Linewidth collapse in three-photon exciton-polariton spectra of CsI under pressure

Unusual phenomena occurring under hydrostatic compression in the near-band-edge structure of CsI crystals were examined using the nonlinear optical technique of three-photon excitation. The lowest one-photon allowed transition to the exciton-polariton A of gamma-4 symmetry redshifts, while the one-photon forbidden paraexciton C of gamma-5 symmetry blueshifts to a crossing of energies at 4.4 kbar. During this process, the A level exhibits strong spectral line narrowing by more than a factor of 10, while the C level intensity increases by about the same factor. We believe this is the first reported observation of pressure induced line narrowing of deep UV electronic states in solids.