Exploring Ultrastrong Light-Matter Interaction with Superconducting Split Ring Resonators

Curdin Maissen(1), Giacomo Scalari(1), Janine Keller(1), Christian Reichel(2), Werner Wegscheider(2), Sara Cibella(3), Roberto Leoni(3), Mattias Beck(1), Jérôme Faist(1)   

(1) Institute of Quantum Electronics, ETH Zürich, Switzerland;
(2) Laboratory for Solid State Physics, ETH Zürich, 8093 Zürich;
(3) Istituto di Fotonica e Nanotecnologie, CNR, Rome;    


Collective excitations in semiconductor heterostructures provide large dipole moments which, together with a strong light confinement, is needed to achieve very large interaction strengths. Metallic Split Ring Resonators in the THz-frequency range allowed us to reach a strongly subwavelength cavity mode volume and to increase the normalized interaction strength Ω/ω of close to unity. With a new resonator design based on the Niobium we were able to reduce both, the radiative and resistive losses of the cavity mode and to further reduce the cavity volume. The new resonator shows a quality factor Q = 54 at 2.9 Kelvin and a normalized coupling strength of 0.66 to the cyclotron transition in a high mobility two-dimensional electron gas.