Optically induced coherences reveal key features of nonlinearities in the optical response of semiconductors and can be employed to obtain information about dynamical processes via such techniques as transient four-wave mixing and differential transmission. This thesis presents experimental observations of three previously unrecognized aspects of coherent nonlinear optical phenomena occurring in gallium arsenide and multiple quantum wells: excitation-induced dephasing, two-photon biexcitonic coherence, and electronic Raman coherence. Results are analyzed using optical Bloch equations, which emphasize the potential generalization to other semiconductor systems. Excitation-induced dephasing is similar to classical line broadening. The work in this thesis shows for the first time that this incoherent effect is directly responsible for the dominant coherent optical nonlinearity in bulk gallium arsenide. Exact analytic models show that excitation-induced dephasing accounts for the previously unexplained polarization and density dependence of four-wave mixing signals obtained at the 1s exciton resonances in gallium arsenide. This thesis also presents the first observation of the two-photon biexcitonic coherence in a gallium arsenide multiple quantum well using a modified Michelson phase-conjugate interferometer to directly observe the 770-Terahertz oscillation of the nonradiative superposition of the electronic ground state and the biexciton state. The contribution of the two-photon coherence to the nonlinear optical response is thereby confirmed, in addition to the well-known stepwise biexcitonic response. Finally, the short-lived electronic Raman coherence between the light- and heavy-hole 1s excitons in gallium arsenide is conclusively demonstrated for the first time. The Raman coherence provides direct access to dephasing processes not detectable in conventional linear spectroscopy. Similar non-radiative coherences are related to population trapping, electromagnetically induced transparency, and lasing without inversion in atomic systems, and the new observations in gallium arsenide suggest that analogous phenomena may occur in semiconductors.
Optically induced coherences reveal key features of nonlinearities in the optical response of semiconductors and can be employed to obtain information about dynamical processes via such techniques as transient four-wave mixing and differential transmission. This thesis presents experimental observations of three previously unrecognized aspects of coherent nonlinear optical phenomena occurring in gallium arsenide and multiple quantum wells: excitation-induced dephasing, two-photon biexcitonic coherence, and electronic Raman coherence. Results are analyzed using optical Bloch equations, which emphasize the potential generalization to other semiconductor systems. Excitation-induced dephasing is similar to classical line broadening. The work in this thesis shows for the first time that this incoherent effect is directly responsible for the dominant coherent optical nonlinearity in bulk gallium arsenide. Exact analytic models show that excitation-induced dephasing accounts for the previously unexplained polarization and density dependence of four-wave mixing signals obtained at the 1s exciton resonances in gallium arsenide. This thesis also presents the first observation of the two-photon biexcitonic coherence in a gallium arsenide multiple quantum well using a modified Michelson phase-conjugate interferometer to directly observe the 770-Terahertz oscillation of the nonradiative superposition of the electronic ground state and the biexciton state. The contribution of the two-photon coherence to the nonlinear optical response is thereby confirmed, in addition to the well-known stepwise biexcitonic response. Finally, the short-lived electronic Raman coherence between the light- and heavy-hole 1s excitons in gallium arsenide is conclusively demonstrated for the first time. The Raman coherence provides direct access to dephasing processes not detectable in conventional linear spectroscopy. Similar non-radiative coherences are related to population trapping, electromagnetically induced transparency, and lasing without inversion in atomic systems, and the new observations in gallium arsenide suggest that analogous phenomena may occur in semiconductors.
The study of the nonlinear optical response of excitons in semiconductors is important for both scientific and technological applications. Although the nonlinear optical properties of bulk semiconductors and heterostructures have been studied for almost three decades, with most of the research focusing on heterostructures, the mechanisms leading to the nonlinearities in these systems are not completely understood. Determining these mechanisms and developing a theoretical picture which describes the most dominant effects remains a challenge and an active area of study. Although optical interactions in semiconductors is a genuine many-body problem, a description of this system similar to the optical Bloch equations used in atomic studies has shown some success, but more work is needed to make this model complete. Experimental work, particularly in the bulk system uncomplicated by the interface disorder present in heterostructures, will provide the guidance to extend this formalism. In this dissertation, the frequency domain resonant nonlinear optical response of excitons in bulk gallium arsenide at low density is studied to gain insight into the dominant mechanisms leading to the response, as well as the dynamics of the nominally three-dimensional excitonic system. Specifically, the nonlinear response was probed with differential transmission (DT) and four-wave mixing (FWM) experiments, which measure to lowest order the third-order susceptibility, $\chi\sp{(3)}.$ Using these various tools, three specific areas were studied and the results will be presented. First, the role of the biexciton, the state of two bound excitons, in determining the bulk nonlinear response was explored. The observation of a red-shifted nonlinear resonance suggests the importance of this state at moderate densities. Second, a new response, characterized by a resonant increase in exciton absorption induced by optical pumping with initial superlinear growth and a transition to behavior described by simple line broadening with increased pump intensity, was studied in detail. A model considering increased polariton absorption with increased exciton-exciton interactions explains these effects. Finally, nonlinear optical spectroscopy was used to study exciton transport, where near-ballistic behavior was revealed. The effect of exciton transport, thermalization and radiative recombination on the nonlinear optical response was explored.
This dissertation addresses both fundamental aspects of Coulomb correlations in semiconductor quantum wells and more practical aspects of theoretical analysis of semiconductor optoelectronic devices. After introducing the subject, we present and evaluate a state-of-the-art theory of the third order coherent optical response of a semiconductor quantum well based on the Dynamics Controlled Truncation (DCT) scheme. Already in the third order (the so-called chi (3)) regime, semiconductors exhibit a number of many-body Coulomb correlation effects. Their manifestation in various multi-pulse experimental configurations, customarily used in ultrafast semiconductor spectroscopy, has been an important component of this thesis. Coherent optical effects in a semiconductor 3-band system based on the heavy-hole, light-hole and conduction bands were investigated. The quantum beats in the time-integrated differential transmission signal were analyzed and compared with experimental data obtained at the University of Iowa. Fundamental differences from corresponding quantum beats in atomic 3-level systems were found. Also, the analysis of experimental data (obtained at the University of Arizona) of the coupled heavy-hole-light-hole optical Stark shift revealed evidence of intervalence band coherences, an analog of Raman coherences in atomic 3-levels systems. A scheme for realization of electromagnetically-induced transparency (EIT) based on the interference of excitonic and biexcitonic coherences was proposed. Corresponding experiments performed at the University of Oregon showed indeed a considerable coherent reduction of excitonic absorption. Furthermore, an extension of the chi(3) analysis revealed an energy renormalization of the biexciton, in good agreement with the corresponding experiment. A microscopic analysis of polarization dynamics in time-resolved four-wave mixing signals was performed, revealing interesting implications for the biexciton dephasing in addition to the significance of many-body correlations. In the case of four-wave mixing in semiconductor microcavities, our theoretical analysis in conjunction with experimental data obtained at the University of Tokyo gave us indications for a significant shortcoming of the second Born approximation (2nd BA) applied to two-exciton Coulomb correlations in a thin semiconductor quantum well, in agreement with the general knowledge of the qualitative failure of the 2nd BA in systems with short-range interaction in two dimensions. We also analyzed a novel all-optical switching technique based on the nonlinear polarization rotation. Apart from identifying the many-particle processes relevant for the switch operation in the chi(3) regime, we proposed ways to further optimize the switch.
The three-dimensional confinement created by the ultrasmall semiconductor structures known as quantum dots greatly modifies the optical properties of the spatially localized carriers. These systems are presently of great interest to the research community, both for the improved understanding of the physics of confined structures, and for the potential applications of these systems to areas such as optoelectronics and quantum computing. Here, the techniques of transient coherent nonlinear optical spectroscopy are applied to the investigation of excitons confined within InAs/GaAs self-assembled quantum dots. These methods provide a powerful tool for the study of dynamic processes in these novel materials. The nonlinear measurements are based on the differential transmission experimental geometry, and utilize ultrafast pulsed lasers in order to temporally resolve the carrier dynamics after excitation. The experiments performed here are resonant with the ground state absorption of the quantum dots, as evidenced by comparison with the photoluminescence measurements which are used to characterize the sample. The incoherent relaxation dynamics of the population of photoexcited excitons are examined, allowing for a determination of both the lifetime of the excitons and the time scale for relaxation between the two polarization eigenstates of the system. The three-dimensional quantum confinement results in the suppression of these relaxation processes, as predicted. The use of coherent spectroscopic techniques allows for the first unambiguous demonstration of an optically-induced nonradiative Raman coherence in these quantum dots. Simultaneous excitation of both polarization states of the exciton results in a coherent superposition state, whose temporal evolution is observed through the homodyne-detection scheme. In these transient experiments, this is manifested as a beating in the nonlinear response, which allows us to directly measure the fine-structure splitting between the polarization states. Utilizing optical pulse shaping techniques, transient spectral hole burning is performed, which allows for the nonlinear response to be spectrally resolved. As expected for the strongly confined quantum dot excitons, the results show that diffusion among the localized states is not a significant effect. Temperature dependent measurements show that at elevated temperatures, extra dephasing processes, potentially related to acoustic phonon scattering, become important. Finally, this technique allows for the first direct observation of the induced absorption from the exciton to biexciton transition in these InAs quantum dots, and a measurement of the associated binding energy. These results lay the foundation for future studies in this material system, and for the possible development of quantum logic devices utilizing quantum dots.
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.
The coherent response of excitons in semiconductor nanostructures measured in four-wave mixing (FWM) depends strongly on the inhomogeneous broadening of the exciton transition. We investigate GaAs/Al0.3Ga0.7As single quantum wells (SQW) and AlxGa1-xAs mixed crystals. Additional to the usual phase-space filling nonlinearity, excitation-induced dephasing (EID) and biexciton formation (BIF) are important. EID leads to a strong dependence of the signal on the angle between the linear input polarizations. We find that EID persists in inhomogeneous systems, showing that the mutual density-dependent dephasing rate difference between two subsystems within the inhomogeneous distribution is strongly dependent on their energy difference. BIF is strongly affecting the cross-linear polarized FWM response. The signal for positive delay is dominated by the transitions from the one-exciton state X to the two-exciton states. Here, the third-order polarization at X-XX is not at the same resonance as the first-order polarization at 0-X. Consequently, the rephasing of the microscopic third-order polarization to the macroscopic FWM photon echo is blurred by the non-perfect correlation of X and XX energies, leading to a fast and non-exponential signal decay in delay time. For inhomogeneous broadenings larger than the biexciton binding energy, we find an enhanced biexciton binding energy, and a quenching of the oscillator strength of the unbound biexciton.
This dissertation presents investigations of fundamental optical nonlinearities in semi-conductor microstructures. Two distinct effects are studied. In the first part the excitonic optical Stark effect in InGaAs and GaAs multiple quantum-well structures is investigated by means of pump-probe spectroscopy. For nonresonant excitation below the excitonic transition the direction of the shift of the resonance depends on the polarization of the pump and probe pulses. In particular, for anti-circular polarization a surprising red-shift is observed. For resonant excitation, induced absorption energetically above and below the exciton transition and bleaching of the resonance is found. Experiments using both resonant and nonresonant excitation reveal the importance of bound and unbound two-exciton states in absorption changes of the 1s heavy-hole exciton resonance. It is found that higher-order Coulomb contributions determine the intensity as well as the time dependence of the differential excitonic absorption. In addition, the influence of light-hole excitons is analyzed. It is shown that the direction of the optical Stark shift for nonresonant excitation depends also sensitively on the heavy-hole to light-hole splitting and the detuning of the pump pulse. For very high pump intensities and nonresonant excitation the absorption is split when a circularly polarized pump and a linearly polarized probe beam are used. For co-circular excitation traces of hyper-Raman gain are observed. In the second part of this dissertation, the nonlinear optical response of semiconductor microcavities in the nonperturbative regime is studied in resonant single-beam transmission and pump-probe experiments. In both types of experiment, a pronounced third transmission peak lying spectrally between the two normal modes is observed. Its dependence on the probe intensity, pump intensity, pump-probe delay, exciton-cavity detuning and pump detuning is investigated. For single-beam transmission, the energy of the third peak parallels the position of the cavity resonance. It is more pronounced for circularly polarized excitation and lasts longer than the two normal modes. For pump-probe experiments, the third peak increases with decreasing probe intensity and increasing pump intensity. Its energy is close to the low-energy side of the pump spectrum and virtually unaffected by the cavity-exciton detuning. The appearance of the third peak requires temporal overlap of pump and probe pulses. The origin of this complex nonlinearity is the quantum nature of light, which induces intraband polarizations in the presence of a coherent driving field and a finite carrier density. It is found that the coupling of the intraband polarizations via guided modes to the polarization of the fundamental longitudinal mode is responsible for the third transmission peak. A fully quantized theory reproduces the experimental observations.