Regensburg 2016 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 33: Photonics and Nanooptics I: Infrared Spectroscopy

O 33.9: Vortrag

Dienstag, 8. März 2016, 12:30–12:45, H4

Second Harmonic Phonon Spectroscopy of α-Quartz — •Christopher Winta, Sandy Gewinner, Wieland Schöllkopf, Martin Wolf, and Alexander Paarmann — Fritz-Haber-Institut der Max-Planck-Gesellschaft

We experimentally study the efficiency of second harmonic generation (SHG) from optical phonon resonances in crystalline α-quartz, employing intense, tunable picosecond mid-infrared laser pulses generated from the FHI free-electron laser (FEL). Due to its numerous optical phonon branches, α-quartz exhibits multiple Reststrahl bands with high reflectivity, located between the respective transversal and longitudinal optical phonon resonances. For each of these spectral regions, the real part of the dielectric function is negative, resulting in rapidly attenuated evanescent waves which make these experiments sensitive to a thin near-surface layer.

In our experiments, we employ a non-collinear, reflective geometry which allows for a background-free detection of the SHG signal, even for strongly absorbing samples. Making use of the large tuning range of the FEL, we can investigate essentially all optical phonon resonances of α-quartz. We observe sharp resonances in the SHG at the frequencies of transverse optical phonons due to a resonant enhancement of the nonlinear susceptibility. At each resonance, the SHG signal also strongly depends on the azimuthal orientation of the crystal as a consequence of its trigonal crystal symmetry. We show that this feature can be exploited to selectively investigate the different contributions of the nonlinear susceptibility tensor to the SHG signal.