Stimulated Raman spectroscopy
Stimulated Raman spectroscopy, also referred to as stimulated Raman scattering (SRS), is a form of spectroscopy employed in physics, chemistry, biology, and other fields.
The basic mechanism resembles that of spontaneous Raman spectroscopy: a pump photon, of the angular frequency
However, SRS, as opposed to spontaneous Raman spectroscopy, is a third-order non-linear phenomenon involving a second photon—the Stokes photon of angular frequency
In SRS, the signal is equivalent to changes in the intensity of the pump and Stokes beams.
The signals are typically rather low, of the order of a part in 10^5, thus calling for modulation-transfer techniques: one beam is modulated in amplitude, and the signal is detected on the other beam via a lock-in amplifier.
Employing a pump laser beam of a constant frequency and a Stokes laser beam of a scanned frequency (or vice versa) allows for unraveling the molecule's spectral fingerprint.
This spectral fingerprint differs from those obtained by other spectroscopy methods, such as Rayleigh scattering, as the Raman transitions confer different exclusion rules than those that apply to Rayleigh transitions.
[1] In their experiment, they introduced a Kerr cell containing nitrobenzene into a ruby laser cavity to study Q-switching processes.
A year later, Garmier et al.[1] introduced a two-wave mixing framework to describe SRS.
The principle of SRS can be intuitively understood by adopting the quantum mechanical description of the molecule's energy levels.
Initially, the molecule lies in the ground state, its lowest electronic energy level.
Then, it simultaneously absorbs both pump and Stokes photons, which causes a vibrational (or rotational) transition with some probability.
However, the simultaneous absorption of two photons might provide a coupling route between the initial and final states.
When the energy difference between both pump and Stokes photons matches the energy difference between some vibrational (or rotational) state and the ground state, the probability for a transition due to this stimulated process is enhanced by orders of magnitude.
Thus, the SRS signal is proportional to the decrease or increase in the pump, or Stokes beams intensities, respectively.
The first-rate equation describes the change in Stokes beam intensity along the SRS interaction length.
The first term on the right-hand side is equivalent to the amount of intensity gained by the Stokes beam due to SRS.
The second rate equation describes the change in pump beam intensity; its form is similar to the former.
In most cases, the experimental conditions support two simplifying assumptions: (1) photon loss along the Raman interaction length,
Mathematically, this corresponds to and (2) the change in beam intensity is linear; mathematically, this corresponds to Accordingly, the SRS signal, that is, the intensity changes in pump and Stokes beams, is approximated by where
Every molecule has some characteristic Raman shifts associated with a specific vibrational (or rotational) transition.
As this difference starts to differ from a specific Raman transition, the Raman gain coefficient's value drops, and the process becomes increasingly less efficient and less detectable.
An SRS experimental setup includes two laser beams (usually co-linear) of the same polarization; one is employed as pump and the other as Stokes.
When designing the experimental setup, one has great liberty when choosing the pump and Stokes lasers, as the Raman condition (shown in the equation above) applies only to the difference in wavelengths.
Since SRS is a resonantly enhanced process, its signal is several orders of magnitude higher than a spontaneous Raman scattering, making it a much more efficient spectroscopic tool.
[2] The latter corresponds to a spontaneous Raman scattering process performed by a laser with a frequency close to the electronic transition of the subject in the study.
However, it requires the use of highly powerful UV or X-ray lasers that might cause photodegradation and might also induce fluorescence.
All applications utilize the ability of SRS to detect a vibrational (or rotational) spectral signature of the subject in the study.
Stimulated Raman scattering (SRS) microscopy allows non-invasive label-free imaging in living tissue.
Employing femtosecond laser pulses, as was done in the Katz, Silberberg,[7] and Xie[8] groups, allows for an instant generation of a substantial portion of the spectral signature by a single laser pulse.
