Resonant third-order materials

Photoinduced processes and resonant third order nonlinearity in poly(3-dodecylthiophene) has been studied by fs time resolved 4-wave mixing . Similar work has been reported for the poly(p-phenylene vinylene) system . Such materials have potential for the use as nonlinear optical switching devices. 

The general task is to trace the evolution of the third order polarization of the material created by each of the above 12 Raman field operators. For brevity, we choose to select only the subset of eight that is based on two colours only—a situation that is connnon to almost all of the Raman spectroscopies. Tliree-coloiir Raman studies are rather rare, but are most interesting, as demonstrated at both third and fifth order by the work in Wright s laboratory [21, 22, 23 and 24]- That work anticipates variations that include infrared resonances and the birth of doubly resonant vibrational spectroscopy (DOVE) and its two-dimensional Fourier transfomi representations analogous to 2D NMR [25]. 

Hill, C. A. S. Underhill, A. E. Winter, C. S. Oliver, S. N. Rush, J. D. Resonance Enhanced Third-order Nonlinearities in Metal Dithiolenes. In Organic Materials for Nonlinear Optics II Hann, R. A., Bloor, D., Eds. Royal Society of Chemistry London, 1991, pp 217-222. 

The components of the third-order nonlinear susceptibility relevant to the CARS process are conveniently subdivided into two terms vibrationally resonant (X, ) and vibrationally nonresonant (Xnr) components. The total response of the material depends on the sum of these two terms ... 

The dispersion of NLO properties is a major source of problems. Measurements are frequently available at one wavelength only, and the degree to which the results are influenced by material resonances close to the measurement wavelength is often difficult to quantify. It is possible to compensate for some of the dispersion effects in certain cases. However, unlike with second-order nonlinearity, the simple two-state model is generally considered insufficient for describing the dispersion of the third-order nonlinearity at least two excited states have to be considered. 

The interest in semiconductor QD s as NLO materials has resulted from the recent theoretical predictions of strong optical nonlinearities for materials having three dimensional quantum confinement (QC) of electrons (e) and holes (h) (2,29,20). QC whether in one, two or three dimensions increases the stability of the exciton compared to the bulk semiconductor and as a result, the exciton resonances remain well resolved at room temperature. The physics framework in which the optical nonlinearities of QD s are couched involves the third order term of the electrical susceptibility (called X )) for semiconductor nanocrystallites (these particles will be referred to as nanocrystallites because of the perfect uniformity in size and shape that distinguishes them from other clusters where these characteriestics may vary, but these crystallites are definitely of molecular size and character and a cluster description is the most appropriate) exhibiting QC in all three dimensions. (Second order nonlinearites are not considered here since they are generally small in the systems under consideration.)... 

The global energy conservation condition, Eq. (4.11), is explicitly demonstrated for resonant processes up to third order (5 3), particularly for resonant passive processes, at exact resonance, where population change can be achieved at a nonquadrature level by the fields. The phase matching condition is assumed, Ak = 0. As before, the material resonance at the one-photon level is taken into account by the complex wave vector, k, = k -I-ik", whose imaginary part is absorbed into the amplitude, ,( ) = sxp[-k" r]. The electrical susceptibility is expressed in terms of the scalar Cartesian component, as given in Eq. (2.17). 

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