Silicon Meta-optics Enabling Edge Detection and Varifocal Thermal Imaging

Abstract

Optical metasurfaces have emerged as an exciting small form factor alternative to traditional refractive imaging systems due to their ability to spatially control amplitude, phase, and polarization of incident light. Propagation phase metasurfaces employ sub-wavelength meta-atoms of varying diameters for wavefront control by inducing specific phase delay spatially across the device. Spatial design freedom gives rise to point spread function (PSF) engineering, which can perform information encoding of incoming light. Specifically, I discuss an optimized broadband Laplacian-of-Gaussian edge detector metasurface, which uses birefringent meta-atoms to create PSFs in both polarization directions. I examine testing results captured with a long wave infrared (LWIR) camera, which detects large amounts of signal from objects between 0-150°C. In the future, similar edge detection metasurfaces could serve as a preprocessing step for computer vision neural networks, minimizing computational power requirements. Computer vision object recognition over this temperature range has applications in medical imaging, autonomous vehicles, and security. Next, I review the design and creation of a Moiré metalens system, which provides continuous focal length adjustment in the LWIR. Alignment of two Moiré metasurfaces supplies tunable varifocal imaging via rotation of one metasurface. This system eliminates traditional axial translation in varifocal imaging systems, significantly decreasing the form factor for applications in endoscopy and compact device imaging. A newly developed silicon photolithography and etching method was used to fabricate metasurfaces for both projects. I explore the potential for high-aspect ratio meta-atoms with this new process.