The Effect of High-Numerical-Aperture Objectives on Polarization Measurements in Micro-Raman Spectrometry

Polarized Raman spectra of the reference materials carbon tetrachloride and α-quartz are obtained with the Raman microprobe MOLE with the use of different high-numerical-aperture objectives. The depolarization ratios of the Raman bands of isotropic samples are in good agreement with those obtained with the use of conventional instruments, provided that the reflection or transmission factor of the beamsplitter is introduced as a correction. Even with the very wide-aperture objectives used, the depolarization effect is not large and can be evaluated theoretically. Correct polarized spectra of anisotropic samples can also be obtained with a Raman microspectrometer. Here again, the depolarization effect induced by the wide-aperture objective is not large and can be evaluated theoretically if the birefringence of the crystal does not play a significant role. The depolarization induced by the birefringence is especially important when incident and scattered light propagate in a direction close to the optical axis of the crystal, but can be minimized by a reduction of the optical path within the sample.