Surprising and novel results are obtained for both the linear and the nonlinear optical response of GaAs under magnetic field. Using a variety of spectroscopic techniques, we measure field dependent effects due to Coulomb correlation

Glutsch, S.

Mycek, M.A.

Siegner, U.

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LBL-37139 UC-404 1 Lawrence Berkeley Laboratory e UNIVERSITY OF CALIFORNIA t Materials Sciences Division To be presented at the Conference for Laser and Electrode Optics/ Quantum Electronics and Laser Science, Baltimore, MD, May 21-26, 1995, and to be published in the Proceedings Ultrafast Spectroscopy of GaAs Under Magnetic Field M.-A. Mycek, U. Siegner, S .  Glutsch, and D.S. Chemla March 1995 Prepared for the U.S. Department of Energy under Contract Number DE-ACO3-76SFooog8 ~ISTR~$~JTION OF THIS DOCUMENT IS UNLIMITED A DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. Neither the United States Government nor any agency thereof, nor The Regents of the University of Califor- nia, nor any of their employees, makes any warranty, express or im- plied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infMge pri- vately owned rights. Reference herein to any specific commeraal product, process, or service by its trade name, trademark, manufac- turer, or otherwise, does not necessarily constitute or imply its en- dorsement, recommendation, or favoring by the United States Gov- ernment or any agency thereof, or The Regents of the University of California, The views and opmions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or The Regents of the University of California and shall not be used for advertising or product endorsement pw- poses. Lawrence Berkeley Laboratory is an equal opportunity employer. DISCLAIMER Portions of this document may be illegible in electronic image produced from the document. products. Images are best available original LBL 37139 uc-404 ULTRAFAST SPECTROSCOPY OF GaAs UNDER MAGNETIC FIELD Mary-Ann Mycek, Uwe Siegner, Stephan Glutsch, and Daniel S. Chemla Department of Physics University of California Berkeley, CA 94720 Materials Sciences Division Lawrence Berkeley Laboratory University of California Berkeley, CA 94720 March 1995 This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Division of Materials Sciences of the U.S. Department of Energy under Contract No. DE-AC03-76SFOOO98. DlSTFUSUTION OF THIS OOCUMENT IS UNtlMtTED Ultrafast Spectroscopy of GaAs Under Magnetic Field Mary-Ann Mycek, Uwe Siegner", Stephan Gluts&, Daniel S. Chemla Department of Physics, University of California at Berkeley, and Materials Sciences Division, Lawrence Berkeley Moratory 1 Cyclotron Rd., MS 2-300, Berkeley CA 94720 Phone: (510)486-5265 FAX= (51 0)-#86-5530 Abstract: Surprising and novel results are obtained for both the linear and the nonlinear optical response of GaAs under magnetic field. Using a variety of spectroscopic techniques, we measure field dependent effects due to Coulomb correlation. (a) Permanent address: University of Marburg, Germany I Ultrafat Spectroscopy of GaAs Under Magnetic Field, M.-A. Mycek, et al. Ultrafast Spectroscopy of GaAs Under Magnetic Field Mary-Ann Mycek, Uwe Siegner'"', Stephan GIutsch, Daniel S. ChemIa Department of Physics, University of California at Berkeley, and Materials Sciences Division, Lawrence Berkeley Laboratory I Cyclotron Rd., MS 2-300, Berkeley CA 94720 Phone: (51 0)-486-5265 FAX: (51 O)-486-5530 (a) Permanent address: University of Marburg, Germany Application of a magnetic field to semiconductors confines and quantizes electronic states with momentum k perpendicular to B, thereby reducing the dimensionality by two. The 2D to OD transition has been investigated in 2-D quantum wells under magnetic field. [1,2] The purpose of our work is to study the 3D to 1D transition in a bulk crystal under field. We observe, in high quality GaAs, surprising effects in both linear and transient nonlinear optical response. We attribute these effects to Coulomb correlation, whose importance is well known. [3] Fig.(la) illustrates the dramatic effect of Coulomb correlation on the linear absorption of GaAs. [4] For B=OT the hh and lh excitons are followed by a smooth continuum of states. When B#O the lowest exciton resonances remain almost unchanged, while pronounced asymmetric resonances appear at higher energy. 1 Ultrafast Spectroscopy of GaAs Under Magnetic Field, M.-A. Mycek, et al. These are Fano resonances arising from coupling between the discrete magnetoexciton at the edge of the n>l Landau transition with the energetically degenerate 1D-continua of n ’ a  Landau levels (LL). Numerical calculations were performed modeling the semiconductor by two parabolic bands in the effective mass approximation. Fig.(lb,c,d) shows results for h e a r  response at B=6T without (b), with intra-LL (c), and with intra- and inter-LL Coulomb interaction (d). It clearly demonstrates that the origin of the Fano interference is a Coulomb correlation induced coupling. Transient nonlinear optical response was measured via four-wave mixing (FWM) experiments using a lOOfs Ti:sapphire laser. Time-integrated (TI), time- resolved (TR) and spectrally resolved measurements of the FWM emission were performed for magnetic fields between B=0-12T. The TI and TR FWM response of the lowest order Lorentzian-magnetoexciton is shown in Fig.(2). With increasing B, the TI-FWM signal clearly develops a strong component at negative time delay, which is a signature of Coulomb correlation. This component disappears for higher excitation density, consistent with increased screening. Further information is obtained from the TR-FWM measurement, which shows a non-exponential temporal evolution with a lengthening decay for increasing B. These results are in striking contrast with those from 2-D systems, where Coulomb correlation is quenched for increasing B. 2 Ultrafmt Spectroscopy of GaAs Under Magnetic Field, M.-A. Mycek, et al. For the first time, we have measured the transient response of a Fano resonance. We find that it is anomalous, when compared to the well-known Lorentzian resonance response. [SI Fig.(3) summarizes this for B=lOT. The TR- FWM measurement decays on a long time-scale -5OOfs, consistent with the linewidth and shape of the power spectrum. Very surprisingly, however, the TI- FWM decays instantaneously, as fast as the autocorrelation trace of the laser. Fano resonances are, in fact, structured continua, but this does not suffice to explain their transient nonlinear behavior. Calculations based on an “atomic” Fano model, Le. with a constant coupling matrix element, indicate that the linear and nonlinear polarizations decay on the same time scale. Again, Coulomb correlation, the cause of the Fano interference in semiconductors, governs their nonlinear response and seems to play a determinant role. Acknowledgments: This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under Contract No. DE-AC-03-76SFoo098. References: [l] C. Stafford, S. Schmitt-Rink, W. Schaeffer, Phys. Rev. B 41,loooO (1990). [2] Stark, Knox, and D.S. Chemla, in “Optics of Semiconductor Nano~tructures,’~ edited by F. Henneberger, S .  Schmitt-Rink, and E.O. Gobel (Akademie Verlag, Berlin, 1993), and the references therein. 3 Ultrafast Spectroscopy of GaAs Under Magnetic Field, M.-A. Mycek, et ai. [3] See, for example, “Optics of Semiconductor Nanostructures,” ibid. [4] S. Glutsch, U. Siegner, M.-A. Mycek, and D.S. Chemla, to appear in Phys. Rev. B. [5] U. Siegner, M.-A. Mycek, S. Glutsch, and D.S. Chemla, to appear in Phys. Rev. Lett. and Phys. Rev. B. 4 Ultrafast Spectroscopy of GaAs Under Magnetic Field, h4.-A. Mycek et al. Captions: Figure 1: (a) Measured linear absorption of GaAs at T=lAK, taken for 0 - circularly polarized light. Here strain has lifted the degeneracy in the valence band, so the light hole (Ih) and heavy hole (hh) exciton appear split Theoretical calculations for B = 6T: (b) without; (c) with intra-LL; (d) with intra- and inter-LL Coulomb interaction. Figure 2: For excitation at the lowest order magnetoexciton: (a) TI-FWM at excitation density N 10'6cm-3 and for field strengths B = 1OT (top), 6T, 2T, OT (bottom); (b) TR-FWM at time delay At4 ,  with N s 3 ~ 1 0 ' ~ c m ~ ,  and for B = 1OT (top), 8T, 4T, OT (bottom). Figure 3: Transient response of a Fano resonance at B = 1OT and excitation density N z 4~10 '~cm-~:  TR-FWM at A t d  vs. TI-FWM, with the laser autocorrelation for reference. (Inset absorption at B = 1OT and laser pulse power spectrum, showing excitation conditions.) 5 cv 0 n 1 I- N I I  v- m s a W 0 I I I 0 v, 0 -?- v) 7 (qun  -qJe) uo!gdJosqv n II W 0 4 0 4 0 n 0 Q W E i= (VUMl I n LL E3 = 10 Tesla 4 2 - time resolved FWM - time integrated FWM I - laser putocorrelFtion 500 fs Time (fs) 

Ultrafast spectroscopy of GaAs under magnetic field

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