  Free-space coupling to strongly subwavelength individual optical elements is
a central theme in quantum optics, as it allows to control and manipulate the
properties of quantum systems. In this work, we show that by combining an
asymmetric immersion lens setup and complementary design of metasurfaces we are
able to perform THz time-domain spectroscopy of an individual, strongly
subwavelength (d/{\lambda}0=1/20) meta-atom. We unravel the linewidth
dependence of planar metamaterials as a function of the meta-atom number
indicating quenching of the Dicke superradiance.
  On these grounds, we investigate ultrastrongly coupled Landau polaritons at
the single resonator level, measuring a normalized coupling ratio of
{\Omega}/{\omega}=0.60 resulting from coupling of the fundamental mode to a few
thousand electrons. Similar measurements on a low loss, less doped two
dimensional electron gas yield a coupling ratio {\Omega}/{\omega}=0.33 with a
cooperativity C=4g^2/{\kappa}{\gamma}= 94. Interestingly, the coupling strength
of a coupled single resonator is the same as of a coupled array. Our findings
pave the way towards the control of light-matter interaction in the ultrastrong
coupling regime at the single electron/single resonator level. The proposed
technique is way more general and can be useful to characterize the complex
conductivity of micron-sized samples in the THz and sub-THz domain.
