Polarization suppression, nonresonant

CARS-CS experiments have been reported in the low-concentration limit ((N) <<1) on freely diffusing submicron-sized polymer spheres of different chemical compositions using both the E-CARS [162, 163] and the polarization-resolved CARS [163] detection scheme for efficient nonresonant background suppression. These experiments have unambiguously demonstrated the vibrational selectivity of CARS-CS, the dependence of its ACF amplitude on the particle concentration, (N), the dependence of lateral diffusion time, Tp, on the sphere size, and the influence of the microviscosity on its Brownian motion. 

Figure 2 (a) Polarization geometries for the suppression of the nonresonant (a), resonant-isotropic (/J),... 

A remedy obviously should be available using polarization tricks. In conventional Raman spectroscopy, the isotropic and anisotropic components are deduced from linear combinations of the polarized and depolarized spectra, while a nonresonant part is not clearly recognized (41). In frequency-domain CARS it is known how to suppress the nonresonant contribution and solely measure resonant scattering (isotropic plus anisotropic part) (42). In time-domain CARS, polarization interference can do an even better job with three magic cases (derived in Refs. 35,39). These authors derived explicit expressions for the coupling factors F in Equations (2)-(4) ... 

The nonresonant background is suppressed by using a circularly polarized pump wave the detailed explanation for this can be found in references [6,30,34], but basically it arises because the nonresonant third-order susceptibility has a higher permutation symmetry than the resonant part leading to complete cancel-... 

RIKES Raman-induced birefringence Modulation of 0)2, is CW Induced change in 0) polarization for 0) -0)2 = ft>Raman Nonresonant background can be suppressed, no phase matching Limited sensitivity, susceptible to turbulence and birefringence from windows, optics, sample... 

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