Abstract

Recently, femtosecond photocurrent transients in quantum wells have been studied in various conditions using excitonic electroabsorption as a femtosecond electrical signal detector.¹ In the present case, we study femtosecond electrical signal generation in quantum wells at 77 K with a tunable 100-fs Ti:sapphire laser for excitation and nonresonant electro-optic sampling² using LiTaO₃. This combination is interesting because we can tune the excitation above and below the excitonic resonance and investigate the resultant photocurrent signal. It has been predicted that subresonant excitation of quantum wells in de bias conditions should result in short electric signals (ac-dc Stark effect).^3,4 In bulk .materials, in principle the same effect exists⁵ (however, excitations into the Franz-Keldysh tail produce a real photocurrent excitation which is undesirable, whereas the Franz-Keldysh tail is supported by quantum confinement in quantum wells.

© 1992 Optical Society of America