Ultrafast nonlinear optical properties

In contrast to the computations of the preceding section we directly calculate the expectation value of the CC dipole operator (finally linearized with respect to the external field) applying the classical path approximation for nuclear dynamics. Such a direct calculation of the dipole operator expectation value becomes of particular interest when focusing on ultrafast nonlinear optical properties (transient absorption, photon echo signal, etc.). 

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. 

The remaining sections outline recent findings on the optical properties of metal-dendrimer nano-composites. The sections have been organized into areas of interest related to the fabrication as well as photophysical properties of these new and important materials. The first section discusses some of the important issues concerning the fabrication of these novel materials and their chemical characterization. The second section discusses the linear and nonlinear optical properties of these materials. The third section provides details related to the ultrafast emission properties of gold- and silver-dendrimer nanocomposites. 

Nonlinear optical properties of PTs which exhibit ultrafast responses and large nonlinearities attributed to one-dimensionality and delocalization of n-electrons along the polymer chains are also described [403,404]. Poly(4,4 -dipentoxy-2,2 -bithiophene) and poly(4,4 -dipentoxy-2,2 5, 2"-terthiophene) show a fast and high third-order nonlinearity [405]. Third-order nonlinearities depend on the nature of the polymer backbone and only slightly on the substituents [406], The optical transparency and the third-order optical nonlinearities can be tailored in random copolymers of 3-methylthiophene and methyl methacrylate [407]. A solution-processable thiophene copolymer with a side... 

NMR. nuclear magnetic resonance non-bridging oxygens (NBO), 25 nonlinear optical properties nonlinear electron polarization, 389 optical Kerr effect, 389 optical susceptibility, 389 second harmonic generation, 389 ultrafast response, 389 non-Newtonian flow, 240 non-oxides, 47... 

For these classes of conjugated molecular and polymer structures, the principal property is that their nonreson-ant, nonlinear optical responses are dominated by ultrafast, virtual excitations of the ir-electron states. This was directly demonstrated by MNA (2-methyl-4-nitroaniline) single crystal measurements of macroscopic second order susceptibilities at do (j 3) and optical frequencies (13-1 ) and combined second harmonic measurements and theo-... 

Figure 2 illustrates the basic concept of a typical pump-probe spectroscopy used in most ultrafast spectroscopy techniques. In its simplest form the output pulse train of an ultrafast laser is divided in two by a beam splitter. One pulse in train (called pump) first excites the sample under investigation. The second pulse train (called probe) will probe the sample with a suitable time delay with respect to the pump by introducing an optical delay in its path and some optical property (e.g., reflectivity, absorption, Raman scattering, luminescence, optical nonlinear responses) of the sample is then detected to investigate the changes produced by the pump. In most of the time-resolved pump-probe experiments, the time resolution is limited only by the pulse width of the laser or the jitter between the laser systems. 

Del Fatti, N., Vallee, F. Ultrafast optical nonlinear properties of metal nanoparticles. Appl. Phys. B 73, 383-390 (2001)... 

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