8:15 AM - EL01.21.02
Diffraction Control with High Q Phase Gradient Metasurfaces for Nonlinear Freespace Optics
Mark Lawrence1,David Barton1,Jefferson Dixon1,Jung-Hwan Song1,Jorik Van de Groep1,Mark Brongersma1,Jennifer Dionne1
Photonic micro-structures, including microring resonators, photonic crystal defect cavities, and whispering gallery resonators, have been by far the most successful platforms for boosting light matter coupling, which is largely thanks to their huge Quality (Q) factors, spanning thousands to millions. The associated long photon residence times and enormous amplification of the local light intensity has led to highly-efficient lasing, frequency comb generation, optical signal modulation, optical isolation, quantum generation, and single molecule biosensing. At the same time, nanoantennas provide an unprecedented level of control over the scattering of freespace optical signals, especially when arranged into metasurface arrays, enabling flat optical elements such as beam steerers, lenses, and holograms. However, researchers typically face a trade-off between antenna size in relation to wavelength and resonant lifetime, with subwavelength structures limited to quality factors (Q) less than 100, keeping more exotic nonlinear phenomena out of reach without the use of extremely high power femtosecond lasers.
By utilising Mie and guided mode resonance (GMR) to control the scattering profile of high Q, Q>1000, resonant nanostructures, here, we demonstrate experimentally that this trade off is not in fact fundamental. We show that GMR in ultrathin dielectric metasurfaces can be employed to bring strong field amplification to a wide class of freespace nanophotonic systems, producing efficient optical nonlinearities in a nanoscale footprint. Combining wavefront shaping with subtle structural symmetry breaking, arbitrary freespace scattering can be achieved alongside GMRs with lifetimes that can be increased almost indefinitely. As a proof of principle, we provide the first theoretical and experimental demonstration of two high Q phase gradient metasurface functions, one capable of efficiently steering an infrared plane wave to a predetermined angle and another splitting a plane wave into two beams, with both devices supporting GMR Q factors greater than 1000.
The metasurfaces were patterned into a silicon on sapphire wafer with a 600nm silicon layer, using electron beam lithography followed by reactive ion etching. To realize the different metasurface phase profiles, nanowires of varying widths were arranged within a 2121nm supercell. Subwavelegnth periodic notches were also etched into particular nanowires, giving rise to GMRs with Q factors controlled via notch depth. Using a home built angle resolved microscope coupled to a grating spectrometer, we report efficient beam steering and beam splitting between 1350-1500nm, accompanied by GMRs with Q factors as high as 2500. To the best of our knowledge, this is the highest Q factor observed to date in a phase gradient device. We have numerically confirmed that the huge electric fields associated with these resonances can excite efficient nonlinearities, including the Kerr effect and stimulated Raman scattering, opening the possibility for novel functionalities such as subwavelength nonreciprocity. While our proof of principle demonstrations involve beam steering and beam splitting, the design principle we present could easily be extending to other types of wavefront shaping applications, such as lensing.
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