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.
Highlights
J-aggregates of polymethine dyes have become a focus of interest in the interdisciplinary field of molecular nonlinear optics [1,2,3,4,5] and Langmuir-Blodgett engineering [6,7,8]
At the incident light intensities used in our experiments the exciton annihilation plays a significant role in decay of exciton population especially close to resonance [ 18, 20, 21, 26]
This theory is adequate for the description of the annihilation processes in the absence of motion-limited diffusion, whereas a time-dependent function should be used for the one-dimensional exciton diffusion within the aggregate
Summary
J-aggregates of polymethine dyes have become a focus of interest in the interdisciplinary field of molecular nonlinear optics [1,2,3,4,5] and Langmuir-Blodgett engineering [6,7,8]. Sharp and intense absorption bands at room temperature provide the possibility of recording optical information by using the photoablative technique [6,7] or photodissociation [8,9] (photodesordering) of J-aggregates. Fullerenes have recently revealed high susceptibilities and no photodegradation at off-resonance case of DFWM with 1064 nm picosecond pulses [10]. Sasaki and Sh. Kobayashi [5] reported an anomalous intensity dependence of both absorption changes and DFWM signal for PIC bromide aggregates at 77 K which was ascribed to coherent exciton-exciton interaction. In the present paper we report further results of DFWM of tuneable picosecond pulses in J-aggregates of PIC in the presence of this complex and strongly intensity dependent relaxation
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