4. Survey of Novel Multiphoton Active Materials 1257 4.1. Multiphoton Absorbing Systems 1257 4.2. Organic Molecules 1257 4.3. Organic Liquids and Liquid Crystals 1259 4.4. Conjugated Polymers 1259 4.4.1. Polydiacetylenes 1261 4.4.2. Polyphenylenevinylenes (PPVs) 1261 4.4.3. Polythiophenes 1263 4.4.4. Other Conjugated Polymers 1265 4.4.5. Dendrimers 1265 4.4.6. Hyperbranched Polymers 1267 4.5. Fullerenes 1267 4.6. Coordination and Organometallic Compounds 1271 4.6.1. Metal Dithiolenes 1271 4.6.2. Pyridine-Based Multidentate Ligands 1272 4.6.3. Other Transition-Metal Complexes 1273 4.6.4. Lanthanide Complexes 1275 4.6.5. Ferrocene Derivatives 1275 4.6.6. Alkynylruthenium Complexes 1279 4.6.7. Platinum Acetylides 1279 4.7. Porphyrins and Metallophophyrins 1279 4.8. Nanoparticles 1281 4.9. Biomolecules and Derivatives 1282 5. Nonlinear Optical Characterizations of Multiphoton Active Materials 1282

Multiphoton Absorbing Materials : Molecular Designs, Characterizations, and Applications

Two types of photomodulation of orientations of liquid crystals (LCs) are reviewed: 1) order–disorder phase transitions of LCs induced by photochemical reactions of photochromic molecules and 2) order–order alignment change of LCs (change in LC directors) induced by photochemical reactions or without photochemical events. Both processes produce a large refractive-index modulation, which forms the basis of a range of photonic applications. Various modes of photomodulation of orientations of LCs with plausible mechanisms and their possible applications in photonics are described.

Photomodulation of liquid crystal orientations for photonic applications

The control of luminous radiation has extremely important implications for modern and future technologies as well as in medicine. In this Review, we detail chemical structures and their relevant photophysical features for various groups of materials, including organic dyes such as metalloporphyrins and metallophthalocyanines (and derivatives), other common organic materials, mixed metal complexes and clusters, fullerenes, dendrimeric nanocomposites, polymeric materials (organic and/or inorganic), inorganic semiconductors, and other nanoscopic materials, utilized or potentially useful for the realization of devices able to filter in a smart way an external radiation. The concept of smart is referred to the characteristic of those materials that are capable to filter the radiation in a dynamic way without the need of an ancillary system for the activation of the required transmission change. In particular, this Review gives emphasis to the nonlinear optical properties of photoactive materials for the functi...

Nonlinear Optical Materials for the Smart Filtering of Optical Radiation.

5.3.2. Traps and Grating Dark Decay 3288 5.3.3. Other Concerns 3288 6. New Photorefractive Materials 3289 6.1. Polymer Composites 3289 6.2. Organic Amorphous Glasses 3292 6.3. Fully Functionalized Polymers 3294 6.4. Polymer-Dispersed Liquid Crystals 3295 6.5. Other Liquid Crystal-Containing Materials 3296 6.6. Near-Infrared-Sensitive Materials 3297 6.7. Other Materials Directions 3298 6.7.1. Hybrid Organic−Inorganic Composites and Glasses 3298

Organic photorefractives: mechanisms, materials, and applications.

Nonlinear optics of liquid crystalline materials

We have observed extraordinarily large optical nonlinearity in Methyl Red–doped nematic liquid-crystal film. Grating diffraction can be generated with an optical intensity as low as 40 µW/cm2, and a refractive-index change coefficient of more than 6 cm2/W is obtained. The effect is attributed to formation of an optically induced dc space-charge field and to the resulting reorientation of the highly birefringent nematic director axis.

Optically induced space-charge fields, dc voltage, and extraordinarily large nonlinearity in dye-doped nematic liquid crystals

Coherent amplification of a signal beam by a strong pump beam is observed in thin films of fullerene-doped nematic liquid crystal. Exponential gain constants as high as 2890  cm-1 with no phase cross talk are achieved at low applied dc bias voltage and pump beam intensity. The underlying mechanism is the electro-optically induced spatially reorientation of the liquid-crystal axis and the resultant phase-shifted index grating required for two-beam coupling.

Coherent beam amplification with a photorefractive liquid crystal.

Optically induced nematic liquid crystal axis reorientation results in extraordinarily large nonlinear refractive index changes that could find practical applications in conjunction with cw or long-pulse lasers. We discuss the origins of these nonlinearities, and present the results of recent experimental studies of image conversion, optical limiting and sensor protection using aligned dye-doped nematic liquid crystal films in all-optical configurations. These processes are characterized by unprecedented low threshold laser powers, thus presenting nonlinear photosensitive nematic liquid crystals as promising next generation image processing and optical switching/limiting material.

Extremely nonlinear photosensitive liquid crystals for image sensing and sensor protection.

We demonstrate a dynamic all-optical image-intensity-inversion technique that uses self- and mutual-phase-modulation effects with a highly nonlinear nematic liquid-crystal film placed in an intermediate focal plane. This process requires submilliwatt optical power, responds in a few milliseconds, and can be realized over a very broad spectral range.

All-optical image processing with a supranonlinear dye-doped liquid-crystal film.

In nematic liquid crystal films doped with methyl-red dye or azobenzene liquid crystal, we have observed extremely efficient optically induced director axis reorientation effects, and refractive index change coefficients >> 1 cm2/W. The basic mechanisms involved are determined to be optically induced space charge fields and flow reorientation, order parameter modification and molecular torques by the photo-excited dopant molecules. These supra-optical nonlinearities (SONs) enable nonlinear image conversion, optical limiting and holographic grating formation with μW power lasers.

P. H. Chen

Papers

Optically induced space-charge fields, dc voltage, and extraordinarily large nonlinearity in dye-doped nematic liquid crystals

Coherent beam amplification with a photorefractive liquid crystal.

Extremely nonlinear photosensitive liquid crystals for image sensing and sensor protection.

All-optical image processing with a supranonlinear dye-doped liquid-crystal film.

Supra Optical Nonlinearities (SON) of Methyl Red- and Azobenzene Liquid Crystal - Doped Nematic Liquid Crystals