THz Ultrastrong Coupling in an Engineered Fabry–Perot Cavity

Abstract

We report a monolithic THz planar Fabry-Perot cavity fabricated with wafer bonding and provided with metallic mirrors with subwavelength apertures. We demonstrate its coupling to the cyclotron resonance of a high-mobility, two-dimensional electron gas. <em>Q</em> factors up to 89 are observed for the first mode at ≃300 GHz, with cooperativity <em>C</em> = 56.4 and a normalized coupling ratio ^Ω/_ω in the first mode of up to 17.5% and the third mode of up to 6.4%.