Z‐Polarization sensitive detection in micro‐Raman spectroscopy by radially polarized incident light

Abstract

AbstractWe demonstrate z‐polarization sensitive detection in micro‐Raman spectroscopy by introducing a radially distributed polarized light and by employing a high numerical aperture objective lens and circular mask. The radial polarization is provided by a radial‐waveplate consisting of an 8‐way segmented half‐waveplate with each segment having a different orientation of the optical axis. The segmented waveplate allows for both z‐polarization (perpendicular to the sample plane) and xy‐polarization (parallel to the sample plane) of the electric field at the focus by selecting the proper polarization of the incident field. The z‐polarization Raman components that are barely detectable in conventional measurements can be clearly observed in this experimental setup. Alpha‐quartz, which is well known in terms of polarized Raman spectroscopy, is utilized as a test sample. We have succeeded in extracting the z‐Raman tensor components, and the intensity ratio of each measured tensor value in our experiment is in good agreement with previously reported work. The Raman spectral intensity ratios are greatly modified when observed under polarization control, and we can clearly see a small isolated Raman peak that usually appears only as a shoulder of a strong neighboring peak. The effect of the confocal detection setup on the accuracy of depolarization measurements is also discussed and the relationship between the depolarization ratio and pinhole size is shown. In this research, we aim to provide a solid basis for z‐polarization imaging in a micro‐Raman configuration under high numerical aperture objective lenses. Copyright © 2008 John Wiley & Sons, Ltd.