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 te chnique of three-photon excitation. The lowest one-photon allowed transitio n to the exciton-polariton A of Gamma(4)(-) symmetry redshifts, while the o ne-photon forbidden paraexciton C of Gamma(5)(-) symmetry blueshifts to a c rossing 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 lev el intensity increases by about the same factor. We believe this is the fir st reported observation of pressure induced line narrowing of deep UV elect ronic states in solids.