Optical Limiting With Neutral Nickel Dithiolene Complexes
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ABSTRACT We report an investigation of optical-limiting behavior in two neutral nickel complexes with multi-sulfur 1,2 dithiolene ligands, [Ni(medt)2J I (medt = 5,6-dihydro-6-methyl-1,4-dithiin-2,3dithiolate) and [Ni(phdt)2 ] 2 (phdt = 5,6-dihydro-5-phenyl-l,4-dithiin-2,3-dithiolate) in benzene solution. The fluence-dependent transmission of the complexes was observed with nanosecond and picosecond laser pulses at 532-nm wavelength. The limiting thresholds of the complexes were -0.3 J/cm2 , when measured with the picosecond pulses. Both picosecond time-resolved pump-probe and Z-scan measurement revealed that the limiting effects should originate from excited-state absorption and refraction. The transparency window (400-900 nm), observed in the linear absorption spectra of the complexes, indicated that their limiting response should cover a wider range than those of fullerenes and phthalocyanines.
INTRODUCTION Recently, reverse saturable absorbers received considerable attention since they can be used as optical-limiting devices [1]. The most frequently reported materials are fullerenes (C60) [1-6], and phthalocyanine complexes [7-10]. But the spectral response of optical-limiting effects in these two types is confined to the visible region. We note that there exists another class of materials, nickel complexes of 1,2-dithiolene ligands. These dithiolenes have been used as Qswitch dyes for near infrared lasers such as Nd:YAG lasers since the early seventies [11]. Recently, they have also been intensively investigated as optical-switching materials for optical communications [12, 13]. Here we report our study on optical-limiting effects in two neutral nickel complexes with multi-sulfur dithiolene ligands, [Ni(medt) 2](medt = 5,6-dihydro-6-methyl1,4-dithiin-2,3-dithiolate) I and [Ni(phdt) 2] (phdt = 5,6-dihydro-5-phenyl-l,4-dithinn-2,3dithiolate) 2. EXPERIMENTAL The preparation and structures of the two dithiolene complexes were reported elsewhere [13]. Their chemical structures are shown schematically in Fig. 1(a). It can be seen that the four sulfur 413 Mat. Res. Soc. Symp. Proc. Vol. 597 0 2000 Materials Research Society
atoms surrounding the nickel atom yield a square planar coordination. The five-membered ring containing the nickel atom forms a de-localized ir-electron system, making it favorable for nonlinear-optical properties. The dithiolene complexes were dissolved in benzene (C6H 6) and contained in 1-mm-thick quartz cells for optical measurements. Their linear absorption spectra were recorded with a spectrophotometer (Hitachi U-3410). Their nonlinear-optical responses were measured with frequency-doubled, pulsed Nd:YAG lasers, which produced laser pulses of nano- and picosecond duration at 532 nm. The lasers were operated at a repetition rate of 10 Hz. The spatial profiles of the pulses were nearly Gaussian after passing through a spatial filter. Three setups of nonlinear-optical measurements are described as follow: (1) The optical-limiting behavior was observed by measuring the fluence-dependent transm
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