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Physical Review Letters

Fano Resonances in Semiconductor Superlattices

C. P. Holfeld, F. Löser, M. Sūdžius, Karl Leo, D. M. Whittaker, Klaus Köhler

Published Jul 27, 1998• 78 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.81.874

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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.
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.
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.
This Letter presents for the first time the observation of the resonant optical Stark effect in a semiconductor. The excitonic system of Cu2{\mathrm{Cu}}_{2}Cu2​O is well suited to the investigation of this effect because of its rather large binding energy and well-resolved resonances at low temperatures. The mixing between the 1S1S1S and 2P2P2P excitons by the electric field of a tunable high-power CO2{\mathrm{O}}_{2}O2​ laser leads to drastic changes in the one-photon spectrum around the 2P2P2P absorption line. Even details of the line shape are consistently described in terms of the frequency-dependent nonlinear susceptibility.
We report the observation of tunneling induced transparency in asymmetric double quantum well structures. Resonant tunneling through a thin barrier is used to coherently couple the two upper states in a three level system of electronic subbands in a GaAs/AlGaAs structure. This creates Fano-type interferences for the collective intersubband excitations in the absorption from the ground state, analogous to electromagnetically induced transparency (EIT) in atomic systems. We observe a 50% reduction in absorption between the subband resonances which can be explained by taking into account the coherent coupling of the upper states. We analyze the bias dependent absorption spectra and determine the relevant lifetime broadening and dephasing rates for the transitions.
This paper investigates the Fano-Feshbach resonance with a two-channel coupled-square-well model in both the frequency and time domains. This system is shown to exhibit Fano line-shape profiles in the energy absorption spectrum. The associated time-dependent transition moment response has a phase shift that has recently been understood to be related to the Fano line-shape asymmetric qqq parameter by $\ensuremath{\varphi}=2arg(q\ensuremath{-}i)$. The present study demonstrates that the phase-qqq correspondence is general for any Fano resonance in the weak-coupling regime, independent of the transition mechanism.
Summary form only given. In the excitonic ac Stark shift in quantum wells the excitonic absorption line is blueshifted by shining on the sample an intense laser pulse, whose frequency is lower than the exciton transition frequency. The effect is bigger when the laser spectrum approaches the resonance with the excitonic absorption. But if the laser and excitonic absorption spectra sensibly overlap, a great amount of electronic population is excited and consequently the Coulomb nonlinearities due to the interaction between excited carries or excitons dramatically increase. One prominent effect under these conditions is the excitation- induced dephasing which can destroy the coherence of the electronic system and therefore the coupling between excitons and light, which is inherently coherent. Some evidence for the resonant Stark effect was already reported, but the direct observation of the spectral splitting was still missing. This observation demonstrates that the coherent exciton light coupling in semiconductors can be strong enough to survive the fast dephasing, and that therefore an efficient ultrafast manipulation of the exciton absorption in the coherent transient is possible. In order to magnify the ac Stark effect and its visibility a special sample was designed: a single In/sub 0.04/Ga/sub 0.96/As quantum well is grown on the top of an AlAs/AlGaAs distributed Bragg reflector; two /spl lambda//2 spacers separate the quantum well from the mirror and from the air. In this configuration the light field incident on the sample and the one reflected by the mirror constructively interfere at the quantum well position, giving a high effective absorption.