Characterization of gap-plasmon based metasurfaces using scanning differential heterodyne microscopy.

Optical phase-gradient metasurfaces, whose unique capabilities are based on the possibility to arbitrarily control the phase of reflected/transmitted light at the subwavelength scale, are seldom characterized with direct measurements of phase gradients. Using numerical simulations and experimental measurements, we exploit the technique of scanning differential heterodyne microscopy (SDHM) for direct phase and amplitude characterization of gap-plasmon based optical metasurfaces. Two metasurface configurations utilizing the third-order gap surface plasmon (GSP) resonance, representing a binary grating and linear phase gradient, are experimentally characterized with the SDHM operating at the light wavelength of 633 nm. Comparing the experimental performances of these GSP metasurfaces with those expected from the phase and amplitude profiles reconstructed from the SDHM measurements, we verify the efficiency and accuracy of the developed SDHM characterization approach for direct inspection of GSP reflective metasurfaces.

Dual-Band Metasurfaces Using Multiple Gap-Surface Plasmon Resonances.

Metasurfaces operating at multiple spectral ranges with integrated diversified functionalities while retaining the flexible design strategy are highly desired within the area of modern flat optics. Here, we propose and demonstrate the use of multiple gap-surface plasmon (GSP) resonances for the realization of dual-band multifunctional metasurfaces by designing GSP meta-atoms that would resonate at two different wavelengths. By tailoring nanobrick dimensions of a simple GSP meta-atom so as to enable both the first-order resonance at 1450 nm and the third-order one at 633 nm, we design phase-gradient GSP metasurfaces for polarization-independent beam steering and polarization-splitting, simultaneously, at telecom (1350-1550 nm) and visible (575-675 nm) wavelengths. The fabricated metasurfaces show good performance with >65% diffraction efficiency at the first-order resonant wavelength of 1450 nm and over 50% efficiency within the telecom range of 1350-1550 nm, while at the third-order resonant wavelength of 633 nm, the diffraction efficiency is 20 and >10% within the visible range of 575-675 nm. Our findings, therefore, demonstrate a flexible and robust approach for the realization of efficient dual-band GSP metasurfaces that can readily be combined with complex integrated designs to implement multiple functionalities highly sought after for diverse applications.