What is the spatial resolution of a Raman microscope?

The achievable spatial resolution is primarily defined by the laser wavelength and microscope objective being used.  The theoretical diffraction limited spatial resolution, according to the laws of physics and optics, is defined by the following equation:

Spatial resolution = 0.61 λ  /  NA

where λ is the wavelength of the laser, and NA is the numerical aperture of the microscope objective being used.

For a 532 nm laser with a 0.90/100x objective this would predict a spatial resolution of 361 nm.  However, whilst this equation is applicable for standard light microscopy, the optical processes occurring during Raman microscopy are much more complex.  For example, scattering of the laser/Raman photons and interaction with interfaces in the sample can reduce this resolution.  Thus, typical Raman spatial resolution is often quoted as being in the order of 1 µm, whilst with ‘good’ samples spatial resolution approaching the diffraction limit can be achieved.

From this equation it can be seen that lower wavelength lasers offer high spatial resolution (e.g., a blue laser at 488 nm will have a smaller spot size than an infra-red laser at 785 nm if the same objective is used), as do high NA objectives (e.g., a 0.90/100x objective will give a smaller spot than a 0.55/50x objective).

Note that the above equation relates to lateral (XY) spatial resolution.  Depth (Z) spatial resolution is more complex, and depends strongly on the confocal design of the Raman microscope being used.  There are several methods in use today, some truly confocal, others pseudo confocal, which work with varying success.  For a true confocal design (which incorporates a fully adjustable confocal pinhole aperture) depth resolution in the order of 1-2 µm is possible, allowing individual layers of a sample to be discretely analysed.  The achievable depth resolution will depend strongly on the laser wavelength, microscope objective, and sample structure.

Cross section of a layered polymer structure, showing the capability of confocal Raman microscopy for analysis of µm thick layers. Total scan depth (Z) is 10 µm.
Cross section of a layered polymer structure, showing the capability of confocal Raman microscopy for analysis of µm thick layers. Total scan depth (Z) is 10 µm.
Raman mapping of 250 nm and 350 nm semiconductor features
Raman mapping of 250 nm and 350 nm semiconductor features Sample provided by ATMEL ROUSSET (Université Paul Cezanne, France)
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