Choosing the most suitable laser wavelength for your Raman Application
Over the years, dispersive Raman spectroscopy has increasingly been implemented for material identification due to its portability and sampling flexibility. When choosing a Raman instrument, one of the primary concerns is the wavelength of the laser that is integrated into the Raman spectrometer system.
Even though the Raman shift of any material is only related to the material’s specific chemical structure and is independent of the excitation wavelength, different wavelengths have their specific strengths and weaknesses. So how does one select a laser excitation wavelength for specific applications? There are many different options, but the three most widely used are 532nm, 785nm and 1064nm. Some important indicators of performance are listed below. The most obvious is the difference in excitation efficiency. Raman scattering efficiency is proportional to ?-4 , where ? is the laser wavelength. For example, Raman scattering at 532nm is a factor of 4.7 more efficient than at 785nm and 16 times better than 1064nm, effectively meaning that scan time is much longer at higher wavelengths as compared to 532nm, assuming that all other conditions remain the same. Detector sensitivity is another concern. Since Stokes Raman is used for most such instruments, Raman signals excited by a 532nm laser are distributed in the visible range, where the response is best for most silicon-based CCD chips. Meanwhile, Raman signals from 785nm fall within the NIR range (750-1050nm), where the response is still relatively good. For 1064nm, however, typically there is no response from the CCD above 1100nm, and therefore an IR sensor InGaAs detector, which has about 1/10 the efficiency of the CCD, is used. >>>