Symmetry Specific Low-Frequency Vibrational Spectra of Crystals Measured by Two-beam 3CBCRS

We report, for the first time, the extension and application of two-beam 3-color broadband coherent Raman spectroscopy to measure symmetry-specific phonon vibrations in a cubic crystal, BGO (Bi₄Ge₃O₁₂) [1]. We measured low-frequency polarization-sensitive coherent Raman spectra of BGO and compared them with polarized Raman measurements. We used polarization arrangements of the laser beams to select vibrational bands with specific symmetries. We tested the fidelity of phase-retrieval methods by comparing retrieved spectra with polarization background-suppressed measurements. The Raman bands followed the allowed transitions predicted by factor-group analysis; however, coherent scattering polarization selection rules were found to be altered. The applied experimental and theoretical method was published earlier [2,3]. To avoid second-order nonlinear effects and to operate below the plasma generation and damage thresholds of the crystal, we optimized the focusing and signal-detection system. Our study opens the way for advanced studies of the low-frequency spectral region in microcrystal phonon and vibrational investigations.

[1] Z. Chen et al., J. Phys: Condens. Matter, vol. 23, no. 38, p. 385402, 2011.

[2] L. Allen, L. Ujj, APS March Meeting 2023, G00.00031, March 5-10

[3] L. Ujj, Spectrochimica Acta Part A, vol 199, pp. 448-454, 2018.