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

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 eta = Omega(R)/omega(0) = 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 2x10(-6)lambda(3)(0), 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.

Automated summary

Continuously graded parabolic quantum wells are used to overcome the limitations of square quantum wells at terahertz frequencies. Microcavity intersubband polaritons are formed at frequencies as low as 1.8 THz, with ultra-strong coupling sustained up to 200 K. The use of sub-wavelength resonators preserves the ultra-strong coupling regime, making it a potential approach for generating non-classical light.