What are the advantages and disadvantages of near infra-red (NIR) lasers for Raman?

Near infra-red (NIR) lasers for Raman typically includes a range of wavelengths greater than 700 nm, such as 785 nm, 830 nm, 980 nm and 1064 nm. The key reason for the use of NIR Raman is for fluorescence suppression, but there are a number of drawbacks which must be considered. Whilst at times NIR Raman is invaluable, it should certainly not be considered the best solution for every sample.

Advantages

  • Fluorescence suppression can often be assisted using NIR lasers.  Fluorescence is a two photon process, which firstly requires absorption of a photon, and is followed by the emission of a fluorescent photon.  On the other hand, Raman is a one photon scattering process, which does not require absorption.  Whilst many materials absorb in the visible region, fewer do so in the NIR region.  Thus, in many cases NIR lasers will not give rise to fluorescence (since absorption does not occurs), but Raman scattering will be present as normal.  In cases where samples strongly fluoresce with visible excitation NIR Raman can provide a solution and allow a good Raman spectrum to still be obtained.

Disadvantages

  • Decreased sensitivity can result from NIR excitation, since Raman scattering efficiency is proportional to λ-4, where λ is the laser wavelength.  Thus, Raman scattering at 785 nm is almost a factor of 5 less efficient than that at 532 nm.  This problem is compounded by decreasing sensitivity of the CCD detector in the NIR Raman range.  As a result, measurement times are often considerably increased to get similar spectral quality to measurements made with visible lasers.
  • NIR lasers often have beam qualities (e.g., beam width and divergence) which are not so well suited for microscopy.  The result is that spatial resolution can be somewhat compromised, and thus achievable results may not meet theoretical predictions.

 

 

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