Influence of laser shape on thermal increase during micro‐Raman spectroscopy analyses
Résumé
Micro-Raman spectrometers are used in various domains, from archaeology to space exploration. These systems use microscope objectives to focus the laser onto the sample to be analysed. Although this method drastically increases the spatial resolution down to a few hundreds of nanometres, it is also commonly admitted that this focused laser beam may heat and alter the studied material. The setting of the laser power is thus a universal problem for any micro-Raman spectroscopy user who has to find a compromise between the Raman signal intensity and the risk of thermal alteration. This parameter is not easy to set based on bibliographic references because, depending on the architecture of the system used, on the laser wavelength and on the sample, damage may occur at very different laser power levels. Here, we describe the different parameters that influence thermal increase induced by the laser and propose easy to make experiments to measure them. A series of experiments is then carried out to study the thermal alteration of various materials when exposed to a laser beam focused with the different microscope objectives of a micro-Raman system. It is shown that, except for very thin samples or powdered materials, the relevant parameters to consider are the laser power at the sample surface and the duration of exposure to the laser beam, whatever the objective used. We also demonstrate that the risk of sample alteration is almost non-existent for thick transparent materials. Interestingly, it is shown that, for semi-transparent materials, alteration may occur at lower laser power for low magnification objectives than for high magnification objectives. These results are explained by the high aperture angle of microscope objectives which induces high dispersion of the laser within the sample.