Coherent Anti-Stokes Raman Spectroscopy (CARS)

Coherent anti-Stokes Raman scattering (CARS) spectroscopy has been used to provide gas-phase quantitative scalar information (e.g. temperature, density, and species concentrations) for more than 5 decades. This technique is renowned for its ability to realize non-intrusive in-situ measurements in harsh environments with excellent spatial and temporal resolution and has become an important tool in multiple energy and combustion science applications, where high-fidelity data are needed. CARS is a non-linear optical process, where the signal originates from the coupling of multiple laser fields to the internal energy states of the probed molecules. This interaction results in excellent chemical specificity, while temperature information is obtained through the direct retrieval of the population distribution on the CARS signal spectrum. CARS thus represents the state-of-the-art in gas-phase thermometry, with unmatched accuracy and precision. The strong “laser-like” signal, which can be detected remotely from where it is generated, makes it also suited for extremely harsh and luminous environments such as flames and plasmas. The present chapter summarizes the fundamentals of gas-phase CARS and discusses a number of most recent advancements: i.e. single-shot CARS imaging, new light sources for ultrabroadband CARS, and simultaneous referencing of the femtosecond (impulsive) excitation efficiency. These recent developments open for interesting possibilities of using CARS in new type of experiments, with coverage of in principle all Raman active modes, obtained with space-time correlated resolution, and improved significance in the delivered data.