20aTJ-2 強レーザーパルス照射下の Wannier-Stark ladder 励起子 Fano 共鳴状態における非対称 Autler-Townes 分裂

尚志 西村, 秀一 田崎

Ken-ichi Hino, Ken Goto, Nobuyuki Toshima

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.

Anindya Rastogi, Erhan Sağlamyürek, Taras Hrushevskyi +2 more

Electromagnetically induced transparency (EIT) and Autler-Townes splitting (ATS) are similar, but different quantum optical phenomena: EIT results from a Fano interference, whereas ATS is described by the ac Stark effect. Likewise, despite their close resemblance, light-storage techniques based on the EIT memory protocol and the recently proposed ATS memory protocol [Saglamyurek et al., Nat. Photon. 12, 774 (2018)] are distinct: The EIT protocol relies on adiabatic elimination of absorption, whereas the ATS protocol is based on absorption. In this article, we elaborate on the distinction between EIT and ATS memory protocols through numerical analysis and experimental demonstrations in a cold rubidium ensemble. We find that their storage characteristics manifest opposite limits of the light-matter interaction due to their inherent adiabatic versus nonadiabatic nature. Furthermore, we determine optimal memory conditions for each protocol and analyze ambiguous regimes in the case of broadband storage, where nonoptimal memory implementations can possess characteristics of both EIT and ATS protocols. We anticipate that this investigation will lead to deeper understanding and improved technical development of quantum memories, while clarifying distinctions between the EIT and ATS protocols.

宗 田中, 真樹 平野, 精二 宮下

C. P. Holfeld, F. Löser, M. Sūdžius +3 more

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.

幸男 橋本