Optical effects, thermal-wave

The nanostructure of a material is its stmcture at an atomic scale. Nanoparticles and nanostructures generally refer to structures that are small enough that chemical and physical properties are observably different from the normal or classical properties of bulk solids. The dimension at which this transformation becomes apparent depends on the phenomenon investigated. In the case of thermal effects, the boundary occurs at approximately the value of thermal energy, kT, which is about 4 X 10 J. In the case of optical effects, nonclas-sical behaviour is noted when the scale of the object illuminated is of the same size as a light wave, say about 5 x 10 m. For particles such as electrons, the scale is determined by the Heisenberg uncertainty principle, at about 3 x 10- m. 

The third-harmonic generation method has the advantage that it probes purely electronic nonlinearity. Therefore, orientational and thermal effects as well as other dynamic nonlinearities derived from excitations under resonance condition are eliminated (7). The THG method, however, does not provide any information on the time-response of optical nonlinearity. Another disadvantage of the method is that one has to consider resonances at oj, 2w and 3o> as opposed to degenerate four wave mixing discussed below which utilizes the intensity dependence of refractive index and where only resonances at a) and 2a) manifest. 

The simple analysis presented above confirms that new formulations are required to produce stable, reliable products for field use. Practical system requirements, as defined by Mil Spec conformity and the use of standard fabrication and assembly processes, definitely require that a electro-optic polymer system with better thermal properties than thermoplastic acrylates be developed. That this is true for optical interconnection boards and modules is not surprising because of their complexity. It is perhaps remarkable that it remains true for even simple devices, such as a packaged, pigtailed traveling-wave modulator. The ultimate success of electro-optic polymers will be their use in cost-effective products that are used by systems designers. 

All-optical bistability angular dependence, 324 experimental setup, 321,323/ output-input curves, 321,324,325/ thermal effect, 324 All-optical guided wave devices examples, 124,125/126... 

In the third-harmonic generation, the third-order susceptibility leads to a nonlinear polarization component which oscillates at the third-harmonic frequency of the incident laser beam. This leads to a light wave at the third-harmonic frequency of the fundamental wave. As optical frequencies are involved and since the output frequency is different from the input frequency only the electronic nonlinearities can participate without any contributions from thermal or orientational effects. Because one needs fast nonlinearities for all-optical signal processing, the main interest is directed towards the fast electronic nonlinearities. Therefore and also due to its simplicity, third-harmonic generation is a very attractive method to characterize newly developed materials. 

Figure 13 Schematic of the setup of the pump-probe experiment with polarization resolution for the probing of the induced change in sample transmission. X/2 half-wave plate P1-P3 polarizers L1-L4 lenses D1-D5 detectors Ch chopper VD optical delay line. The sample is permanently moved in a plane perpendicular to the beams in order to avoid accumulative thermal effects.

The structures that have evolved for ablative-mode optical discs make use of interference effects to minimize the reflectance (R) of the disc in the absence of a hole. A typical ablative-mode optical disc has the structure shown in Figure 5.51. The substrate is an optically transparent material such as polycarbonate, poly(methyl methacrylate), poly(ethylene terephthalate), or poly(vinyl chloride), topped by a subbing layer to provide an optically smooth (to within a fi-action of a nanometer) surface for the recording layer. A metal reflector (typically aluminum) is then incorporated next to a transparent dielectric medium such as spin-coated poly(a-methyl styrene) or plasma-polymerized fluoropolymers. This dielectric spacing layer serves both to satisfy the quarter-wave (2/4) antireflection conditions and to insulate thermally the A1 reflector from the top absorbing layer where the information pits are created. 

---