Berlin 2008 – scientific programme

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DF: Fachverband Dielektrische Festkörper

DF 5: Scanning and diffraction methods

DF 5.7: Talk

Monday, February 25, 2008, 16:50–17:10, EB 407

Phonon resonances in ferroelectrics probed with scattering scanning near-field optical microscopy (s-SNOM) using a free-electron laser — •Lukas Eng¹, Susanne Schneider¹, Stefan Grafström¹, Stephan Winnerl², and Manfred Helm² — ¹Institute of Applied Photophysics, TU Dresden — ²Forschungszentrum Dresden-Rossendorf

s-SNOM is based on the interaction between an optically scattering nanoparticle (AFM tip) and a dielectric sample. The size of the scatterer defines the resolution of the microscope, which is on the order of a few nanometers. We use here sample-enhanced phonon resonances to study the local optical properties of such anisotropic ferroelectrics.

We present the spectroscopic near-field examination of optically anisotropic ferroelectrics, namely lithium niobate (LiNbO₃) [1] and barium titanate (BaTiO₃). In these samples we excite phonon resonances in the IR regime. As we need to tune the incident wavelength exactly to the sample resonance, we use a free-electron laser at the Forschungszentrum Dresden-Rossendorf as a precisely tunable light source in the IR with a wavelength range 4-200 µm. Furthermore, we show near-field images of ferroelectric domains of BaTiO₃ representing a purely anisotropic near-field contrast. We are presenting the first tunable IR near-field measurements on ferroelectric single crystals, which are furthermore in excellent accordance with recent calculations of optical anisotropy in such systems [2].

[1] S.C. Schneider et al., Appl. Phys. Lett. 90, 143101 (2007)

[2] S.C. Schneider et al., Phys. Rev. B 71, 115418 (2007).