Coherent anti-Stokes Raman spectroscopy

Of the non-linear Raman spectroscopy is a group of spectroscopic analysis method, based on the non-linear Raman scattering light of solids or gases. For excitation ( narrow-band ) lasers are used, which is why this method belong to laser spectroscopy.

The group of non-linear Raman spectroscopies can be divided into three main methods of the exploited Effect:

Operation

The difference with the (linear) Raman spectroscopy lies in the particular type of excitation. In this case, either of two laser beams of different frequency ( pump and Stokes laser ) having appropriate frequencies required (eg for CARS) or more photons of a single laser beam (e.g., SRS ). The photons are superimposed in the Raman medium so-called ( depending on the application, a solid, or atoms or molecules in the gaseous state ). The interaction of photons with matter of the medium thereby creating a laser -like output beam. The emitted beam is precisely then resonantly enhanced when the overlap of the frequencies of the input corresponds to a photon Raman resonance. The signal appears at the magnitude of the resonance Raman shifted to lower or higher frequencies relative to the frequency of the input photons. These beams are referred to as the Stokes or anti-Stokes radiation.

Coherent Anti -Stokes Raman Scattering

The CARS spectroscopy is used, inter alia, for the study of material properties, thermodynamic properties (e.g. temperature) or to the species - selective microscopy. It is used among others in the molecule spectroscopy, plasma and combustion diagnostics, microscopy and quality assurance of diamonds. It can also be used in the investigation of biological systems for the visualization of almost any arbitrary molecular species by excitation of characteristic vibrational modes ( CH2 vibration of lipids Amidschwingung for proteins, phosphate vibration for DNA), which therefore does not need to be labeled by fluorescent dyes.

Stimulated Anti -Stokes Raman Scattering

The anti-Stokes Stimulated Raman scattering (English stimulated Anti -Stokes Raman scattering, SARS ) is also based on the principle of non-linear overlay of input photons in a Raman medium. Unlike CARS are not superimposed photons of different frequencies with SARS, but several photons of the same frequency. These photons are superimposed in a so-called four-photon process, and it to resonate in the Raman medium, for example by the energy difference between two states of vibration of a molecule occurs. The resulting photons are either shifted to lower frequencies ( Stokes ) or higher frequencies ( anti-Stokes ).

In general the method is used to shift the frequency of the laser at higher frequencies ( the anti-Stokes beam ), which are not achievable with existing lasers. The shifted radiation passes into the UV and vacuum UV range, and is used in molecular spectroscopy.