Stimulated Raman Scattering

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Molecules (and crystals) have their characteristic vibrations that depend on the composition, atomic arrangement, topology etc. of the system. This also means that characteristic vibrations are fingerprints of a given molecule and can be used for its identification and detection.
The energy of these vibrational modes is in the infrared region and can be studied directly through infrared absorption. Another method, called Raman scattering, however, is based on specific interaction of monochromatic light with matter: the inelastic scattering. During scattering most of the incident photons will have the same energy after being scattered by a molecule (this is the so called elastic scattering), but a minor part of them will lose or gain energy during the process (this is the so called inelastic light scattering). This energy difference is equal to the energy of a characteristic vibration mode of the molecule. So by recording the spectrum of the scattered light in the region different from the wavelength of the incident monochromatic light it is possible to detect the characteristic vibrations and to identify the molecule involved in the scattering. While Raman scattering is a spontaneous process of low probability, it is widely used to study different objects from small molecules through crystals and nanoparticles to viruses, cells and DNA.
The sensitivity of Raman spectroscopy can be increased remarkably by replacing the spontaneous process with a stimulated one. In this case two monochromatic light sources (lasers) are used instead of one and when their energy difference matches a Raman transition of the molecule, stimulated excitation of molecular transition rate occurs. This results in amplification of the Raman signal and the intensity of the two lasers changes. Stimulated Raman scattering (SRS) is a non-linear process requiring high light intensities (achieved by pulsed laser sources) and the spatial and temporal synchronization of the two laser beams.
The sensitivity of SRS is a few orders of magnitude higher than that of spontaneous Raman scattering. This means a SRS measurement requires a very short time (below milliseconds) and can be used for real time monitoring and imaging of different processes in biological systems.


Wigner Research Center fro Physics

The Wigner Research Centre for Physics of the Hungarian Academy of Sciences is the leading research institution in physics in Hungary. It has numerous grants from the European Framework programmes and H2020. The Applied and Nonlinear Optics Department deals with the development of optical measuring systems, vibrational and optical spectroscopy, study of femtosecond lasers, and ultrafast and attosecond physics, and crystal physics research.