THz ultra-strong light-matter coupling up to 200K with continuously-graded parabolic quantum wells

TL;DR

This paper demonstrates the achievement of ultra-strong light-matter coupling at terahertz frequencies up to 200K using graded quantum wells, enabling potential advancements in quantum vacuum radiation and few-electron polaritonic systems.

Contribution

It introduces a novel use of graded parabolic quantum wells to sustain ultra-strong coupling at higher temperatures and in sub-wavelength resonators, surpassing previous limitations.

Findings

Ultra-strong coupling achieved up to 200K.

Microcavity intersubband polaritons at 1.8 THz.

Fewer than 3000 electrons per resonator are strongly coupled.

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 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 200K. 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 $2.10^{-6} \lambda_0^3$, 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.