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Journal of Physics Conference Series

Dynamic Stark effects in a semiconductor quantum dot exhibiting a two-hole species valence band

Swati Goyal, P. K. Sen

Published May 18, 2012• 0 citations
Publisher

Abstract

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.

DOI
10.1088/1742-6596/365/1/012050

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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.
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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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.
A theoretical study of the coherently coupled heavy-hole--light-hole optical Stark effect in semiconductor quantum wells is presented. Based on a detailed investigation of the various many-body contributions to the Stark shifts, we identify a parameter regime in which the existence of intervalence band coherences (analogous to the nonradiative Raman coherence in three-level atoms) can be inferred from the coherently coupled heavy-hole--light-hole excitonic optical Stark shift. The analysis is performed for the third-order nonlinear optical regime $({\ensuremath{\chi}}^{3}$ regime), and is based on the dynamics-controlled truncation (DCT) formalism. It includes the relevant optical selection rules, excitonic and two-exciton-correlation (e.g., biexcitonic) effects.
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.
We report the first experimental study of the optical Stark effect in single semiconductor quantum dots (QD). For below band gap excitation, two-color pump-probe spectra show dispersive line shapes caused by a light-induced blueshift of the excitonic resonance. The line shape depends strongly on the excitation field strength and is determined by the pump-induced phase shift of the coherent QD polarization. Transient spectral oscillations can be understood as rotations of the QD polarization phase with negligible population change. Ultrafast control of the QD polarization is demonstrated.