Nanoparticle optical limits

J.cm ) delivered by a picosecond laser, tJie absorbance of gold nanoparticles, for instance, increases suddently, particularly for large nanoparticles (15 nm mean radius), indicating a non-linear optical behavior. The metal nanoparticles behave then as ultra-fast optical limiters or shutters, indicating their ability to be used as a shielding against powerful lasers. 

Whatever the nonlinear process involved, the metal/dielectric nanocomposites have been often inquired into for their optical limiting (OL) properties [120, 123, 124, 140-150], owing to the important stake that such a functionality represents for civil and military applications in human eye or detector protection. Indeed, as for telecom applications, metal nanoparticles present the advantages of both intense and fast nonlinear response. 

Figure 3 compiles selected results obtained for optical limiting properties of materials containing gold or silver nanoparticles. Note that the ordinate axis shows the normalized transmission - that is, the transmission normalized to unity at low input fluence - and not the absolute one. All kinds of materials roughly present the... 

Zhan, C., Li, D., Zhang, D., Xu, W., Nie, Y., Zhu, D. The excited-state absorption and third-order optical nonlinearity from 1-dodecanethiol protected gold nanoparticles Application for optical limiting. Opt. Mat. 26, 11-15 (2004)... 

Koudoumas, E., Kokkinaki, O., Konstantaki, M. et al., 2002, Onion-like carbon and diamond nanoparticles for optical limiting, Chem. Phys. Lett. 357, 336-340. 

The role of the Ag became obvious when the optical-limiting results for the Ag-containing nanomaterials were compared with those for other nanoparticles, including nanoscale CdS, PbS, and Ni particles in stable suspensions (257). The nanoparticles that contained no Ag were found to be considerably weaker optical limiters. For example, the Ni nanoparticles in a stable transparent suspension exhibited only marginal optical-Umiting response to 5-ns laser pulses at 532 nm (Figure 50) (257). 

Table 4 Physical and Structural Parameters of the Metal and Metal Sulfide Nanoparticles for Optical Limiting Measurements (257)...

Figure 49 Optical limiting responses of the nanocrystalline Ag2S particles in a PVP polymer-stabilized ethanol suspension (O) of 90% linear transmittance at 532 nm are compared with those of Csq in toluene ( ) and chloroaluminum phthalocyanine in DMF (V) of the same linear transmittance and those of the CdS nanoparticle suspension (O) of 81% linear transmittance and the PbS nanoparticle suspension (A) of 90% linear transmittance. (From Ref. 257.)...

Recently, Sun and coworkers evaluated the optical-limiting properties of Ag nanoparticles produced via RESOLV with supercritical ammonia as opposed to water-in-C02 microemulsion and with hydrazine reduction as opposed to NaBH4 reduction (287). The nanoparticles obtained with the rapid expansion of a water-in-C02 microemulsion had a significantly broader particle size distribution than those with the supercritical ammonia solution and the nanoparticles obtained from the hydrazine reduction were on average 50% larger than those... 

Figure 51 Optical limiting responses of the Ag2S nanoparticles prepared via RESOLV with the rapid expansion of a supercritical/ammonia solution (narrow particle size distribution) ( ) and a water-in-C02 microemulsion (broader particle size distribution) (A).

Porel, S., Singh, S., Harsha, S. S., Rao, D. N., and Radhakrishnan, T. P. (2005). Nanoparticle-embedded polymer In situ S5mthesis, free-standing films with highly monodisperse silver nanoparticles and optical limiting. Chem. Mater. 17, 9-12. 

Another approach is to use the LB film as a template to limit the size of growing colloids such as the Q-state semiconductors that have applications in nonlinear optical devices. Furlong and co-workers have successfully synthesized CdSe [186] and CdS [187] nanoparticles (<5 nm in radius) in Cd arachidate LB films. Finally, as a low-temperature ceramic process, LB films can be converted to oxide layers by UV and ozone treatment examples are polydimethylsiloxane films to make SiO [188] and Cd arachidate to make CdOjt [189]. 

The optical properties of metal nanoparticles have traditionally relied on Mie tlieory, a purely classical electromagnetic scattering tlieory for particles witli known dielectrics [172]. For particles whose size is comparable to or larger tlian tire wavelengtli of the incident radiation, tliis calculation is ratlier cumbersome. However, if tire scatterers are smaller tlian -10% of tire wavelengtli, as in nearly all nanocrystals, tire lowest-order tenn of Mie tlieory is sufficient to describe tire absorjDtion and scattering of radiation. In tliis limit, tire absorjDtion is detennined solely by tire frequency-dependent dielectric function of tire metal particles and the dielectric of tire background matrix in which tliey are... 

Figure C2.17.13. A model calculation of the optical absorjDtion of gold nanocrystals. The fonnalism outlined in the text is used to calculate the absorjDtion cross section of bulk gold (solid curve) and of gold nanoparticles of 3 mn (long dashes), 2 mn (short dashes) and 1 mn (dots) radius. The bulk dielectric properties are obtained from a cubic spline fit to the data of [237]. The small blue shift and substantial broadening which result from the mean free path limitation are...

The broadband analysis was confirmed by the experimental results mentioned in Sect. 5.4.1. This method can also be further enhanced by some of the techniques described in Sects. 5.4.2 and 5.4.3. The conclusion is that these methods of microcavity-enhanced optical absorption sensing provide compact, inexpensive, and sensitive detectors for molecular species in the ambient gas or liquid, and that further increases in sensitivity can be implemented to make them even more competitive. The molecular-transition specificity that is implicit in absorption spectroscopy is a limiting restriction, but the surface-enhanced Raman sensing that is enabled by metallic nanoparticles on the microresonator surface can significantly increase the number of molecular species that could be detected. 

When the size of metals is comparable or smaller than the electron mean free path, for example in metal nanoparticles, then the motion of electrons becomes limited by the size of the nanoparticle and interactions are expected to be mostly with the surface. This gives rise to surface plasmon resonance effects, in which the optical properties are determined by the collective oscillation of conduction electrons resulting from the interaction with light. Plasmonic metal nanoparticles and nanostructures are known to absorb light strongly, but they typically are not or only weakly luminescent [22-24]. 

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