Damping of Rabi oscillations in intensity-dependent photon echoes from exciton complexes in a CdTe/(Cd,Mg)Te single quantum well

Abstract

We study Rabi oscillations detected in the coherent optical response from various exciton complexes in a 20 nm-thick CdTe/(Cd,Mg)Te quantum well using time-resolved photon echoes. In order to evaluate the role of exciton localization and inhomogeneous broadening we use selective excitation with spectrally narrow ps-pulses. We demonstrate that the transient profile of the photon echo from the localized trion (X − ) and the donor-bound exciton (D 0 X) transitions strongly depends on the strength of the first pulse. It acquires a non-Gaussian shape and experiences significant advancement for pulse areas larger than π due to non-negligible inhomogeneity-induced dephasing of the oscillators during the optical excitation. Next, we observe that an increase of the area of either the first (excitation) or the second (rephasing) pulse leads to a significant damping of the photon echo signal, which is strongest for the neutral excitons and less pronounced for the donor-bound exciton complex (D 0 X). The measurements are analyzed using a theoretical model based on the optical Bloch equations which accounts for the inhomogeneity of optical transitions in order to reproduce the complex shape of the photon echo transients. In addition, the spreading of Rabi frequencies within the ensemble due to the spatial variation of the intensity of the focused Gaussian beams and excitation-induced dephasing are required to explain the fading and damping of Rabi oscillations. By analyzing the results of the simulation for the X − and the D 0 X complexes we are able to establish a correlation between the degree of localization and the transition dipole moments determined as µ(X − )=73 D and µ(D 0 X)=58 D. PACS numbers:Introduction. Coherent control of excitonic states in semiconductor nanostructures under resonant excitation with intense optical pulses attracts a lot of attention in relation with possible applications in quantum information [1]. These ideas exploit coherent rotations of the Bloch vector in the photoexcited two-level system (TLS), which depends on the area of the exciting pulse via Rabi oscillations [2][3][4]. Since stronger localization of excitons is in favor of longer decoherence times, most of the studies of coherent control have concentrated on quantum dots (QD) [3,5,6]. However, the strong localization in QDs is accompanied by large variations in QD size, shape, and composition which consequently leads to the large inhomogeneous broadening of the optical transitions when an ensemble of emitters is used. Therefore, most Rabi oscillation studies were performed on single QDs [7-10].In semiconductor quantum well (QW) structures the inhomogeneous broadening of the optical transitions is significantly smaller as compared to QD systems, i.e. it is possible to selectively address different exciton complexes, such as free and localized excitons, localized charged excitons (trions, X − ), and donor-bound excitons (D 0 X). Therefore, QW structures can be considered as a model system for the investigation of Rabi oscillations and t...

Polarimetry of photon echo on charged and neutral excitons in semiconductor quantum wells

“…In the present paper, we have chosen the model system of a CdTe/(Cd,Mg)Te QW with spectrally well isolated optical transitions, that was previously studied by other FWM-based techniques [21,26]. In particular, to precisely distinguish between the donor-bound exciton and trion transitions three-pulse photon echo spectroscopy in an external magnetic field was carried out [26].…”

“…Details of the structure growth can be found in Ref. [21]. The structure was not intentionally doped with donors and the resident electron density n d 10 10 cm 2 is due to an unavoidable background of impurities.…”

“…In the following we study the PE amplitude when the reference pulse is centered exactly at 2τ delay. It is known from previous studies carried out on the same QW structure that optical excitation at elevated power densities may result in coherent Rabi flops of the PE amplitude accompanied by temporal shifts of the echo pulse, which were recently studied using the timeresolved FWM technique [21]. In order to avoid this effect and stay in the quasi-linear χ (3) -regime we employ low-power pulses providing a total pulse power below 20 nJ/cm 2 .…”

“…In order to avoid this effect and stay in the quasi-linear χ (3) -regime we employ low-power pulses providing a total pulse power below 20 nJ/cm 2 . The X hh , T, and D 0 X optical transitions in this QW were also studied with temporal and spectroscopic resolutions, and the spectral dependence of the coherence time T 2 was measured [21]. Here we focus mostly on the polarization properties of the photon echoes from these transitions.…”

Quantum beats in the polarization of the spin-dependent photon echo from donor-bound excitons in CdTe/(Cd,Mg)Te quantum wells

“…For measuring spin-dependent PE we have chosen the D 0 X transition since it shows better coherence properties than the X − transition. These include longer optical coherence times up to T 2 = 100 ps (∼ 80 ps for X − ) and more robust optical Rabi oscillations in the PE amplitude [27]. As a result, we are able to apply higher pulse pow-ers and to set substantially longer delay times τ (200 ps), avoiding strong excitation-induced dephasing.…”