OP: Spatial Light Modulation using Reconfigurable Phase Change Material Metasurfaces

许多新兴的应用，包括自动驾驶、增强现实面罩和无玻璃3D显示器，都依赖于光束转向。虽然大多数现有的解决方案依赖于机械移动，如在无人驾驶汽车顶部旋转光源，但此类移动部件需要大量能源，而且往往会限制可靠性和速度。没有移动部件的转向灯可以非常节能、快速，并且几乎具有无限的寿命。这种非机械光束扫描技术的核心是光学相移器：一种通过改变材料的折射率来改变光路长度的装置。遗憾的是，大多数现有材料的折射率变化很小。该项目旨在探索一种新的材料，称为相变材料，与大多数已知材料相比，它可以提供几乎1000倍的折射率变化。此外，变化是非易失性的，即，一旦材料改变，状态就保持不变。这可以降低能耗，降低控制电路的复杂性。电子界已经在探索这种材料，以制造下一代闪存。然而，这个项目研究的是这种材料的光电特性。为了进一步增强相移，该项目正在开发毛发般薄的光学结构，也被称为超表面。这些超表面由数百万个可以改变入射光的纳米结构组成，通过使这些结构不再是相变材料，光束可以被引导。除了推进当前的光束控制状态，该项目还在新型材料表征以及光学纳米结构的设计和纳米制造方面培训了一支多样化的跨学科劳动力队伍。塑造具有亚波长空间分辨率的光学波前对于具有深远科学和技术影响的各种应用(例如，在混浊、无序介质中的自适应光学和成像)和商业利益(例如，用于自主运输和像素化全息的光检测和测距)都是重要的。实现这种能力的主要技术是紧凑型光学移相器，它可以在低能量(皮焦耳)和高频(MHz)下将入射光的相位完全改变360度。现有的可调谐光学技术无法提供这一功能；机械可调谐调制器只能达到几千赫的速度，而基于液晶的调制器工作在100‘S赫兹。空间光调制器的像素大小也在几十个波长的数量级，这增加了每个像素的能量消耗。为此，该项目研究了新出现的非挥发性硫化物基相变材料和纳米光子亚表面结构，目标是创造快速、低功率的空间光调制器。准表面中的亚波长散射体通过将复杂的曲率转换为离散的空间相位轮廓，将复杂的曲率映射到平坦的、波长尺度的厚表面上。除了紧凑的尺寸和重量外，亚表面还采用半导体行业开发的成熟的、高度可扩展的纳米制造技术，采用一步光刻工艺制造。相变材料可以在相邻像素之间以最小的串扰提供大的、非易失性的折射率变化，因为只有当达到一定的阈值温度时才会发生转变。非易失性变化还可以显著简化空间光调制器的控制复杂性。该项目结合了超表面的数值电磁模拟、纳米制造以及相变材料及其相变的表征。该研究小组正在开发新的亚分子像素和亚表面结构，并表征新的非挥发性相变材料，以展示亚表面的电子重新配置。对新型相变材料及其电子重构的研究对于加深我们对这些材料的认识，为可重构光电材料的研究增添新的材料具有重要意义。通过准分子和光学谐振器增强光学相移可以从根本上揭示可调谐纳米光子结构及其设计原理的新知识。这样的设计原则可以很容易地转化为其他可调光子材料。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.