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Coherent transient effects in a InGaAs1-x/GaAs semiconductor quantum dot

Manish K. Bafna, Pratima Sen

Published Jan 1, 2013• 0 citations
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Abstract

We present a theoretical analysis to examine the strain effect on light and heavy hole energy levels and role of exciton and bi-excitons on the absorptive optical mutation in InxGa1-xAs/GaAs quantum dots (QDs) near the band edge. The a nalysis is based upon the strain induced Luttinger Hamiltonian and time dependent perturbation technique of the coherent radiation-semiconductor interaction model for a three-level system under near band gap resonant excitation regime. The relaxation and dephasing mechanisms phenomenological have been incorporated. Numerical estimations of refractive and absorptive optical mutation have been made for a InGaAs/GaAs quantum dot duly shined by femtosecond pulse Ti: sapphire laser. Our results are found to agree qualitatively with available experimental observations.

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•Jan 1, 2002•0 citations
We present a theoretical analysis to examine the strain effect on light and heavy hole energy levels and role of exciton and bi-excitons on the absorptive optical mutation in InxGa1-xAs/GaAs quantum dots (QDs) near the band edge. The a nalysis is based upon the strain induced Luttinger Hamiltonian and time dependent perturbation technique of the coherent radiation-semiconductor interaction model for a three-level system under near band gap resonant excitation regime. The relaxation and dephasing mechanisms phenomenological have been incorporated. Numerical estimations of refractive and absorptive optical mutation have been made for a InGaAs/GaAs quantum dot duly shined by femtosecond pulse Ti: sapphire laser. Our results are found to agree qualitatively with available experimental observations.
We have proposed a simplified theoretical model to examine the optical nutation in one-dimensional semiconductor quantum wires. We explored the role of excitons, biexcitons, and the effect of confinement in enhancing transient optical processes in such semiconductor nanostructures. We apply the time dependent perturbation technique to the coherent radiation-semiconductor interaction for a three-level system in the near band gap resonant excitation regime. We have incorporated the relaxation and dephasing mechanisms phenomenologically. The induced polarization is calculated theoretically and the role of biexciton density on the polarization is analyzed. Numerical estimations of refractive and absorptive optical nutation have been made for a GaAs∕AlxGa1−xAs quantum wire duly shined by femtosecond pulse Ti: sapphire laser. Our results agree qualitatively with available experimental observations.
We have analytically explored the nature of Stark shift and self energy correction in the presence of two discrete excitonic states i.e, the heavy-hole and light-hole valence bands, below the fundamental absorption edge of semiconductor quantum dots. The time dependent perturbation technique for a three-level system, under the near band gap resonant excitation regime, has been employed; with the incorporation of the relaxation and dephasing mechanisms phenomenologically. The detuning spectra of Stark shift and self energy correction and their dependence on a moderately strong electric field, have been analyzed. The numerical and analytical estimations in a weak noncentrosymmetric CdTe quantum dot show a decrease in Stark shift and an enhancement in self energy correction for both heavy-hole and light-hole excitons.
Transient optical nonlinearities of semiconductor quantum dots in the strongly confined regime are studied, in which the exciton, biexciton and magnetic field effects are taken into account. The photo induced band-to-band electronic transitions are considered from both the heavy-hole (hh) and light-hole (lh) valence bands to the lowest (1s) exciton state below the fundamental absorption edge. The time dependent perturbation technique for a three-level system, under the near band gap resonant excitation regime, has been employed; with the incorporation of the relaxation and dephasing mechanisms phenomenologically. The induced polarization is calculated from where the real and imaginary parts of the third-order nonlinear optical susceptibility, responsible for the occurrence of nonlinear refraction and nonlinear absorption respectively, have been derived. The temporal and detuning spectra of nonlinear refraction and absorption; and their dependence on density of biexcitons and a moderately strong magnetic field, have been analyzed. It is observed that the nonlinear refraction and nonlinear absorption enhance with increasing magnetic field in small CdS quantum dots.
Using straightforward time-dependent perturbation treatment, transient coherent radiation-semiconductor interaction mechanisms are investigated analytically in a quantum well structure (QWS) duly irradiated by a moderate power femtosecond laser with photon energy slightly less than the crystal bandgap energy. The transition energy is calculated by recognizing the finite potential well depth of the QWS. The effects of higher excitonic states on the Stark shift and self-energy correction are examined in GaAs/AlGaAs QWS. The study shows a red shift with increasing excitation intensity and the shift is also found to be less when all the discrete excitonic states below the band edge are considered together. An increased self-energy correction accompanies this red shift.
The effect of an external magnetic field on optical coherent transient phenomena has been analyzed for semiconductor quantum well structures (QWS). The Lüttinger Hamiltonian is used to incorporate the complex valence band structure which is modified due to the application of the magnetic field. We have considered a GaAs/AlGaAs QWS with magnetic field applied in the direction perpendicular to the plane of the well. We get two quantum levels due to confinement, and doubly degenerate heavy-hole and light-hole bands corresponding to each quantum level, thus leading to eight hole subbands. This results in an 8×8 Hamiltonian in an appropriately chosen basis. Energy eigenvalues and eigenvectors have been calculated by numerically diagonalizing the above Hamiltonian. The absorption characteristics and the optical coherent transient phenomena have been studied by solving the effective semiconductor Bloch equations in the weak excitation regime. The polarization induced in conjunction with the results obtained by solving the modified Lüttinger Hamiltonian has been used to analyze the transient characteristics. We observe a blueshift in transition energy with an increase in magnetic field. The features are changed dramatically when one includes band mixing. The crossing of subbands in the absence of band-mixing effects changes over to anticrossing behavior when band mixing is included. Multiple oscillations are observed in the optical nutation signal that can be attributed to band-mixing effects. Absorption spectra also show a significant blueshift with an increase in magnetic field and additional peaks appear when band mixing is included.
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.