Dispersion of Third Order Optical Nonlinearity of PseudoisocyanineJ‐Aggregates

Abstract

Dispersion of degenerate four-wave mixing (DFWM) efficiency near the exciton resonance of aggregates of 1,1′-diethy1-2-2′-cyanine (pseudoisocyanine, PIC) chloride is reported. Correlation with the spectrum of imaginary parts of nonlinearity obtained by pump and probe techniques is discussed. Third order nonlinear susceptibility of PIC aggregates has been calculated supposing highest density packing attainable by Langmuir-Blodgett technique. The nonlinear figure-of-merit of aggregates was evaluated, which is intensity dependent due to exciton annihilation. Observed intensity and wavelength dependence of mixing efficiency is qualitatively explained in terms of absorption saturation, exciton annihilation, and subsequent generation of an excess diagonal disorder by vibrational energy accumulated in aggregates.

Near‐Resonant Nonlinear OpticalProperties of Pseudoisocyanine J‐Aggregates in Langmuir‐BlodgettLayers

“…are comparable to those obtained for J-aggregates in solution in 0.2 mm cuvette at concentration 10 -2 M [14]. Furthermore, the spectrum of the DFWM efficiency is well correlated with the absorption spectrum (see Fig.…”

“…5). It shows that the nonlinearity of PIC Jaggregates in LB film near resonance is mainly determined by real excitations like in J-aggregates in solution [14]. From the results the similar ultrafast processes contributing to the nonlinearity of Jaggregates can be expected in both, solution and LB film.…”

“…Figure 3 shows that in an LB film the intensity dependence of the DFWM signal does not follow strictly the third order intensity dependence which is characteristic for off resonance case of DFWM and has been shown for PIC J-aggregates in solution [15,18]. The third order dependence of the DFWM signal intensity on the input is observed only at applied intensities corresponding to less than one absorbed photon per 30 monomer molecules. Surprizingly, the logarithmic plot results not in a straight line dependence with one power over the whole intensity range as for all investigated wavelengths in J-aggregate solutions received [13,14]. At low intensities the ascent of the curve is 3.0 as expected for low intensities (annihilation and saturation free conditions) whereas at higher intensities the ascent decreases to 1.6.…”

“…On the other hand, the dynamics of Frenkel excitons under restricted geometries is one of important fields of fundamental research increasingly exploited with the understanding of the role of confinement in the giant nonlinearities and their time responses [10][11][12]. J-aggregates of polymethine dyes are the self organizing systems well suitable for the study ofthe dynamics of 1-D confined excitons [12][13][14]. J-aggregates are linear chains of coupled chromophores with high size-enhanced resonance third order non-linear optical susceptibility [15].…”

“…Comparison of third order nonlinear susceptibilities IX(a)[ of pseudoisocyanine J-aggregates in solution and in Langmuir-Blodgett layers measured at the used concentration or layer thickness and calculated for highest packing density.The values for J-aggregates in aqueous solution were taken from[14,15] …”