Bulk materials, nonlinear optical properties

On the other hand, the nonlinear optical properties of nanometer-sized materials are also known to be different from the bulk, and such properties are strongly dependent on size and shape [11]. In 1992, Wang and Herron reported that the third-order nonlinear susceptibility, of silicon nanocrystals increased with decreasing size [12]. In contrast to silicon nanocrystals, of CdS nanocrystals decreased with decreasing size [ 13 ]. These results stimulated the investigation of the nonlinear optical properties of other semiconductor QDs. For the CdTe QDs that we are concentrating on, there have been few studies of nonresonant third-order nonlinear parameters. 

This paper is a tutorial overview of the techniques used to characterize the nonlinear optical properties of bulk materials and molecules. Methods that are commonly used for characterization of second- and third-order nonlinear optical properties are covered. Several techniques are described briefly and then followed by a more detailed discussion of the determination of molecular hyperpolarizabilities using third harmonic generation. 

The extensive jt-delocalized system of metal-dithiolene complexes is also responsible for the nonlinear optical properties (NLO) which have been recently reviewed . The interaction of radiation with the matter induces an instantaneous displacement (polarization Pq = /X = aE, where a is the linear polarizability) of the electronic density away from the nucleus at small field (linear optics). At high fields (laser light) the polarizability of the molecule can be driven beyond the linear regime and a nonlinear polarization is induced (NLO) = aE + fiE" + y E + and for the bulk material... 

Importantly, physical and chemical properties of nanoparticulate films are markedly different from those of the bulk materials. For example, magnetic nanoparticles can be prepared where only one magnetic domain is present, so that the rotation or alignment of the whole particle implies the rotation or alignment of the magnetic moment [53]. Semiconductor nanoparticles possess strong nonlinear optical properties due to increased oscillator strength within excitonic transitions [54-56]. Electrooptical shifts can be induced in metal particles because the surface plasmon band position depends on the free electron concentration electron injection can be used to modulate the peak position [57]. 

In this paper it has been attempted to provide an introductory overview of some of the various nonlinear optical characterization techniques that chemists are likely to encounter in studies of bulk materials and molecular structure-property relationships. It has also been attempted to provide a relatively more detailed coverage on one topic to provide some insight into the connection between the macroscopic quantities measured and the nonlinear polarization of molecules. It is hoped that chemists will find this tutorial useful in their efforts to conduct fruitful research on nonlinear optical materials. 

Transition metal acetylides combine the properties of acetylenes with those of the transition metals, offering flexibility in the tuning of structural and electronic properties of both the organic and inorganic constituents. Optimization of the molecular and bulk crystalline properties is envisaged to lead to a new class of useful nonlinear optical materials. 

Phosphates showing a bulk polarization (i.e. ferroelectric phases) may be used for nonlinear optical processes see Nonlinear Optical Materials) such as second harmonic generation and electro-optic switching. KTP (Section 5.2.2) and related phases (NH4T10P04 and KTi0As04) are very efficient nonlinear materials. The ferroic phosphates described above also show nonlinear properties. KDP materials are inferior to KTP types but they find use in electro-optics as they are very transparent over a wide frequency range. 

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