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.

Nonlinear absorption at 532 nm in a C60–toluene solution by using 8-ns and 30-ps laser pulses is reported. The transmittance for both pulse widths is fluence dependent. A five-level model of C60 is described that yields excellent agreement with both pulse-width data sets for incident fluences as high as approximately 1 J/cm2. Additional phenomena observed at higher fluences indicate that other mechanisms may be active and contribute to optical limiting in this regime. The application of C60 as an optical limiter material is discussed.

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 sigma2 and second hyperpolarizability gamma of the solute molecules. Applying this experimental technique to diphenylbutadiene in chloroform, we obtain sigma2 = (40 +/- 8) X 10(-50) cm-4 s/photon-molecule and \gamma\ = (420 +/- 80) X 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.

We have observed double-peaked phase-conjugate pulses in the degenerate four-wave mixing of nanosecond laser pulses in solutions of diphenyl polyenes. We attribute this to the superposition of fast and slow gratings, where the slow grating is due to two-photon absorption. This is supported by the results of time delay and polarization experiments as well as evidence of a second spatial harmonic grating.

Recently, optical limiting has been demonstrated in solutions of diphenyl polyenes, and χ(3) values an order of magnitude larger than CS2 have been measured.1 In this report, degenerate four-wave mixing (DFWM) has been used to investigate the origin of the nonlinear response in solutions of diphenyl polyenes. The DFWM measurements were made using a frequency doubled Q-switched Nd: YAG laser producing 7 nsec pulses at 532 nm. The temporal profile of the phase-conjugate signal observed contains two features separated by ~5 ns. These results indicate that the nonlinear response of these materials contains both a fast and a slow component. The fast component is attributed to intrinsic nonlinearities. The slow component is attributed to slowly responding processes such as thermal effects and has been studied using polarization and time delay experiments. The results of these experiments indicate that the slow component is not simply explained and may consist of more than one nonlinear response. The molecules are known to have two-photon absorption near the laser wavelength that may be contributing to the slow component that is observed.