Metasurfaces have traditionally employed building blocks with physically intuitive optical responses. Many of these concepts utilize dielectric posts, which serve as waveguides and are stitched together with sufficient spacing to minimize coupling between adjacent elements. These design principles can produce single wavelength devices with good performance, but are difficult to generalize to multi-wavelength devices with exceptional performance. We show that concepts in inverse design can be used to produce dielectric metasurfaces with capabilities that exceed the current state-of-the-art. With swarm and topology optimization, we incorporate optical coupling between waveguide elements in the device designs, which yield physically non-intuitive mode profiles and coupling dynamics. To demonstrate the power and versatility of our design approach, we fabricate silicon devices that can efficiently deflect light to 75 degree angles and multi-functional devices that can steer beams to different diffraction orders based on wavelength. We also show that single crystal silicon can be used to realize efficient metasurface devices across the entire visible spectrum, ranging from 480 to 700 nanometers. Alternative forms of silicon, such as polycrystalline and amorphous silicon, suffer from higher absorption losses and do not yield efficient metasurfaces across this wavelength range. We envision that metasurfaces based on inverse design will serve as a hardware platform for the efficient routing, sorting, and manipulation of single or few photons for quantum optics applications.
Jonathan A. Fan, "High performance metasurfaces based on inverse design (Conference Presentation)," Proc. SPIE 10359, Quantum Nanophotonics, 103590G (Presented at SPIE Nanoscience + Engineering: August 08, 2017; Published: 29 September 2017); https://doi.org/10.1117/12.2275568.5593176551001.
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