At the present day, many types of spectroscopic techniques are available to researchers. Standard spectroscopy, such as absorption and emission spectroscopy, is linear and uses one incident light beam. In this case, the observed light interactions have a tendency to be weak and the spectra produced can be indefinite.
Nonlinear spectroscopy, such as Coherent Raman Scattering and Stimulated Raman Scattering, uses two or more light beams. Many nonlinear spectroscopic techniques can vary the lights optical parameters such as its amplitude, frequency, polarization and phase.
There are several advantages of using nonlinear techniques. Nonlinear spectroscopy uses multiple sources of light, meaning more information is obtained from samples tested using a nonlinear technique.
Nonlinear spectroscopy can be used to study interfacial and surface processes. It can be used to study interactions in areas of the electromagnetic spectrum not accessible to linear spectroscopic methods. Nonlinear spectroscopy is also useful for studying dynamic processes and can be used to understand nanoparticles and their unique optical properties.
Raman spectroscopy provides information about molecular vibrations that can be used for sample identification and quantitation. The technique involves shining a monochromatic light source (i.e. laser) on a sample and detecting the scattered light.
Laser wavelengths ranging from ultraviolet through visible to near infrared can be used for Raman spectroscopy. The choice of laser wavelength has an important impact on experimental capabilities.
- Sensitivity. For example, an infra-red laser results in a decrease in scattering intensity by a factor of 15 or more, when compared with blue/green visible lasers.
- Spatial resolution. Thus, the achievable spatial resolution is partially dependent on the choice of the laser.
- Optimization of resulting based on sample behavior. For example Blue or green lasers can be good for inorganic materials and resonance Raman experiments and surface-enhanced Raman scattering (SERS). Red or near infra-red (660-830 nm) are good for fluorescence suppression. Ultraviolet lasers for resonance Raman on biomolecules and fluorescence suppression.
Raman spectroscopy is used in many varied fields – in fact, any application where non-destructive, microscopic, chemical analysis and imaging is required. Whether the goal is qualitative or quantitative data, Raman analysis can provide key information easily and quickly. It can be used to rapidly characterize the chemical composition and structure of a sample, whether solid, liquid, gas, gel, slurry or powder.