China
Africa
Explore chapters and articles related to this topic
Gap Solitons in Photorefractive Optical Lattices
Published in Arpan Deyasi, Pampa Debnath, Asit K. Datta, Siddhartha Bhattacharyya, Photonics, Plasmonics and Information Optics, 2021
A centrosymmetric crystal is one which possesses inversion symmetry or an inversion centre as a symmetry element. The first discovery of stable optical spatial solitons in PR materials was in noncentrosymmetric crystals like SBN. The mediating nonlinearity was because of the linear electro-optic effect or the Pockels’ effect. The Pockels’ effect mandates the change in refractive index is proportional to the electric field. Optical spatial solitons in centrosymmetric PR crystals were later predicted by Segev et al.[13]. Centrosymmetric PR crystals exhibit quadratic electro-optic effect or the dc Kerr effect in which the refractive index changes as the square of field, i.e.Δn α E2. The gap solitons in such centrosymmetricPR materials have been studied in [12] and we shall follow the same approach to introduce the topic to the reader.
C
Published in Philip A. Laplante, Comprehensive Dictionary of Electrical Engineering, 2018
centripetal force force that is present during the robot motion. The force depends upon the square of the joint velocities of the robot and tends to reduce the power available from the actuators. centroid (1) the center of a mass. (2) description of the center of a particle beam profile. (3) a region in the pattern space to which a remarkable number of patterns belong. centroid defuzzification a defuzzification scheme that builds the weighted sum of the peak values of fuzzy subsets with respect to the firing degree of each fuzzy subset. Also called height defuzzification. centroid method a widely used method of defuzzification whereby the centroid of the membership function of the fuzzy set is used as the defuzzified or crisp value. It is also known as the center of gravity method or the composite moments method. centroidal profile a method for characterizing and analyzing the shape of an object having a well defined boundary. The centroid of the shape is first determined. Then a polar (r, ) plot of the boundary is computed relative to this origin: this plot is the centroidal profile, and has the advantage of permitting template matching for a 2-D shape to be performed relatively efficiently as a 1-D process. centrosymmetric medium a material that possesses a center of inversion symmetry. Of importance because, for example, second-order nonlinear optical processes are forbidden in such a material. cepstrum inverse Fourier transform of the logarithm of the Fourier power spectrum of a signal. The complex cepstrum is the inverse Fourier transform of the complex logarithm of the Fourier transform of the signal.
Measurement Techniques for Refractive Index and Second-Order Optical Nonlinearities
Published in Hari Singh Nalwa, Seizo Miyata, Nonlinear Optics of Organic Molecules and Polymers, 2020
Toshiyuki Watanabe, Hari Singh Nalwa, Seizo Miyata
NLO properties such as SHG and electro-optic Pockel's effect are associated closely with the chemical and crystalline structures of the polymeric materials. Of the 32 different crystal point group symmetries sketched below in which all crystalline materials have been classified, II possess a center of symmetry; hence, they are called "centrosymmetric" materials. On the other hand, the 21 that do not have a center of symmetry arc called "noncentrosymmetric" crystal structures. Of the noncentrosymmetric classes, 20 exhibit piezoelectric behavior; hence, they are inherently active to the nonlinear optical SHG and electro-optic Pockel's effects. The NLO properties of ferroelectric polymers will be discussed in detail in another section. If molecular dipoles are aligned in an organic polymer in such a fashion that selfcancellation does not occur, then spontaneous polarization appears because of its unique chemical and morphological structures. As discussed earlier, polarization can be induced by applying a poling technique, although that polarization is electrically quite different from spontaneous polarization. Based on the dielectric properties of the poled polymer, SHG and electro-optic Pockel's effects are also affected by its elastic behavior, because these NLO parameters increase rapidly near the glass transition temperature (Tg) with an increase in the electrostriction constant and a decrease in the dynamic modulus. Therefore, it is important to study the effect of the poling field and temperature on the SHG and electrooptic properties of polymers and to optimize them because they depend strongly on poling conditions.
Torsional vibration of a flexoelectric nanotube with micro-inertia effect
Published in Mechanics of Advanced Materials and Structures, 2023
O. Hrytsyna, J. Sladek, V. Sladek, Q. Deng, M. Hrytsyna
Piezoelectric materials are frequantly utilized in macro- and/or micro/nano-electromechanical systems (MEMS/NEMS) [1]. Recently, the flexoelectric effect has been started to be utilized too. The flexoelectricity describes the coupling between the gradient of a mechanical strain and the electric polarization. Piezoelectric and flexoelectric material properties play an important role in polarized elastic structures at the nanoscale [2–5]. Piezoelectricity describes the electromechanical coupling between the electric polarization and a uniform strain. Piezoelectric effect is absent in centrosymmetric materials. In such materials only direct and converse flexoelectric effects can contribute to the electromechanical coupling phenomena. The ability to transform the mechanical strain gradient to electric polarization is known as the direct flexoelectric effect (the mechanical-to-electrical energy transformation). The converse flexoelectricity is the phenomenon of induction of mechanical stresses and deformations by the electric field gradient (the electrical-to-mechanical energy transformation). Contrary to the piezoelectricity, the flexoelectricity might exist in crystals of any symmetry (including the centrosymmetric materials).
Reflection of plane waves on the stress-free and rigid boundary surfaces of pre-stressed piezoelectric-orthotropic substrate: A comparative approach
Published in Mechanics of Advanced Materials and Structures, 2022
Mukesh Kumar Pal, Abhishek Kumar Singh, Richa Kumari
In Piezoelectric material, there is a generation of electric potential (EP) in a non-centrosymmetric crystal when they are under mechanical stress such as compression, stretching and bending. Few piezoelectric materials like PZT (piezoelectric ceramics) and PVDF (piezoelectric polymers) which are frequently used in piezoelectric actuators and sensors for structural health monitoring and structural repair. In last two decades, extensive studies have been conducted on piezoelectric materials with an aim to make it more commercial through manufacturing smart material devices. Magnificent work on wave propagation aspects has been accomplished by some researchers viz. Cheng and Sun [1], Jin et al. [2], Singh [3], Gaur and Rana [4], Lata et al. [5], Lata [6], Kaur and Lata [7], Kumar et al. [8] and Singh et al. [9] are few among others. The study of propagation, reflection and transmission of plane waves plays a vital role for providing information of the internal structure of the material medium. In present time, the problems concerned with reflection of plane waves are of great interest in the field of acoustics. Moreover, the plane wave propagation in an anisotropic media differs from the propagation of said waves in isotropic media.