Semiconductor excitons in strong terahertz fields

Band diagram of a semiconductor quantum well excited by a NIR laser (left). Electrons and holes are bound by the Coulomb attraction into excitons with a hydrogen-like spectrum (middle). Strong THz pumping of the 1s-2p intraexcitonic transition results in a characteristic energy splitting (right).

http://www.hzdr.de/db/Cms?pOid=29931&pNid=2803

Optical transitions between exciton states in semiconductors — intraexcitonic transitions — usually fall into the terahertz (THz) range and can be resonantly excited with narrowband, intense THz radiation. The Free Electron laser FELBE located at the HZDR, in combination with a synchronized near-infrared (NIR) picosecond or femtosecond laser, is ideally suited to study these excitations.

We have observed signatures of intraexcitonic transitions in various different experiments.

·                                 Nonlinear optical mixing between the NIR and THz optical fields gives rise to optical sidebands, i.e., new NIR signals appear at the sum or difference of the NIR and THz photon energies involved. The intensity of these sidebands can be resonantly enhanced due to the influence of exciton levels.

·                                 The NIR excitonic photoluminescence shows characteristic spectral and temporal signatures if the THz field drives the population into a different excitonic state. Time- and spectrally resolved photoluminescence measurements allow us to investigate the dynamics of intraexcitonic relaxation and scattering.

·                                 The NIR excitonic absorption spectrum in the vicinity of the band edge is affected by strong THz fields. In particular, the  1s exciton ground state exhibits a dynamical Stark — or Autler–Townes — splitting, which also reveals characteristic dependencies on THz photon energy and field strength.

---

Publications

J. Bhattacharyya, S. Zybell, F. Eßer, M. Helm, H. Schneider, L. Schneebeli, C. N. Böttge, B. Breddermann, M. Kira, S. W. Koch, A. M. Andrews, G. Strasser, Magnetic control of Coulomb scattering and terahertz transitions among excitons, arXiv:1309.6426 [cond-mat.mes-hall]

M. Teich, M. Wagner, H. Schneider, and M. Helm, Semiconductor quantum well excitons in strong, narrowband terahertz fields, New J. Phys. 15, 065007 (2013)

W. D. Rice, J. Kono, S. Zybell, S. Winnerl, J. Bhattacharyya, H. Schneider, M. Helm, B. Ewers, A. Chernikov, M. Koch, S. Chatterjee, G. Khitrova, H. M. Gibbs, L. Schneebeli, B. Breddermann, M. Kira, and S. W. Koch, Observation of forbidden exciton transitions mediated by Coulomb interactions in photoexcited semiconductor quantum wells, Phys. Rev. Lett. 110, 137404 (2013)

M. Wagner, M. Teich, M. Helm, and D. Stehr, Temperature dependence of the intraexcitonic AC Stark effect in semiconductor quantum wells, Appl. Phys. Lett. 100, 152101 (2012)

M. Wagner, H. Schneider, D. Stehr, S. Winnerl, A. M. Andrews, S. Schartner, G. Strasser and M. Helm, Terahertz nonlinear optics using intra-excitonic quantum-well transitions: Sideband generation and AC Stark splitting, physica status solidi (b) 248, 859 (2011)

M. Wagner, H. Schneider, D. Stehr, S. Winnerl, A. M. Andrews, S. Schartner, G. Strasser, and M. Helm, Observation of the intraexciton Autler-Townes effect in GaAs/AlGaAs semiconductor quantum wells, Phys. Rev. Lett. 105, 167401 (2010)

M. Wagner, H. Schneider, S. Winnerl, M. Helm, T. Roch, A. M. Andrews, S. Schartner and G. Strasser, Resonant enhancement of second order sideband generation for intra-excitonic transitions in GaAs/AlGaAs multiple quantum wells, Appl. Phys. Lett. 94, 241105 (2009)