New insights into Raman thermometry
Among its diverse applications, Raman spectroscopy is a well-established tool for contactless thermometry, enabling non-destructive measurements in molecules and solids. Traditionally, the temperature dependence of the Raman cross-section has been described using Bose-Einstein statistics, which governs the thermal distribution among the vibrational degrees of freedom, and scales linearly with the Bose occupation factor.
In recent experiments, featured in Physical Review Letters, the Femtoscopy group has shown striking deviations from this behavior and highlighted a fundamental difference between spontaneous and stimulated (coherent) responses. In contrast with spontaneous spectroscopy, coherent Raman exploits the joint action of multiple light pulses to stimulate and probe vibrational excitations. This approach offers several advantages, such as fluorescence-free spectra, enhanced cross sections and improved spectro-temporal resolutions. However, it also introduces additional energy transfer channels between light and matter, which ultimately factor out the Bose dependence.
This directly reveals a striking temperature-dependence of the molecular polarizability - the fundamental material property underlying the Raman response - and explains the unexpected deviations from the classical Bose model, commonly used in spontaneous Raman spectroscopy.
Therefore, mapping the polarizability's temperature dependence via stimulated Raman experiments represents a powerful tool to investigate vibrational energy redistribution processes, unlocking insights that remain elusive to conventional methods.
Authors: Giovanni Batignani, Emanuele Mai, Miles Martinati, Mohanan M. Neethish, Shaul Mukamel, Tullio Scopigno
Magazine: Physical Review Letters Article