Current Research
Dynamical process of an electron spin and nuclear spin ensemble in a single quantum dot
Studies on localized spins have been attracting a lot of interests. One of the recent topics is
the spin interaction between an electron and nuclei (contact-type hyperfine interaction), which comes into focus
in the field of single electron spin manipulation in semiconductor nanostructures.
Since the localization suppresses significantly the electron spin relaxation mechanism based on the spin-orbit interaction,
an electron may interact with nuclei in the wave function for a long time, and can make a large nuclear
spin polarization via mutual spin flip (i.e. flip-flop) process. The averaged nuclear spin
polarization acts back on an spin-unpaired electron as an effective magnetic field (Overhauser field)
and induces the energy shift of Zeeman splitting spectral lines (Overhauser shift). Since, with the
localized electrons, the nuclear spins show the interesting nonlinear response to the externally
controlled parameters, the interaction can offer a powerful tool to change the electron spin properties.
In particular, semiconductor quantum dots serve the advantageous platform in order to utilize this
interaction because of the strong confinement of an electron and the controllability.
One of the central topics in electron and nuclear spin interaction is the influence of the nuclear fluctuation ΔBN on the electron spin relaxation.
For this subject,
we investigated the e-n spin dynamics in QD
structures by using the DCP of the positively charged exciton
(X+). The DCP of X+ PL changed in synchronization with
the OHS or the energy splitting of the e-spin levels, and this
phenomenon provides the possibility of the sensitive probing
of the QD-NSP. By taking advantage of this feature, the key
quantities (ΔBN and TΔ) were evaluated directly from the
experimental data. In addition, we extended the dynamics
model of NSP by including the dynamics of the X+ states,
and we confirmed the validity of the e-spin relaxation model
by comparing the time-resolved OHS and DCP measurements
with the calculated results.
Optical anisotropy of self-assembled InAlAs quantum dots
Studies on localized spins in semiconductor QDs have been attracting considerable interest.
This is because the discrete electronic levels involved in the optical
transitions serve the fascinating applications in which QDs are used as emitters of single,
indistinguishable, and entangled photons. For these applications, it is crucial to study
the polarization of the emitted photons associated with exciton annihilation.
For ideal QDs as artificial atoms, the relevant eigenstates are bright excitons with the angular momentum of, and the circularly polarized photons or are to be absorbed emitted to from the eigenstates.
However, actual QDs have the anisotropic distributions of shape and strain, and as a result, the confinement potential
symmetry is reduced from D2d to C2v or lower. It is well
known that the shape anisotropy induces the change in the
emission polarization as well as the level splitting as an exciton
fine structure. Also, QDs formed by self-assembly in
the Stransky.Krastanov SK growth mode is believed to
have a large strain originating from the QD formation process,
and the strain with the anisotropic distribution more or
less remains inside a QD even after QD formation is complete.
Consequently, the emission polarization is affected by
the anisotropic exchange interaction(AEI) and the straininduced
valence-band mixing (SI-VBM). The former originates
from the QD shape asymmetry and the latter comes
mainly from the in-plane anisotropic relaxation of strains.
Accordingly, the polarization of the QD emissions is one of
valuable probes for the origin of the QD symmetry lowering.
Since the strain anisotropy may be different largely from QD
to QD, the investigation of the polarization is necessary for
the individual QDs.
PL polarization conversion in self-assembled InAlAs quantum dots
The polarization conversion from optical orientation to alignment in single InAlAs/AlGaAs quantum dots
(QDs) was studied in detail under zero and nonzero magnetic fields. Under the influence of the effective
magnetic field, bright exciton doublets precess in pseudospin space, where the torque vector is composed of the
external magnetic field and the anisotropic exchange field. For a number of QDs, we measured the angle
difference of the polarization axes obtained with circularly polarized excitations, which is an indicator of the
conversion efficiency under a zero magnetic field. By applying a longitudinal magnetic field, a high conversion
efficiency of 50% was achieved. Additionally, the exciton spin-relaxation time and the magnitude of built-in
linear dichroism were estimated from the exciton spin dynamics using the three-dimensional pseudospin
precession model, and a long spin-relaxation time exceeding the recombination lifetime was obtained. Finally,
we discussed the influence of a nuclear magnetic field on the polarization conversion.
