What laser wavelengths are used for Raman spectroscopy?

Laser wavelengths ranging from ultra-violet through visible to near infra-red can be used for Raman spectroscopy.  Typical examples include (but are not limited to):

  • Ultra-violet:  244 nm, 257 nm, 325 nm, 364 nm
  • Visible:  457 nm, 473 nm, 488 nm, 514 nm, 532 nm, 633 nm, 660 nm
  • Near infra-red:  785 nm, 830 nm, 980 nm, 1064 nm

The choice of laser wavelength has an important impact on experimental capabilities:

  • Sensitivity.  Raman scattering intensity is proportional to λ-4 where λ is the laser wavelength.  Thus an infra-red laser results in a decrease in scattering intensity by a factor of 15 or more, when compared with blue/green visible lasers.
  • Spatial resolution.  The diffraction limited laser spot diameter can be calculated according to the equation, diameter = 1.22 λ  /  NA (where λ is the wavelength of the laser, and NA is the numerical aperture of the microscope objective being used).  For example, with a 532 nm laser, and a 0.90/100x objective, the theoretical spot diameter will be 0.72 µm – with the same objective, a 785 nm laser would yield a theoretical spot diameter of 1.1 µm.  Thus, achievable spatial resolution is partially dependent on choice of laser.
  • Optimisation of resulting based on sample behaviour.  For example:

    • Blue or green lasers can be good for inorganic materials and resonance Raman experiments (e.g., for carbon nanotubes and other carbon materials) and surface enhanced Raman scattering (SERS).
    • Red or near infra-red (660-830 nm) are good for fluorescence suppression.
    • Ultra-violet lasers for resonance Raman on bio-molecules (such as proteins, DNA, and RNA), and fluorescence suppression.

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