The influence of disorder and localization on optical dephasing of excitons in the semiconductor mixed crystals ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$
Sex and
Alx${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As has been investigated by means of time-resolved four-wave mixing and photon echo experiments. A dephasing time of several hundreds of picoseconds is found for resonantly excited localized excitons in ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$
Sex while the dephasing time in
Alx${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As amounts to only a few picoseconds. In ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$
Sex dephasing results mainly from hopping processes, i.e., exciton-phonon interaction. The contribution of disorder is negligible in terms of phase relaxation in ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$
Sex. In contrast, in
Alx${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As elastic disorder scattering yields an essential contribution to the dephasing rate. We present a theoretical model, which treats dephasing of optical excitations in a disordered semiconductor, including the influence of disorder as well as exciton-phonon interaction. On the base of this model, the experimentally observed differences in the dephasing behavior of excitons in ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$
Sex and
Alx${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As are related to the microscopic structure of the disorder potential and the mechanism of exciton localization.
Excitons, the bound states of an electron and a hole, in semiconductors are interesting
objects and they continue to attract attention of researchers from all over the world. Despite intense research on exciton physics over last three-four
decades, many interesting phenomena related to the dynamics and the nonlinear optical
properties of excitons in semiconductors and their nanostructures in the coherent and
incoherent regimes are still to be explored. This thesis presents our effort to study the
nonlinear optical properties as well as the exciton dynamics in coherent and incoherent
regime in semiconductor nanostructures including GaAs quantum wells (QWs) and InAs
quantum dots (QDs).
We have set up several time-resolved and steady-state optical spectroscopic measurement
techniques in our laboratory to carry out our study on excitons dynamics in
semiconductor QWs and QDs. We work out a clear and detailed theory of the pump-probe signal taking account of all aspect of the measurement, namely,
the lock-in detection, spectral selection by the monochromator and time-integration at
the slow photodetector. We have used the formalism of optical Bloch equations conventionally
used to analyse wave mixing experiments. Our analysis puts the pump-probe
technique on the same footing as the wave mixing experiments. We show that pumpprobe
experiments can be used to study both coherent and incoherent dynamics. We
also present a clear physical picture of the measured signal using the Lorentz harmonic
oscillator model.
We report the investigation of the coherent exciton dephasing dynamics using pumpprobe
reflectivity measurement in GaAs QWs for resonant excitation. Two GaAs/AlGaAs
QWs with well thicknesses 8 nm and 17.5 nm are used. We found a strong dependence of
the dephasing rate of the excitons with increasing exciton density due to exciton-exciton
scattering. We found that the exciton dephasing rate of 8 nm QWs at low excitation density
and at low lattice temperature is larger than the corresponding dephasing rate of the
17.5 nm QWs, possibly due to larger effect of inhomogeneous broadening. We also report
that the dephasing rates for both QWs are linear with the sample temperature below the
30 K.
We report the study of ionization equilibrium between the exciton and the electronhole
plasma population in the temperature and pressure (excitation density) phase space.
Non-resonant excitation of semiconductor creates a nonequilibrium state of electron-hole
plasma. We investigate this ionization equilibrium using steady state photoluminescence
and time-resolved excitation correlation PL measurements. We present a
clear phase diagram (in temperature and carrier density phase space) of this ionization process.A clear density-dependent increase in the ionization ratio is observed and its
connection with the Mott transition is discussed.
Semiconductor nanocrystals and QDs are another important area of research, mainly
due to their huge potential for device application. We present a simple
model which combines carrier loss kinetics, detailed balance principle in thermodynamic
equilibrium and Pauli exclusion principle to quantitatively explain the observed behaviour
of the experimental data.
We report on the optical preparation of coherent superpositions of exciton and biexciton states manifested in temporal nonlinear oscillations in interacting exciton gases.The effect is illustrated for atomically thin semiconductors, where the reflected light reveals these interactions in a unique way.The occurring nonlinear coherent oscillations are counteracted by incoherent excitation-induced dephasing, a phenomenon which originates from a new type of quantum interference between excitons and the twoexciton scattering continuum.To improve the experimental accessibility, we discuss different methods to control the oscillation modulation depth by modifying the mutual interplay of the exciton-biexciton superposition and excitation-induced dephasing.We find that the coherent optical response can be manipulated by the polarization degree of the exciting light field, the laser detuning, external magnetic fields, and quantum coherent feedback.The extraordinary temporal behavior and its control distinguishes the nonlinear coherent oscillations from atomic Rabi oscillations and enables their engineering based on our proposed control schemes.