Femtosecond Optical Nonlinearity in the Squarylium Dye J-Aggregates
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with two propyl groups substituted on both ends of the chromophore form J-aggregates [8]. Upon them, a Langmuir film of bis[4-(N-propylhexylamino)phenyl]squarylium (SQ36) showed a most intense J-band at 777 nm. For the purpose of estimating the proper nonlinear optical properties, characterizations of the morphology observed by fluorescence microscopy and the linear optical properties determined by the transmission and reflection spectra of the Langmuir film are discussed in this paper. An analysis of the femtosecond nonlinear responses suggests delocalization of the Frenkel excitons in these two-dimensional J-aggregates. EXPERIMENT A solution of squarylium dye was prepared by dissolving SQ36 ( Fig. 1) in chloroform at a concentration of 8.31 X 10-4 M. Langmuir films of squarylium dye J-aggregate were formed by spreading this chloroform solution on a water surface [8]. For samples of fluorescence micrographs, thinly spread water on a glass slide was used as a substrate for Langmuir films. H13C6 + Q 0 OC61H13 H7 C3
\C 3 H7
Fig. 1. Chemical formula of the SQ36. 777 Mat. Res. Soc. Symp. Proc. Vol. 488 0 1998 Materials Research Society
The monolayer was excited by a filtered Xenon lamp through a band-pass filter from 700 to 750 nm. A fluorescence emission image was taken by a CCD camera though a long-pass filter from 770 nm. Polarized images were taken with the polarization of both the excitation and emission light in the same direction. A transparent Langmuir trough formed on a glass substrate with Teflon side walls having a surface diameter of 20 mm and a depth of 20 mm was used to measure both the linear and nonlinear optical properties. For preparing the subphase of a monolayer, the temperature of pure water was adjusted to 5 'C by a peltier cooler with a water-circulating heat sink. For investigating of the linear optical properties, a spectro-photometer of a linear CCD array with a bundle-type optical fiber was used. The transmission spectra were measured under the illumination of white light from a halogen lump through a facing individual optical fiber. Reflection spectra were measured by using a branched bundle optical fiber connected to a detector and the light source to the blanched ends, with the other end facing normally to the water surface. The transmittance and reflectance of the trough and water without a monolayer was used for a reference of the absorption spectra, respectively. The femtosecond excited-state dynamics was investigated by measuring the transient absorption change spectra using pump pulses with a center wavelength of 780 nm and probe pulses of a white light continuum with 200 fs duration under a I kHz repetition rate. The diameters of both the pump and probe beam were 300 g.tm. Before conducting pump-probe measurements, the formation of Jaggregates was confirmed by measuring the absorption spectra using femtosecond pulses of the white-light continuum [9]. RESULTS AND DISCUSSIONS After spreading of a chloroform solution of squarylium dye, the formation of J-aggregates was observed with
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