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ADVANCES IN RAMAN TECHNIQUES19euMicroscopyandAnalysis | July/August 2017INTRODUCTIONRaman scattering as a probe of vibrational transitions has made leaps and bounds since its discovery, and various schemes based on this phenomenon have been developed with great success.Applications range from basic scientific research, to medical and industrial instrumentation. Some schemes utilise linear Raman scattering, whilst others take advantage of high peak-power fields to probe nonlinear Raman responses.This article intends to provide a brief overview of the differences and benefits, together with the laser source requirements and the advancements in techniques enabled by recent developments in lasers. LINEAR RAMANThe advent of the laser in providing a high-intensity coherent light source has helped to make Raman scattering a useful method for spectroscopy by increasing signal levels of the spontaneous event to enable use of readily available detectors.The most widely used method to date is linear Raman, which takes advantage of commercial continuous-wave lasers. The choice of excitation wavelength depends on the sample used, where in general, a shorter wavelength would yield a higher efficiency yet suffers from higher scattering and may induce damage in biological samples within the UV region.A diode pumped solid state (DPSS) laser with a wavelength between 473 nm and 1064 nm, a narrow bandwidth output of few tens of GHz or below 1 MHz if needed within the linewidth of vibrational transitions for high resolution, low noise (less than 0.02%) and excellent beam quality (fundamental transversal electromagnetic mode TEM00) provides optimised performance for the resolution of the Raman measurement needed.The wavelength is chosen based on the sample under investigation, with 532 nm being commonly used for the necessary virtual electronic transition. In the following section, four examples from different areas of Raman applications show the diverse applications of linear Raman and what advances have been achieved.An example of studying a real-world application, the successful control of food quality using Raman spectroscopy and multivariate analysis, is described by Jernshøj et al.1. Examining the quality of meat and vegetables is discussed, together with the source specification, when constructing a system for conditions outside of a laboratory environment. For real-world applications, the laser source needs not only to work at the right output parameters, but also it necessitates a robust design making it portable and highly efficient for integration into instruments and being able to interface with operating software.The two next experiments benefit from the high resolution that a narrow linewidth laser source offers as well as its high beam quality.Laser requirements and advances for Raman techniquesAndreas IsemannLaser Quantum GmbH, 78467 Konstanz, GermanyFigure 1 An example of the RR microfluidic device counting of photosynthetic microorganisms. As the cells of the model strain Synechocystis sp. PCC 6803 flow through the Raman detection area of the microfluidic device, RR spectra were acquired continuously about 27 times every second. The v1 RR band was used to differentiate 12 C- and 13 C-cells. The intensity of this band was plotted against the temporal axis and displayed in green and red for 12 C- and 13 C-cells, respectively. A part of the figure near 23.3 s was enlarged to show a 13 C- and 12 C-cell passed through the Raman detection area sequentially only about 0.1 s apart from each other; the untreated RR spectra of those two cells show a distinctive red shift of all of the carotenoids RR bands. Reprinted (adapted) with permission from Li et al2. Copyright (2012) American Chemical Society