Third-order nonlinearities, optimization

In general, the optimization of organic molecules for third order nonlinear optical applications has enjoyed much less success than for second order optical nonlinearities. The major reason for this has been the questionable validity of the two-level model for y, and the difficult assessment of the contribution of two-photon states for the more acceptable three-level model. 

The development of highly active third-order nonlinear optical materials is important for all-optical signal processing. In contrast to second-order nonlinear optical molecular systems, there are few rational strategies for optimizing the third-order nonlinear optical response of molecular materials. Unlike second-order materials, there exist no molecular symmetry restrictions for the observation of a third-order nonlinear optical response. It is the instantaneous... 

In third-order nonlinear optics, guidelines for the optimization of the second-order hyperpolarizability y of a molecule have been steadily improving, but the understanding is far less developed than for Experimental and theoretical observations have shown that for molecules with an extended conjugation in a single direction, asymmetric substitution yields the largest second-order hyperpolarizabilities y [56,57] similar to the case of /3 in several systems. This issue will be discussed in this Section for a selection of molecules that is by no means complete. 

We have discussed recent developments in the molecular design for third-order nonlinear optical applications. The following optimization guidelines can be derived from the above discussions. 

Electrodeposited sol-gel-based composite films also showed optical applications. Gu and coworkers [85,86] co-electrodeposited Te0 > -Si02 hybrid films from the TEOS-Te(i-PrO)4 niixed precursor for nonlinear optics. Te(IV) was partially reduced during electrodeposition, as characterized by EDX of the obtained films. The as-prepared films had third-order nonlinear susceptibility ix ) of 5.9 X10 to 4.29 X 10 esu, and the films had of 1.551 X 10 esu after posttreatment annealing. Mandler and coworkers co-electrodeposited TMOS with multiwalled carbon nanotubes (MWCNTs) on ITO and silver. The optimized films electrodeposited on ITO showed transparency of about 50% with nonlinear optical properties, and the optimized films electrodeposited on silver had specular reflectance lower than 0.5% in the wavelength range of 400-15 000 nm, which can be used as antireflection coatings. 

Recently, other attempts have been reported by Dai et al. (439) using the complexes 31 (Scheme 26). These authors admit that the efficiency of their materials needs to be improved by optimizing the balance between absorption and nonlinearity. Third-order NLO properties of M(dmit)2 and M(mnt)2 complexes with sandwiched organometallic cations [CpFe(r -C6H6)]+ have also been reported very recently (440). 

Thus, in both cases, the molecular unit can be tailored to meet a specific requirement. A second crucial step in engineering a molecular structure for nonlinear applications is to optimize the crystal structure. For second-order effects, a noncentrosymmetrical geometry is essential. Anisotropic features, such as parallel conjugated chains, are also useful for third-order effects. An important factor in the optimization process is to shape the material for a specific device so as to enhance the nonlinear efficiency of a given structure. A thin-film geometry is normally preferred because nonlinear interactions, linear filtering, and transmission functions can be integrated into one precise monolithic structure. 

Organic compounds with delocalized 7r-electron systems are leading candidates for nonlinear optical (NLO) materials, and interest in these materials has grown tremendously in the past decade [108-118]. Reliable structure-property relationships—where property here refers to first-order (linear) polarizability a, second-order polarizability and third-order polarizability y—are required for the rational design of optimized materials for photonic devices such as electro-optic modulators and all-optical switches. Here also, quantum-chemical calculations can contribute a great deal to the establishment of such relationships. In this section, we illustrate their usefulness in the description of the NLO response of donor-acceptor substituted polymethines, which are representative of an important class of organic NLO chromophores. We also show how much the nonlinear optical response depends on the interconnection between the geometric and electronic structures, as was the case of the properties discussed in the previous sections [ 119]. 

Third- and higher-order frequency conversion are often done with beams that are tightly focused within the nonlinear medium to increase the peak intensity. In this situation, optimal performance can require either a positive or neg-... 

---