Optical-limiting response

Fig. 7 Optical limiting responses to 8 ns, 532 nm optical pulses, of DCM solutions (0.86mg/mL) of hb-PAs. Data for a toluene solution of C6o (0.16mg/mL) is shown for comparison...

Non-linear optics in molecular materials has long been restricted to the investigation of organic chromophores. Most of the tin-based coordination compounds have been proposed only recently. Additionally, heavy atoms, such as tin, have also been used to enhance the optical-limiting response of metal phthalocyanines." Although this property derives from the cubic (a E ) NLO response, and falls outside the scope of the present review, it provides additional insights suggesting that tin-based coordination compounds could deserve more attention in the search for future NLO chromophores. 

Figure 6.2 Comparison of the optical-limiting response of P26 and its purely organic polyyne (T = 82%).

P35 as photoconductors was also investigated in a single-layer device.67 A recent study also showed that polymer P35 displays a large optical-limiting response to a nanosecond laser pulse with satisfactory threshold value at 0.12J/cm2, and its behavior is superior to that for poly(4,4 -diethynyl-5,5 -bithiazole) organic polymer.60... 

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.)...

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).

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. 

Due to the limited response time of suitable sensors fast sorption or gas transport processes on a time scale below a second are hard to monitor. To significantly improve the resolution in time an interferometric pressure sensor can be applied. The central part of the interferometric pressure sensor presented is a Michelson-interferometer this set-up is sensitive to changes in gas pressure as the index of refraction, and thus the optical path length for a laser beam within the interferometer, is a function of the gas density. 

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. 

For many optical limiting applications, solid films are desirable. Further work is underway to elucidate the response of fullerene films to optical pulses. ... 

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. 

The proposed excited state fullerene-fullerene interactions are consistent with the results of picosecond transient absorption measurements, in which excimer-like transients were observed [181]. The transient absorption is probably responsible for the significant optical limiting effects of the CgQ-styrene copolymers [182],... 

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