Best suited Laser wavelength - The correct selection of the laser wavelength can be an important consideration for Raman spectroscopy. With modern equipment, often several laser wavelengths may be employed so as to achieve the best detection of the Raman signal:
For instance, many samples, especially those of an 'organic' or 'biological' nature will be quite fluorescent species. Exciting these samples with a laser in the green (532 nm) may promote this fluorescence, and may swamp any underlying Raman spectrum to such an extent that it is no longer detectable.
In this instance, the use of a laser in the red (633 nm) or NIR (785 nm) may provide a solution. With the lower photon energy, a red or NIR laser may not promote the electronic transition (and hence the fluorescence) and so the Raman scatter may be far easier to detect.
Conversely, as one increases the wavelength, from green to red to NIR, the scattering efficiency will decrease, so longer integration times or higher power lasers may be required.
Thus, it is often most practical to have a number of laser wavelengths available to match the various sample properties one may encounter, be it resonance enhancements, penetration depth of fluorescence.
Green, Red and NIR 785 nm laser excitation of a fluorescent sample.
The strong background seen with the green and red lasers swamps the Raman signal, whereas the 785 nm excitation is outside of the fluorescence range, enabling the Raman to be detected.
Details on Raman Lasers