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 ${\ensuremath{\Gamma}}_{4}^{\ensuremath{-}}$ symmetry redshifts, while the one-photon forbidden paraexciton $C$ of ${\ensuremath{\Gamma}}_{5}^{\ensuremath{-}}$ symmetry blueshiftsto a crossing of energies at 4.4 kbar. During this process, the $A$ level exhibits strong spectral linenarrowing 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.