructure, rather than their chemical composition. Metamaterials can be designed and manufactured as structural units to achieve desired properties or functions, such as focusing light with low loss. It is known in the art that transparent metamaterials, such as certain low-loss dielectrics, can be specifically fabricated as metastructures or arrays of metaelements to form metasurfaces or metalenses, providing ultrathin planar optics for visible and infrared imaging. Metamaterials for these purposes can include periodically arranged dielectric pillars, rings, crosses, etc., on the order of 50–200 nanometers in size on a transparent or reflective substrate, allowing optics to be as thin as 10–20 microns. It has been demonstrated in the art that optical-quality, broadband planar metalenses and metamirrors with good chromatic aberration correction performance can be fabricated using conventional microelectronics fabrication techniques.

[0004] One of the challenges facing practical applications of these metalenses and metamirrors is their size. Typical optical systems for these applications have lenses on the order of 1 mm in diameter, and current semiconductor fabrication techniques limit their size to the sub-centimeter range. While this works well for microlens applications such as cell phones, many other applications require significantly larger apertures. One solution is to "tile" many mirrors to create large segmented apertures. This has been done for some large reflecting telescopes, but even then, the individual segments are orders of magnitude larger than a typical metalense. Furthermore, current fabrication techniques limit the phase contrast that can be achieved in metamirrors, thus necessitating the use of Fresnel mirrors and / or very small tiles for larger apertures. Therefore, achieving large-aperture metamirrors requires a combination of improving the size of each individual tiled mirror, fabricating segmented mirrors with a larger number of segments, and developing computational techniques to correct for the optical errors introduced by the Fresnel configuration and segmentation.

[0005] Adaptive optics systems are known in the art for a variety of optical applications that employ small actuators, such as piezoelectric actuators, to deform mirrors. These types of compensating mirrors have a variety of uses, such as in telescopes with optics that need to be corrected in real time when observing celestial objects under changing atmospheric conditions. Such active adaptive optics systems, which provide beam correction using conventional optics and mirror technology, such as compensating mirror technology, are currently available from AOA Xinetics. Summary of the Invention [Problem to be solved by the invention]

[0006] The following discussion discloses and describes an optical assembly including a plurality of metamirrors, each metamirror including a substrate, a reflective layer formed on the substrate, an array of optical meta-elements extending from the reflective layer, and an array of microactuators coupled to the substrate opposite the reflective layer. Combinations of the microactuators are controlled to modify the orientation and curvature of the metamirror and to set how the meta-elements focus a light beam reflected from the reflective layer.

[0007]

[0007] Additional features of the present disclosure will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a front view of an optical metamirror assembly having multiple tiled metamirrors. [Figure 2] FIG. 2 is a side view of the optical metamirror assembly shown in FIG. 1. [Figure 3] FIG. 1 is an isometric view of a metamirror. [Figure 4] FIG. 1 illustrates the curved geometry of a metamirror versus focal length. DETAILED DESCRIPTION OF THE INVENTION

[0009]

[0012] The following discussion of embodiments of the present disclosure directed to actively deformable metamirrors is merely exemplary in nature and is in no way intended to limit the disclosure or its application or uses.

[0010]

[0013] This disclosure proposes an optical metamirror that includes an array of metaelements formed on a reflective surface and a microactuator to form an adaptive optics system, for example for use in telescopes. Light incident on the metamirror is altered by the metaelements, reflected by the reflective surface, and altered again by the metaelements to focus the light. The microactuators are controlled to tilt the metamirror and / or curve the reflective surface to control the focusing effect of the metaelements.

[0011]

[0014] FIG. 1 is a front view of an optical metamirror assembly 10 for focusing a beam of light, including a metamirror of the type generally mentioned above. Assembly 10 includes multiple hexagonal metamirrors 12 arranged in a tiled configuration, with assembly 10 defining a round aperture 14, such as the aperture of a telescope. It should be noted that the size, number, and shape of metamirrors 12 and the shape of aperture 14 are illustrated merely by way of example and can generally be selected for any suitable application or manufacturing capability. Each metamirror 12 includes a substrate 20 having a reflective surface, such as a metallized reflective layer 22, formed thereon, and multiple metaelements 24 formed thereon by any suitable process, such as a lithography process used to fabricate semiconductor devices. It should be noted that metaelements 24 are illustrated much larger than they actually are for illustrative purposes only. It should also be noted that in alternative embodiments, due to manufacturing limitations, for example, each metamirror 12 can include a combination of multiple smaller sub-metamirrors of any suitable shape and size.

[0012]

[0015] The meta-elements 24 may extend to any suitable height, such as 50-200 nm, from the substrate 20 and have any suitable shape to deflect or focus light at a desired wavelength consistent with the discussion herein, with the assembly configured to operate over a range of wavelengths. Additionally, the meta-elements 24 may have different shapes across the meta-mirror 12. Light incident on the meta-mirror 12 is first deflected by the meta-elements 24, then reflected by the reflective layer 22, and then deflected again by the meta-elements 24 and focused to a focal point. As discussed in more detail below, each meta-mirror 12 is actuable relative to the other meta-mirrors 12, thereby allowing the meta-mirrors 12 to be tilted relative to one another and controlling the focusing of light.

[0013]

[0016] FIG. 2 is a side view of the assembly 10. Each metamirror 12 includes a microactuator assembly 30 having a base 32 and a dense array 34 of controllably variable-length microactuators 36 extending therefrom, with the ends of the microactuators 36 formed on the opposite side of the substrate 20 from the reflective layer 22. In one non-limiting example, the microactuators 36 are piezoelectric elements that can be shortened or extended by applying appropriate electrical signals, for example, from a controller 38. Thus, each metamirror 12 can be tilted or bent independently of the other metamirrors 12 to selectively focus light. Some of the metamirrors 12 can be pre-tilted to approximately focus light reflected from the array 10 to a common focal point, with the outer metamirrors 12 having a larger tilt, and the microactuators 36 used to focus all of the metamirrors 12 to a common, high-precision focal point. In addition to focus control, active adaptive optics control is used to correct imperfections and errors in the individual metamirrors 12 and to mitigate the effects of segmentation and chromatic aberration. Further benefits of metamirror 12 actuation include the ability to adjust optical performance in real time, for example to compensate for environmental, mechanical stresses, and atmospheric disturbances. Challenges include mitigating chromatic aberration and controlling dispersion from the metamirror 12.

[0014]

[0017] While metamirror 12 is hexagonal in the above-described embodiment, other shapes may be applicable. FIG. 3 is an isometric view of metamirror 40, similar to metamirror 12, but having a rectangular shape to illustrate that other shapes are also applicable. As described above, metamirror 40 includes a microactuator assembly 42 having a base 44 and an array 46 of microactuators 48 extending therefrom. A substrate 50 is formed on the microactuators 48 and includes a reflective surface 52. An array of metaelements 54 is formed on and extends from substrate 50.

[0015]

[0018] There are two expected effects of active mirror deformation on optical properties, including geometric optical effects and changes in light dispersion caused by displacement of the meta-mirror structure. Figure 4 shows the curved geometry of a mirror 60 with focal length f, where line 62 represents the mirror 60 when it is flat and line 64 represents the mirror 60 when it is curved; the meta-element is not shown. While the following discussion is in two dimensions, it should be understood that the mirror 60 is three-dimensional and, in this non-limiting embodiment, circular. Distances d and d I represents the relative phase, i.e., modulo the wavelength, imparted by the flat and curved lines 62 and 64 of the mirror 60, respectively, at a position r along the flat and curved lines 62 and 64 of the mirror 60, where r is assumed to be the same. These are the phase changes required to reflect a vertical ray of light impinging downward to a focal point at point O. The value t is the displacement at point r between a point on the mirror 60 when the mirror 60 is flat and the corresponding point on the mirror 60 when the mirror 60 is curved. For small displacements (micrometers), this is equivalent to a small circular arc, as shown in this example. Also, the deformation of the mirror 60 is assumed to be circular, with a radius of curvature R much larger than the diameter D of the mirror 60. In these conditions, the radius r of the mirror 60 m , and the edge t of the mirror 60 m As a function of the maximum displacement at the focal point f, it is given by:

number

[0016]

[0019] As an example, we analyze the case of a flat metamirror with a diameter of 1 cm, with a numerical aperture NA=0.2 and a focal length f=2.4495 cm. The edge t of the mirror 60 m With a maximum displacement (upward) of 10 micrometers, a focal length reduction of approximately 0.5 mm is obtained, which demonstrates significant focus control with the active optics approach discussed herein and makes the optical metamirror assembly 10 very easy to assemble and focus.

[0017]

[0020] Another important advantage of using this active optical correction approach for a segmented metamirror is that it allows relaxing the condition imposed by the constraint imposed by the maximum phase change required for a metamirror of a given diameter, where the condition is given by: D×NA×Δω<2cΔΦ (2) where D is the metamirror diameter, NA is the numerical aperture, Δω is the bandwidth, c is the speed of light, and ΔΦ is the phase change.

[0018]

[0021] By employing the physical deformation available from the actuator array, it is possible to compensate for the violation of the inequality in equation (2) that occurs when using metamirrors with significantly larger diameters. From equation (1), it is possible to determine the required change in the edge t of the mirror 60 to significantly increase the effective diameter D or bandwidth of the mirror 60, if desired, while keeping the metamirror phase profile fixed as fabricated. m For the focusing example given above, the required maximum deformation of the edge t of the mirror 60 can be calculated as mis increased to about 230 micrometers, the diameter D of mirror 60 can be increased by substantially 20%, or can span nearly the entire visible spectrum. This can enable the fabrication of metamirrors with diameters of several centimeters without using very large phase changes. Such actively corrected metamirror arrays can also be combined with computational image correction techniques to improve overall optical performance.

[0019]

[0022] The remaining effects of metamirror deformation on optical performance include expected changes in the spacing and orientation of the metamirror nanostructures. This involves coupled near-field Mie scattering of light from two or more adjacent dielectric nanostructures, including the mirror 60. These nanostructures, typically rectangular dielectric structures up to 1 μm in height, are expected to have their relative spacing and orientation modified by the curvature of the mirror 60. The distance between elements varies by an amount close to half the ratio of the mirror thickness to the radius of curvature, depending on the elastic moduli of the various materials in the mirror 60. Approximate calculations indicate that a 10 μm displacement in a 1 cm metamirror corresponds to a 3 m radius of curvature. If the mirror 60 is 500 μm thick, a typical wafer thickness, the change in spacing is approximately 0.02%, which has a negligible effect on performance.

[0020]

[0023] The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. Those skilled in the art will readily appreciate from such discussion, the accompanying drawings, and the appended claims that various changes, modifications, and variations can be made without departing from the spirit and scope of the present disclosure, as defined in the appended claims.

Claims

1. a plurality of metamirrors, each including a substrate, a reflective layer formed on one side of the substrate, an array of optical meta-elements formed on the reflective layer, and an array of microactuators coupled to another side of the substrate opposite the reflective layer, each microactuator associated with a respective region of the meta-elements, the meta-elements in all of the metamirrors combining to focus a light beam reflected from the reflective layer to a common focal point; a controller for controlling each of the micro-actuators to control the continuous curvature of the reflective layer and to control the focusing effect of the meta-element; An optical assembly comprising:

2. The optical assembly of claim 1 , wherein each of the metamirrors includes a plurality of sub-metamirrors.

3. The optical assembly of claim 1 , wherein the controller varies a length of the micro-actuator to curve the reflective layer.

4. The optical assembly of claim 3 , wherein the microactuator is a piezoelectric element.

5. The optical assembly of claim 1 , wherein at least some of the reflective layers of the metamirror are pre-tilted with respect to each other.

6. The optical assembly of claim 1 , which is part of a telescope.

7. The optical assembly of claim 1 , wherein the plurality of metamirrors are arranged together in a tiled configuration.

8. The optical assembly of claim 7 , wherein the metamirror has a hexagonal shape.

9. An optical assembly comprising a plurality of metamirrors, each metamirror including a substrate, a reflective layer formed on one side of the substrate, an array of optical metaelements extending from the reflective layer, and an actuator assembly having an array of microactuators coupled to another side of the substrate opposite the reflective layer, each of the microactuators corresponding to a respective region of the metaelement, and each of the microactuators being controlled to control the orientation of the metamirror and the continuous curvature of the reflective layer, thereby controlling how the combination of the metaelements focuses a light beam reflected from the reflective layer.

10. The optical assembly of claim 9 , wherein the microactuator is a piezoelectric element.

11. The optical assembly of claim 9 , wherein at least some of the substrates of the metamirror are pre-tilted with respect to one another.

12. The optical assembly of claim 9 , wherein the plurality of metamirrors are arranged together in a tiled configuration.

13. The optical assembly of claim 12 , wherein the metamirror has a hexagonal shape.

14. 10. The optical assembly of claim 9, which is part of a telescope.

15. A metamirror comprising a substrate, a reflective layer formed on one side of the substrate, an array of optical meta-elements formed on the reflective layer, and an array of actuators coupled to another side of the substrate opposite the reflective layer, wherein each of the actuators corresponds to a respective region of the meta-elements, the meta-elements combine to focus a light beam reflected from the reflective layer to a focal point, and each of the actuators is controlled to control the orientation of the meta-mirror and the continuous curvature of the reflective layer, thereby controlling how the meta-elements focus the light beam.

16. The meta-mirror of claim 15 , wherein the array of actuators comprises an array of micro-actuators, the lengths of which are controlled to cause the reflective layer to bend.

17. The meta-mirror of claim 16 , wherein the microactuator is a piezoelectric element.

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