Raman spectroscopy is one of the most informative probes for studies of material properties under extreme conditions of high pressure. The Raman techniques have become more versatile over the last decades as a new generation of optical filters and multichannel detectors become available. Here, recent progress in the Raman techniques for high-pressure research and its applications in numerous scientific disciplines including physics and chemistry of materials under extremes, earth and planetary science, new materials synthesis, and high-pressure metrology will be discussed. 1. Introduction Raman spectroscopy probes elementary excitations in materials by utilizing inelastic scattering processes of a near ultraviolet, visible, and near infrared monochromatic light source (commonly a laser). The scattered radiation forms a spectrum near that of the excitation laser wavelength. The results are easy accessible for recording as very sensitive detectors are available in this spectral range. The main strength of the Raman spectroscopy is the ability to provide a great wealth of easily analyzable information very rapidly. The Raman spectra can be used to characterize the elastic, vibrational, electronic, and magnetic subsystems through the observations of the corresponding elementary excitations. The Raman spectra of phonons (lattice and molecular vibrations) have a very high selectivity, thus permitting finger-printing analysis of the materials phase that can include its composition and state. The information about the crystal structure is provided through the vibrational selection rules, which govern the Raman activity of phonon modes depending on their symmetry and the wave vector. The access to electronic and magnetic states is attained through the coupling to the vibrational states, and through the observations of the spectra of the electronic and magnetic excitations. As the spectra of elemental excitation change with the application of pressure, the associated phenomena can be effectively studied by in situ Raman spectroscopy. These include changes in the energy of the vibrational excitations, phase transformations (including melting), chemical reactivity, and magnetic and electronic transitions. This information can be used for multiple applications because these transformations are of interest for fundamental physics and chemistry, materials, and earth and planetary science. Last (but not least), Raman spectroscopy is a great tool for measurements of pressure at both extremely low and extremely high temperatures. Previous comprehensive reviews on Raman
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