In materials showing reverse saturable absorption (RSA), the optical absorbance increases as the power of the light incident on them increases. To date, RSA has only been observed when very intense light sources, such as short-pulse lasers, are used. Here, we show that hydroxyl steroidal matrices embedding properly designed aromatic molecules as acceptors and transition-metal complexes as donors exhibit high RSA on exposure to weak incoherent light at room temperature and in air. Accumulation by photosensitization of long-lived room-temperature triplet excitons in acceptors with a large triplet-triplet absorption coefficient allows a nonlinear increase in absorbance also under low-power irradiation conditions. As a consequence, continuous exposure to weak light significantly decreases the transmittance of thin films fabricated with these compounds. These optical limiting properties may be used to protect eyes and light sensors from exposure to intense radiation generated by incoherent sources and for other light-absorption applications that have not been realized with conventional RSA materials.

Large reverse saturable absorption under weak continuous incoherent light

POSMAT - Programa de Pos-Graduacao em Ciencia e Tecnologia de Materiais UNESP - Universidade Estadual Paulista, Bauru, SP

Effect of chemical modifications on the electronic structure of poly(3‐hexylthiophene)

Nonlinear optical activity in Bridgman growth layered compounds

Substituted phenylacetylenes are polymerized using in situ generated (arene) M(CO) 3 complexes as catalysts. The catalytic activity of the complexes is discussed with reference to the metals and arene ligands. A ladder polymer of phenylacetylene formed via cycloaddition mechanism as a possible precursor to linear polyphenylacetylenes, in bulk polymerization conditions, has been isolated and characterized. Some of the properties of the polymers are also described.

Metathetical polymerization of substituted phenylacetylenes using in situ generated (arene)M(CO)3 complexes (arene = toluene, o-xylene or mesitylene, M Cr, Mo or W) as catalysts

Optical power limiting and transmitting properties of cadmium iodide single crystals: Temperature dependence

Conjugated polymers with sidegroups such as methyl, trimethylsilyl, p-methoxy phenyl, o-methoxy phenyl, and hexyl groups on a polyphenylacetylene chain are studied in an attempt to understand the contribution of factors such as the backbone, conformation of the chain, sidegroups and polymerization catalyst on the optical nonlinearities. The results of degenerate four-wave mixing experiments at 532 nm with nanosecond pulses indicate that the substitution of an electron-donating group away from the backbone can enhance the third-order susceptibility. Some of the polymers show optical limiting action at this wavelength. The thermal contribution to the measured susceptibility is also determined.

T. A. Vijayaraj

Papers

Optical nonlinearities in substituted conjugated polymers

Optical limiting action in poly(para-methoxy phenylacetylene)