Ultrafast nonlinear absorption of quantum well excitons periodically distributed in a Bragg reflector

Ultrafast absorptive nonlinearity of excitons is enhanced in a Bragg reflector in which quantum wells are periodically distributed. Absorptive excitonic nonlinearity in the exciton Bragg reflector is characterised by femtosecond transient transmission and reflectance measurements with e-h pair density far above the exciton saturation density. The absorptive excitonic nonlinearity is dominated by two-photon absorption enhanced under the optical confinement in this Bragg reflector. Optical frequency shift caused by ultrafast refractive index change due to the enhanced two-photon absorption, is evaluated from frequency-resolved absorption transients. Spectral modulation of optical pulses and the resultant pulse chirping are produced by this frequency shift and the optical pulses reflected from the exciton Bragg reflector are compressed by compensating the pulse chirping. (C) 1999 Elsevier Science B.V. All rights reserved.