Thz Ultrastrong Coupling In An Engineered Fabry-Perot Cavity

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. Q factors up to 89 are observed for the first mode at similar or equal to 300 GHz, with cooperativity C = 56.4 and a normalized coupling ratio Omega/omega in the first mode of up to 17.5% and the third mode of up to 6.4%.