Berlin 2015 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 22: Ultrafast Surface Dynamics

O 22.5: Vortrag

Dienstag, 17. März 2015, 11:45–12:00, MA 042

Second Harmonic Spectroscopy in the Reststrahlen Band of SiC Using an Infrared Free Electron Laser — •Alexander Paarmann, Alexey Melnikov, Sandy Gewinner, Wieland Schoellkopf, and Martin Wolf — Fritz Haber Institute of the Max Planck Society, Berlin

We experimentally study the efficiency of optical second harmonic generation in the highly reflective Reststrahlen spectral region of the wide-band gap semiconductor 6H-SiC, employing tunable picosecond mid-infrared laser pulses generated from the FHI free electron laser. The Reststrahlen region, located between longitudinal and transversal optical phonon resonances, provides a unique window to study nonlinear effects in the phononic response of a crystal. We observe sharp resonances of the second harmonic signal at both transverse and longitudinal optical phonon fundamental frequencies, the latter being unexpected since no resonance in the optical nonlinearity is predicted for longitudinal modes. We discuss the underlying mechanism of local field enhancement, which is tightly linked to the largely dispersing linear optical properties in the Reststrahlen band. The negative real part of the dielectric function results in rapidly attenuated evanescent waves in this region, making these experiments sensitive to a thin near-surface layer. The resulting large amplitude atomic motions induced by intense mid-infrared laser excitation opens the door to future studies of nonlinear phononic response at semiconductor surfaces and interfaces.