Applied Physics Letters

Ultrafast phase dynamics of coherent carriers in GaAs

Xiangyang Yu, Qi Luo, Wen‐lan Li, Q. Li, Zhiren Qiu, Jianying Zhou

Published Dec 7, 1998• 13 citations

Abstract

Ultrafast phase dynamics of free-induction decay for carriers in bulk GaAs is studied with differential-phase spectroscopy. The instantaneous phase shifts of the free-induction decay with respect to the excitation pulses are extracted from simultaneously recorded laser pulse autocorrelation and free-induction decay in GaAs. Ultrafast phase dynamics during and immediately after the femtosecond pulse excitation are numerically evaluated with optical Bloch equations using pump pulse and semiconductor exciton parameters as input. Good agreement of the theory with experiment is obtained.

DOI: 10.1063/1.122708

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Applied Physics Letters

Ultrafast phase dynamics of coherent carriers in GaAs

Xiangyang Yu, Qi Luo, Wen‐lan Li, Q. Li, Zhiren Qiu, Jianying Zhou

Published Dec 7, 1998• 13 citations

Publisher

Abstract

DOI: 10.1063/1.122708

Related papers

DIFFERENTIAL-PHASE SPECTROSCOPY FOR COHERENT CARRIERS WITH LASER FEMTOSECOND PULSE PAIRS

Yun Fang

It is proved that both amplitude and phase in instantaneous optical processes can be detected from the number density of the excited state populations of coherent carriers when excited with femtosecond laser pulse pairs. Experimental technique of phase spectroscopy is used for investigations of semiconductor GaAs and rare earth solids. Phase spectra of GaAs, crystal Nd:YVO 4 and powdered Nd:YAG are obtained. Dephasing time of GaAs exciton is 190fs at 77K and the Nd:YVO 4 and Nd:YAG are 300fs at room temperature.It is proved that both amplitude and phase in instantaneous optical processes can be detected from the number density of the excited state populations of coherent carriers when excited with femtosecond laser pulse pairs. Experimental technique of phase spectroscopy is used for investigations of semiconductor GaAs and rare earth solids. Phase spectra of GaAs, crystal Nd:YVO 4 and powdered Nd:YAG are obtained. Dephasing time of GaAs exciton is 190fs at 77K and the Nd:YVO 4 and Nd:YAG are 300fs at room temperature.

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Ultrafast phase dynamics of coherent emission from excitons in GaAs quantum wells

D. S. Chemla, J.‐Y. Bigot, Mary‐Ann Mycek +2 more

We investigate the temporal evolution of the instantaneous frequency of ultrashort-pulse four-wave mixing in GaAs/ ${\mathrm{Ga}}_{\mathit{x}}$ ${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well structures. We find that the coherent light emitted by excitons exhibits a complex phase behavior which depends critically on the density of excitons and the central frequency of the exciting pulses. Depending on the excitation conditions, we observe nonlinear shifts of the instantaneous frequency within one ultrashort pulsed emission or quantum beats. In the latter case we determine that the beat \ensuremath{\pi}-phase shift is very fast, only 40% above the fundamental quantum limit. We also present elaborate numerical simulations of the experiments based on a six-band generalization of the semiconductor Bloch equations. This formalism takes into account Coulomb many-body interactions and Pauli exclusion. It reproduces the salient features of the experimental data. It fails to account for important details revealed by the very sensitive phase measurements. The discrepancies can be traced back to two approximations in the theory: the statical treatment of screening, and the Lorentzian description of dephasing. In both cases, this indicates the need for theoretical refinements requiring a microscopic description of Coulomb scattering and dephasing processes in a non-Markovian theory.We investigate the temporal evolution of the instantaneous frequency of ultrashort-pulse four-wave mixing in GaAs/${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well structures. We find that the coherent light emitted by excitons exhibits a complex phase behavior which depends critically on the density of excitons and the central frequency of the exciting pulses. Depending on the excitation conditions, we observe nonlinear shifts of the instantaneous frequency within one ultrashort pulsed emission or quantum beats. In the latter case we determine that the beat \ensuremath{\pi}-phase shift is very fast, only 40% above the fundamental quantum limit. We also present elaborate numerical simulations of the experiments based on a six-band generalization of the semiconductor Bloch equations. This formalism takes into account Coulomb many-body interactions and Pauli exclusion. It reproduces the salient features of the experimental data. It fails to account for important details revealed by the very sensitive phase measurements. The discrepancies can be traced back to two approximations in the theory: the statical treatment of screening, and the Lorentzian description of dephasing. In both cases, this indicates the need for theoretical refinements requiring a microscopic description of Coulomb scattering and dephasing processes in a non-Markovian theory.

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The ultrafast relaxation of excitons in GaAs, coherently excited by a short optical pulse and subjected to collisions with free carriers and noncoherent excitons which are independently created by a second synchronized light pulse, is studied directly in the time domain by a probing of the excitonic phase coherence. The relaxation rates reveal strong exciton-exciton scattering with a collision efficiency of 1.6 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\ensuremath{-}1}$ and even 10 times more efficient exciton-free-carrier scattering.The ultrafast relaxation of excitons in GaAs, coherently excited by a short optical pulse and subjected to collisions with free carriers and noncoherent excitons which are independently created by a second synchronized light pulse, is studied directly in the time domain by a probing of the excitonic phase coherence. The relaxation rates reveal strong exciton-exciton scattering with a collision efficiency of 1.6 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\ensuremath{-}1}$ and even 10 times more efficient exciton-free-carrier scattering.

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Coherent optical polarization of bulk GaAs studied by femtosecond photon-echo spectroscopy

A. Lohner, Kai Rick, P. Leisching +5 more

The nonlinear polarization close to the band gap of GaAs is studied by spectrally and temporally resolved four-wave mixing. Excitonic and free carrier contributions both excited within the bandwidth of the 100 fs pulses are distinguished for the first time. The excitonic part dominates at carrier densities below ${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. At higher density, nonthermalized free carriers give rise to an additional component resonant to the pulse that shows a photon-echo-like time behavior. Monte Carlo simulations including the coherent polarization and the scattering dynamics of the carriers account for the data.The nonlinear polarization close to the band gap of GaAs is studied by spectrally and temporally resolved four-wave mixing. Excitonic and free carrier contributions both excited within the bandwidth of the 100 fs pulses are distinguished for the first time. The excitonic part dominates at carrier densities below ${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. At higher density, nonthermalized free carriers give rise to an additional component resonant to the pulse that shows a photon-echo-like time behavior. Monte Carlo simulations including the coherent polarization and the scattering dynamics of the carriers account for the data.

Physical Review LettersJul 5, 199380 citations

We investigate the temporal evolution of the instantaneous frequency of ultrashort-pulse four-wave mixing in GaAs/

{\mathrm{Ga}}_{\mathit{x}}

${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well structures. We find that the coherent light emitted by excitons exhibits a complex phase behavior which depends critically on the density of excitons and the central frequency of the exciting pulses. Depending on the excitation conditions, we observe nonlinear shifts of the instantaneous frequency within one ultrashort pulsed emission or quantum beats. In the latter case we determine that the beat \ensuremath{\pi}-phase shift is very fast, only 40% above the fundamental quantum limit. We also present elaborate numerical simulations of the experiments based on a six-band generalization of the semiconductor Bloch equations. This formalism takes into account Coulomb many-body interactions and Pauli exclusion. It reproduces the salient features of the experimental data. It fails to account for important details revealed by the very sensitive phase measurements. The discrepancies can be traced back to two approximations in the theory: the statical treatment of screening, and the Lorentzian description of dephasing. In both cases, this indicates the need for theoretical refinements requiring a microscopic description of Coulomb scattering and dephasing processes in a non-Markovian theory.

{\mathrm{Al}}_{0.06}{\mathrm{Ga}}_{0.94}\mathrm{As}.

The decay of the FWM signal for positive time delays unambiguously reflects the dephasing of the continuum, irrespective of the excitation of excitonic transitions. We find identical ultrafast sub-20-fs decay times of the coherent emission from continuum transitions in GaAs and

{\mathrm{Al}}_{0.06}{\mathrm{Ga}}_{0.94}\mathrm{As},

independent of the excess excitation energy. These decay times depend only weakly on the carrier density for densities below ${10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}.$ The continuum dephasing is analyzed considering carrier-carrier and carrier-LO-phonon scattering in a relaxation rate approach. The excitonic transitions in

{\mathrm{Al}}_{0.06}{\mathrm{Ga}}_{0.94}\mathrm{As}

dominate the FWM signal at negative time delays.

{\mathrm{cm}}^{3}

${\mathrm{s}}^{\ensuremath{-}1}$ and even 10 times more efficient exciton-free-carrier scattering.

{10}^{16}

${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. At higher density, nonthermalized free carriers give rise to an additional component resonant to the pulse that shows a photon-echo-like time behavior. Monte Carlo simulations including the coherent polarization and the scattering dynamics of the carriers account for the data.