Influence of resonator design on ultrastrong coupling between a two-dimensional electron gas and a THz metamaterial

Abstract

THz metamaterials have been shown to be a promising candidate for CQED experiments, i.e. Ultrastrong coupling has been demonstrated.¹ Modes of split ring resonators (SRRs) have been coupled via the AC-electric field to inter landau level transitions in two dimensional electron gases (2DEGs). SRR typically exhibit two distinct electromagnetic modes.² One mode consists of an electric field confined in the slit of the resonator, while current in the resonator ring stores the magnetic field. This mode is often called LC-mode, in analogy to the lumped circuit representation, and has a nearly Lorentzian line shape. The other mode is a dipole mode localized physically on the resonator edges. The line shape of this resonance is more sensitive to the actual resonator geometry. We studied extensively the influence of the resonator geometry on the coupling strength. The resonances studied span the frequency range from 250 GHz to 1.3 THz for the LC type and 0.8 THz to 2.5 THz for the dipole resonance. The number and position of gaps in the SRR has been varied, putting them both in series as well as in parallel. These modifications influence the mode volume of the LC-mode which changes the coupling strength in two opposite ways. The electric field magnitude increases and thus the coupling. However, the number of participating electrons in the 2DEG is reduced which reduces the &radic;<i>N</i> coupling enhancement. The largest coupling we measured so far is =&Omega;/&omega;₀ = 0.58.¹