Optical heterodyning of the phase-tuned femtosecond optical Kerr gate signal for the determination of complex third-order susceptibilities

We show that ultrafast optically stimulated birefringence and dichroism may be conveniently investigated by combining polarization sensitive optically heterodyned detection with phase tune-up between the optical Kerr gate signal and the local oscillator. The real and the imaginary parts of complex third-order optical nonlinearity can be effectively separated and their values and signs determined. 60 fs pulses at 620 nm were used in experiments carried on tetrahydrofuran solutions of canthaxanthin, a carotenoid important for photobiology. The values of both parts of the complex second hyperpolarizability γ as well as the sign of its real part determined by this method compare well with that obtained from the concentration dependence method employing the homodyne-detection optical Kerr gate technique.