Sub-diffraction-limit resonators operating on the fundamental monopolar resonance: Application to THz polari tons

We demonstrate semiconductor resonators operating on the fundamental monopolar resonance which corresponds to the fundamental oscillation mode of split-ring resonators. Their size is considerably below the optical diffraction limit in all three dimensions of space. The maximum confinement is obtained for resonators operating at a wavelength of λ ~ 272 μm (v ~ 1.1 THz): they have a diameter of 13 μm and a thickness of 1 μm. This corresponds to a X_eff/6 confinement in all 3 directions of space (X_eff is the wavelength inside the material). As a proof-of-principle of cavity-QED application, we employ these resonators for intersubband polaritonics.