  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.
