Fig. 1: Polaritonic saturable absorber structure.

a Reflectivity spectra of the coupled structure measured at T = 300 K (black curve) and at 6 K (red curve). The blue curve shows the transmission through the QW slab with three internal reflections, measured in a transmission geometry as shown in the inset. b Calculated reflectivity R as a function of the grating period. The red, dashed line highlights the configuration of our sample with a grating period of 16 µm and polariton resonances located at frequencies of ν_LP = 2.52 THz and ν_UP = 2.85 THz, respectively. c Schematics of possible options of the QCL design showing the laser structure and the SA replacing one of the laser’s end mirrors. The coordinate systems define the directions x, y and z used throughout the manuscript. d z-component of the near-field enhancement relative to the incident field amplitude, η, within a cross-section of the cavity (top panel), and magnification of the area highlighted by the black rectangle (bottom panel). e Population inversion Δn = n₁ − n₂, where n₁ and n₂ are the populations of first and second subband, respectively, as a function of pump intensity and pump frequency, ν, calculated using a rate equation model. Red dashed line, ideal pump frequency for saturation. f Population inversion Δn as a function of pump intensity, extracted from the data shown in e, at 2.75 THz.