Dual Comb CARS

Coherent Anti-Stokes Raman Scattering (CARS) is a relative of conventional Raman techniques which are sensitive to molecular vibrations associated with chemical bonds. CARS has the advantage of being orders of magnitude more sensitive due to the coherent nature of the signal.

CARS microscopy is a third order (4 wave mixing) non-linear process that involves the interaction of three lasers of differing frequencies (ω); the pump beam (ωp), that elevates the molecule from its ground state to a 1st virtual state, the stokes beam (ωs) that coherently returns the molecule to a first vibrational excitation level of the ground state with a corresponding vibrational excitation at frequency ωvib, and the probe beam (ωpr) that raises the molecule to a higher 2nd virtual state from which it returns to the ground state by emitting the high frequency shifted anti-Stokes radiation. As the molecule returns to the ground state it emits a photon of ωCARS = ωppr - ωs, which is blue shifted by ωvib with respect to the probe laser. As this is a coherent process the emitted signal can be 100,000 times larger than conventional spontaneous Raman.

The anti-Stokes signal is enhanced when the difference between (ωp) & (ωs) corresponds to a Raman active resonance frequency of a vibration. By tuning the pump beam frequency, a range of specific molecules can be monitored and imaged. If however many different molecules are present or there are unknown species in the sample, this technique can have difficulties.

Dual comb CARS uses two laser frequency combs which simultaneously measure all spectral elements over a wide bandwidth with high resolution in microseconds on a single photo-detector. This allows compound identification to be included in imaging and monitoring of fast events such as chemical reactions.

By using GHz repetition rate lasers to generate the frequency combs, the experimental dead-time is greatly reduced and hence the required acquisition time of the system can be much shorter. The timing between the two modelocked lasers and hence the experimental frequency calibration can be precisely predetermined by using a TL-1000 or TL-1000-ASOPS unit.