The authors report a spectroscopic near-field investigation using a tunable free-electron laser in combination with a scattering near-field optical microscope. They excite optically uniaxial LiNbO_3 close to a phonon resonance in the infrared regime, thereby exciting a phonon polariton resonance in the coupled tip-sample system. They find that the resonance shows a clear dependence on the orientation of the optical axis of the birefringent crystal within the surface plane. This provides evidence that in addition to the dominant contribution of the dipole moment parallel to the tip axis, also the component along the surface is sensed in such a scattering experiment.
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