Nonlinear Optical Properties of Modified Carbocyanines
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INTRODUCTION Carbocyanine and its derivatives are the subjects of many studies, mainly due to their applications in photographic and laser industries. 1-2 Studies in 3,3'-diethylthiadicarbocyanine iodide (DTCI) have shown very efficient photoisomerization, which also leads to change in the absorption spectra between the isomers. I This presents a possible way to introduce RSA action for optical limiting purpose. On the other hand, if a heavy atom is introduced at the meso position of DTCI (X-DTCI), intersystem crossing can be dramatically 3 improved to very close to unity, essentially eliminating the isomerization and fluorescence. The shorter coherent time in chirped pulses effectively enhances the time resolution in a pump-probe experiment. Several groups recently reported progress on utilizing chirped pulses to resolve nonlinearities much shorter than the probing laser pulses.4- 6 While only refractive nonlinearity exists in the transparent media, it is possible in the resonant regime that both the absorptive and the refractive nonlinearities are present. In complicated systems, the co-existence of more than one physical process leading to optical nonlinearities may be realized. In view of the two points raised above, it is interesting to examine the optical properties of a longer version of the X-DTCI, the dicarbocyanines (X-DTDCI). EXPERIMENT are generated by a cw mode-locking/regenerative-amplifier The laser pulses, at 532 mrn, combination. The duration of them is measured by a streak camera system to be (108±13) ps
17 Mat. Res. Soc. Symp. Proc. Vol. 479 © 1 99 7 Materials Research Society
(FWHM in the intensity). The streak camera system also reveals double pulsing in our laser system, with a separation of (180±9) ps between the two pulses in each shot. The pump-probe experiment employs the co-propagating scheme with a crossing angle of 9'. A mechanical delay line controlled by a PC is introduced into the probe arm to provide the relative time delay. An energy meter with silicon detectors, controlled by a PC, is used to collect data. A typical transmission is the average over 20 shots. The pump is at least 104 times stronger than the probe. Samples are prepared by dissolving weighed powder into DMSO solvent (HPLC Grade) and subsequently transferred into 1mm photometer cells. The solid powder of the three samples is synthesized in-house. NMR is performed on each sample to confirm the structure. Absorption spectra indicate that no ion or dimer absorption is present in any of our samples. RESULTS The absorption spectra of all three samples are shown in Fig. 1. Our laser wavelength of 532 nm is situated at the shorter side of the So->S 1 transition. A slight blue shift of the absorption peak with respect to H-DTDCI is evident on Br-DTDCI. The absorption maxima show redshift from around 560 nm in X-DTCI to 660 nm in X-DTDCI. 1 This is in consistent with an extended conjugation of the de-localized 7t-electrons in the polymethine portion of the X-DTDCI.
3 2.5 2
CI-DTDCI
Br-DTDCI
S
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