OP: Spatial Light Modulation using Reconfigurable Phase Change Material Metasurfaces

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Many emerging applications, including autonomous driving, augmented reality visors and glass-free 3D displays rely on optical beam steering. While most existing solutions rely on mechanical movements, such as rotating a light source on top of a driverless cars, such moving parts require a large amount of energy and often limits reliability and speed. Steering light without moving parts can be extremely energy efficient, fast, and with virtually an infinite lifetime. At the heart of such a non-mechanical beam scanning technology is an optical phase shifter: a device that changes the optical path length by changing the refractive index of the material. Unfortunately, the index change of most existing materials is very small. This project aims to explore a new class of materials, called phase-change materials, which can provide almost 1000 times larger index change compared to most known materials. Moreover, the change is non-volatile, i.e., once the material is changed, the state is retained. This can reduce the energy consumption, and the complexity of the control circuit. Such materials are already being explored in the electronics community to create next-generation flash memory. This project, however, studies the optoelectronic properties of this material. To further enhance the phase shift, the project is developing hair-thin optical structures, also known as metasurfaces. These metasurfaces consist of millions of nanoscale structures that can modify incident light, and by making these structures out of phase-change materials the light beam can be steered. Along with advancing the current state of optical beam steering, this project trains a diverse, interdisciplinary workforce on novel material characterization, as well as design and nanofabrication of optical nanostructures.Shaping an optical wavefront with sub-wavelength spatial resolution is important for various applications with far-reaching scientific and technological impacts (e.g., in adaptive optics and imaging through turbid, disordered media) and commercial interests (e.g., Light Detection and Ranging for autonomous transportation and pixelated holography). The primary enabling technology for such capability is a compact optical phase shifter, which can change the phase of the incident light by a full 360 degrees at low energy (pico-Joule) and high frequency (MHz). Existing tunable optical technologies cannot provide this functionality; mechanically tunable modulators can reach a speed of only a few kHz, whereas liquid-crystal based modulators operate at 100’s of Hz. The pixel size of the spatial light modulator is also on the order of tens of wavelengths, which increases the energy consumption per pixel. To that end, this project studies emerging, non-volatile, chalcogenide-based phase-change materials and nanophotonic metasurface architectures with the goal of creating fast, low-power spatial light modulators. The sub-wavelength scatterers in a metasurface enable mapping complex curvatures onto a flat, wavelength-scale thick surface by converting them into a discretized spatial phase profile. In addition to their compact size and weight, metasurfaces are fabricated using a single-step lithography procedure with mature, highly scalable nanofabrication technology developed by the semiconductor industry. Phase-change materials can provide a large, non-volatile change in their refractive index with minimal crosstalk between neighboring pixels, as the transition only happens when a certain threshold temperature is reached. The non-volatile change also can significantly simplify the control complexity of spatial light modulators. This project combines numerical electromagnetic simulation of metasurfaces, nanofabrication, and characterization of phase-change materials and their phase transitions. The research team is developing novel metamolecule pixels and metasurface architectures and characterizing new non-volatile phase-change materials to demonstrate electronic reconfiguration of metasurfaces. This research on novel phase-change materials and their electronic reconfiguration are important to enhance our understanding of these materials and add new materials to the gamut of reconfigurable optoelectronic materials. Enhancing optical phase shifts via metamolecules and optical resonators can uncover fundamentally new knowledge on tunable nanophotonic structures and their design principles. Such design principles can be easily translated to other tunable photonic materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.