Optical limiters nonlinear refraction

The discussion in this chapter is limited to cyanine-like NIR conjugated molecules, and further, is limited to discussing their two-photon absorption spectra with little emphasis on their excited state absorption properties. In principle, if the quantum mechanical states are known, the ultrafast nonlinear refraction may also be determined, but that is outside the scope of this chapter. The extent to which the results discussed here can be transferred to describe the nonlinear optical properties of other classes of molecules is debatable, but there are certain results that are clear. Designing molecules with large transition dipole moments that take advantage of intermediate state resonance and double resonance enhancements are definitely important approaches to obtain large two-photon absorption cross sections. 

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]. 

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],... 

The reflectivity and width of the band gaps increase as the refractive index modulation increases. The basic principle for optical limiting by a ID nonlinear photonic crystal material can be understood from Figure 2. Initially, (A), the refractive indices of the different layers are matched, and the sample... 

Nonlinear absorption provides the most useful mechanism for optical limiting, so it is desirable to use the induced reflectivity in conjunction with nonlinear absorption. The target material is then one in which alternate layers contain a dye that possesses both nonlinear absorption and nonlinear refraction. This material has a modulation in both the real and imaginary (absorptive) parts of the index. This paper describes the progress towards such a material. 

A new type of nano-layered polymeric nonlinear optical material for optical limiting applications has been fabricated. It comprises alternating polymer layers with thousands of layers and a layer thickness down to 30 nm or less. By introducing appropriate dyes into alternate polymer layers, a material with a modulation in the real and/or imaginary part of the nonlinear refractive index in the direction normal to the surface is possible. 

Another nonlinear nanolayered material was fabricated with the phthalocyanine dye, PbPc(CP)4, in alternate layers. This dye possesses a strong nonlinear absorption and a nonlinear refraction. In the layered material, both are modulated at a spatial frequency corresponding to the layer thickness, which was an average of 87 nm. The material showed both a nonlinear absorption and a nonlinear reflection and provided optical limiting. The modeling of this nonlinear response of this material was not attempted here. To our knowledge, there is little available theory on the response of such nonlinear structures to pulsed lasers. 

This work demonstrates that these new materials have the potential to provide enhanced optical limiting by the more efficient use of nonlinear refractive processes. Clearly, it is necessary to understand in more detail how the magnitude of the different components, real and imaginary (absorptive), of the nonlinearities in the layers and the distribution of layer thickness influence the nonlinear response. Experiments to this end are in progress, however, a better theoretical model for the response of multilayer structures with both nonlinear absorption and refraction is desirable. Such a model would aid in the design of more efficient layer structures. 

The nonlinear optical properties and the role of the surface plasmon resonance (SPR) in optical limiting (OL) properties of Ag-Cu nanoclusters co-doped in Si02 films were investigated [322]. These films show a self-defocusing nonlinearity and the nonlinear refractive index decreased with decreasing particle size, whereas the nonlinear absorption increased with an increase in Cu concentration. 

Especially at high excitation densities, the refractive index of the material depends on the intensity of the light. Materials with large nonlinear refraction may potentially be used in applications such as optical switching, amplification, and limiting. As we have already described the interaction of an optical beam with the nonlinear optical medium in terms of the nonlinear polarization, we can express the polarization that influences the propagation of the optical beam of frequency (o as... 

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