Two-photon absorption and second hyperpolarizability measurements in diphenylbutadiene by degenerate four-wave mixing

We have investigated two-photon absorption (TPA) in diphenylbutadiene at 532 nm using degenerate four-wave mixing (DFWM) of nanosecond laser pulses. We present a theory which describes the development of double-peaked phase conjugate pulses produced in DFWM due to TPA-induced gratings superimposed on the usual electronic third-order processes. Our analysis suggests a novel technique for measuring both the real and imaginary parts of the third-order susceptibility of a molecular solution, hence leading to a determination of the TPA cross section σ2 and second hyperpolarizability γ of the solute molecules. Applying this experimental technique to diphenylbutadiene in chloroform, we obtain σ2=(40±8)×10−50 cm−4 s/photon–molecule and ‖γ‖=(420±80)×10−36 esu for this diphenyl polyene. We show that these values are in agreement with related measurements in diphenylbutadiene, and that they are consistent with a two-photon resonance near 39 500 cm−1. The DFWM measurement technique we describe is very sensitive and should be applicable for measuring the two-photon spectra of a variety of molecules.