Abstract

We have investigated both the linear and nonlinear absorption of CuBr microcrystallites (MC’s). The linear absorption spectrum is dominated by pronounced exciton peaks similar to bulk CuBr. The quantum confinement is evidenced by the increasing blue shift of the peaks with decreasing MC radius. We concentrate on the situation of R_MC≫R_EXCITON (experiment : factor 10), where fluctuations of R_MC are smaller than 10% as found from X-ray studies and where the experimental spectra can be well explained in terms of the exciton centre-of-mass quantum confinement. At resonant excitation with dye laser pulses we observe a pronounced bleaching of the exciton absorption with rising intensity accompanied by a blue shift of the absorption maximum. We have calculated the energy-shift of the exciton resonance for a high-density exciton gas confined to a MC in Hartree-Fock approximation and good agreement with the experimental data is obtained. At 77K we find a Lorentzian saturation intensity of about 100 kW/cm² and a contrast as large as 30. Operation of the nonlinearity at room temperature with no significant difference to 77K is demonstrated. The results evidence that excitons in MC's give rise to larger optical nonlinearities than in the bulk material.

© 1988 Optical Society of America