Delay and distortion of light pulses by exciton resonances in wide-gap semiconductors

Abstract

Propagation of light pulses in wide-gap semiconductors, GaN and ZnO, has been studied by time-of-flight spectroscopy. It has been demonstrated that significant light delay exists near an exciton resonance and that this delay is accompanied by pulse distortion and attenuation. Simulation of the pulse shape in the time-energy plane has permitted us to refine excitonic parameters inherent for bulk materials. The time-of-flight studies done over a wide temperature range demonstrate the strong influence of resonant scattering by phonons on the light delay in polar semiconductors. Optical dispersion at exciton resonances in semiconductors results in a delay of light transfer, which is induced by the variation of the group velocity — a phenomenon studied long ago(1-3). Thisdelayhasbeenobservedbytime-of-flightspec- troscopy in different semiconductors (4-6). The current inter- estinsuchretardationisenhancedbecauseitisoneoftheways toobtaintheso-calledslowlight,whichisneededforoptical communication and data processing. Wide-gap semiconduc- tors, like ZnO and GaN, possess strong excitonic resonances thatmakesthemsuitableforsuchapplication. Lightvelocities as low as 6000 km/s in ZnO (4) and 2100 km/s in GaN (5) have been recently measured. In view of these findings, the distortion of a pulse shape has attracted special attention (7). It is worth mentioning that early experiments showed no sig- nificant changes in the shape of a light pulse at the excitonic resonances in GaAs (8) and GaP (9), while the optical dis- persion near the resonances implies a different group velocity and attenuation for the higher- and lower-energy edges of the pulses (10), that inevitably means their distortion in the time- energy plane. The optical pulse modification is also important for a set of optoelectronic devices — high-frequency ones or those, where the light should propagate a long distance. The