Jj. Baumberg et al., DYNAMIC CONTRIBUTIONS TO THE OPTICAL STARK-EFFECT IN SEMICONDUCTORS, Physical review. B, Condensed matter, 48(7), 1993, pp. 4695-4706
We Present femtosecond time-resolved optical pump-probe measurements o
f the coherent excitonic nonlinear optical response in GaAs quantum we
lls. The data exhibit features that cannot be explained by the convent
ional description of the optical Stark effect as a rigid blueshift of
the exciton absorption spectrum when the pump field is present. Using
the semiconductor Bloch equations we develop a model that permits the
pump-induced changes in the probe transmission to be calculated for ar
bitrary laser pulse profiles, both chirped and unchirped. The results
demonstrate that the amplitude and phase spectral structure of the pro
be strongly influence the detection signal when the pulses are ultrafa
st (i.e., faster than the polarization decay time), in marked contrast
to the normal assumptions made in transient optical spectroscopy. We
examine these dynamic contributions in terms of four-wave mixing of di
fferent frequencies in the probe spectrum, and show that in addition t
o the optical Stark shift normally measured, there is an additional co
mponent arising from dynamic changes in the refractive index due to re
sonantly enhanced cross-phase modulation.