We will present experiments on ultrastrongly coupled, strongly subwavelength resonators in arrays down to single element. We will discuss the limitations of the planar resonators approach when employing extremely subwavelength gaps and we will present the new developments towards single-object, few electrons ultrastrongly coupled systems.

In this work, by combining an asymmetric Silicon immersion lens configuration and a complementary resonator design, far-field transmission measurement of a single subwavelength split-ring resonator, ultra-strongly coupled to inter Landau level transitions in a single quantum well, is resolved. The highest coupling is 60%, achieved for only 2000 coupled electrons for a single resonator on an InSb quantum well. This technique can be useful to characterize the complex conductivity of micron-sized samples, such as exfoliated graphene or transition metal dichalcogenide flakes, in the THz domain.