Birkedal, Dan

Hvam, Jørn Märcher

Sayed, Karim El

Vadim, Lyssenko

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Continuum Contribution to Excitonic Four-Wave Mixing

Summary form only given. We present an experimental and theoretical investigation of ultrafast transient four-wave mixing (TFWM) of GaAs-AlGaAs quantum wells for simultaneous excitation of exciton and continuum states. Recent TFWM experiments on semiconductors have shown unexpected results when both discrete exciton states and continuum states have been excited simultaneously by ultrafast laser pulses. The decay of the signal, as a function of delay, is significantly faster than the dephasing time of the excitons

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Downloaded from orbit.dtu.dk on: Dec 18, 2017Continuum contribution to excitonic four-wave mixingBirkedal, Dan; Sayed, Karim El; Vadim, Lyssenko; Hvam, Jørn MarcherPublished in:Summaries of Papers Presented at the Quantum Electronics and Laser Science ConferenceLink to article, DOI:10.1109/QELS.1996.865629Publication date:1996Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Birkedal, D., Sayed, K. E., Vadim, L., & Hvam, J. M. (1996). Continuum contribution to excitonic four-wavemixing. In Summaries of Papers Presented at the Quantum Electronics and Laser Science Conference (pp. 107-107). IEEE. DOI: 10.1109/QELS.1996.865629WEDNESDAY MORNING 1 QELS'96 1 107 QWB5 915 am Continuum contribution to excitonic four- wave mixing D. Birkedal, K. E. Sayed, V. G. Lyssenko, J. M. Hvam, Mikroelektronik Centref, The Technical University of Denmark, DK-2800 Lyngby, Denmark We present an experimental and theoret- ical investigation of ultrafast transient four-wave mixing (TFWM) of GaAs/ AlGaAs quantum wells for simultaneous excitation of exaton and continuum states. Recent TFWM experiments on semiconductors have shown unexpected results when both discrete exciton states and continuum states have been excited simultaneously by ultrafast laser pulses.') The decay of the signal, as a function of delay T, is significantly faster than the dephasing time of the excitons. However, the width of the spectrally re- solved excitonic TFWM response and the real-time evolution of the TFWM signal is reflecting the instrinsic exatonic de- phasing time. We have investigated a GaAs/Ab3- Ga, ,As multiple-quantum-well sample with spectrally resolved TFWM. The 1.6s 1.M 1.66 1 . 1  1.67 Photon energy lev] QWB5 Fig. 1 Transient four-wave mixing spectrum at zero time delay. The spectrum shows the heavy (HH) and light hole (LH) excitonic resonances for the 160,130,100, and 80 8, quantum wells. 10 d '  0.1 0 01 0 w1 r [PSI QW65 Fig. 2 Intensity of the transient four-wave mixing signal at the heavy hole excitonic resonances of the 160,130, and 100 quantum wells. The curves have been displaced for darity and the lines are guides for the eye. The fast initial component of the traces for the 160 and 130 quantum wells is due to continuum excitation. MBE-grown sample consists of wells of the thicknesses 80, 100, 130, and 160 and was kept at 5 K during the experi- ments. The sample was excited by 100-fs pulses from a self-modelocked Tisap- phire laser in the two-beam self- diffraction geometry? The spectrum of the TFWM signal for zero time delay is shown in Fig. 1 for resonant excitation of the heavy hole exciton of the 100 quan- tum well. The width of the excitonic res- onances are similar to those observed in the absortion spectra of the sample (c1 meV), indicating dephasing times of sev- eral picoseconds. The 130 and 160 quantum wells are excited several tens of meV into the continuum and the delay dependence of the excitonic TFWM in- tensities clearly reflects this. Figure 2 shows the intensity of the TFWM signal at the exatonic resonances as a function of delay. The response from the 100 I% quantum well shows quantum beats as- sociated with the light and heavy hole ex- citons and a decay of several picoseconds in agreement with the width of the reso- nance. The responses from the 130 and 160 R wells show a fast initial component of the order of the laser pulse followed by a slower component on the picosec- ond timescale. While the initial fast com- ponent is of the same magnitude as the response from the 100 A quantum well, the slow component is seen to be several orders of magnitude less intense. We show that these observations are well explained using a model based on the semiconductor Bloch equations, tak- ing populations generated by local fields into account. It is demonstrated that, for continuum excitation, the main signal at the spectral position of the exciton is due to diffraction in a second-order popula- tion grating of excitons generated by the action of the local field of the continuum states on the first order polarization of the exciton states. However, the local field of the continuum states is only sig- nificant within a time corresponding to the inverse width of the excitation. This is shown to result from interference of the first-order polarization of the continuum states. 1. J. Feldmann, T. Meier, G. von Ples- sen et al., Phys. Rev. Lett. 70, 3027 (1993). 2. U. Siegner, M.-A. Mycek, S. Glutsch, D. S. Chemla, Phys. Rev. Lett. 74, 470 (1995). T. Yajima, Y. Taira, J. Phys. Soc. Jap. 47, 1620 (1979). 3. QWBC 930  am Three-beam four-wave mixing on CaAs using 1 5 4  pulses M. U. Wehner, D. Steinbach, M. Wegener, lnstitut f i r  Angewundte Physik, Univ. Karlsruhe, 0-76128 Karlsruhe, Germany Two-beam four-wave mixing (FWM) ex- periments on bulk GaAs at low temper- atures using pulses of 15 fs in duration have recently revealed two observations: i) non-Markovian relaxation resulting from exciton-LO-phonon interaction' at QWB6 Fig. 1 Experiment: Diffracted signal as a function of tzl and fa, GaAs, T = 77 K. QW66 Fig. 2 Model Diffracted signal as a function of tZ1 and fa.  intermediate carrier densities and ii) an apparent contradiction between the observed decay in the time delay domain and the FWM-linewidth' at densities as low as a few 1014 ~ m - ~ .  Thisapparent contradiction, which has previ- ously only been observed on Fano-reso- nances: has had no explanation up to To further investigate these aspects we have performed three-beam experi- ments under identical conditions. The three 15-fs pulses, tuned to the band edge, with wave vectors q1, qa and q3 ar- rive at times fl, fa  and fS, respectively. The $*acted signal in direction q3 + q2 - 91 is detected as a function of the time delays tZl = t2 - tl and fa = f3 - f2. Figure 1 shows the diffracted signal as a func- tion of the two time delays tZ1 and f32 at rather low total carrier density (9.5. lo1' ~ m - ~ ) .  The half width at half maximum of the FWM spectra (not shown) is 1 meV (resolution limited), which corresponds to a dephasing time T2 of T2 > 658 fs, which is in apparent contradiction with the rapid decay (-10 fs) in the tZ1 do- main. However, the decay for tZ1 = 0 and negative f32 exhibits a time constant of -1 ps, which is consistent with the observed linewidth. This behavior can neither be understood within the opitcal Bloch equations nor within the semiconductor Bloch equations together with a constant T2. Here we present a model for the dom- inant nonlinear optical contribution un- der ultrafast excitation that accounts for the scattering of 1-s excitons with contin- this point. Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on July 24,2010 at 13:57:10 UTC from IEEE Xplore.  Restrictions apply. 

Continuum contribution to excitonic four-wave mixing

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Continuum contribution to excitonic four-wave mixing

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