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

• Published in: Advanced optical materials 2023-05, Vol.11 (9), p.n/a
• Main Authors: Goulain, Paul, Deimert, Chris, Jeannin, Mathieu, Pirotta, Stefano, Pasek, Wojciech Julian, Wasilewski, Zbigniew, Colombelli, Raffaele, Manceau, Jean‐Michel
• Format: Article
• Language: English
• Subjects: Condensed Matter; Coupling; Electrons; High temperature; intersubband; Intersubband transitions; Materials science; metamaterials; Optics; Physics; Polaritons; Quantum theory; Quantum wells; Resonators; terahertz
• ISSN: 2195-1071; 2195-1071
• DOI: 10.1002/adom.202202724

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 = ΩR/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 2×10−6λ03$2{\bm{ \times }}{10^{{\bm{ - }}6}}\lambda _0^3$, 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. By faithfully implementing continuously graded alloy semiconductor, a stack of doped parabolic quantum wells is grown. It overcomes the classic limitation of square quantum wells and shows excellent optical performances in terms of linewidth and temperature operation. When placed at the vicinity of sub‐wavelength resonators, it operates in the ultra‐strong coupling regime up to 200 K.