Linear and Nonlinear Optical Properties of Metal Nanocluster-Silica Composites Formed by Sequential Implantation of Ag A
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Metal nanocluster-glass composites demonstrate unique optical properties and significant nonlinear response. In this work nanometer dimension metal colloids were formed in silica by sequential implantation of Ag and Cu ions. The Ag and Cu were implanted with relative ratios of Ag to Cu of 9:3, 6:6, and 3:9 at a total nominal dose of 12x10 16 ions/cm 2. TEM techniques were used to examine colloid size and size distributions. The linear optical response was measured from 200 to 900 nm, while the nonlinear optical properties were measured using the z-scan technique at a wavelength of 570 nm. The linear and nonlinear optical properties were found to be dependent upon the ratio of sequentially implanted Ag to Cu and are consistent with effective medium theory. INTRODUCTION
Glasses containing nanometer dimension metal colloids have been suggested as potentially useful materials for nonlinear optical devices. 1 ,2,3 Both the linear and nonlinear optical properties depend on the electronic structure of the colloids. 1-5 Effective medium theory can be used to describe the optical response of nanometer dimension metal particles embedded in a dielectric medium as a function of the properties of the metal colloids and the host dielectric. 1.2,3 The linear response for colloids with diameters less than 20, where Xis the wavelength of the incident 45 radiation, is reasonably described by Mie scattering theory in the electric dipole approximation, , and is given by
(X= 18xnn,
Pe2
S [e + 2nd+
(1)
E2
where a is the absorption coefficient, e(.) = el + i e2 is the dielectric constant of the metal, p is the volume fraction of the metal particles and nd is the index of refraction of the dielectric host. The absorption is expected to exhibit a peak at the surface plasmon resonance frequency for which the condition el+2n3 = 0 is met. The surface plasmon resonance frequency depends explicitly on the electronic properties of the metal colloids and on the index of refraction of the host dielectric, nd, while depending implicitly on particle sizeA5 The third order nonlinear susceptibility, ) , of small non interacting particles in a dielectric can also be expressed using effective medium theory as 1 e X(3)= p•(co))lf(wo)12X(m3)
and fj(ow)
3n( 1 +rn= el + 2nd
(2)
is the nonlinear susceptibility of the metal clusters. where fc(co) is the local field factor and X(m3) There is a potentially large enhancement of the effective nonlinear susceptibility due to local field effects at the surface plasmon resonance frequency. Equations 1 and 2 express the dependence of the optical properties of the composite on the electronic structure of the nanosize metal colloids. 6 The index of refraction and the intensity dependent term are related to the above quantities by 153 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
no+n 21
n2 = 2n-oRe[XffJ]
and
(3)
where no is the linear index of refraction and n2 is the intensity dependent component. Ion implantation offers a method of forming nanometer dimension metal particles in
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