Influence of two-photon absorption on third-order nonlinear optical processes as studied by degenerate four-wave mixing: The study of soluble didecyloxy substituted polyphenyls

Abstract

We have investigated the influence of two-photon absorption on the third-order nonlinear optical properties of model organic molecules using the technique of degenerate four-wave mixing (DFWM). A theoretical formulation developed here shows that the presence of two-photon absorption, which is related to the imaginary part of the third-order susceptibility χ(3), leads to an enhancement of the effective third-order nonlinearity and to the appearance of effects caused by the formation of two-photon generated excited states. The dynamic behavior of the nonlinearity is then governed by the properties of excited molecules. The nonlinear effects also involve contributions which depend on the fifth power of the electric field. We have performed a systematic study of third-order nonlinear optical properties of alkoxy (–C10H21OCH) substituted p-polyphenyl oligomers using the technique of time-resolved degenerate four-wave mixing with subpicosecond pulses at 602 nm. Experimentally determined values of the second-order hyperpolarizability γ for the oligomers increase smoothly from the monomer to the trimer, with a more rapid increase to the pentamer and to the heptamer. In addition, the hyperpolarizabilities for the pentamer and the heptamer appear to be complex. A smooth increase of the γ value is expected from an increase of the π conjugation from a shorter chain oligomer to a longer chain oligomer. The more rapid increase of the γ value in the pentamer, and especially in the heptamer, however, cannot be explained satisfactorily by only taking into account the π-conjugation length. Two-photon absorption for the pentamer and the heptamer at the measurement wavelength of 602 nm is suggested to be important as the observed dynamic behavior is satisfactorily explained by the predictions of the theoretical model presented here. It is shown that the effective γ value for a two-photon absorbing material is a function of optical intensity, pulse width, and sample length if one uses the conventional degenerate four-wave mixing description.

Third-order optical nonlinearity studies ofp-heptaphenyl derivatives-doped sol-gel processed composite glass and THF solution by degenerate four-wave mixing and optical Kerr gate measurements

“…4(a). Compared to the signal obtained previously for the DDOPPH [10], no significant long decay was observed, despite that a strong blue emission was observed under excitation at 602 nm. This can be understood since we already showed [13] that the contribution of the (fifth-order) population grating term becomes significant at high pumping power and depends on the efficiency of the .5…”

“…In a previous paper [10] we already presented th e temporal time-resolved DFWM profile of DDOPPH (called the heptamer, molecular structure shown in Fig. 1 (b)) in THF solution.…”

“…The dyes are didecyl and didecyloxy substitutedpara-polyphenyl heptamers (DDPPH and DDOPPH hereafter, respectively, shown in Fig. 1), that were synthesised earlier by Reinhardt's group [ 10]. Our group has already reported on a degenerate four-wave mixing (DFWM) study of DDOPPH which shows promising signal due to two-photon absorption at the wavelength used in the measurement [ I0].…”

“…The full theoretical solution is given in Ref. [10]. In this paper, we present measurements of the magnitudes and the signs of the real and the imaginary components of the complex third-order optical susceptibility of DDPPH and DDOPPH in THF solution and in composite-glass using degenerate four-wave mixing (DFWM) [13] and optical Kerr gate (OKG) methods [14].…”

“…In our previous papers [10], we found it convenient to express the polarization as a sum of components which include the response to instantaneous field amplitudes and that due to the changes of linear susceptibility brought about by the presence of excited states population grating. Thus the polarization can be expressed as P = X~I)E Jr-X(3)EEE+ xr I/N) [v(1)~, Ae -^gv(l)'~EI -4-9 9 9 (1)…”