Introduction to Raman Microspectroscopy

In principle, Raman microspectroscopy is attractive because the practical diffraction limit is on the order of the excitation wavelength, which is about lOx 

Even though nothing can be done to make the Raman cross section of vibrational bands any greater without the application of such techniques as resonance Raman spectroscopy or surface-enhanced Raman scattering (SERS), several important technological developments led to the design of todays truly powerful Raman spectrometers. These included (in no particular historical order) the development of the following  

The development of CCD-Raman spectrometers revolutionized Raman spectroscopy. Within the space of about 5 years, about 10 Raman spectrometers based on multichannel technologies were introduced commercially. Several of the CCD-Raman spectrometers were either designed for or could be readily modified for microspectroscopy. Although FT-Raman microspectrometers have been reported (e.g., [22]), they have not proved very popular for three reasons safety, sample heating, and sensitivity. In practice, therefore, CCD-Raman spectrometers have proved to be far more successful for Raman microspectroscopy than FT-Raman spectrometers and most instruments are based on this concept. 

Raman microspectroscopy is not a new concept. In 1966, Delhaye and Migeon [32] showed that a laser beam could be tightly focused at a sample, and that Raman-scattered light could be collected and transferred to a spectrometer with minimal loss. Their calculations showed that the increased irradiance more than compensated for the decrease in the size of the irradiated volume. The first Raman microscope was reported by Delhaye and Dhamelincourt in 1975 [33]. An instrument based on these principles (the MOLE ) was introduced by Jobin-Yvon at about the same time however, the optical scheme used for imaging, which employed global illumination, was inefficient and it was not until the advent of CCD-Raman spectrometers that the advantages of Raman microscopy became apparent. 

In principle, Raman microspectroscopy is attractive because the practical diffraction limit is on the order of the excitation wavelength, which is about 10-fold smaller for Raman spectroscopy with a visible laser than for mid-lR spectroscopy. It is therefore possible to focus visible or NIR laser light to much smaller spot 

In practice, therefore, CCD-Raman spectrometers have proved to be far more successful for Raman microspectroscopy than FT-Raman spectrometers, and most instruments are based on this concept. 

---