Optical limiting

Optical limiting is by definition an optical nonlinear phenomenon that results in an increase of the optical absorption as the incident irradiation increases. Due to the fact that the triplet photoexcited state of C60 can absorb light more efficiently than its ground state, as the intensity of the incoming irradiation increases the intensity of the transmitted irradiation does not increase linearly [263,264]. The latter, together with both the extension of optical absorption to the near-IR of mono-organofullerene adducts and the resulting reduction of symmetry of the functionalized materials, as compared with the intact C60, has allowed several 

A fulleropyrrolidine derivative has been synthesized and covalently linked to HPLC silica gel and the resultant material was used to separate calixarenes, cy-clodextrins, and protected peptides [271]. Another type of organofullerene material containing polysiloxane was recently synthesized. It was found to be suitable for separating high-boiling-point compounds, and can be found applications as the stationary phase in capillary gas chromatography [272]. 

Poly(l,l-silole)s, SCPs catenated through the ring silicon atom, can be regarded as a new class of polysilanes. It was found that PL intensities of the toluene solution of a poly(l,l-silole) 24 (Fig. 12) could be quenched by the addition of tiny amounts of 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenol (picric acid), 2,4-dinitrotoluene (DNT), and nitrobenzene, demonstrating that poly(l,l-silole)s are potential chemosensors for explosives.41 TNT could also be detected using the polymer film. In an air stream containing 4 ppb TNT, 8.2% decrease of the PL intensity was found from the film. PL quenching can also be detected when the film contacts a 50 ppb TNT-water solution. 

Dithienosilole-thiophene alternating copolymer 21 is the important example for the electrical conduction of SCPs.42 When the copolymer is doped with iodine, a high electrical conductivity of 400 S cm-1 can be achieved. This value is the highest among SCPs and is also close to that of well-defined poly(3-alkylthiophene). 

A hyperbranched polysilole 25 (Fig. 13) is nonlinear and optically active and can strongly attenuate the optical power of intense laser pulses.43 

The dichloromethane solution of the polysilole (0.7 mg/mL) starts to limit the optical power at a low threshold of 185 mJ/cm2 and suppresses the optical signals to a great extent (81% power cutoff). 

This work was partly supported by the National Science Foundation of China, the Ministry of Science and Technology of China, and the Research Grants Council of Hong Kong, China. 

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Materials are also classified according to a particular phenomenon being considered. AppHcations exploiting off-resonance optical nonlinearities include electrooptic modulation, frequency generation, optical parametric oscillation, and optical self-focusing. AppHcations exploiting resonant optical nonlinearities include sensor protection and optical limiting, optical memory appHcations, etc. Because different appHcations have different transparency requirements, distinction between resonant and off-resonance phenomena are thus appHcation specific and somewhat arbitrary. 

Reverse saturable absorption is an increase in the absorption coefficient of a material that is proportional to pump intensity. This phenomenon typically involves the population of a strongly absorbing excited state and is the basis of optical limiters or sensor protection elements. A variety of electronic and molecular reorientation processes can give rise to reverse saturable absorption many materials exhibit this phenomenon, including fuUerenes, phthalocyanine compounds (qv), and organometaUic complexes. 

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

The above measurements all rely on force and displacement data to evaluate adhesion and mechanical properties. As mentioned in the introduction, a very useful piece of information to have about a nanoscale contact would be its area (or radius). Since the scale of the contacts is below the optical limit, the techniques available are somewhat limited. Electrical resistance has been used in early contact studies on clean metal surfaces [62], but is limited to conducting interfaces. Recently, Enachescu et al. [63] used conductance measurements to examine adhesion in an ideally hard contact (diamond vs. tungsten carbide). In the limit of contact size below the electronic mean free path, but above that of quantized conductance, the contact area scales linearly with contact conductance. They used these measurements to demonstrate that friction was proportional to contact area, and the area vs. load data were best-fit to a DMT model. 

Light filters for colorimeters, see Filters, optical Limiting cathode potential 509 see also Controlled potential electro-analysis Linear regression 145 Lion intoximeter 747 Liquid amalgams applications of, 412 apparatus for reductions, 413 general discussion, 412 reductions with, (T) 413 zinc amalgam, 413 Liquid ion exchangers structure, 204 uses, 204, 560... 

Keywords. Dendrimers, Fullerenes, Optical limitation. Photophysical properties, Thin films... 

Ehrlich JE, Wu XL, Lee IS, Hu ZY, Rockel H, Marder SR, Perry JW (1997) Two-photon absorption and broadband optical limiting with bis-donor stilbenes. Opt Lett 22 1843-1845... 

Perry, J. W. Organic and Metal-containing Reverse Saturable Absorbers for Optical Limiters. In Nonlinear Optics of Organic Molecules and Polymers, Nalwa, H. S. Miyata, S., Eds CRC Boca Raton FL 1997, pp 813-840. 

One promising application for C60 is as an optical limiter. Optical limiters are used to protect people and materials from damage by high light intensities usually associated with intense pulsed sources. Optical limiting is accom-... 

Understanding the mechanisms of the optical limiting effect in metal dendrimer nanocomposites may also require understanding the timescale of the effect. In general, for optical excitation close to the linear absorption band, such... 

Indeed, the timescale of the optical limiting effect in dendrimer nanocomposites is somewhat different than that found in other materials and this may be crucial to the understanding of the mechanism. Recent reports have investigated... 

He GS, Yong K, Zheng Q, Sahoo Y, Baev A, Ryasnyanskiy Al, Prasad PN (2007) Multiphoton excitation properties of CdSe quantum dots solutions and optical limiting behavior in infrared range. Opt Exp 15 12818-12833... 

Nonlinear optical organic materials such as porphyrins, dyes, and phthalocyanines provide optical limiting properties for photonic devices to control light frequency and intensity in a predictable manner. The optical limit of CNTs composites is saturated at CNTs exceeding 3.8wt% relative to the polymer mass (Chen et al., 2002). Polymer/ CNT composites could also be used to protect human eyes, for example, optical elements, optical sensors, and optical switching (Cao et al., 2002). 

Goh HW, Goh SH, Xu GQ, Lee KY, Yang GY, Lee YW, Zhang WD (2003). Optical limiting properties of double-C60-end-capped poly(ethylene oxide), double-C -end-capped polyethylene oxide)/poly(ethylene oxide) blend, and double-C -end-capped poly(ethylene oxide)/multiwalled carbon nanotube composite. J. Phys. Chem. B 107 6056-6062. 

Xu JF, Xiao M, Czerw R, Carroll DL (2004) Optical limiting and enhanced optical nonlinearity in boron-doped carbon nanotubes. Chemical Physics Letters 389 247-250. 

Although there have been great advances in covalent functionalization of fullerenes to obtain surface-modified fullerene derivatives or fullerene polymers, the application of these compounds in composites still remains unexplored, basically because of the low availability of these compounds [132]. However, until now, modified fullerene derivatives have been used to prepare composites with different polymers, including acrylic [133,134] or vinyl polymers [135], polystyrene [136], polyethylene [137], and polyimide [138,139], amongst others. These composite materials have found applications especially in the field of optoelectronics [140] in which the most important applications of the fullerene-polymer composites have been in the field of photovoltaic and optical-limiting materials [141]. The methods to covalently functionalize fullerenes and their application for composites or hybrid materials are very well established and they have set the foundations that later were applied to the covalent functionalization of other carbon nanostructures including CNTs and graphene. 

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