Nonlinear Raman Spectroscopy
CARS: Coherent Anti-Stokes Raman Scattering
Coherent Anti-Stokes Raman Scattering is a four-wave mixing process involving three different excitation pulses, the so called pump, Stokes and probe pulse which interact with a sample. In case that the difference frequency between pump and Stokes beam matches a vibrational resonance the resonance is excited and is probed by the third pulse. Thus a new blue shifted fourth wavelength is generated. In case no resonance is excited there will still occur a nonresonant background. The advantage about spontaneous Raman Scattering is the coherent excitation of vibrational modes which results in a many orders higher intensity of the generated signal. The disadvantage is the need for the suppression of the background to maintain spectral selectivity.
SRS: Stimulated Raman Scattering
In stimulated Raman scattering two laser beams Pump and Stokes beam at wp and ws coincide on a sample. Only in case the difference frequency matches a particular molecular vibrational frequency, stimulated excitation of the vibrational transition occurs. The intensity of the scattered light at the pump wavelength experiences a loss (SRL) and the Stokes wavelength a gain (SRG). If the frequencies do not match a vibrational resonance no signal occurs. No background like during the CARS process is generated.
In our group there are several laser and amplifier systems available which are well suitable for experiments with nonlinear Raman spectroscopy since they deliver high repletion rates, wide spectral ranges and high pulse energies.
Optical Coherence Tomography (OCT) is an interferometric imaging techniques which allows for non-destructive imaging with high resolution. It is widely used in medical in vivo applications such as dermatology or ophthalmology. The achievable axial resolution depends on the spectral bandwidth of the used light source. Several techniques have been developed to obtain OCT pictures in video rate.
With our home-built Ti:sapphire oscillators it is possible to achieve an axial resolutions of 1µm in air (Link auf Gallerie).
Due to the high peak intensities of the ultrashort laser pulses, it is also possible to increase the chemical selectivity by nonlinear OCT techniques such as CARS-OCT or SHG-OCT. Here the nonlinear signal generated in the sample , i.e. the second harmonic or Anti-Stokes signal respectively, is interferometrically detected and allows for additional structural or chemical information in the recorded image.