Fig. 3. 2D perovskite single crystal grown on a glass substrate.

(A) Schematic representation of the TIR configuration adopted for resonant blueshift measurements: An immersion oil objective (60×) is used to focus the excitation beam on a 2D PEAI single-crystal flake grown on a glass substrate; the same objective is used to collect the reflected light. (B) Energy and in-plane momentum k resolved reflectivity spectra of a thick single-crystal slab of PEAI; the dips in reflectivity correspond to lower polariton modes resulting from the coupling of the exciton mode to different optical modes; the white line represents the energy of the bare exciton mode; the red dashed lines represent lower polariton modes; the enhanced intensity for k ⩾ 10 ${\mathrm{\mu m}}^{-1}$ corresponds to angles of incidence beyond the light line between air and the perovskite slab.