The "inelastic scattering of light," or Raman effect, was observed in practice for the first time in 1928 by C.V. Raman for
which he was awarded the Nobel Prize in 1930. It is only in the last two decades, however, that Raman spectroscopy has begun
to realize its potential as an almost universally applicable analytical technique from materials and life sciences applications
to point of care analysis. This is primarily thanks to the availability of compact laser sources, high sensitivity cameras,
and high resolution compact spectrometers.
Wavelength is Key
The physical basis of the technique offers huge flexibility and advantages in comparison with its sister technique, infrared
spectroscopy, but at the same time presents a key challenge: the excitation needs to be highly (i) monochromatic (the Raman
bands have the same shape as the light source) (ii) collimated and (iii) intense (due to the low probability of inelastic
scattering, < 1 in 106 photons). Hence, it is the advent of lasers that has brought Raman spectroscopy — literarily — into the field.
In selecting the correct laser, the wavelength is important and depends on the application. For example, fluorescence is usually
much stronger than the Raman scattering signal but unlike the fluorescence, the Raman scattering signal is observed even when
exciting at wavelengths outside the absorption spectrum. Figure 1 illustrates this by showing that the fluorescence that swamps
the Raman spectrum of a boron based fluorophore when the Raman spectrum is recorded at 785 nm (where absorption/fluorescence
competes with the Raman process) is completely absent in the spectrum obtained at 1064 nm. Thus highlighting the importance
of selecting the right excitation wavelength.
Figure 1: Near-IR dispersive Raman spectrosocpy. Comparison of the spectrum of a solid sample of an organic fluorophore at
785 nm (red) with that obtained at 1064 nm (blue). Excitation with a Cobolt Rumba™ 1064 nm (100 mW at sample), detection
4 by 5 s exposure, Andor idus163 spectrograph with an iDus-InGaAs detector.
DPSS Lasers for Raman Spectroscopy
Until recently, ion gas lasers (Ar, He, HeCd, and Kr) have been the first choice for Raman spectroscopy. However, the ever
increasing number of wavelengths available with continuous wave diode pumped solid state (DPSS) lasers together with the high
average powers (> 1 W) and compact footprint means that multi-wavelength Raman spectroscopy can be implemented as a turnkey
low maintenance solution in any laboratory as well as in field portable applications. For example, Cobolt DPSS lasers qualify
as excellent Raman excitation sources, thanks to their extremely narrow linewidth (<1 MHz), excellent wavelength stability,
and high level of spectral purity (-60 dB).