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 …