Ultrastrong Light-Matter Coupling with a single THz Split Ring Resonator and monolayer graphene

Getting a better understanding of ultrastrong light matter interaction is not only interesting regarding fundamental research, but also relevant to gain insights on new properties of matter, possible new different phase transitions, and for applications in Quantum Information Processing. For all these purposes, it is most beneficial to study the interaction at a single electron level.

Moving towards the single electron regime, we attempted to modify our conventional samples consisting of metamaterials with arrays of evenly spaced complementary split ring resonators (cSRRs). They intrinsically enhance electric fields at volumes much smaller than the illuminating wavelength, at resonant frequencies corresponding to their equivalent LC circuit. One of the possible modifications to couple only to a few electrons was to reduce the number of resonators down to a single meta-atom. However far field measurements of these samples did not show the expected resonant behavior, as the LC mode of the cSRR could not be excited. In our previous work (ref 1), we presented a solution to this problem: An asymmetric solid immersion lens (aSIL) setup where two Si lenses focus the THz beam onto the single resonator, exciting the desired LC resonant mode and making its far field resolution possible.

Having the possibility to probe a single resonator, rather than an array thereof, is very important as a first step towards measuring high-quality, exfoliated 2D materials like monolayer graphene. The material flakes are usually very small (∼20 · 20 μm) and sensitive during processing, thereby making it unfeasible to place them under multiple resonators on the same sample. Another imperative step towards measuring ultrastrong coupling (USC) between the electrons in a single layer graphene flake is being able to electrically contact the sample during the measurement to shift the Fermi level. As a first step for this we present the measurements of USC coupling to electrons in a gated GaAs-AlGaAs quantum well.

By placing a stack of monolayer graphene and hBN in the gap of the resonator and making use of the above mentioned components, we are therefore able to present the first optical measurement of electrons in graphene coupled to the mode of a single THz resonator.