Electro-optically tunable active metasurfaces that enable dynamic modulation of reflection amplitude, phase, and polarization using resonantly excited materials and phenomena are powerful design elements for meta-imaging and computation. As flat, low-profile optical elements, active metasurfaces have potential serve as cascadable, programmable components in optical meta-imaging systems, such as lens-less cameras and single-photon imaging systems. Active metasurfaces that enable dynamic complex index modulation to vary the amplitude, phase and polarization have been recently explored using several active materials and modulation phenomena, including carrier index in plasmonic ENZ structures, reorientation of liquid crystal molecules, electrooptic effects in quantum well heterostructures and polar perovskite materials, as well as index changes in phase change materials. Recently also, metasurfaces that employ dielectric phase-gradient elements with high quality factor non-local as well as local resonances have expanded the design space for active metasurfaces by enabling silicon and other more conventional dielectric materials with modest values of index change to be utilized. We can develop a taxonomy for active metasurfaces based on the attainable degree of spatial and temporal control. The spatial phase gradient arising from phase-reconfigurable array elements can enable continuous phase gradient tuning for beam steering or varifocal lensing. The degree of temporal control is connected to the reconfiguration timescale: In quasistatic metasurfaces, temporal gradients are slow compared to the period of electromagnetic waves, while time-modulated structures feature temporal gradients modulated fast enough to alter the frequency of the scattered beam. This opens the possibility to frequency multiplex scattered beams from active metasurfaces to facilitate harmonic beam steering in which a single active aperture steers scattered beams for different frequencies at independent angles. In this talk, I will discuss metasurfaces with high quality factor, local, resonant elements capable of two-dimensional phase gradient generation, in both passive and active metasurface designs. I will also describe active metasurfaces with both spatial and temporal phase gradients, and an active metasurface as a lens-less imaging system, and compare the characteristics to conventional lens-coupled image sensors.
Harry Atwater is the Otis Booth Leadership Chair of the Division of Engineering and Applied Science, and the Howard Hughes Professor of Applied Physics and Materials Science at the California Institute of Technology. Atwater’s scientific effort focuses on nanophotonic light-matter interactions. His work spans fundamental nanophotonic phenomena and applications, including active wavefront shaping of light using metasurfaces, optical propulsion of lightsails, quantum and 2D nanophotonics as well as solar energy conversion, on earth and in space. Atwater was an early pioneer in nanophotonics and plasmonics and gave a name to the field of plasmonics in 2001. He is Chair of the LightSail Committee for the Breakthrough Starshot program. Currently Atwater is also the Director for the Liquid Sunlight Alliance (LiSA), a Department of Energy Hub program for solar fuels, and was also the founding Editor in Chief of the journal ACS Photonics. Atwater is a Member of the US National Academy of Engineering, a Web of Science Highly Cited Researcher, and the recipient of the 2021 von Hippel Award of the Materials Research Society.