Molecules, resonant nonlinearity

A clear-cut distinction between the two processes is the time response, which is ultrafast in the former (essentially instantaneous), and finite (associated with the lifetime of real excitations) in the latter. Here we limit our discussion to resonant nonlinearities, which are crucial for describing the fundamental working mechanisms of organic photovoltaic cells. Examples will be reported concerning isolated molecules and the condensed phase. 

There are three different types of C—N bonds in di-cyandiamide, NCNC(NH2)2, and the molecule is nonlinear. Of all the resonance structures you drew for this molecule, predict which should be the most important. 

P. N. Prasad and G. S. He, Multiphoton Resonant Nonlinear Optical Processes in Organic Molecules. In Nonlinear Optical Materials. Theory and Modeling, Vol. 628, S. P. Kama and A. T. Yeates, Eds., American Chemical Society, Washington, DC, 1995, p. 225. 

In the above discussion, we have only considered the effects due to the CTE-CTE repulsion, which contribute to the resonant nonlinear absorption (as well as to other resonant nonlinearities) by the CTE themselves. Here, however, we want to mention a more general mechanism by which the nonlinear optical properties of media containing CTEs in the excited state can be enhanced. This influence is due to the strong static electric field arising in the vicinity of an excited CTE, If, for example, the CTE (or CT complex) static electric dipole moment is 20 Debye, at a distance of 0.5 nm it creates a field Ecte of order 107 V/cm. Such strong electric fields have to be taken into account in the calculation of the nonlinear susceptibilities, because they change the hyperpolarizabilities a, / , 7, etc. of all molecules close to the CTE. For instance, in the presence of these CTE induced static fields, the microscopic molecular hyperpolarizabilities are modified as follows... 

With the basic mechanism understood, the resonant nonlinearity of semiconductor clusters can now be quantitatively analyzed. Since one trapped electron-hole pair can bleach the exciton absorption of the whole cluster, the bleaching efficiency per absorbed photon of a nanocluster is the same as that of a molecule, as described by Eq. (20). For a given rp and r, the resonant third-order optical nonlinearity of a nanocluster is simply determined by the (a - ax) term. 

One major difference between a semiconductor nanocluster and molecule is the size dependence of a. As discussed in Section II.B and illustrated in Figure 5, there is a rapid rise in the absorption cross section of the first excited state as the cluster size is reduced below the exciton size. The resonant third-order nonlinearity of a nanocluster is therefore predicted to increase with decreasing cluster size. In reality it is limited by the cluster size dispersion and the presence of surface defects [15]. To maximize the resonant nonlinearity, one needs (1) a sharp exciton absorption band (which means smaller and monodisperse clusters), and (2) semiconductor clusters with larger absorption coefficients, such as GaAs or PbS. 

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. 

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