Terahertz LC Microcavities: From Quantum Cascade Lasers to Ultrastrong Light-Matter Coupling

We review research on the physics of intersubband transitions in the THz range in a sub wavelength microcavity environment. Laser action was achieved at 1.5 THz by inserting quantum cascade gain material between the capacitor plates of a new resonant LC cavity, achieving a normalized mode volume ratio of only $V_{eff}/(\lambda /2n)^{3}=0.12$ of the cavity mode $V_{eff}$ and the normalized optical volume $(\lambda /2n)^{3}$ . By using the same cavity as the constituting meta-atom of a THz metamaterial, strong and ultra strong light matter coupling was observed up to room temperature. Finally, the same metamaterial coupled to parabolic semiconductor quantum wells was investigated in the regime of electrical in-plane pumping, showing THz emission in the ultra strong coupling regim