Optical studies of low-dimensional semiconductors

During my graduate studies, I have identified for the first time the Negatively Charged Exciton line in GaAs quantum wells. This and other results are summarized below.


Charged excitons:
We have identified the negatively charged excitons in a dilute 2D electron gas [G. Finkelstein et al., Phys. Rev. Lett. 74, p. 976 (1995)]. It was the first correct identification of this complex in GaAs. In fact, the charged exciton line had been reported in many earlier publications on nominally undoped GaAs quantum wells, but was erroneously ascribed to biexcitons, impurity or well-width-fluctuations bound excitons, "dark" excitons etc. Later on we completed the list of the charged intrinsic complexes in quantum wells by the observation of the positively charged excitons [G. Finkelstein et al., Phys. Rev. B53, p. R1709 (1996)]. We extensively studied the properties of these complexes at zero and high magnetic fields. In particular, we compared the radiative lifetimes of the neutral and negatively charged excitons by the resonantly excited time-resolved photoluminescence. This unique technique allowed us to investigate in detail the highly nonequilibrium dynamics of the neutral-charged excitons system and to extract the formation time of the charged excitons [G. Finkelstein et al., Phys. Rev. B58, p. 12637 (1998)].

Photoluminescence spectra of a gated 2D
electron gas at different gate voltages (and 
hence electron densities). The localization
transition at -1.5 V shows up as an abrupt
change in the spectra.
2DEG at the transition to the localized regime:
We simultaneously studied optical and transport properties of a gated 2D electron gas at low densities. We observed a strong correlation between changes in the local characteristics (as probed by photoluminescence) and abrupt drop of the electron conductivity [G. Finkelstein et al., Phys. Rev. Lett. 74, p. 976 (1995)]. This correlation indicates that in the 2D electron gas the change of the microscopic properties gives rise to the onset of localization.

Shakeup processes in the photoluminescence of a 2DEG:
We have studied the shakeups in the photoluminescence spectra of a 2D electron gas at high magnetic fields. In shakeup processes the recombining electron-hole pair creates excitations (magnetoplasmons) in the 2DEG. These processes, proved out to be extremely sensitive to the details of the many-body interactions in the 2D electron gas [G. Finkelstein et al., Phys. Rev. B56, p. 10326 (1997)]. Observation of the shakeup lines in the recombination spectra of the negatively charged exciton directly proves that it is indeed a three-particle complex [G. Finkelstein et al., Phys. Rev. B53, p. 12593 (1996)].