Ultrastrong Light–Matter Coupling in Deeply Subwavelength THz LC Resonators

The ultrastrong light–matter coupling regime has been demonstrated in a novel three-dimensional inductor–capacitor (LC) circuit resonator, embedding a semiconductor two-dimensional electron gas in the capacitive part. The fundamental resonance of the LC circuit interacts with the intersubband plasmon excitation of the electron gas at ω_c = 3.3 THz with a normalized coupling strength 2Ω_R/ω_c = 0.27. Light–matter interaction is driven by the quasi-static electric field in the capacitors and takes place in a highly subwavelength effective volume V_eff = 10^–6λ₀³. This enables the observation of the ultrastrong light–matter coupling with 2.4 × 10³ electrons only. Notably, our fabrication protocol can be applied to the integration of a semiconductor region into arbitrary nanoengineered three-dimensional meta-atoms. This circuit architecture can be considered the building block of metamaterials for ultralow dark current detectors.