Here, in our article in Laser Focus World August 2018, we discuss the lasers that can be used for Raman spectroscopy and why.
Numerous different wavelengths of light are commonly used in, ranging from the ultraviolet (UV) through the visible, and into the near-infrared (near-IR). Choosing the best illumination wavelength for a given application is not always obvious. Many system variables must be considered to optimize a , and several of them are connected to the wavelength selection.
Why is the choice of laser wavelength important for Raman Spectroscopy?
To start with, the Raman signal is inherently very weak. It relies on the photon-phonon interaction in the sample material, which is typically a one-in-a-million event. In addition, the Raman scattering intensity is inversely proportional to the 4th order of the illumination wavelength, which means that illumination at longer wavelengths results in a decreased Raman signal.
The detector sensitivity is also dependent on the wavelength range. CCD‘s are commonly used for detection of the Raman signal. The quantum efficiency of these CCD devices rolls off fairly quickly beyond 800 nm. For illumination beyond 800 nm, it is possible to use InGaAs array devices, but those are associated with higher noise levels, lower sensitivity and higher cost. The wavelength dependence of the Raman signal strength and the detection sensitivity all seem to point towards the use of shorter wavelength illumination (UV and visible) as opposed to longer wavelengths (in the near-IR). However, there is still a challenge to overcome with shorter wavelength illumination: Fluorescence emission. Many materials emit fluorescence when excited with UV-visible light, which can swamp the weak Raman signal. Even so, the most commonly used wavelength in Raman spectroscopy is 785 nm. It offers the best balance between scattering efficiency, influence of fluorescence, detector efficiency and availability of cost-efficient and compact, high-quality laser sources. However, the use of visible lasers in the blue and green (in particular at 532 nm) is increasing.
What performance characteristics should be considered when selecting a laser for Raman spectroscopy experiments?
In addition to wavelength, there are a number of important performance parameters that should be taken into account when choosing the best laser sources for a Raman spectrometer. Key performance parameters are: spectral linewidth, frequency stability, spectral purity, beam quality, output power and power stability, and optical isolation. In addition, the compactness, robustness, reliability, lifetime and cost structure should be considered.
The compactness, robustness, reliability, lifetime and cost structure are also very important parameters to be considered in the selection of the optimum illumination source for a Raman system. Raman instrumentation has progressed into becoming a standard analytical tool in many scientific and industrial applications. Users expect to run routine experiments or process monitoring measurements for years without the need for service or exchange of the laser source. In a growing number of cases, the instrument must also operate in harsh, industrial environments.
For these reasons, most Raman systems nowadays are equipped with solid-state based laser sources rather than gas lasers. Today, compact solid-state lasers with proven operation lifetimes of several 10,000 hours which meet the most advanced optical performance requirements are available in all wavelength ranges commonly used for Raman spectroscopy.