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

摘要

Continuously graded parabolic quantum wells with excellent optical performancesare used to overcome the low-frequency and thermal limitationsof square quantum wells at terahertz (THz) frequencies. The formation ofmicrocavity intersubband polaritons at frequencies as low as 1.8 THz is demonstrated,with a sustained ultra-strong coupling regime up to a temperatureof 200 K. Thanks to the excellent intersubband transition linewidth, polaritonspresent quality factors up to 17. It is additionally shown that the ultra-strongcoupling regime is preserved when the active region is embedded in subwavelengthresonators, with an estimated relative strength η = Ω_R/ω_0 = 0.12.This represents an important milestone for future studies of quantumvacuum radiation because such resonators can be optically modulated atultrafast rates, possibly leading to the generation of non-classical light via thedynamic Casimir effect. Finally, with an effective volume of 2 ×10~(-6) λ_0~3, it isestimated that fewer than 3000 electrons per resonator are ultra-strongly coupledto the quantized electromagnetic mode, proving it is also a promisingapproach to explore few-electron polaritonic systems operating at relativelyhigh temperatures.