Fig. 1. Polariton bleach effect at pump-probe delay time, t₂= 0 ps.

(A) The central concept for optical nonlinearity manipulation via changes in the cavity thickness is that by decreasing the cavity longitudinal length, and consequently the mode volume, the pump-induced polariton density is increased along with the corresponding optical nonlinearity. The wave-like feature represents the microcavity standing waves in resonance with the molecular modes. The resonant cavity mode decreases when the cavity length is reduced (e.g., in our experiment, the 10th- and 5th-order cavity modes are nearly resonant with the molecular polarization when the cavity longitudinal length is 25 and 12 μm, respectively). The red and blue rays entering and leaving the FP cavities provide a pictorial representation of input and output (detected) electromagnetic fields, and do not imply any focusing or laser profile. Further description of the experimental setup is given in sections S1.1 and S1.2. (B) Intensity of the bleach is presented by comparing the transmission intensity when the pump is on (green) and off (red) in the 12-μm cavity. (C) Pump-probe spectra showing the polariton bleach with various cavity longitudinal lengths (5, 12, and 25 μm). It is found that the polariton bleach (reduction of transmission) becomes weaker as the cavity longitudinal length increases. All pump-probe spectra are normalized to the maximum intensity of the pump-off transmission spectra. All measurements were performed with a fixed molecular concentration, Rabi splitting, and pump input (~10 mJ/cm²).