Ultrastrong Coupling of Si1–xGex Parabolic Quantum Wells to Terahertz Microcavities

Control and manipulation of quantum states by light are increasingly important for both fundamental research and applications. This can be achieved through the strong coupling between light and semiconductor devices, typically observed at THz frequencies in 2D electron gases embedded in lithographic optical cavities. Here, we explore the possibility of achieving ultrastrong coupling between conduction sub-band states in Si1–xGex heterostructures and THz cavity photons fabricated with a potentially silicon-CMOS-compliant process. We developed Si1–xGex parabolic quantum wells with a transition at ω0 = 3.1 THz and hybrid metal-plasmonic THz patch-antenna microcavities resonating between 2 and 5 THz depending on the antenna length. In this first demonstration, we achieved anticrossing around 3 THz with spectroscopically measured Rabi frequency ΩR ≃ 0.7 THz (ΩR/ω0 ≃ 0.2, i.e., ultrastrong coupling). The present group-IV semiconductor material platform can be extended to the 5–12 THz range, where these semiconductors are transparent, as opposed to the III–V compound semiconductors plagued by strong THz optical phonon absorption. Moreover, the intersubband transition in parabolic quantum wells hosted by the nonpolar Si1–xGex crystal lattice is robust against carrier density and temperature variations, making the strength of the coupling only weakly temperature-dependent from 10 to 300 K. These results pave the way for the employment of the Si1–xGex material platform to perform fundamental research in ultrastrong light–matter coupling, fully exploiting the plasmonic character of the cavity mirror, as well as in ultrafast modulators and saturable absorbers for THz laser research.