Definition of coherent anti-Stokes Raman spectroscopy

Photonics Dictionary

coherent anti-Stokes Raman spectroscopy: Coherent anti-Stokes Raman spectroscopy (CARS) is a powerful technique used in spectroscopy to probe molecular vibrations in a sample. It is based on the Raman effect, which involves the inelastic scattering of light by molecules, resulting in a shift in the energy (frequency) of the scattered photons.

In CARS, two laser beams, known as the pump beam and the Stokes beam, are overlapped in the sample. These laser beams have slightly different frequencies, \( \omega_p \) and \( \omega_s \), respectively, and the frequency difference, \( \omega_p - \omega_s \), matches the frequency of a molecular vibration of interest in the sample.

When the pump and Stokes beams interact with the sample, they generate a coherent optical signal at a new frequency, known as the anti-Stokes frequency, \( \omega_{as} = 2\omega_p - \omega_s \). This anti-Stokes signal is proportional to the intensity of the molecular vibration being probed. By detecting and analyzing the intensity of the anti-Stokes signal, information about the vibrational modes and molecular composition of the sample can be obtained.

Nonlinear process: CARS is a nonlinear optical process, meaning that the intensity of the anti-Stokes signal is not directly proportional to the intensity of the pump and Stokes beams. Instead, it depends quadratically on the intensity of the incident laser beams.

High sensitivity: CARS offers high sensitivity and selectivity for detecting specific molecular vibrations in a sample. By tuning the frequencies of the pump and Stokes beams to match the desired Raman transitions, researchers can selectively probe particular vibrational modes within complex molecular systems.

Fast imaging: CARS can be used for rapid imaging of molecular distributions in biological samples and materials. By raster scanning the laser beams across the sample, spatially resolved maps of molecular concentrations and distributions can be generated with high spatial resolution and minimal sample damage.

CARS spectroscopy finds applications in various fields, including chemistry, biology, materials science, and pharmaceuticals, where it is used to study molecular structure, dynamics, and interactions in diverse systems ranging from biological tissues and cells to complex organic molecules and materials. Its ability to provide label-free, non-invasive, and real-time molecular imaging makes it a valuable tool for fundamental research and applied science.