CARS is a spectroscopy method that has been known for decades and was used in particular to study combustion processes such as in engines and car engines. The CARS effect is a third-order nonlinear effect. Here, two laser beams intersect (pump beam ωP and Stokes beam ωS) in a medium and the antistoke ray ωaS is generated. With this, the chemical structure, the temperature and the pressure can be determined based on the Raman selection rules.
So far, these experiments have been complicated and required expensive fs laser setups. Leukos has now developed a white-light laser that enables the construction of inexpensive, easy-to-use and quickly adjustable broadband CARS systems.
CARS/Raman setup
The figure above shows the schematic structure of our CARS system, which is coupled to a high-performance Raman system from Renishaw. A supercontinuum source from Leukos consisting of a sub-ns pulsed microChip laser (Nd:YAG @ 1064nm) and a photonic crystal fiber (PCF) serves as excitation for CARS. Part of the beam goes into the PCF, which broadens the pulse spectrally to 400 to 2000 nm, with wavelengths <1064 nm being blocked by means of a long-pass filter, which would otherwise overlay the anti-Stokes signal. The remaining longer-wave portion >1064nm represents the broadband Stokes source. The narrow-band pump beam is radiated into a delay line to compensate for the path length difference of the PCF. The two beams are superimposed using a dichroic mirror. However, these parts could be used for other processes, such as the stimulated Raman effect or the stimulated CARS effect. The lens is used to focus on the sample, thus generating the CARS signal in transmitted light. The light is captured with the lens of the ›Raman Renishaw inVia‹ and analyzed using the spectrometer and CCD of the Raman system.
In this way, the CARS signal traverses the same optical path as the Raman signal. A motorized change of the filter sets in the Raman system in a matter of seconds enables the user to carry out CARS and Raman measurements at the same location and almost simultaneously. This allows a direct comparison of both methods and a direct decision to be made as to which measurement method is more effective.
CARS versus Raman
Due to the non-linearity, CARS measurements are significantly stronger than Raman measurements with comparatively similar parameters. This applies above all to strong Raman scatterers and, to a limited extent, to the weak ones. The non-linearity of the CARS effect also leads to a higher local resolution, which is illustrated in the graphic below. With objectives of 20x magnification, CARS achieves a factor of ≈4 better depth resolution than Raman, although the resolution shown here is limited by the sample structure. With suitable lenses, resolutions can be achieved that are clearly in the sub-μ range and are not subject to Abbe's resolution limitation.
In addition, the anti-Stokes signal is blue-shifted and therefore has a shorter wavelength than the irradiated laser. In contrast to Raman, with CARS there are no disruptive fluorescence effects that would otherwise be superimposed on the signal. With more intensive CARS signals, measurement times can be drastically reduced while the resolution is increased at the same time. This means that fast 2D and 3D mappings can be implemented on dynamic processes, such as concentration measurements in microchannels.
However, a disadvantage is the interference with a non-resonant background caused by non-resonant four-photon interaction, which changes the line shape and complicates the analysis of the spectra. This can be corrected using mathematical methods or suppressed experimentally using polarization techniques.
Folded BoxCARS, in which the pump beam is split into two beams, is particularly suitable for this. This leads to a further narrowing of the scattering volume due to the smaller overlapping of the beams. With this method, even higher spatial resolutions can be achieved, with which volumes in the attoliter range can be examined. Another advantage results from the anti-Stokes beam, which is spatially separated from the other beams, which simplifies the spectral analysis with regard to the choice of filters.
Example of fast mapping showing shape based on selected Raman bands.
Because of the new structure and the reduced costs, we see completely new application possibilities in addition to the study of combustion processes mentioned above:
- Shaping in biophysics (e.g. cell structure). (Advantage: fast mapping and no fluorescence)
- Study of rapid biophysical processes (e.g. observation of cell death down to the molecular structure)
- Analysis of materials from the environmental sector such as food, wood derivatives, dyes (e.g. in clothes) that cannot be directly measured by Raman because of their fluorescence.
- Analysis of nanoparticles that are too small for direct Raman measurements.
Not all of these new applications have yet been implemented and we are eagerly awaiting interested parties who would like to work here.
We see a great advantage in the CARS integration into the high-quality Raman system, since research in these areas always requires good Raman spectra.
