Fig. 3: Experimental visualization of cavity modes in SnO₂-NBs.

a Schematic of the SINS experiment for broadband IR-THz (Δλ) hyperspectral imaging. b SINS amplitude S₂(ω) narrowband maps (10 cm⁻¹ spectral width) reconstructed from a full hyperspectral image of SnO₂-NB1/Au for the frequencies 520, 529, 539, and 565 cm⁻¹. Horizontal white-dashed lines delimit crystalline and amorphous phases of the NB. Blue arrows guide the eyes highlighting the interference fringes formed by HPhP waves inside the SnO₂-NB1 crystal. Scale bar represents 200 nm. c Spectral linescan (false-color map) superposed by SINS amplitude profiles (white circles) extracted from Fig. 2e. Red solid lines represent the FP model fitting (Eq. 5) of the experimental profiles at 510, 520, 529, and 539 cm⁻¹. Amplitude profiles are in arbitrary units and were vertically offset for clarity. d Dispersion relation for HPhP volume fundamental mode (M₀) calculated analytically from Eq. 2 through the imaginary part of the complex reflectivity \({\mathrm{Im}}[r_{\mathrm{p}}]\) (white-dashed line). White circles represent frequency–momentum (ω–q) values model-extracted from the fittings in c. Inset illustrates the cavity modes (FP model) for a NB with thickness t and width w.