Abstract

Recently, femtosecond photocurrent transients in quantum wells have been studied under 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 sampling^[2] 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 sub-resonant excitation of quantum wells under DC bias conditions should result in electrical signals which are as short as the optical pulse (AC-DC stark effect).^[3,4] In bulk materials, in principle the same effect exists,^[5] however excitations into the Franz-Keldysh tail produce a real photocurrent excitation which is undesirable, whereas the Franz-Keldysh tail is suppressed by quantum confinement in quantum wells.

© 1992 IQEC