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Physical Review B

Asymmetric Autler-Townes-like doublets in laser-driven excitonic Fano resonance in biased superlattices: Role of many-body Coulomb exchange

Ken-ichi Hino, Ken Goto, Nobuyuki Toshima

Published Jan 28, 2004• 8 citations
Publisher

Abstract

Transient four-wave mixing (FWM) spectra from excitonic Fano resonance (FR) states in biased superlattices are theoretically investigated by numerically solving the semiconductor Bloch equations adopting FR wave functions of the excitonic Wannier-Stark ladder as an expansion basis set. It is found that an Aulter-Townes-like doublet arises from resonant excitation of FR excitons, and shows conspicuous asymmetry in spectrally resolved FWM signals, the degree of which depends strongly upon a Fano-$q$ parameter. Furthermore, the many-body Coulomb exchange effects markedly modify the spectral profiles, so that either lobe of the doublet is considerably narrowed with accompanying reversal of the asymmetry pattern. The effects also shift the overall resonance spectra toward the high-energy side. The three findings of spectral narrowing, asymmetry reversal, and blue shift feature the present dressed FR system. The corresponding time-resolved spectra are also discussed from the viewpoint of the Rabi oscillation.

DOI
10.1103/physrevb.69.035322

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Transient four-wave mixing (FWM) spectra from excitonic Fano resonance (FR) states in biased superlattices are theoretically investigated by numerically solving the semiconductor Bloch equations adopting FR wave functions of the excitonic Wannier-Stark ladder as an expansion basis set. It is found that an Aulter-Townes-like doublet arises from resonant excitation of FR excitons, and shows conspicuous asymmetry in spectrally resolved FWM signals, the degree of which depends strongly upon a Fano-qqq parameter. Furthermore, the many-body Coulomb exchange effects markedly modify the spectral profiles, so that either lobe of the doublet is considerably narrowed with accompanying reversal of the asymmetry pattern. The effects also shift the overall resonance spectra toward the high-energy side. The three findings of spectral narrowing, asymmetry reversal, and blue shift feature the present dressed FR system. The corresponding time-resolved spectra are also discussed from the viewpoint of the Rabi oscillation.
Summary form only given.Fano resonances arise due to quantum mechanical coupling of a discrete state to a degenerate continuum. The line shape in optical absorption spectra is characterized by an intrinsic broadening of the line, strong asymmetry, and a reduced absorption at one side of the peak falling well below the pure continuum level. Intensive theoretical investigations predicted Fano resonances as a general feature of a large variety of low-dimensional semiconductors. We present the observation of Fano resonances in semiconductor superlattices (SL). In SL, discrete excitonic states of the Wannier-Stark ladder are in resonance with subband continua of lower ladder states. The coupling between discrete and continuous states is mediated by Coulomb interaction, which is also responsible for the formation of the discrete excitonic states. As a unique feature of Fano resonances in SL, the Fano coupling strength can be continuously changed in the experiment by tuning the applied electric field. We have also performed time-integrated four-wave-mixing experiments to investigate the influence of Fano coupling on the polarization decay of a Wannier-Stark exciton. We have been able to resolve the polarization decay of a Fano resonance in the time-integrated signal and to follow its changes with field.
Abstract A four-level atomic model is proposed to exhibit the evolution of Fano-like interference effect in the spontaneous emission spectrum of Autler-Townes (AT) doublet when the atom is initially prepared in the phase-dependent superposition of quantum states. By using this model, phase-dependent generation of a narrow spectral line within the AT-doublet is predicted. This feature is shown to be modified by the AT-peak-splitting effect. In addition, it has been also shown that the latter may be accompanied by an anomalous peak shifting effect, which is found to be obtained in a typical phase-coherent atomic ensemble or phaseonium under consideration. As a whole, the emission spectrum can be obtained with three peaks instead of two peaks associated with a sharp spectral hole as a consequence of the emergence of asymmetric Fano-like resonance. This is the most interesting outcome of the present study.
We report the first observation of Fano resonances in biased semiconductor superlattices: The excitonic Wannier-Stark ladder transitions show asymmetric absorption due to the coupling to the continua of lower transitions. In contrast to other known examples of Fano resonances, the Fano coupling can be continuously tuned in this system by changing the static field across the superlattice. The line shapes and their coupling dependence are in excellent agreement with theory. We also investigate the dephasing dynamics of the resonances and observe an increase in dephasing with increasing Fano coupling.
Under the conditions of the rotating wave approximation (RWA), a transition strongly driven by a resonant oscillating field displays the well known symmetric Autler - Townes doublet. However, if the counter-rotating component, neglected in the RWA, is taken into account, the Bloch - Siegert shift gives rise to an Autler - Townes doublet of unequal intensity even in the case of a resonant driving field. This effect is investigated theoretically in a V-shaped three-level double-resonance configuration and the results are presented in this paper. An interesting observation is that the level of asymmetry not only depends on the driving-field intensity but also on the characteristics of the driven system including relaxation rates and equilibrium population distributions.
While the advanced coherent control of qubits is now routinely carried out in low frequency (GHz) systems like single spins, it is far more challenging to achieve for two-level systems in the optical domain. This is because the latter evolve typically in the THz range, calling for tools of ultrafast, coherent, nonlinear optics. Using four-wave mixing micro-spectroscopy, we here measure the optically driven dynamics of a single exciton quantum state confined in a semiconductor quantum dot. In a combined experimental and theoretical approach, we reveal the intrinsic Rabi oscillation dynamics by monitoring both central exciton quantities, i.e., its occupation and the microscopic coherence, as resolved by the four-wave mixing technique. In the frequency domain this oscillation generates the Autler-Townes splitting of the light-exciton dressed states, directly seen in the four-wave mixing spectra. We further demonstrate that the coupling to acoustic phonons strongly influences the FWM dynamics on the picosecond timescale, because it leads to transitions between the dressed states.