Reverse saturable Absorption in Organically Doped Xerogels
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ABSTRACT Pd and Zn - tetraphenyltetrabenzoporphyrins (PdTPTBP and ZnTPTBP), hexacyanin 3 (HITCI), and substituted phthalocyanines were incorporated in solid-state matrices (xerogels) using a sol-gel process. Nonlinear reverse saturable absorption was observed with those materials when they were illuminated with nanosecond laser pulses at 532 nm, or other wavelengths in the visible spectrum (between 450 nm and 630 nm). PdTPTBP doped xerogels exhibit a nonlinear2 activation threshold of about 10 mJ/cm , which is much lower than the value of 80 mJ/cm obtained under similar conditions with classical Al phthalocyanine chloride, or HITCI molecules. Solid state "red active" reverse saturable absorbers can be obtained with substituted phthalocyanines doped xerogels. The different experimental results are discussed using classical 4energy level diagrams. INTRODUCTION Various classes of organic and inorganic compounds exhibit a reverse saturable absorption effect [1]. Such compounds dispersed in appropriate transparent medium can act as optical limiters and thus can be used to protect optical sensors or even human eye from the damaging effects of the
ultrabright light pulses. Optical limiting phenomenon has been previously described for a large number of compounds in liquid solutions, e.g. indanthrone derivatives [2], metal clusters [3], fullerenes [4], phthalocyanines [5], metallated porphyrins [6] and tetrabenzoporphyrins [7]. From a practical point of view, however, solid-state optical limiters should substantially simplify the design of the corresponding components for optical devices. Transparent matrices capable of hosting different types of organic and/or inorganic molecules can be conveniently prepared using sol-gel process [8]. This process does not require highly elevated temperatures which is especially useful to incorporate thermally unstable organic molecules into transparent inorganic hosts. In the present paper, we describe the synthesis and properties of new solid-state materials, namely ZnTPTBP, PdTPTBP, HITCI and substituted phthalocyanines-doped xerogels, which exhibit optical limiting in different ranges of the visible spectrum. BACKGROUND: ABSORBERS
NONLINEAR
THRESHOLD OF REVERSE SATURABLE
For a given wavelength within the visible spectral range some organic molecules can be relatively low absorbent in the ground state and much more absorbent in the first excited state. When illuminated at this wavelength such molecules exhibit reverse saturableabsorption. At low incident fluences most of the molecules within illuminated volume occupy low absorbent ground state So, while at higher incident fluences a significant number of molecules is pumped to the 97 Mat. Res. Soc. Symp. Proc. Vol. 479 01997 Materials Research Society
excited state with higher absorption coefficient. Thus, the overall absorption by the system increases. Obviously, a necessary condition for this effect is a wavelength overlap between the absorption bands for both states. Depending on the type of molecular system and efficiency of t
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