Theoretical Research On Methods To Separate And Supress Nonresonant Background In Cars Spectrum

As a typical Third-Order nonlinear spectral technology, CARS is prominent for its intensive signal, good orientation and highly interference-proof advantage from fluorescent and is widely used in research of molecular vibrational state decay process and macromolecules. In the meantime, it can measure temperature and concentration in combustion system and is used as imaging tool owing to its intensive vibration signal and no need of fluorescent label. However, those vibrational states and background signals in organic molecules make CARS spectrum hard to recognize and separate. The background signal also interfere the CARS spectrum and its imaging function.This paper describes discovery, development, the latest research and application of CARS and makes detailed analysis to CARS process theory. From classical Maxwell approach and semi-classical quantum theory, the paper also deduces relation between CARS strength and third-order nonlinear susceptibility. Moreover, it clarifies the way to achieve detailed expression of third-order nonlinear susceptibility from pellucid Hellwarth diagrams which can clearly distinguish contributions from resonance and non-resonance in third-order nonlinear susceptibility.Multiplex CARS (MCARS) is used to detect the vibrational states in complex system concerning the relatively high signal-to-noise ration, basing on its advantage to stimulate many vabrational modes simultaneously, be wide in working range, not time consuming and not affected by laser power fluctuation. However, the non-resonant background signal prevents us to correctly determine the concentrations from vabrational subjects by way of MCARS spectrum.Making use of relation between the resonant part of nonlinear susceptibility and spontaneous Raman line shape, we perform a least-squares fitting on the experimental multiplex CARS results of Polycarbonate and successfully separate resonant and non-resonant component in the spectrum. Although this approach was only used in analysis of protein solutions before, here we apply it in the MCARS analysis in organic material film.