Optical limiters experimental

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 transmissivity decreases with the input intensity (Fig. 3.107) and dopant concentration (Fig. 3.108) [529]. The good agreement between experimental data and the theoretical predictions given by Eqs. (86) and (87) demonstrates the dominant role of TPA in causing the observed optical limiting properties of compound 94a [529]. 

Tlie usual experimental techniques developed to study the optical Kerr effect in materials have already been described in a preceding chapter of this book. We only mention here the methods which have especially been used for nanocomposite materials as colloidal solutions or thin films Degenerate four-wave mixing (DFWM) and optical phase conjugation, which provide the modulus of x only and may be completed by Interferometry techniques to get its phase as well, optical limiting, optical Kerr shutter, and z-scan, which is probably the most common technique used in recent years due to its ability to provide simultaneously the nonlinear refraction and absorption coefficients of the same sample point [118],... 

Wavelength Dependence of Optical Properties. The relationship between the optical properties n and k and the other molecular-crystalline properties of the solid are discussed by Siegel and Howell [45], The theoretical treatments of the prediction of these properties are also discussed by them. Here, we examine some of the limited experimental data on n(X) and k(A.) for solids. 

Recently, the commercial availability of PDA and other optical drop-sizing methods have resulted in more rapid and accurate measurements. However, to be practical these devices require a continuous spray process so are not handy for single drop experiments. This is because their measurement volumes are typically small compared to the region through which fragments pass. As a result, only limited experimental data exist and more research is warranted. 

Figure 3.23 Experimental setup for the measurement of optical-limiting properties...

Figure 8.39. Performance of an optical limiter made of P30T/[6,6]PCBCa (1 1) composite film (30 /xm thick) at 760 nm solid circles, experimental data broken line, linear transmission (44%) solid line, calculated response [33],...

Figure 11.14. An experimental arrangement for aehieving the optical limiting effect using the self-phase modulation or self-focusing/self-defocusing effect. See chapter 12, however, for a fiber array limiting device that does not suffers from small field of view associated with this configuration involving pin-hole.

Figure 12.3. Experimental set up for optical limiting action using laser induced nematic liquid crystal axis realignment effect. Upper diagram shows the geometry of the twisted nematic cell. The focused spot diameter of the Argon laser on the film is 150 microns. Sample thickness 25 microns. Dye concentration 0.5 %.

Using C6o(Cl) (n = 16 to 20) [88] as a multifunctional initiator under ATRP conditions (CuCl/2,2 -bipyridine, toluene, 130 °C), styrene [89] and N-vinylcarbazole [90] could be polymerized (Scheme 5.15). There is experimental evidence that the resulting compounds are star shaped, but the number and length of the arms or the number of residual Cl on the fullerene could not be determined. A thermal polymerization, producing a non-negligible amount of polymer containing no fullerene, is always observed. Nevertheless, the optical-limiting behavior of (PVK)j,C6o(Cl). y approached that of pristine Ca> [90]. This work was further extended to a Ni-based ATRP catalyst system and it was estimated that about two PS arms were attached to C6o(Cl) [91]. 

Although experimental studies of DNA and RNA structure have revealed the significant structural diversity of oligonucleotides, there are limitations to these approaches. X-ray crystallographic structures are limited to relatively small DNA duplexes, and the crystal lattice can impact the three-dimensional conformation [4]. NMR-based structural studies allow for the determination of structures in solution however, the limited amount of nuclear overhauser effect (NOE) data between nonadjacent stacked basepairs makes the determination of the overall structure of DNA difficult [5]. In addition, nanotechnology-based experiments, such as the use of optical tweezers and atomic force microscopy [6], have revealed that the forces required to distort DNA are relatively small, consistent with the structural heterogeneity observed in both DNA and RNA. 

---