TECHNOLOGY RamaN SpECTROSCOpY
From lab to fab
Stephen mounsey discovers some new approaches to bringing Raman spectroscopy from the research laboratory to industrial production lines
R
aman spectroscopy can offer incredible capabilities in industrial quality control and analysis,
particularly in the semiconductor and pharmaceutical markets. Though the technique is not new, it is only beginning to find its feet outside of research laboratories. A Raman spectrometer analyses a sample by using a laser to excite the electrons surrounding its molecules from their ground state to so-called virtual energy levels. The majority of these excited molecules fall back down to their original energy state, but a small proportion of them (approximately one in ten million) fall to a different, non-ground state. The difference in energy between the original and new state can be measured by a Raman spectrometer, and although the signal is very weak relative to other scattered light, the differences in wavelength observed are very characteristic of the chemical composition of the sample. The Raman signal can provide
16 ELECTRO OpTiCS l june 2011
a chemical fingerprint, and one that can give particularly useful information about the overall structure of larger molecules or bulk materials. Two samples may, for example, be chemically identical, with their molecules arranged in different phases; here Raman spectroscopy is able to quickly determine the phase of each, whereas techniques such as FT-IR or UV-Vis are not. Applications for this kind of analysis exist in the pharmaceutical industry, where drug developers need to know exactly in which phase chemicals exist within their products; such factors can have an impact on the speed at which the drug is absorbed into the body. Raman spectroscopy can allow drug companies to ensure that the correct phase is present – and, because the technique is very quick, pill-by-pill checking is also possible.
In semiconductor manufacturing,
stresses in the material must be carefully controlled during wafer processing. Raman spectroscopy can provide a quick way of checking the stress in a material, as the Raman signal of a known material will change depending on the type of stress it is under. These measurements can be taken instantly, and from a stand-off distance of up to 2m.
Keeping it narrow Photonics companies produce lasers for Raman spectroscopy in a wide range of specifications and capabilities. The wavelength of the laser is selected based on each application, and can range all the way from near-UV to near-IR. The linewidth of the laser is a more
By combining Raman imaging with confocal microscopy, users are able to produce accurate maps of chemical distributions within samples, such as the drug distribution within this tablet Image courtesy of WiTec
stringent criterion than its wavelength, however, as Stuart Nunn, technical sales engineer at Laser Components, explains: ‘With Raman, we are exciting the molecule from a low energy state to a higher virtual energy state. Ideally, this excitation should be done with a monochromatic light source, which means it should have a well-defined wavelength and energy.’ Laser Components produces a range of diode lasers for use as Raman excitation sources, optimised to achieve the narrow linewidth, monochromatic light required. ‘Laser diodes are quite monochromatic anyway,’ adds Nunn, ‘but we improve this by combining them with volume
although
spectral linewidths in the range of 1GHz are sufficient for most Raman applications, being sharper than that does not hurt
Bragg gratings. These gratings stabilise the wavelength with respect to the temperature of the diode, and also decreases the spectral linewidth, so as to make the laser even more well- defined and monochromatic.’ The diode laser packages can also include a thermoelectric cooler to further stabilise wavelength. According to Nunn, Laser
Components tries to keep the costs of its Raman lasers down by supplying them in simple packaging. ‘Fibre-delivered lasers for Raman are becoming increasingly popular,’ he adds. ‘These are easy for us to make, as they use the standard butterfly package, which customers recognise; it is fibre delivered, thermoelectrically cooled, and easy to integrate.’ The company is planning on selling these lasers to system integrators producing hand-held Raman analysers for quality control applications. Although effective and relatively cheap, diode lasers are not capable of supplying the narrow linewidth light required for more sensitive Raman spectroscopy. Volker Pfeufer, senior product line manager at Coherent, says that the narrowness of laser linewidth required depends on the type of material being analysed, as well as the resolution of the analysis. ‘When analysing liquids and solids, the linewidth of the laser used is not all that important, as the spectral lines we’re looking at are wide – somewhere in the region of 1GHz,’ he says, noting also that a linewidth of 1GHz is 1,000 times wider than a linewidth of 1MHz. ‘When analysing gasses, on the other hand, we’re looking at drastically smaller linewidths in the range of 100MHz to 1GHz, depending on temperature. Here we need very narrow linewidth lasers, so that the atomic transitions can be resolved.’ Coherent’s offering for the Raman market is a single type of laser suitable for all Raman applications.
www.electrooptics.com
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