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Chinese Physics Letters

Optical Bistability in Graded Core-Shell Granular Composites

Yamin Wu, Guoqing Chen, Chaoqun Ma, Xue S, Zhuo-wei Zhu

Published Mar 1, 2012• 4 citations
Publisher

Abstract

The intrinsic optical bistability (OB) of graded core-shell granular composites is investigated. The coated particles are made of cores with gradient dielectric function ∊ c (r) = A(r/a) k and nonlinear shells. In view of the exponential distribution of the core dielectric constant, the potential functions of each region are obtained by solving the Maxwell equations, and the mathematical expressions of electric field in the shells and cores are determined. Numerical study reveals that the optical bistable threshold and the threshold width of the composite medium are dependent on the shell thickness, core dielectric exponent, and power function coefficient. The optical bistable width increases with the decreasing shell thickness and the power exponent and with the increasing power function coefficient.

DOI
10.1088/0256-307x/29/3/037802

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The intrinsic optical bistability (OB) of graded core-shell granular composites is investigated. The coated particles are made of cores with gradient dielectric function ∊ c (r) = A(r/a) k and nonlinear shells. In view of the exponential distribution of the core dielectric constant, the potential functions of each region are obtained by solving the Maxwell equations, and the mathematical expressions of electric field in the shells and cores are determined. Numerical study reveals that the optical bistable threshold and the threshold width of the composite medium are dependent on the shell thickness, core dielectric exponent, and power function coefficient. The optical bistable width increases with the decreasing shell thickness and the power exponent and with the increasing power function coefficient.
We studied the intrinsic optical bistability(OB) of coated spherical particles with graded dielectric profiles.According to Maxwell equations, we obtained the spatial average of the local field in the core as a function of the external applied field. Numerical results for power-law graded profiles show that the optical bistable behavior is dependent on the shell thickness and the power-laws exponent k. The interval of optical bistability increases with increasing shell thickness or increasing k. In addition, the field-dependent reflectance at normal incidence R in random composites is also investigated, and a hysteretic loop is observed.
Optical bistability has been investigated in granular metal-dielectric composite media at high temperatures. Based on Maxwell-Garnett approximation and the spe ctralrepresentation, we calculate numerically the electric field as a function of the applied electric field E２０,and thus the opticalbistability can be studied.We show that the width of optical bistability decreases with increasing temperature.Furthermore, we study the dependence of the refractive index and the extinction coefficient on the temperature.
In this paper, the effect of a tunable dielectric core on local field enhancement, induced optical bistability, and the optical bistability domain in cylindrical core–shell nanoparticle composites are studied. The local field enhancement factor increases significantly at two resonant frequencies. The results demonstrate that the local field enhancement factor in the cylindrical core–shell nanoparticle increases when the natural attribute of the dielectric function of the dielectric core is varied by adding a dielectric function to it. Furthermore, we demonstrated that the magnitude of the imaginary part of the active dielectric core increases as the onset and offset input intensities increase. This indicates that the optical bistability or threshold width range widens as the imaginary part of the dielectric function of the dielectric core increases, thereby enlarging the threshold domain to improve system activation.
We present results on the nonlinear-optical response for a series of dielectric particles having nanometer sizes that are coated with a metallic shell. Using the freedom of changing particle shapes and coating thicknesses, we optimize the threshold intensity for intrinsic optical bistability. The metallic shell creates an enhanced local field in the nonlinear core. We report threshold intensities for silver-coated CdS as low as 12 W/cm2{\mathrm{cm}}^{2}cm2.
We study the interaction of an electromagnetic field with a core-shell nanostructure and propose a nonlocal effective medium theory for a core-shell structure made of a nonlocal core and a magneto-optical shell cylinder embedded in the host medium. In addition to the appearance of optical bistability and tristability in this system, it is found that the threshold and window of bistability are strongly dependent on the nonlocality and magneto-optical properties of the core-shell nanoparticle, the size of the nanoparticle, and the incident wavelength. By fine modification of these parameters, it is possible to achieve maximal enhancement of optical nonlinearity from the proposed system. Our study offers a theoretical direction to further structure design and optimization and also applications in the high speed all-optical signal processing.
We examine the nonlinear dielectric response of small spherical grains ϵ m embedded in a linear dielectric ϵ d . The models for the spherical grains are a Kerr medium1,2 and a two-level system.3 The expression for the dielectric function ϵ m is dependent of the grain size; in addition it also depends on the local electric field E L . This field is related to the external applied field E0 by the equation E L = (3ϵ d /ϵ m + 2ϵ d )E0. The steady-state equation for the averaged external field is ∇2E0 + (ω2/c2)ϵ*E0 = 0, where ω is the frequency of the field, ϵ* is the dielectric function from the effective medium approximation, and it is obtained from the self-consistency relation4 〈ϵ(r) − ϵ*/ϵ(r) + 2ϵ*〉 = 0; the angular brackets denote an average taken over the ensemble of dielectric spheres distributed in a small volume. Using the slowly varying-envelope approximation we solve these equations for a distribution of grain radii and the affect this has on the effective optical nonlinearities. In particular, we show its effect on the optical bistability for these composite systems.