Variation of self-energy and exciton-phonon coupling in KI and RbI with pressure

We measured the 1s exciton energies in KI and RbI versus temperature at three different pressures via three-photon-excitation spectroscopy. The observed excitations display a Lorentzian line shape, which can be viewed as evidence that excitons and phonons in KI and RbI are only weakly coupling. Using the weak-coupling model we were able to find values for the real part of the temperature-dependent self-energy of the 1s exciton (peak-shift parameter {Δ}). We find that the absolute value of {Δ} and the strength of the exciton-phonon coupling increase with pressure. We also studied the behavior of the linewidth of the 1s exciton-polariton resonance with pressure and temperature and found that it decreases with pressure. However, we do not think that the linewidth parameter {Γ} of the polariton can be rigorously interpreted as the imaginary part of the exciton self-energy.