Fig. 1: Design and operation of the metasurface.

a Schematic of the polaritonic metasurface. An incoming field (\({s}_{+}\)) feeds a nanoantenna, which in turn couples to the ISB transitions of the MWQ stack with a Rabi rate \(g\), and determines the outgoing field (\({s}_{-}\)). b In the strong coupling regime, the low-power reflection spectrum of the system (blue line) displays the characteristic polaritonic splitting, leading to very large reflection at \(\omega ={\omega }_{0}\). For high impinging intensities (red line) the ISB transitions in all quantum wells are fully saturated, and the reflection spectrum approaches the one of the bare nanoantenna, featuring very low reflection at \(\omega ={\omega }_{0}\). In these calculations we assumed \({\gamma }_{a}={\gamma }_{r}={\gamma }_{2}=\Omega /10\). c Reflection spectra of the metasurface versus the saturation level of the MQW for two different impinging frequencies: \(\omega ={\omega }_{0}\) (orange line in (c), also marked in (b) with the orange arrow) and \(\omega ={\omega }_{0}+\Omega\) (green line in (c), also marked in (b) with the green arrow). d Reflection contrast \(\Delta R\) defined as the difference in the low- and high-intensity reflection coefficients of the metasurface versus \({\gamma }_{2}/\Omega\), for \(\omega ={\omega }_{0}\). In this plot, the value of \({\gamma }_{2}\) is varied while all other parameters are the same as in (b). e Measured absorption spectra (black line) of the bare InGaAs/GaAsSb MQW material. By fitting the main peak with an analytical model (red dashed line) we extract a transition energy of about 153 meV and a linewidth equal to FWHM = \(2\hslash {\gamma }_{2}\) = 7.4 meV.