Terahertz Electroabsorption Spectroscopy: The AC Franz-Keldysh Effect

Bulk and confined semiconductors driven by intense THz electric fields are predicted to exhibit an AC analogue of the well-known Franz Keldysh Effect (FKE). This ACFKE alters the absorption spectrum and causes mixing of near-bandgap probe radiation and the applied THz field. (Y. Yacoby, Phys Rev. 169), 610 (1968). ^, (A.P. Jauho, K. Johnsen, Phys. Rev. Lett. 76), 4576 (1996). We report on an experiment to observe the ACFKE in both bulk GaAs and an InGaAs/GaAs multiple quantum well. A tunable Ti:Sapphire laser provides near-bandgap NIR radiation, while the UCSB Free-Electron Laser provides coherent, intense THz radiation tunable from 0.1 - 5THz. We observe a red-shift of the bulk absorption edge, and broadening of absorption features associated with confined states. We also observe sideband generation on the NIR probe at even multiples of the applied THz frequency. We will describe the dependence of these effects on NIR and THz frequency and THz power. Supported by ONR, JSTC, and QUEST (an NSF Sci. & Tech. Ctr.)