Ultra-low frequency Stokes and anti-Stokes Raman spectroscopy at 785nm with volume holographic grating filters

We report the first results of ultra-low frequency Stokes and anti-Stokes Raman spectra at 785nm showing clearly resolved frequency shifts down to 10cm(-1) from the excitation line, using commercially available ultra-narrow band notch and ASE suppression filters, and a single stage spectrometer. Near infra-red (NIR) wavelengths are of particular interest for Raman spectroscopy due to the reduced fluorescence observed for most materials. Previously reported attempts to produce ultra-low frequency Raman spectra at 785nm with volume holographic notch filters were largely unsuccessful, due to the fact that these ultra-narrow line notch filters and the wavelength of the laser must be very well matched to be effective. Otherwise, if the filters have any manufacturing errors or the laser wavelength is unstable, insufficient suppression of the Rayleigh scattered light will allow it to overwhelm the Raman signal. Recent improvements in both notch and ASE filters, wavelength-stabilized lasers, and optical system design have enabled low-frequency Raman spectra to be successfully taken at 785nm for several typical materials. Two ultra-narrow line notch filters formed as volume holographic gratings (VHGs) in glass with individually measured optical densities of 4.5 were used to block the Rayleigh scattered light from a matched VHG wavelength stabilized laser. Five discrete peaks below 100cm(-1) were simultaneously observed for sulfur in both the Stokes and anti-Stokes regions at 28, 44, 52, 62, and 83cm(-1). With no degradation in filter performance over time and extremely narrow spectral transition widths of less than 10cm(-1), this relatively simple system is able to make ultra-low frequency Stokes and anti-Stokes Raman measurements at a fraction of the size and cost of traditional triple monochromator systems.