Abstract

Femtosecond coherent phonon experiments are performed on a series of GaAs/Al^Ga^As multiple quantum wells (MQWs) with varying X, at fixed well and barrier width = 100 A. For X = 0.2, we have found clear beating in time domain (Fig. la). Fourier transformation reveals two distinct peaks at the LO and TO phonon positions (Fig. lb), which is indicative of bulk-like plasmon-phonon oscillations in the growth direction.¹ This is surprising, for the supposed carrier confinement due to the relatively large barrier width has been thought to forbid such oscillations. At x = 0.5, little beating is seen (Fig. la) and only the LO phonon frequency is found in Fourier transformation (Fig. lb), in agreement with earlier results on a series of GaAs/Al^Ga^^As samples with fixed x = 0.35.

© 1998 Optical Society of America