Fig. 3: Frequency- and time-resolved spectra of the perovskite-cavity system.

a Schematic of the perovskite-cavity system, with the cavity longitudinal m = 1 mode at ~1 THz. The field amplitudes of the m = 1 and 2 modes (with ~1 and ~2 THz frequency) appear in green and pink. b Simulations of the spectral transmission of the cavity-perovskite system as a function of the sample position in the cavity, with experimentally determined cavity and perovskite parameters. Depending on the position of the perovskite in the cavity, the local field strength (shaded lines) causes polariton formation at ~1 and ~2 THz. c Experimental (circles) and simulated transmission spectra, for x_gap = 135 μm and x = 63 μm (red line), x_gap = 160 μm and x = 46 μm (blue line). Estimated error bars are produced from four measurements. d Optical pump-THz probe signal of MAPI in the cavity with the same parameters as in (c), and also off-resonant cavity (x_gap = 1 cm, yellow line). Note the enhanced photoconductivity signal for the on-resonant case (red). e Experimental (circles) and simulated (thin lines) differential photoconductivity response for the same parameters as in (c). Estimated error bars are produced from four measurements. f Refractive index of (photoexcited) MAPI outside the cavity, used to calculate the response in (c) and (e). All optical pump-THz probe measurements (inside as well as outside the cavity) are performed with an identical pump fluence of around 1 × 10¹⁸ photons m⁻²