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Coherent Anti-Stokes Raman Scattering (CARS) is a spectroscopic method sensitive to vibrational signatures of molecules similar to Raman spectroscopy. CARS uses photons of two different colours to address the molecular vibration, and creates a resonantly enhanced signal at a third wavelength when the frequency difference of these two photons coincides with a Raman resonance giving vibrational contrast.

These two photons need to be present at the sample position at the same time, i.e. overlapped in space as well as in time. To address the different Raman resonances, the frequency difference between the two photons needs to be adapted, i.e. one of the laser wavelengths has to be tuned.

CARS measurements have initially been performed using two synchronized femtosecond or picosecond laser beams. CARS spectroscopy thus needs to be done by tuning the wavelength of either of these beams, which however, has constraints regarding tuning speed. Any analysis relying on the need of a CARS spectrum would be limited by this low imaging and update rate. Multiplex CARS (M-CARS) offers a much faster acquisition of a complete CARS spectrum by simultaneous detection of a wide range of Raman shifts. This is accomplished by using a ps pump pulse (determining the spectral resolution) and a broadband fs Stokes pulse covering many Raman shifts at the same time. This allows a detection of a CARS spectrum without the need to tune the laser source.

By chirping the fs Stokes pulse (spectral focussing, or C-CARS), it is possible to still obtain again a high spectral resolution, depending on the amount of chirping. A slightly different scheme is Stimulated Raman Scattering (SRS) that also makes use of two photons of different colour, however, there is no signal at a third frequency. The signal is rather experienced as stimulated Raman loss, or stimulated Raman gain, which typically is detected via lock-in detection.

One advantage of SRS over CARS is that it is background free, and also, the line shapes are the same as in continuous wave Raman and existing data bases of molecular fingerprints may be reused. SRS can make use of basically the same laser sources as CARS. SRS also comes in the variant of multiplex SRS with the same advantage of increased acquisition speed of a complete spectrum.

The venteon OPCPA, with an additional green output, offers two optically synchronized beams, one to be used as the pump beam, and the output from the OPCPA as the broadband Stokes beam. The high pulse energy in the µJ range at MHz repetition rate allows for ample spectral intensity over the large bandwidth of several thousand wavenumbers. Differential CARS (D-CARS) is another variant to obtain high-speed spectra, utilizing a broadband oscillator and using different parts of the broad spectrum as pump and Stokes. Some variants also use pulse shaping to tailor the individual parts of the spectrum.

The venteon ultra, offering an octave spanning spectrum, is an ideal laser source for these CARS/SRS variants.


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