Condensed
Matter & Surface Sciences
COLLOQUIUM
Joseph G. Tischler
Naval Research Laboratory
Single spin spectroscopy of neutral and
charged
excitons in GaAs quantum wells and dots
With optical techniques applied to individual quantum
dots, we now have the opportunity to probe, orient, and eventually to
manipulate single spins. This can be
achieved through the optically-excited state of a single electron—the
singly-charged exciton (trion). The energy level structure and dynamics of
carriers, carrier complexes, and nuclei reflect a rich combination of
interactions between charge, spin, and electric and magnetic fields. We have studied these interactions for trions and excitons in individual
quantum dots using photoluminescence spectroscopy in the presence of magnetic
fields. These observations require a
self-consistent treatment of exchange, Zeeman, and
hyperfine interactions.
We first establish a correspondence between earlier
work in wide quantum wells, where the system exhibits nearly 2-dimensional
behavior, and in narrow wells, where interface roughness confines carriers
within the plane. Trion binding energies
increase faster than predicted for strictly two-dimensional wells,
demonstrating the importance of lateral confinement in trion
energetics. In ensemble spectra, excitons
and trions are readily distinguished by this energy
difference and by their characteristic dependence on temperature and excitation
energy.
At the level of individual dots, trion
and exciton spectra show a rich variation in behavior
that is not readily apparent in the ensemble spectra. The rules of thumb established for ensembles
(based on their temperature and excitation energy dependence) do not hold
uniformly for all dots, and thus magnetic fine structure becomes an important
feature to distinguish trions and excitons. Because they contain two unpaired carrier
spins, excitons display exchange splittings
that may be observed directly when a magnetic field is applied parallel to the
sample plane. On the other hand, the trion singlet state contains only a single unpaired spin,
so there is no exchange splitting.
The detailed behavior of individual quantum dots is
influenced by their structure and by the local charge and spin environment of
each dot. Sharp features in excitation
spectra coincide with a large-scale charge rearrangement in the quantum well,
which can dramatically affect the spectra of individual dots. Excitation at these quasi-resonances changes
the formation probability of trions and excitons, reduces single-dot linewidths,
inhibits the Overhauser effect, and enhances optical
polarization at low temperatures. These
and other properties make this system a well-equipped laboratory for studying
the energetics and dynamics of spin in semiconductor
nanostructures.
Thursday, September 8, 2005