Coherent anti-Stokes Raman spectroscopy

Unlike Raman spectroscopy, CARS employs multiple photons to address the molecular vibrations, and produces a coherent signal.

In 1965, a paper was published by two researchers of the Scientific Laboratory at the Ford Motor Company, P. D. Maker and R. W. Terhune, in which the CARS phenomenon was reported for the first time.

The name coherent anti-Stokes Raman spectroscopy was assigned almost ten years later, by Begley et al. at Stanford University in 1974.

Classically, the Raman active vibrator is modeled as a (damped) harmonic oscillator with a characteristic frequency of ωv.

In CARS, this oscillator is not driven by a single optical wave, but by the difference in frequency (ωp-ωS) between the pump and the Stokes beams instead.

On a molecular level, this implies that the electron cloud surrounding the chemical bond is vigorously oscillating with the frequency ωp-ωS.

These electron motions alter the optical properties of the sample, i.e. there is a periodic modulation of the refractive index of the material.

While intuitive, this classical picture does not take into account the quantum mechanical energy levels of the molecule.

If a Stokes beam is simultaneously present along with the pump, the virtual state can be used as an instantaneous gateway to address a vibrational eigenstate of the molecule.

Instead, the molecule acts like a medium for converting the frequencies of the three incoming waves into a CARS signal (a parametric process).

There are, however, related coherent Raman processes that occur simultaneously which do deposit energy into the molecule.

The Raman signal is detected on the red side of the incoming radiation where it might have to compete with other fluorescent processes.

The spectroscopic line shape of the CARS intensity therefore resembles a Fano profile which is shifted with respect to the Raman signal.

However, given the limits on input power (damage threshold) and detector noise (integration time), the signal from a single transition can be collected much faster in practical situations (a factor of 105) using CARS.

However, at very low concentrations, the advantages of the coherent addition for the CARS signal are reduced and the presence of the incoherent background becomes an increasing problem.

Those molecules together generate a Raman signal in the order of 2×10−11 W (20 pW) or roughly one hundred million photons/sec (over 4π steradians).

More recently, CARS microscopy has been utilized as a method for non-invasive imaging of lipids in biological samples, both in vivo and in vitro.

Moreover, RP-CARS, a particular implementation of the Coherent anti-Stokes Raman spectroscopy microscopy, is used to study myelin and myelopathies.

In 2020, Scully and team used femtosecond adaptive spectroscopic techniques via CARS to identify individual virus particles.