Doped Gels for Optical Limiting Applications.
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high doping concentration and a chemically stable material. In this communication, we present reverse saturable absorption properties of these doped xerogels. Previous fluorescent decay measurements lead to model the process by three energy level absorption model, allowing indirect measurement of the excited state absorption. SAMPLE PREPARATION Aluminum phthalocyanine chloride (AIPcC1) doped xerogels AlPcCl doped xerogels were obtained by hydrolysis-condensation of tetraethoxysilane (TEOS) or modified silicon alkoxide precursors RSi(OEt) 3 (MTEOS and VTEOS). Hydrolysis was performed under acid-catalyzed conditions with ethanol as common solvent. The molar ratios of silicon : water : ethanol were respectively 1 : 3 : 3. HCl acidified water-ethanol mixture was added to the alkoxide-ethanol mixture. Solution of A1PcCl in ethanol was used as solvent. The solution was stirred vigorously for two hours at room temperature to ensure homogeneous mixing. After complete hydrolysis, a small amount of pyridine was added as basic medium to neutralize the acidity and to increase the condensation reaction rate. The resulting clear sols were cast into cylindrical-shaped moulds which were then left for gelation, in a drying oven, at 40'C. After several weeks, the samples attained an atmospherically stable condition. Such samples have a typical size of 60 mm diameter by 1.5 mm thickness. Fullerene functionalized xerogels In the grafting experiment a mixture of fullerene C60 and C 70 was functionalized with a nucleophilic addition of 3-aminopropyltriethoxysilane (APTES) to the C60 /70 double bonds as published elsewhere [15,16]. C60 /70 molecules were dissolved in excess of APTES alone (10-2 mole/l) at 50'C for several hours. After complete dissolution, the resulting solution had become orange-brown in color. The schematic reaction for APTES with C60 is shown in figure 1. The solution of the alkoxysilane-C 60/70 derivative in APTES was used to prepare fullerene functionalized xerogels. Sols prepared from MTEOS or VTEOS following the procedure described above were mixed with a small amount of fullerene modified silane solution. Under such conditions (basic-catalyzed condensation) the gelation times were very short (2 minutes). After gelation the gel samples were aged at room temperature for one week.
(EtO)3 Si-(CH2)3 +
n [(EtO) 3Si-(CH2) 3NH2 ]
H
H
--
APTES C60 Figure I : Grafting of C601 70 on a sol-gelprecursor EXPERIMENTAL SET-UP The optical experiments were carried out at 532 nm with frequency doubled Q-switched Nd:YAG lasers emitting nanosecond or picosecond optical pulses, of duration TPns or rpps. These pulses were focused on the samples using a 10 cm focus length lens L 1. The laser waists, measured with a CCD camera, were about a hundred micrometers. The transmitted energy is collected by a second lens of variable aperture. In the first case with a large aperture (optical set-up n' 1) we attempt to measure all the transmitted energy, while in the second case with a smaller aperture (optical set-up n' 2) we only wish to me
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