Second-harmonic generation , nonlinear optics

In the single-domain state, many ferroelectric crystals also exhibit high optical nonlinearity and this, coupled with the large standing optical anisotropies (birefringences) that are often available, makes the ferroelectrics interesting candidates for phase-matched optical second harmonic generation (SHG). 

Optical second harmonic generation (SHG), which stems from the conversion of two photons of frequency to to a single photon of frequency 2(o, is an inherently surface-sensitive technique. Whereas no optical second harmonic wave is generated in the centrosymmetric bulk of a liquid, molecules participating in the asymmetry of the interface between two liquids (noncentrosymmetric environ-ment) contribute to SHG. Since the square root of SHG signal intensity, is proportional to the number N (per unit area), the molecular orientation (I) and the second order nonlinear polarizability of the SHG active species at the interface... 

The effect of the kiex2 term on the motion of Xe is written as Xeee, for example, and physically represents optical second harmonic generation (SHG), to which the polar phonons do not contribute because they are not able to drive or follow high-frequency responses. This nonlinearity... 

The confinement property of waveguides also permits one to observe nonlinear optical process (e.g., optical second harmonic generation, stimulated Raman scattering, etc.) with relatively modest optical powers (19). 

Y.R. Shen, The Principles of Nonlinear Optics, Wiley, (1984) Optical Second Harmonic Generation at Interfaces, Ann. Rev. Phys. Chem. 40 (1989) 327. (Theoretical basis of SHG, experimental set-up, examples of applications to show what types of information are obtained from SHG measurements.)... 

Ferroelectric crystals (especially oxides in the form of ceramics) are important basic materials for technological applications in capacitors and in piezoelectric, pyroelectric, and optical devices. In many cases their nonlinear characteristics turn out to be very useful, for example in optical second-harmonic generators and other nonlinear optical devices. In recent decades, ceramic thin-film ferroelectrics have been utilized intensively as parts of memory devices. Liquid crystal and polymer ferroelectrics are utilized in the broad field of fast displays in electronic equipment. 

Temperature phase synchronism is obtained in optical second-harmonic generation in a ferroelectric smectic C whose helicoid is untwisted by a constant electric field. The nonlinear second-harmonic susceptibility due to spontaneous polar ordering of the molecules is estimated. 

The nonlinear properties of FLCs attracted considerable attention both from the fundamental and technical points of view [132-134]. Since, in ferroelectric phases the center of inversion is absent (Fig. 7.1), the amplitude of the optical second harmonic generation (SHG) ought to be fairly strong, especially in commercially available FLC mixtures with high spontaneous polarization [133]. Reference [134] shows the ways of designing new FLC substances with the increased molecular second-order hyperpolarizability comparable to that of solid electrooptical crystals such as LINbOs. 

Twieg, R. and Jain, K., Organic materials for optical second harmonic generation, in Nonlinear Optical Properties of Organic and Polymeric Materials (ACS Symp. Sen, 233), Williams, D. J., Ed., American Chemical Society, Washington, D.C., 1983, 57. 

In addition to field enhancement via localized plasmon excitation, nonlinear optical methods, such as optical second harmonic generation (SHG), deliver significant contrast enhancement which can be used for near-field exper-... 

Vollmer R 1998 Magnetization-induced second harmonic generation from surfaces and ultrathin films Nonlinear Optics in Metals e6 K H Bennemann (Oxford Clarendon) pp 42-131... 

Campagnola P J, Wei M D, Lewis A and Loew L M 1999 High-resolution nonlinear optical imaging of live cells by second harmonic generation Biophys. J. 77 3341-9... 

Only certain types of crystalline materials can exhibit second harmonic generation (61). Because of symmetry considerations, the coefficient must be identically equal to zero in any material having a center of symmetry. Thus the only candidates for second harmonic generation are materials that lack a center of symmetry. Some common materials which are used in nonlinear optics include barium sodium niobate [12323-03-4] Ba2NaNb O lithium niobate [12031 -63-9] LiNbO potassium titanyl phosphate [12690-20-9], KTiOPO beta-barium borate [13701 -59-2], p-BaB204 and lithium triborate... 

Barium sodium niobium oxide [12323-03-4] Ba2NaNb 02, finds appHcation for its dielectric, pie2oelectric, nonlinear crystal and electro-optic properties (35,36). It has been used in conjunction with lasers for second harmonic generation and frequency doubling. The crystalline material can be grown at high temperature, mp ca 1450°C (37). 

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