An ultrastrongly coupled single terahertz meta-atom

Free-space coupling to subwavelength individual optical elements is a central theme in quantum optics, as it allows the control over individual quantum systems. Here we show that, by combining an asymmetric immersion lens setup and a complementary resonating meta-surface we are able to perform terahertz time-domain spectroscopy of an individual, strongly subwavelength meta-atom. We unravel the linewidth dependence as a function of the meta-atom number indicating quenching of the superradiant coupling. On these grounds, we investigate ultrastrongly coupled Landau polaritons at the single resonator level, measuring a normalized coupling ratio Omega/omega = 0.6. Similar measurements on a lower density two dimensional electron gas yield a coupling ratio Omega/omega = 0.33 with a cooperativity C= 94. Our findings pave the way towards the control of ultrastrong light-matter interaction at the single electron/resonator level. The proposed technique is way more general and can be useful to characterize the complex conductivity of micron-sized samples in the terahertz domain.

Automated summary

This study demonstrates terahertz time-domain spectroscopy of an individual subwavelength meta-atom using an asymmetric immersion lens setup and a complementary resonating meta-surface. The linewidth dependence and ultrastrongly coupled Landau polaritons are investigated, providing insights into the control of light-matter interaction at the single electron/resonator level. The proposed technique has potential applications in characterizing complex conductivity in micron-sized samples in the terahertz domain.