Fig. 3: Effects of confinement and quasi-resonant excitation on the phonon-polaritons.
From: Probing hyperbolic and surface phonon-polaritons in 2D materials using Raman spectroscopy
a Upper Extraordinary (UEp, blue squares) and Lower Surface polaritons (LSp, yellow squares) as a function of sample thickness. Around 750 nm (vertical dashed box), two UEp are experimentally resolved, as shown in the upper spectrum in panel (b). The color map shows the calculated Raman efficiency corrected for the finite acceptance angle. The incidence angle is 5°. Horizontal lines indicate lattice phonon frequencies. b Raman spectra from 750-nm GaSe at 293 and 77 K. Black symbols show the measured data, shaded regions illustrate the Lorentzian contribution from phonons (green) or polariton (purple), and the solid green and red lines represent the fitted spectra for 532 nm and 633 nm laser excitation, respectively. Lattice phonon frequencies are indicated by vertical dashed lines, and the energy difference between the laser excitation (ℏωi) and the 1s exciton (EX) is shown on the left. LOp represents Lower Ordinary polariton.
