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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Pump-probe absorption spectra of small quantum dots

J. T. Andrews, Pranay K. Sen

Published Mar 23, 2000• 0 citations
Publisher

Abstract

Density matrix approach has been employed to analyze the pump-probe spectroscopic absorption spectra of small semiconductor quantum dots (QDs) under strong confinement regime (SCR) with sizes smaller than the bulk exciton Bohr radius such that the Columbic interaction energy becomes negligible in comparison to the confinement energy. The average time rate of absorption has been obtained by incorporating the radiative and nonradiative decay processes as well as the inhomogeneous broadening arising due to nonuniform QD sizes. The analytical results are obtained for QDs duly irradiated by a strong near resonant pump and broadband weak probe. Numerical estimations have been made for (i) isolated QDs and (ii) QD-arrays of GaAs and CdS. The results agree very well with the available experimental observations in CdS QDs. The results in case of GaAs QDs can lead one to experimentally estimate absorption/gain spectra in the important III-V semiconducting mesoscopic structures.

DOI
10.1117/12.379924

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Density matrix approach has been employed to analyze the pump-probe spectroscopic absorption spectra of small semiconductor quantum dots (QDs) under strong confinement regime (SCR) with sizes smaller than the bulk exciton Bohr radius such that the Columbic interaction energy becomes negligible in comparison to the confinement energy. The average time rate of absorption has been obtained by incorporating the radiative and nonradiative decay processes as well as the inhomogeneous broadening arising due to nonuniform QD sizes. The analytical results are obtained for QDs duly irradiated by a strong near resonant pump and broadband weak probe. Numerical estimations have been made for (i) isolated QDs and (ii) QD-arrays of GaAs and CdS. The results agree very well with the available experimental observations in CdS QDs. The results in case of GaAs QDs can lead one to experimentally estimate absorption/gain spectra in the important III-V semiconducting mesoscopic structures.
Density matrix approach has been employed to analyze the pump-probe spectroscopic absorption spectra of small semiconductor quantum dots (QDs) under strong confinement regime with sizes smaller than the bulk exciton Bohr radius such that the Coulombic interaction energy becomes negligible in comparison to the confinement energy. The average time rate of absorption has been obtained by incorporating the radiative and nonradiative decay processes as well as the inhomogeneous broadening arising due to nonuniform QD sizes. The analytical results are obtained for QDs duly irradiated by a strong near resonant pump and broadband weak probe. Numerical estimations have been made for (1) isolated QDs and (2) QD-arrays of GaAs and CdS. The results agree very well with the available experimental observations in CdS QDs. The results in case of GaAs QDs can lead one to experimentally estimate absorption/gain spectra in the important III-V semiconducting mesoscopic structures.
Exact quantum electrodynamical results are obtained for a semiconductor quantum dot placed inside a microcavity of arbitrary photon leakage ( ), and radiative ( ) and nonradiative ( c ) decay rates. Analytical results are obtained for the density matrix elements. The absorption spectra thus obtained for arbitrary values of , and c exhibit the solid-state analogue of the vacuum Rabi splitting when the system decay parameters are much smaller than the quantum dot-cavity-field coupling parameter. Numerical estimates are made for samples of CdS and GaAs quantum dots of dimensions 19 Å and 56 Å, respectively. The results are in qualitative agreement with the experimental observations.
The density matrix approach has been employed to study analytically the property of dispersion and absorption in semiconductor single quantum dot under the confinement regime.The numerical simulation results show that under the condition of weak probe field and adjusting the two coupling field relative magnitude,the magnitude of normal dispersion is changed and the abrupt change of normal and anomalous dispersion takes place.Enhancement the magnitude of probe field and change the angle between probe field and dipole,the steep change of dispersion states occur.Comparing the absorption curves we find the absorption under weak probe is more power than the strong case.The stronger probe field and change the angle between probe field and dipole,the absorption peak is higher between the resonance point and the space increases.
We numerically investigate the influence of the pump-probe wavelength on the gain and phase recovery dynamics of quantum-dot (QD) semiconductor optical amplifiers (SOAs). The temporal variations of the gain and phase recovery response are calculated at various pump-probe wavelengths, covering the emission wavelength of both the QD ground and excited states. The gain recovery becomes faster at the longer pump-probe wavelength due to the enlarged carrier escape time. The initial phase change turns from a negative to a positive value as the pump-probe wavelength becomes smaller, which is determined by the spectral position of the pump-probe wavelength in reference to the Gaussian-shaped inhomogeneous broadening. The calculated results agree with the previous experimental results measured by other experimental groups.
Biexciton and triexciton states in CuCl quantum dots were studied by means of time-resolved size-selective pump-and-probe technique. A clear induced absorption band is observed on the high-energy side of the excitation photon energy. The new induced absorption is assigned to the transition from the exciton ground state to one of the weakly correlated exciton pair states which were theoretically predicted to exist and to play an important role in nonlinear optical processes. Its pump energy dependence and temporal evolution strongly support this assignment. Under high-density or two-color excitation condition, a triexciton state in quantum dots is observed for the first time.