Eliminating Nearfield Coupling in Dense High Quality Factor Phase Gradient Metasurfaces.

Abstract

High Q phase gradient metasurfaces are promising for revolutionizing light manipulation, but near-field coupling typically forces a trade-off between quality factor and resolution. Here, a strategy for eliminating coupling-based nonlocal effects in wave shaping metasurfaces composed of meta-pixels is presented with arbitrarily long resonant lifetimes arranged with sub-diffraction spatial resolution. By working at a zero-coupling regime introduced by the interference between enhanced longitudinal and transverse electric fields, the tradeoff between Q and resolution no longer exists. Numerical demonstrations show that metasurfaces with quality factors of a few million and resolution <λ/1.6 can produce beam-splitting to angles of ±53° and beam-steering to an angle of 33° with diffraction efficiencies over 90% via refractive index modulations of just 2 × 10^-6 and 7 × 10^-6, respectively. Experimentally, the signature of a zero-coupling regime is discovered in the form of a sign flip in the angular dispersion with resonant wavelength, which validates the scheme. Aside from triangulating a perfect decoupling configuration, one of the fabricated nanofin-isolated metasurfaces with Q-factor >870 has a resonant wavelength that stays within the half linewidth for incident angles of -20° to 20°. This platform paves the way for combining precise wavefront shaping with highly efficient nonlinearity and rapid programmability.