On-chip integration of metasurfaces with optoelectronic components enables the manufacture of reliable and low-cost systems that do not require post-fabrication alignment and assembly. Here we present an on-chip miniature beam scanning metasystem by integrating a large 2D array of VCSELs and a metalens. The beam scanner measures 4 mm*4 mm*3 mm and scans a ~1 mW laser beam at 940 nm with <1 degree divergence angle over a 140 degree*140 degree field of view with <30 mW of power consumption. The scanner switches between two angles in <1 µs, enabling the realization of fully-integrated miniature imaging lidar systems.
KEYWORDS: Vertical cavity surface emitting lasers, LIDAR, Three dimensional sensing, Temperature metrology, Sensing systems, Oxidation, Near field optics, Near field, Internet, Consumer electronics
Lumentum’s short pulse single-junction VCSEL arrays are currently being deployed in Time of Flight (ToF) sensors for short-range Light Detection and Ranging (LIDAR) applications in consumer electronics. However, single-junction VCSEL devices have a slope efficiency (SE) of only around 1.1 W/A making them unsuitable for longer range, higher power applications. With the rise of autonomous vehicle market, there is a need for ultra-short pulse, high peak power VCSELs for medium to long-range LIDAR systems. Multi-junction VCSELs are ideal candidates for this segment. The multi-junction VCSEL consists of stacked gain regions connected by highly doped tunnel junctions. An electron which radiatively recombines with a hole in one gain region, generating a photon, can then tunnel back into the conduction band via the highly doped region and is again available to generate another photon in the subsequent gain region, and so on. The total photons or output power scales with the number of junctions in the device. The optical power density and chip size are critical parameters of a LIDAR module. In this paper, we report the results of compact multi-junction arrays capable of delivering very high peak power.
We describe a high throughput approach to all-inorganic metalens manufacturing using a single step nanoimprint lithography process and titania nanoparticle-based inks. The process yields a refractive index of 1.9, lenses with critical dimensions below 60 nm, feature aspect ratios greater than 8, and efficiencies greater than 55% and consistent device performance across 15 lenses printed within 30 minutes. We further describe pathways to fabricating all-inorganic lenses with RI of 2.1.
We present a technique for designing efficient and robust metasurfaces that optimizes the metasurface design curves instead of individual metasurface elements and thus does not suffer from the size limitations of conventional optimization techniques. Spatially varying design curves are parametrized and optimized using the grating averaging technique. We present simulation and experimental results of highly efficient metasurface beam deflectors and lenses that are robust to fabrication errors. In particular, we present an 80° beam deflector with absolute efficiency of 75% and a metalens with NA of 0.8 and an efficiency of 86% that is robust to fabrication errors.
I will present different techniques developed to design and fabricate visible and near-infrared metasurface with large dimensions. In particular, I will present metasurface design using adjoint optimization and introduce a novel technique for designing arbitrarily large metasurfaces using optimized smaller metasurfaces. I will also discuss metasurface design using grating averaging and nonlinear optimization. Scalable and low-cost fabrication of large diameter visible metasurfaces using different nanoimprint techniques will also be presented.
The recent development of efficient dielectric metasurfaces has enabled practical optical components and systems composed of multiple cascaded metasurfaces. In this talk, I present an overview of our work on modeling, design, and implementation of cascaded metasurface components and systems. In particular, I present accurate system-level models for metasurfaces, techniques for designing efficient metasurfaces, multifunctional cascaded metasurfaces, and bilayer birefringent metasurfaces that provide the ultimate control over the wavefront and polarization of light. Furthermore, I will introduce a novel technique for engineering chromatic dispersion by cascading and briefly discuss a single-snapshot hyperspectral imager enabled by cascading multiple metasurfaces.
We present a new class of grating-integrated microdisk resonators that directly and efficiently couple to free space and can be excited by top illumination. We discuss the theory and design of such devices and present characterization results of 1530-nm-resonators with 0.8 µm to 1.2 µm radii, which are fabricated using amorphous silicon on glass. A 1.2-µm-radius resonator has a measured Q of ~16,000 and is efficiently excited by top illumination as evidenced by an observed thermally-induced bistability threshold of 0.7 mW. The small footprint and ease of coupling enable dense resonator arrays for applications in free space and flat optics.
Standard designs for dielectric metasurfaces suffer from significant chromatic dispersion, impeding their use in broadband systems. We present a fundamental relation between ray trajectories in an optical system and its chromatic dispersion, and describe an associated design procedure to create cascaded optical systems with arbitrary dispersion. We use this procedure to design cascaded metasurface systems with various dispersive characteristics, including an achromatic metalens exploiting the orbital angular momentum of light. As experimental validation, we demonstrate beam deflectors exhibiting several different chromatic dispersions.
Multifunctional metasurfaces perform different functions depending on the wavelength, polarization, or wavefront of the incident light. Designing such metasurfaces require more degrees of freedom (DOF) than what is available in a single layer metasurface, and stacking metasurface layers is one of the approaches for achieving the required DOF for realizing multifunctional metasurfaces. In the conventional metasurface design technique used for designing single layer metasurfaces, the couplings among the meta-atoms are ignored; however, the meta-atoms in multi-layer metasurfaces exhibit significant mutual couplings and multiple scattering phenomena are not negligible. As a result, multi-layer metasurfaces designed using the conventional techniques have low efficiencies. In this talk, we will present an inverse design technique that is suitable for designing efficient large-scale multi-layer metasurfaces. The method is based on a combination of the gradient descent optimization and the adjoint sensitivity techniques and is used to design efficient parametrized multifunctional metasurfaces. The design of multifunctional metasurfaces is cast as a multi-objective optimization problem and the optimal values of meta-atom geometrical parameters are found through an iterative approach. The sensitivities of the objective function and the metasurface response are computed using full-wave simulations; therefore, the mutual interactions and the multiple scattering effects are accurately considered. To demonstrate the effectiveness of the method, we present a bi-layer double-wavelength metasurface composed of more than 2,000 amorphous silicon nano-posts that are embedded in silicon dioxide and arranged in two stacked layers. The bi-layer metasurface projects two different patterns with more than 65% efficiency when illuminated with two different wavelengths.
Miniaturized optical systems with planar form factors and low power consumption have many applications in wearable and mobile electronics, health monitoring devices, and as integral parts of medical and industrial equipment. Flat optical devices based on dielectric metasurfaces introduce a new approach for realization of such systems at low cost using conventional nanofabrication techniques. In this talk, I will present a summary of our recent work on dielectric metasurfaces that enable precise control of both polarization and phase with large transmission and high spatial resolution. Optical metasurface components such as high numerical aperture lenses, efficient wave plates, components with novel functionalities, and their potential applications will be discussed. I will also present the results of our efforts on optimizing and increasing the diffraction efficiency of metasurfaces. Furthermore, by using metasurface cameras and planar retroreflectors as examples, I will discuss a vertical on-chip integration platform that introduces a new architecture for the on-chip integration of conventional and novel optical systems and enables their low-cost manufacturing.
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