Free-electron laser studies of direct and indirect two-photon absorption in germanium

E. Tuncel; J. L. Staehli; C. Coluzza; G. Margaritondo; J. T. McKinley; R. G. Albridge; A. V. Barnes; A. Ueda; X. Yang; N. H. Tolk

doi:10.1103/PhysRevLett.70.4146

Free-electron laser studies of direct and indirect two-photon absorption in germanium

E. Tuncel, J. L. Staehli, C. Coluzza, G. Margaritondo, J. T. McKinley, R. G. Albridge, A. V. Barnes, A. Ueda, X. Yang, and N. H. Tolk

Phys. Rev. Lett. 70, 4146 – Published 28 June 1993

Abstract

We succeeded in observing the two-photon absorption of germanium for both the direct and the indirect gaps, using the Vanderbilt free-electron laser. The two-photon absorption for the indirect gap was found to be 3 orders of magnetude smaller than the direct gap. The indirect-gap threshold position indicates that the two-photon absorption is assisted by an LO phonon, providing a long-delayed positive test of the Bassani-Hassan predictions.