Linear and nonlinear pulse propagation at bound excitons in CdS

Abstract

Pulse propagation experiments are performed with coherent picosecond laser pulses in resonance with the donor bound A-exciton (I2) state in CdS. Using bandwidth-limited time-resolved spectroscopy, we investigate the intensity of the transmitted laser pulse as a function of monochromator energy position and time. This method permits the determination of both the amplitude and the phase of the complex electric-field amplitude. Studying the dependence of pulse propagation on the input intensity, for small-area pulses we found at the rear of the sample the spectral hole formation and beating characteristic of a 0π pulse. At large pulse areas close to 2π the pulse propagates nearly unperturbed, showing no oscillatory reshaping or significant absorption losses. This provides clear evidence for self-induced transparency at an excitonic state in solids. Our interpretation is substantiated by an analysis within the framework of the combined Maxwell and optical Bloch equations.