Optical nonlinearity enhancement via geometric anisotropy

Abstract

We investigate the effects of geometric anisotropy on the optical nonlinearity of metal nanocrystals in a dielectric host. By invoking the effective medium approximation for anisotropic composite media, we calculate the spectral density as a function of volume fraction and anisotropy. The results show that anisotropy not only changes the spectral function substantially and thereby has a pronounced effect on the nonlinearity, but also separates the absorption peak from the nonlinearity enhancement peak so that the figure of merit may be increased by orders of magnitude. We propose the use of electrorheological effect ͑e.g., during annealing͒ to realize these gains in optical nonlinearity. ͓S1063-651X͑97͒51008-7͔

Enhanced nonlinear optical responses of materials: Composite effects

“…In looking for experimental support, we find that the separation between absorption peak and nonlinearity enhancement peak proposed by one of us [90] has recently been confirmed experimentally by Guan et al [142]. In the same article by Guan et al [142], they also experimentally confirmed our theoretical prediction [54] that the peak of third order nonlinear susceptibilities shifts to longer wavelengths with an increasing applied electric field (which, during fabrication, causes metallic particles to have the form of prolate spheroid).…”

“…Such a broad absorption band has been shown to be responsible for the enhanced nonlinear optical response as well as an attractive FOM. Yuen et al [90] pointed out that such an absorption spectrum, being related to the imaginary part of the effective dielectric constant, should equally well be reflected in the Bergman-Milton spectral representation of the effective dielectric constant [3,126,127].…”

“…Furthermore, in case of a large size aspect ratio, the plasmon bands may shift into the infrared, thus enabling the use of metal nanostructures in telecommunication applications in this wavelength range. Compared with spherically shaped particles, anisotropically shaped metallic particles can show reduced plasmon relaxation times [89] as well as enhanced nonlinear responses [90], and may thus be used as building blocks in a variety of optical devices. Some techniques have been developed to fabricate rod-shaped metallic nanoparticles by using lithographic means [91] or .…”

“…The presence of both gradation and dielectric anisotropy is shown to be helpful to achieve the large figure of merit, but unable to realize the separation of the absorption peak from the nonlinearity enhancement peak. In this regard, we may intentionally manipulate composite microstructures, e.g., by using the shape distribution of graded inclusions [7], and by using fractal [6] and anisotropic microstructures [90] with large volume fractions. When the volume fraction of graded inclusions is large, percolation behaviors can occur.…”

Optics of subwavelength gradient nanofilms

“…Unlike the spherically shaped inclusions, the resonant plasmon bands for anisotropically shaped metallic nanoparticles are split for orientations along major and minor axes [147]; moreover, such particles can show a reduced plasmon relaxation time [148]. In contrast to the model discussed above (metallic core and the dielectric shell), analysis [139] is devoted to the dielectric spheroidal core and confocal metallic shell, characterized by an arbitrary ratio of the minor to major axes.…”