学术报告：Terahertz spectroscopy of semiconductor nanostructures with the free-electron laser

题目：Terahertz spectroscopy of semiconductor nanostructures with the free-electron laser

报告人：Professor Manfred Helm

时间：2012.09.18，上午10点

地点：田家炳楼南205

报告人简介：

Manfred Helm (born 1959 in Austria) studied physics at the University of Innsbruck, Austria, where he received the M.Sc. degree in physics in 1983 and his Ph.D. degree in 1987, the latter with research on far infrared (THz) magnetospectroscopy of semiconductors. Afterwards he was postdoc and staff member at Bellcore, USA (1987-1990). In 1990 he joined the University of Linz, Austria as an assistant professor. In 1993 he achieved the "Habilitation" and was later promoted to associate professor (tenure). In 2000 he became director at the Institute of Ion Beam Physics and Materials Science at the Forschungszentrum Rossendorf in Dresden, Germany (now Helmholtz-Zentrum Dresden-Rossendorf), and was appointed at the same time to full professor at the Technical University of Dresden.

His main research areas are the physics of semiconductor quantum structures, linear and nonlinear as well as ultrafast infrared and THz spectroscopy of semiconductors, novel infrared/THz detectors and sources, magnetotransport and magnetooptics, as well as silicon photonics and magnetic semiconductors. He is author or co-author of more than 250 refereed scientific publications with over 3000 citations.

Presently he is in charge of using a terahertz free-electron laser for semiconductor spectroscopy.

See also http://www.hzdr.de/db/Cms?pNid=141&pOid=11485

摘要：

Free electron lasers (FEL) can deliver high-intensity narrow-band radiation in otherwise not easily accessible spectral ranges,  such as the THz range.

After briefly introducing the FEL at HZDR, I will discuss a quantum optical experiment, where we use the FEL to drive the 1s-2p intra-excitonic transition in semiconductor quantum wells and observe the so-called Autler-Townes splitting, a manifestation of electronic states dressed by the radiation field [1]. Our results indicate that we enter a regime well beyond the rotating-wave and two-level approximations. As a second example I will discuss pump-probe experiments on multilayer graphene over a wide range of photon energies (10-250 meV), hereby unveiling the relevant electronic relaxation mechanisms [2]. Remarkably we observe a sign change of the probe signal, i.e. a crossover from induced bleaching to induced absorption, which is related to the interplay between inter- and intraband absorption.

[1] M. Wagner, H. Schneider, D. Stehr, S. Winnerl, A. M. Andrews, S. Schartner, G. Strasser,  M. Helm: „Observation of the intra-exciton Autler-Townes effect in GaAs/AlGaAs semiconductor quantum wells”, Phys. Rev. Lett. 105, 167401 (2010)

[2] S. Winnerl, M. Orlita, P. Plochocka, P. Kossacki, M. Potemski, T. Winzer, E. Malic, A. Knorr, M. Sprinkle, C. Berger, W. A. de Heer, H. Schneider, M. Helm: "Carrier relaxation in epitaxial graphene photoexcited near the Dirac point", Phys. Rev. Lett. 107, 237401 (2011)