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Raman spectroscopy allows for the investigation of vibrational modes of a molecule, by measuring the frequency shift of scattered laser light. This works because the difference in energy between the incident wavelength and the scattered wavelength is equal to the amount of energy required to induce a particular vibrational mode. As a result, higher frequency vibrations such as CH and NH stretches are shifted further from the laser wavelength than lower frequency modes like aromatic ring vibrations allowing you to correlate the spectral bands to the molecular bond structure of the molecule. When attempting to measure ultra-low-frequency vibrations (< 150 cm-1), such as lattice modes, it can become quite challenging to measure the spectrum of the scattered light because of the proximity to the laser. In this blog post, we will examine how holographic filters can be used in conjunction with single frequency lasers to get around these difficulties.
The Raman effect is an extremely weak process, with the Raman shifted light intensities on the order of 60 dB to 70 dB below the Rayleigh scattered light. As a result, when close to the laser line, the Raman signal is completely drowned out by the laser line saturating the detection system. In order to mitigate this issue, all Raman spectroscopy systems include an edge or notch filter (commonly referred to as a Rayleigh filter) that is placed in the collection path to block the laser line without blocking the Raman shifted light. Traditional Rayleigh filters tend to cut-on between 50 cm-1 and 200 cm-1, meaning that the ultra-low-frequency Raman bands are blocked as well. Fortunately, by using volume holographic (VHG) notch filters, like the example shown below from Ondax, you can achieve both extremely high blocking at the laser line and very sharp cut-on’s. 
In order to take advantage of these filters though, you need to use a laser with extremely low sidebands and repeatable center wavelength. For these reasons the 532nm single longitudinal mode (SLM) diode pumped solid-state (DPSS) laser from Oxxius, with a side mode suppression ratio (SMSR) > 70 dB, is ideal for ultra-low-frequency Raman applications. This laser has a 1 MHz line width with a center wavelength accuracy of less than 1 pm, this corresponds to a laser linewidth of 3.33 x 10-5 cm-1 and a center wavelength accuracy of 0.035 cm-1 which falls well within the bandwidth of the VHG notch filter. For this reason, the Oxxius 532nm SLM DPSS laser is the preferred laser source for investigating structural and crystalline modes of non-fluorescing samples. The figure below shows the ultra-low-frequency Raman spectrum of L-Cysteine, collected using the Oxxius SLM 532nm laser with an Ondax VHG filter. From this data, Raman bands are clearly viable at 15 cm-1 with a corresponding anti-Stokes mode at -15 cm-1. 
At RPMC Lasers we offer a wide variety of SLM high stability DPSS lasers from Oxxius with > 100 m coherence length, available at wavelengths ranging from 532 nm to 1064 nm. The high stability and reputability of the Oxxius design are derived from their proprietary, alignment-free monolithic resonator (AMR). The AMR elements are assembled into a single ultra-low-loss optical subsystem, using a proprietary crystal bonding technique. This, in turn, offers the highest spectral quality on the market and high robustness over time.
For detailed technical specifications on our SLM DPSS lasers from Oxxius click here or talk to one of our laser experts today by calling 1-636-272-7227.