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Coherent Anti-Stokes Raman Scattering (CARS)

CARS is the most recent MPM imaging contrast. CARS microscopy derives contrast directly from Raman- active vibrational modes within molecules and requires two synchronised laser pulses of different wavelengths.

CARS is a third order nonlinear optical process in which a pump field Ep and a Stokes field Es interact with a sample to generate a signal field Eas at the anti-Stokes frequency of ωas = 2ωp - ωs. When ωp - ωs is tuned to be resonant with a molecular vibration (&Omega), the CARS signal can be significantly enhanced, producing a vibrational contrast.

Unlike spontaneous Raman scattering, CARS produces a highly directional field. The two excitation beams (ωp and ωs) form a beating field with frequency ωp - ωs. When ωp - ωs matches &Omega all the molecules within the interaction volume vibrate in-phase.


CARS microscopy permits biological imaging with several advantages:
  • Raman resonance enhancement provides chemical selectivity without the need for labelling.
  • There is little scattering of the near-infrared excitation beams, allowing deep penetration in tissues.
  • Due to the anti-Stokes shift, the CARS signal is of shorter wavelength than one-photon fluorescence. This allows detection in the presence of a strong fluorescent background.
  • Coherent addition of CARS fields generates a large signal;
  • Nonlinear dependence on excitation intensities produces inherent 3D resolution.
  • Low absorption of the near-infrared excitation beams, significantly reduces the photodamage in biological samples.

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