THz Ultra‐Strong Light–Matter Coupling up to 200 K with Continuously‐Graded Parabolic Quantum Wells

Abstract Continuously graded parabolic quantum wells with excellent optical performances are used to overcome the low‐frequency and thermal limitations of square quantum wells at terahertz (THz) frequencies. The formation of microcavity intersubband polaritons at frequencies as low as 1.8 THz is demonstrated, with a sustained ultra‐strong coupling regime up to a temperature of 200 K. Thanks to the excellent intersubband transition linewidth, polaritons present quality factors up to 17. It is additionally shown that the ultra‐strong coupling regime is preserved when the active region is embedded in sub‐wavelength resonators, with an estimated relative strength η = Ω_R/ω₀ = 0.12. This represents an important milestone for future studies of quantum vacuum radiation because such resonators can be optically modulated at ultrafast rates, possibly leading to the generation of non‐classical light via the dynamic Casimir effect. Finally, with an effective volume of , it is estimated that fewer than 3000 electrons per resonator are ultra‐strongly coupled to the quantized electromagnetic mode, proving it is also a promising approach to explore few‐electron polaritonic systems operating at relatively high temperatures.