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= Coherent Stoke Raman Spectroscopy =

Introduction
Coherent Stokes Raman Scattering (CSRS) is a nonlinear optical process that involves the generation of coherent anti-Stokes Raman scattering (CARS) signals, but it uses a Stokes beam instead of an anti-Stokes beam. In CSRS, two laser beams are used: a pump beam and a Stokes beam, both tuned to specific Raman resonances of the sample. The interaction of these beams in the sample produces a coherent Stokes Raman scattering signal at a different frequency than the pump and Stokes beams, which can be detected and analyzed.

CSRS is a powerful technique for chemical imaging and molecular spectroscopy, particularly in the study of biological tissues and cells. It provides high spatial resolution and sensitivity to molecular vibrations, making it useful for understanding the structure and dynamics of complex molecules and materials. CSRS is a valuable tool in various fields of science and technology, and its applications range from materials science and environmental science to combustion research. Its ability to provide detailed molecular information in real-time makes it a useful technique for studying complex systems and phenomena.

=== CSRS Generating Process === Coherent Stokes Raman Scattering (CSRS) is a four-wave-mixing process that has been overshadowed by other Raman scattering spectroscopy techniques, such as Coherent Anti-Stokes Raman Scattering (CARS) and Stimulated Raman Scattering (SRS). However, CSRS provides a near-identical mapping of molecular oscillators to CARS and SRS, making it a valuable tool for chemical imaging and molecular spectroscopy.

In CSRS, the radiation generated (2𝜔𝑆 − 𝜔𝑝) is red-shifted with respect to the excitation frequencies of the pump (𝜔𝑝) and Stokes beams (𝜔𝑆), similar to the Stokes emission in linear Raman microscopy. Despite its potential benefits, CSRS has not yet been widely implemented for laser scanning microscopy.

One reason for this neglect may be the high degree of resemblance between CARS and CSRS spectra, as reported in some studies, making it difficult to distinguish between the two techniques. Additionally, using visible light sources for CSRS can result in an added fluorescence background, while using near-infrared (NIR) excitation can result in a radiation wavelength that is offside the high quantum yields of common detectors.

Principle
Coherent Stokes Raman Scattering (CSRS) is a nonlinear optical technique that involves the interaction of two coherent laser beams, a pump beam and a Stokes beam, with a sample. The pump and Stokes beams are tuned to the same frequency, with the Stokes beam typically at a lower energy than the pump beam.

When the pump and Stokes beams interact with the sample, they generate a signal beam at a frequency that corresponds to the energy difference between the pump and Stokes beams, which is also the frequency of a Raman resonance in the sample. This signal beam is called the Stokes signal. By detecting and analyzing the Stokes signal, CSRS can provide information about the Raman-active vibrational modes of the sample, allowing for chemical imaging and molecular spectroscopy.

To reduce the background noise in CSRS, there are two commonly used approaches. The first involves using two angle-tuned narrow bandpass filters that are covered with films. These filters are set up at specific angles to block out unwanted wavelengths and only allow the target wavelength range to pass through, reducing the amount of background fluorescence noise.

The second approach is to modulate the frequencies of the pump and Stokes beams. This is because the intensity of fluorescence noise is dependent on either the Stokes or pump frequency, whereas the CSRS signal is dependent on both frequencies. By modulating the frequencies of both beams and demodulating the difference frequency between them, it is possible to eliminate or significantly reduce the background noise.

Applications
Coherent Stokes Raman Scattering (CSRS) has several advantages over other Raman scattering techniques, such as CARS and SRS. These advantages include high sensitivity and spatial resolution, as well as the ability to probe specific vibrational modes of the sample. As a result, CSRS has potential applications in various fields, such as biology, materials science, and environmental science, and continues to be an active area of research and development.

One advantage of CSRS is its ability to combine with near-infrared (NIR) light, which provides low absorption and scattering coefficients, enabling deep tissue imaging. In addition, the use of long-wavelength pump and Stokes beams in CSRS allows for the detection of lower-energy vibrational modes, providing more comprehensive molecular information.

Furthermore, CSRS microscopy can be configured with a modified phase-matching geometry to radiate more light in the backward direction. This feature can be particularly beneficial for the investigation of thick samples, real-time spectroscopy, multi-focus imaging, and endoscopy applications.