Fig. 1. (a) Energy diagram of CARS, where ω_p is frequency of pump light, ω_s is frequency of Stokes light,ω_as is the anti-Stokes frequency at 2ω_p-ω_s and ω_R is molecular vibrational frequency; (b) The energy diagram of SRS

Fig. 2. (a) Polarization figure of pump and Stokes beams for linear polarization. ep an es are unit vector of polarization of pump and Stokes beams and θ is cross angle between them; (b) Polarization figure for linearly polarized Stokes beamand circularly polarized pump beam

Fig. 3. (a) Experimental setup for polarized PARS; (b) Photoacoustic signal under the action of single pump beam, single Stokes beam and the two beams, in which the photoacoustic signal is obtained under the condition of parallel polarizations of the two beams

Fig. 4. Dependence of SRS intensity on the polarization cross angle θ between pump and Stokes beams for CH₄ molecule.(a) υ1 mode; (b) υ3 mode

Fig. 5. SRS spectra of υ₁ and υ₃ vibrational modes of CH₄ under different polarization conditions. (a) Polarizations of pump and Stokes beams are perpendicular to each other; (b) Polarizations of pump and Stokes beams are parallel to each other; (c) Circular polarization of pump beam and linear polarization of Stokes beam