Gas detection sensitivity of hybrid fs/ps and fs/ns CARS

Coherent anti-Stokes Raman scattering (CARS) is commonly used for thermometry and concentration measurement of major species. The quadratic scaling of CARS signal with number density has limited the use of CARS for detection of minor species, where more sensitive approaches may be more attractive. However, significant advancements in ultrafast CARS approaches have been made over the past two decades, including the development of hybrid CARS demonstrated to yield greatly increased excitation efficiencies. Yet, detailed detection limits of hybrid CARS have not been well established. In this Letter, detection limits for N₂, H₂, CO, and C₂H₄ by point-wise hybrid femtosecond (fs)/picosecond (ps) CARS are determined to be of the order of 10¹⁵ molecules/cm³. The possible benefit of fs/nanosecond (ns) hybrid CARS is also discussed.