Question

Discuss CARS (Coherent Anti-Stokes Raman Scattering) microscopy and its applications.

I dont understand the process which two lasers (stokes & pump) interact together in the sample and causing a vibrational mode. Please describe this part well. Thank you. and how is the signal transferred so the imaged can be created?

Answer 1

CARS process can be physically explained by using either a classical oscillator model or by using a quantum mechanical mode.

1) Classical - Classically, the Raman active vibrator is modeled as a harmonic oscillator with a characteristic frequency of ω_v. The difference frequency (ω_p-ω_S) between the pump and the Stokes beams drives this oscillator instead of single optical wave. When the difference frequency ω_p-ω_S approaches ω_v, the oscillator is driven very efficiently. On a molecular level, this implies that the electron cloud surrounding the chemical bond is vigorously oscillating with the frequency ω_p-ω_S. These electron motions alter the optical properties of the sample, i.e. there is a periodic modulation of the refractive index of the material. This periodic modulation can be probed by a third laser beam, the probe beam. When the probe beam is propagating through the periodically altered medium, it acquires the same modulation. Part of the probe, originally at ω_pr will now get modified to ω_pr+ω_p-ω_S, which is the observed anti-Stokes emission. Under certain beam geometries, the anti-Stokes emission may diffract away from the probe beam, and can be detected in a separate direction.

2)Quantum Mechanical - Quantum mechanically, Our molecule is initially in the ground state. The pump beam excites the molecule to a virtual state. A virtual state allow transitions between otherwise unoccupied real states. 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. The joint action of the pump and the Stokes has effectively established a coupling between the ground state and the vibrationally excited state of the molecule. The molecule is now in two states at the same time(coherent superposition of states). This coherence between the states can be probed by the probe beam, which promotes the system to a virtual state. Again, the molecule cannot stay in the virtual state and will fall back instantaneously to the ground state under the emission of a photon at the anti-Stokes frequency. The molecule is no longer in a superposition, as it resides again in one state, the ground state. In the quantum mechanical model, no energy is deposited in the molecule during the CARS process. Instead, the molecule acts like a medium for converting the frequencies of the three incoming waves into a CARS signal.

CARS microscopy have greatly benefited two fields -

Cell biology and Tissue imaging. Now it is possible to perform chemically specific at the sub-micron scale. It has shown its potential in studies of lipid metabolism, organelle transport, and drug diffusion in living tissue.