N24B-T030 TITLE: Wide Field-of-View, Compact Compound Meta-lenses for Visible-to-Near-Infrared Spectral Range and with 100X Size and Weight Reduction

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Advanced Materials; Microelectronics; Quantum Science

OBJECTIVE: Develop a novel, extremely compact, and lightweight compound lens composed of multiple metasurfaces that permits an ultrawide field of view (FOV) for various imaging and surveillance applications in the visible and near-infrared spectral ranges.

DESCRIPTION: Wide-angle compound lenses, that can provide expanded FOV and keep scenes near and far in focus (large depth of focus), are important for military applications, such as surveillance and vision-based navigation. However, wide-angle lenses are notoriously difficult to create because they have relatively short focal lengths and relatively large lens components, compared to other types of compound lenses. To form images of scenes over a large solid angle while minimizing monochromatic aberrations, existing solutions typically utilize a large stack of aspherical refractive lenses. Even with sophisticated designs, wide-angle of view cameras with FOVs between 60°–110° often require mechanically moving components in order to provide a more comprehensive angular coverage.

It is the focus of this STTR topic to seek a much more promising disruptive technological solution to mitigate the legacy technology shortfalls of size, weight, and robustness issues of wide-angle of view cameras by exploring a wide-angle of coverage compound lens based on metasurface technology. Metasurfaces have recently emerged as a promising platform to realize advanced imaging functionalities [Ref 1]. A metasurface enables a designer to control light by exploiting strong interactions between light and 2D nanostructured thin films [Ref 2]. A metasurface is usually composed of a 2D array of densely-packed, nanoscale optical scattering elements ("meta-units"). The geometric degrees of freedom in the meta-units allow a designer to control a multitude of optical parameters, including the phase delay, amplitude, and polarization state. Therefore, a metasurface can engineer the optical wavefront in a predetermined fashion for the specific applications via the collective action of a 2D array of meta-units. Compared to a simple interface between two optical media, metasurfaces have superb capabilities to bend light beams by large angles with high efficiency [Ref 3], which makes them ideally suited for creating wide-angle of coverage and extremely compact imaging optics. In addition, dispersion engineering of meta-units allows metasurfaces to provide distinct phase profiles at different wavelengths [Ref 4], making it possible to create compound lenses that operate simultaneously at visible and near-infrared spectral bands for various daytime and nighttime operating conditions. The flat form factor of game changing metasurfaces can substantially decrease the weight of optical systems to as small as ~1 % of that of conventional systems based on traditional bulky refractive lenses. Metasurfaces can be fabricated with mature planar wafer-scale fabrication technologies pioneered by the semiconductor industry. That metasurface fabrications can leverage semiconductor manufacturing technology and its concomitant economy of scale represents a revolutionary improvement in low-cost scalable production, a marked departure from the very time consuming and costly legacy grinding and polishing processes currently used for lens manufacturing.

Despite their unique merits as an imaging platform, metasurfaces must overcome a couple of challenges to provide usable performance as wide-angle of coverage imaging optics. Metasurfaces rely on a spatial distribution of phase delay introduced by 2D arrays of subwavelength meta-units. The latter are typically designed without considering the near-field coupling between neighboring meta-units. In reality, a meta-unit is surrounded within subwavelength distances by distinct meta-units and the near-field interactions between them via optical evanescent waves can substantially perturb the local phase and amplitude responses of the meta-unit. This will lead to a deviation from the desired phase and amplitude profiles and could thus severely reduce the transmission efficiency of light through the metasurfaces and degrade the quality of the formed images. In addition, typical meta-unit designs assume light incidence at a normal angle to the metasurface plane; however, the angular optical response of meta-units can be far from that of a simple point source: the optical modes excited within a meta-unit vary as a function of incident angle, which will result in angularly dependent local phase and amplitude responses, with the ultimate consequence that a metasurface lens designed for normal incidence will fail to function properly at oblique incident angles. Thus, this STTR topic seeks an advanced design methodology where the near-field interactions and angular response of meta-units are taken into consideration while modeling optical response of metasurfaces [Ref 5] and an efficient algorithm is devised to determine the optimal arrangement of meta-units over the metasurface plane to minimize phase and amplitude errors due to near-field coupling over a wide range of incident angles.

Specifically, the meta-lens system should satisfy the following criteria: (a) for a collimated incident beam at a near-infrared wavelength (lambda = 940 nm) over an angular range of 50° (i.e., 100° FOV) in both the transverse directions, the focal spot produced by the meta-lens system should be diffraction limited (Strehl ratio > 0.8); (b) optical transmission through the meta-lens system should be higher than 75 %; (c) maximizing the focusing efficiency at the design wavelengths ranging from 450 nm to 750 nm in steps of 50 nm; (d) the first meta-lens layer of the system (i.e., optical aperture) should have a diameter of 1 mm, the focal distance (between the last meta-lens layer and the focal plane) should be 1 mm, and the entire meta-lens system should be less than 5 mm in thickness; and (e) the weight of the meta-lens system should be below 100 mg. The focusing efficiency, defined as the ratio of the integrated power over a circular aperture with diameter 18 µm (microns) in the focal plane to the total power over the lens aperture as a function wavelength.

Specifically, the camera system equipped with the meta-lens system should satisfy the following criteria: (a) for a collimated incident beam at three visible wavelengths (lambda=450, 550, and 650 nm) and one near-infrared wavelength (lambda=940 nm) over an angular range of 60° in both the transverse directions, the meta-lens should provide the same focal length and the focal spots should be diffraction limited (Strehl ratio > 0.8); (b) optical transmission through the meta-lens system should be higher than 85 % at the near-infrared wavelength and higher than 75 % at the visible wavelengths; (c) maximizing the focusing efficiency at the design wavelengths ranging from 450 nm to 750 nm in steps of 50 nm; (d) the first meta-lens layer of the system should have a diameter of 5 mm, the focal distance at both the visible and near-infrared wavelengths should be 2 mm, and the entire meta-lens system should be less than 7 mm in thickness; (e) the weight of the meta-lens alone should be below 500 mg; and (e) resolution of the camera should be at least 10 MP. The focusing efficiency, defined as the ratio of the integrated power over a circular aperture with diameter 18 µm (microns) in the focal plane to the total power over the lens aperture as a function wavelength.

In summary, this STTR topic seeks a solution to create wide-angle of coverage meta-lenses based on metasurfaces that offer the highest quality wide FOV lens for surveillance high-definition charged-coupled device (CCD) or Complementary metal–oxide–semiconductor (CMOS) cameras but with two orders of magnitude reduction in size and weight.

PHASE I: Demonstrate the efficacy of the new metasurface design methodology and the feasibility of a compound wide-FOV meta-lens system as described in the Description. Demonstrated quantitative agreement between the optical model and experiment, and completed a trade-space analysis that identified the optimal method for meta-lens system. Characterize component meta-atoms used in meta-lens system design. The Phase I effort should include prototype plans to be developed under Phase II.

PHASE II: Design, build prototypes, and demonstrate a high-definition CCD or CMOS camera system integrated with a compound wide-FOV meta-lens with dispersion engineered meta-units as described in the Description under natural sunlight and other broadband illumination conditions in Advanced Naval Technology Exercise (ANTX) events. Produce a final report that includes a discussion of potential near-term and long-term development efforts that would improve technology performance and/or ease of fabrication; and also an evaluation of the cost of fabrication and how that might be reduced in the future. The prototypes should be delivered by the end of Phase II.

PHASE III DUAL USE APPLICATIONS: Design and demonstrate a producible metalens camera from Phase II and validate its resulting manufacturing readiness to be transitioned to a Program of Record. Support the Navy in transitioning the technology to Navy use.

The development of the optoelectronic image sensor has been a significant step towards the on-chip integration of cameras; however, the camera lenses are yet to be fully integrated with the image sensor. The freedom in controlling the metasurface phase profiles has enabled the implementation of spherical-aberration-free flat lenses that focus normally incident light to diffraction limited spots. Metasurface flat lenses are able to correct chromatic aberration over broad wavelength range, and to some degree reduce spherical aberration, coma, and other monochromatic aberrations, that would most likely revolutionize optical instrumentation. The prospect of largely shrinking the complexity and size of optical instruments (e.g., replacing the entire set of compound lenses in a camera lens with a few or even a single dispersion less and aberration-corrected flat lens) seems feasible in view of recent developments of metasurface lenses. Metasurface flat lenses will impact computational imaging, active wavefront manipulation, ultrafast spatiotemporal control of light, quantum communications, thermal emission management, novel display technologies, and sensing.

REFERENCES:

1. Yu, N. and Capasso, F. "Flat optics with designer metasurfaces." Nature materials, 13(2), 2014, pp. 139-150. https://doi.org/10.1038/nmat3839
2. Yu, N.; Genevet, P.; Kats, M. A.; Aieta, F.; Tetienne, J. P.; Capasso, F. and Gaburro, Z. "Light propagation with phase discontinuities: generalized laws of reflection and refraction." Science, 334(6054), 2011, pp. 333-337. https://doi.org/10.1126/science.1210713
3. Sell, D.; Yang, J.;Doshay, S.; Yang, R. and Fan, J. A. "Large-angle, multifunctional metagratings based on freeform multimode geometries." Nano letters, 17(6), 2017, pp. 3752-3757. https://doi.org/10.1021/acs.nanolett.7b01082
4. Shrestha, S.; Overvig, A. C.; Lu, M.; Stein, A. and Yu, N. "Broadband achromatic dielectric metalenses." Light: Science & Applications, 7(1), 2018, pp. 1-11. https://doi.org/10.1038/s41377-018-0078-x
5. Zhou, M.; Liu, D.; Belling, S. W.; Cheng, H.; Kats, M. A.; Fan, S.; Povinelli, M. L. and Yu, Z. "Inverse design of metasurfaces based on coupled-mode theory and adjoint optimization." ACS Photonics, 8(8), 2021, pp. 2265-2273. https://doi.org/10.1021/acsphotonics.1c00100

KEYWORDS: metalenses; spectral; metasurfaces; imaging; surveillance; visible; near-infrared

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