In 1928, the Indian scientist Ch.V. Raman discovered in studies on light scattering the, so called, Raman effect. Raman received the 1930 Nobel Prize in Physics “for his work on the diffusion of light and the discovery of the effect named after him” for the experimental proof of Raman effect. Initially, interest in Raman spectroscopy was immense, as instrumentation was not a major challenge at that time and was present in many laboratories.
The excitation radiation was generated by powerful mercury vapor lamps that analyzed Raman scattering with photographic plates. However, after the Second World War, sensitive infrared detectors were developed and soon thereafter the first automatic IR spectrometer came on the market, so that the IR spectroscopy became a routine method and thus Raman spectroscopy became less prevalent. Only with the development of the laser in 1960 did Raman spectroscopy experienced a renaissance. Nonetheless, Raman spectroscopy did not become routine at the time, since the biggest obstacle – fluorescence – did not make this measurement method attractive. Two major developments made in the late 1980s / early 1990s brought Raman spectroscopy back to the spotlight:
The Fourier transformed Raman spectroscopy with excitation wavelengths in the NIR range, where excitation of the fluorescence is extremely rare, and CCD detectors for dispersive Raman devices, which make it possible to record complete Raman spectra in just a few seconds.
The acquisition of Raman spectra requires an intense and monochromatic light source. These properties are fulfilled by the laser radiation. Furthermore, laser radiation is temporally and spatially coherent, parallel and well focusable.
For Raman spectroscopy, laser radiation from the UV to the visible to the NIR spectral range is used.
Nd-YAG laser are solid-state lasers which are used predominantly in FT-Raman devices with a wavelength of 1064 nm. By frequency multiplication, the Nd-YAG laser can also be operated at wavelengths of 532 nm or 266 nm.
UniKLasers is producing single-frequency solid state lasers with line widths of < 500kHz which means a coherence length of > 100m. Because of its brilliant beam parameters the M² is typically better than 1.1, this means Gaussian intensity profile across the beam, collimation and circularity of the beam. This is especially important to those companies that have microscope and/or imaging accessories, since it determines how small a spot size of the laser can be focused onto the sample.
Lasers suitable for Raman spectroscopy must have:
– high power and frequency stability
– narrow bandwidth
– no side lines
– very high beam quality