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Physical review. B, Condensed matter

Subpicosecond photon-echo spectroscopy on GaAs/AlAs short-period superlattices

Martín Koch, D. Weber, Jochen Feldmann, Ernst Otto Göbel, T. Meier, A. Schulze, P. Thomas, Stefan Schmitt-Rink, Klaus Ploog

Published Jan 15, 1993• 59 citations
Publisher

Abstract

We present subpicosecond time-resolved four-wave-mixing studies on GaAs/AlAs short-period superlattices having different miniband widths. Photon echoes are observed as a consequence of the inhomogeneous broadening of the excitonic resonance. In addition, the photon-echo signal exhibits periodic modulations as a function of the time delay between the excitation laser pulses. Free excitons and excitons bound to neutral acceptors are identified as the underlying three-level system leading to the observed photon-echo quantum beats. From the beat period we can directly infer the dissociation energy of the bound exciton. In accordance with theory, we find that the dissociation energy decreases with increasing miniband width, i.e., when the excitons become less confined. In addition, we find that the dephasing rate of bound excitons is about one order of magnitude less than that of free excitons and is mainly determined by energy relaxation.

DOI
10.1103/physrevb.47.1532

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We present subpicosecond time-resolved four-wave-mixing studies on GaAs/AlAs short-period superlattices having different miniband widths. Photon echoes are observed as a consequence of the inhomogeneous broadening of the excitonic resonance. In addition, the photon-echo signal exhibits periodic modulations as a function of the time delay between the excitation laser pulses. Free excitons and excitons bound to neutral acceptors are identified as the underlying three-level system leading to the observed photon-echo quantum beats. From the beat period we can directly infer the dissociation energy of the bound exciton. In accordance with theory, we find that the dissociation energy decreases with increasing miniband width, i.e., when the excitons become less confined. In addition, we find that the dephasing rate of bound excitons is about one order of magnitude less than that of free excitons and is mainly determined by energy relaxation.
We investigate the properties of coherent exciton states in GaAs/Alx{\mathrm{Al}}_{\mathit{x}}Alx​${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells using the four-wave-mixing technique with subpicosecond time resolution. In the first part of this work, we show that many-body interactions in a solid manifest themselves in a line shape of the four-wave-mixing signal that is substantially different from the predictions of the generally used simple two-level model. In particular, the coherent interaction of the quasiparticles leads to a signal at negative delay times, in agreement with recent theoretical predictions. In the second part, we report in detail about experiments where energetically close exciton transitions are excited coherently with a short laser pulse. The quantum or polarization interference leads to the observation of quantum beats in the decay of the diffracted signal. These results demonstrate that quantum-beat spectroscopy can be applied to study intrinsic excitations in solids.
The coherent optical response from 140 nm and 65 nm thick ZnO epitaxial layers is studied using four-wave-mixing spectroscopy with picosecond temporal resolution. Resonant excitation of neutral donor-bound excitons results in two-pulse and three-pulse photon echoes. For the donor-bound A exciton (D0XA{\mathrm{D}}^{0}{\mathrm{X}}_{\text{A}}D0XA​) at temperature of 1.8 K we evaluate optical coherence times T2=33−−50{T}_{2}=33--50T2​=33−−50 ps corresponding to homogeneous line widths of $13--19\phantom{\rule{0.222222em}{0ex}}\ensuremath{\mu}\mathrm{eV}$, about two orders of magnitude smaller as compared with the inhomogeneous broadening of the optical transitions. The coherent dynamics is determined mainly by the population decay with time T1=30−−40{T}_{1}=30--40T1​=30−−40 ps, while pure dephasing is negligible. Temperature increase leads to a significant shortening of T2{T}_{2}T2​ due to interaction with acoustic phonons. In contrast, the loss of coherence of the donor-bound B exciton (D0XB{\mathrm{D}}^{0}{\mathrm{X}}_{\text{B}}D0XB​) is significantly faster (T2=3.6ps{T}_{2}=3.6\phantom{\rule{0.16em}{0ex}}\mathrm{ps}T2​=3.6ps) and governed by pure dephasing processes.
We have studied picosecond two-pulse degenerate four-wave mixing in GaAs/AlAs quantum wells under different polarization configurations of the two pump beams and the analyzed optical polarization. We observe two components, a prompt free polarization decay (FPD) and a delayed photon echo (PE), which show different polarization rules and dephasing times. The observed polarization rules are consistently explained in a theoretical model. Within this model, the two-component behavior is a direct consequence of short-range disorder. The PE dephasing rate is associated with the homogeneous broadening of the excitons while the FPD corresponds to the total dephasing rate, consisting of the homogeneous and inhomogeneous (short-range and long-range) broadenings.
The degenerate four-wave mixing technique in the self-diffraction geometry is employed to investigate a novel GaAs/AlAs quantum well microstructure. Time-integrated transient measurements are carried out to study the coherent dynamics of excitons in a GaAs quantum well with AlAs islands inserted at the center of the well (IIQW) in excitation densities from 6×109 to 1.5×1011 cm-2. The decay of the time-integrated self-diffracted signal shows slower dephasing for the IIQW compared to that of a normal quantum well (NQW) without island insertion. The dephasing rate is found to increase slightly in the IIQW, while it is found to increase sharply in the NQW with the excitation density. A deep beat with a period of 1.45 ps, which disappears for co-circular polarization configuration, is observed in the decay of the self-diffracted signal in the IIQW. The detection-energy-resolved measurement reveals that the modulation of the beat increases up to full depth at low detection energy with respect to the peak energy of the self-diffracted signal. The dependence on detection energy and on polarization configuration shows that the beat is a quantum beat due to biexcitonic effect, which is enhanced by localization due to island insertion.
An overview on photon echo spectroscopy under resonant excitation of the exciton complexes in semiconductor nanostructures is presented. The use of four-wave-mixing technique with the pulsed excitation and heterodyne detection allowed us to measure the coherent response of the system with the picosecond time resolution. It is shown that, for resonant selective pulsed excitation of the localized exciton complexes, the coherent signal is represented by the photon echoes due to the inhomogeneous broadening of the optical transitions. In case of resonant excitation of the trions or donor-bound excitons, the Zeeman splitting of the resident electron ground state levels under the applied transverse magnetic field results in quantum beats of photon echo amplitude at the Larmor precession frequency. Application of magnetic field makes it possible to transfer coherently the optical excitation into the spin ensemble of the resident electrons and to observe a long-lived photon echo signal. The described technique can be used as a high-resolution spectroscopy of the energy splittings in the ground state of the system. Next, we consider the Rabi oscillations and their damping under excitation with intensive optical pulses for the excitons complexes with a different degree of localization. It is shown that damping of the echo signal with increase of the excitation pulse intensity is strongly manifested for excitons, while on trions and donor-bound excitons this effect is substantially weaker.