Optical limiters mechanism

Tutt, L. W., and T. F. Boggess. 1993. Review of optical limiting mechanisms and devices using organics, fullerenes, semiconductors and other materials. Prog. Quantum Electron. 17 299-338. 

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. 

Prototropic interconversions have been the subject of much detailed study, as they lend themselves particularly well to investigation by deuterium labelling, both in solvent and substrate, and by charting the stereochemical fate of optically active substrates having a chiral centre at the site of proton departure. Possible limiting mechanisms (cf. SNl/SN2) are those (a) in which proton removal and proton acceptance (from the solvent) are separate operations, and a carbanion intermediate is involved, i.e. an intermolecular pathway and (b) in which one and the same proton is transferred intramolecularly ... 

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... 

N.V. Kamanina, Mechanisms of optical limiting in p-conjugated organic system fullerene doped polyimide, Synthetic Metals, vol. 127, pp. 121-128, 2002. 

The baseline noise as offered by many UV-Vis detectors is in the range 1 to 2 x 10 5 AU and much lower than the limit of detection and quantitation required for most applications. This value is achieved under optimum conditions, such as with a reasonably new lamp, an ultraclean flow cell, stable ambient temperature, HPLC-grade solvents, and no microleaks in the entire HPLC system. These conditions are always valid at the manufacturer s final test and probably at the time of installation in the user s laboratory. However, after some time, optical and mechanical parts deteriorate (e.g., the lamp loses intensity and the flow cell may become contaminated). If we repeat the test after 3, 6, or 12 months, the noise of 1 x 10 5 AU may no longer be obtained. The recommendation is to select acceptance criteria according to the intended use of the system. 

There is a close relation between NLO and optical limiting (OL) properties. The main mechanisms to achieve OL are nonlinear absorption (NLA) and nonlinear refraction (NLR), but other effects such as nonlinear scattering can also contribute to OL. Materials with a positive NLA coefficient exhibit reverse saturable absorption (RSA), causing a decrease in transmittance at high intensity levels, and so operate as optical limiters [14]. 

High quantum yield photochemical reactions of condensed-phase species may become useful for future optical applications such as molecular switches, optical limiters, and read-write data storage media. Toward these ends, much research has been conducted on novel nonlinear chemical-based materials such as conducting polymers and metal-organic species. Monitoring the early time-dependent processes of these photochemical reactions is key to understanding the fundamental mechanisms and rates that control the outcome of these reactions, and this could lead to improved speed and efficiencies of devices. 

As noted in Section VC, access to the excited states can be enhanced through photoinduced electron transfer, thereby leading to enhanced NLO response. For example, photoinduced charge transfer provides a mechanism for optical limiting [232]. When the excited states are created by photoexcitation the response time of Xinc is determined by the natural decay of the excited state. However, both the magnitude and the lifetime of xSc can be varied by utilizing photoinduced electron transfer. 

However, the disadvantages of optical rotation detectors may be limited by shot or flicker noise, which are dependent on the optical and mechanical properties of the system or by noise in the detector electronics. Generally, the usefulness of this technique has been limited by the lack of sensitivity of commercially available instruments. 

Francois L., Mostafavi M., Belloni J., Delaire J., Optical limitation of gold clusters. Mechanism and Efficiency, Phys. Chem. Chem. Phys., 2001,3,4965-4971. 

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