Electrooptical absorption

Nemkovich, N. A. Reis, H. Baumaim, W. Ground and excited state dipole moments of coumarin laser dyes Investigation by electrooptical absorption and emission methods. J. Luminesc. 1997, 71, 255-263. 

This chapter gives an overview of the origin of NLO effects and focuses on the design of chromophores for two current applications of technological interest poled polymer materials for electrooptic switching and two-photon absorption. 

Since this chapter is focused on electrooptic materials and two-photon absorption, we will not consider measurement techniques for y and y(31 in detail, but refer the interested reader to Ref. [19]. In this section, we will briefly describe some of the techniques that have been used for the measurement of the two-photon cross-section, d. We first recall that the 2PA behavior of a molecule is characterized by a spectrum entirely analogous to a one-photon absorption (1PA) spectrum. Although, from the point of view of an application at a particular wavelength, measurements of d at only that wavelength may contribute to a figure-of-merit for that application, more generally, to understand structure-property relationships, it is helpful to acquire 2 PA data over as wide a frequency range as permitted by the lasers available and by the onset of 1PA. 

Modulation can be accomplished in a variety of ways. For example, the output of a laser can be modulated directly or external modulation can be applied using electro-optic or electro-absorptive modulators. Currently, commercial electrooptic modulators are fabricated from lithium niobate. The performance characteristics of one family of lithium niobate modulators, produced by Lucent Technologies (Breinigsville, PA), are given in Table 1. 

Recently, it was theoretically shown, on the basis of standard density-matrix formalism of nonlinear optics, that chiral isotropic media can possess an electrooptic response [174]. Such materials would be inherently stable and could therefore be extremely useful for the development of electro-optic devices. Contrary to usual electro-optic materials, index (absorption) modulation in such media is due to the imaginary (real) part of the electro-optic susceptibility. The response relies on the damping of the material response. 

X < >o, cofc,-cofc) AC electrooptic effect, AC Kerr effect, Brillouin scattering, Raman scattering. Two-photon absorption High-speed optical gates... 

Electrooptical methods (electrochromism) which are more accurate but which are experimentally more difficult include electric polarization of fluorescence or phosphorescence, electric dichroism, and absorption spectra in the vapor phase in an electric field (Stark effect). 

Most of the metal alkoxides of interest for electrooptical ceramics are solids (less often liquids) that can be purified by recrystallization, sublimation, or distillation. They are all moisture sensitive, and handling under an inert atmosphere and with anhydrous solvents is thus required. Their unequivocal characterization and formulation are best achieved by x-ray diffraction studies (on monocrystals). Studies on solutions (molecular weight data, nuclear magnetic resonance, NMR, with H, or metal nuclei) are a means either to establish whether the solid-state structure is retained or, in the absence of x-ray data, to establish the molecular structure and eventually stereolability [48]. Mass spectrometry provides information on the stability of the oligomers or the het-erometallic species in the vapor phase. The information gained by infrared spectroscopy is limited the technique is mostly useful for the identification of solvates M(OR) (ROH)x (vOH absorption 3400-3100 cm-l or of chemically modified (heteroleptic) alkoxides (probe for the vCO stretching of P-diketonate or carboxylate ligands, for instance). 

Kang, H., P. Zhu, A. Facchetti, and T.J. Marks. 2004. Self-assembled electrooptic thin films with remarkably blue-shifted optical absorption based on an X-shaped chromophore. J Am Chem Soc 126 15974-15975. 

Most of the electrooptic effects are based on light scattering or on light absorption by polarizers or by dissolved dyes. Electrooptic effects which belong to the second group are tunable birefringence, the twisted nematic effect and the guest-host effect. 

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