Polarization-Controlled Nanogap Cavity with Dual-Band and Spatially Overlapped Resonances

Metasurfaces are ultrathin, two-dimensional arrays of sub-wavelength resonators, which can possess optical properties unobtainable naturally. One such desirable property is dual-band absorption occurring at the same spatial location, which could enable the enhancement of multiple processes simultaneously. However, demonstrations of multiband absorption to date have been limited to the mid-infrared and microwave regimes. Here, metasurfaces are demonstrated to have spatially overlapped, dual-band absorption in the visible to near-infrared using arrays of plasmonic nanogap cavities that consist of a sub-10 nm dielectric layer sandwiched between gold rectangles and a gold film. The relative strength of the two modes is tuned dynamically by varying the incident polarization, and the period between elements is used to tune the spectral bandwidth. Additionally, these near-perfect absorber structures fabricated by electron beam lithography (EBL) compared to nanogap geometries fabricated using colloidally synthesized nanoparticles. This comparison reveals that EBL nanogap structures can achieve similar absorption performance to colloidal nanoparticles while enabling a much greater control of the nanoparticle shape, size, and relative position. These widely tunable, dual-band metasurfaces may find applications in spectrally multiplexed photodetectors and for enhancement of optical processes, such as nonlinear generation and Stokes-shifted absorption and emission processes.