Ultrathin Metasurface Wavelength-Selective Mirror for Millimeter/Terahertz Wave Fabry-Perot Cavities

We present an ultrathin metasurface working as both a highly reflective wavelength-selective mirror and a high transmittance output coupler for a semiconfocal Fabry-Perot cavity. The numerically optimized 2-μ m-thick metasurface achieves a peak reflectivity of R = 99.35% at ω0 = 94 GHz and peak transmittance of T ≈ 90% at the fourth harmonic (4ω0). Experimental results show that the metasurface FP cavity exhibits comparable quality factors to cavities enclosed by either a copper mirror or Bragg grating mirror. Our work highlights the potential for ultrathin metasurfaces to function as frequency selective mirrors in millimeter or terahertz wave cavities for various applications, such as nonlinear terahertz wave generation, material characterization, and imaging.