Orbital angular momentum impact on light scattering by phonons tested experimentally

TL;DR

This study experimentally tested the influence of orbital angular momentum of vortex light on Raman scattering in a piezoelectric crystal, finding no observable effects and challenging some theoretical predictions.

Contribution

It provides the first experimental verification of the impact of orbital angular momentum on Raman scattering, negating previous theoretical predictions.

Findings

No influence of orbital momentum on Raman spectra observed.

Silent optic modes frequencies were not revealed by vortex light.

No evidence of symmetry-lowering effects in Raman tensors.

Abstract

Polarized Raman spectra of piezoelectric bismuth germanate single crystal were recorded using vortex light in order to verify the predicted symmetry selection rules for Raman scattering of photons with nonzero orbital angular momentum. At first, the phonon spectrum is fully characterized by the infrared reflectivity and Raman scattering with ordinary light. Then, vortex light Raman spectra were compared with the ordinary ones, but no influence of the orbital momentum on the Raman spectra of Bi$_4$Ge$_3$O$_{12}$ was observed. In particular, we experimentally negate the theoretical prediction that Raman scattering using vortex light can yield frequencies of the silent optic modes of Bi$_4$Ge$_3$O$_{12}$. We also have not seen evidence for the predicted symmetry-lowering of the Raman scattering tensors of $E$ modes. We propose that the orbital momentum effects are inaccessible to the ordinary Raman spectroscopy experiments because of the tiny magnitudes of typical phonon coherence lengths.