We present experimental scattering data at microwave frequencies on a structured metamaterial that exhibits a frequency band where the effective index of refraction (n) is negative. The material consists of a two-dimensional array of repeated unit cells of copper strips and split ring resonators on interlocking strips of standard circuit board material. By measuring the scattering angle of the transmitted beam through a prism fabricated from this material, we determine the effective n, appropriate to Snell's law. These experiments directly confirm the predictions of Maxwell's equations that n is given by the negative square root of epsilon.mu for the frequencies where both the permittivity (epsilon) and the permeability (mu) are negative. Configurations of geometrical optical designs are now possible that could not be realized by positive index materials.

We demonstrate a composite medium, based on a periodic array of interspaced conducting nonmagnetic split ring resonators and continuous wires, that exhibits a frequency region in the microwave regime with simultaneously negative values of effective permeability &mgr;(eff)(omega) and permittivity varepsilon(eff)(omega). This structure forms a "left-handed" medium, for which it has been predicted that such phenomena as the Doppler effect, Cherenkov radiation, and even Snell's law are inverted. It is now possible through microwave experiments to test for these effects using this new metamaterial.

We analyze the reflection and transmission coefficients calculated from transfer matrix simulations on finite lengths of electromagnetic metamaterials, to determine the effective permittivity (ε) and permeability (µ). We perform this analysis on structures composed of periodic arrangements of wires, split ring resonators (SRRs) and both wires and SRRs. We find the recovered frequency-dependent ε and µ are entirely consistent with analytic expressions predicted by effective medium arguments. Of particular relevance are that a wire medium exhibits a frequency region in which the real part of ε is negative, and SRRs produce a frequency region in which the real part of µ is negative. In the combination structure, at frequencies where both the recovered real parts of ε and µ are simultaneously negative, the real part of the index-of-refraction is found also to be unambiguously negative. * Permanent address: Institute of Physics, Slovak Academy of Sciences, Dúbravska cesta 9, 842 28 Bratislava, Slovakia.It has been proposed that electromagnetic metamaterials-composite structured materials, formed from either periodic or random arrays of scattering elements-should respond to electromagnetic radiation as continuous materials, at least in the long wavelength limit [1,2]. In recent experiments and simulations [3,4], it has been demonstrated that certain metamaterial configurations exhibit scattering behavior consistent with the assumption of approximate frequencydependent forms for ε and µ. However, the techniques applied in those studies probed the materials indirectly, and did not provide an explicit measurement that would assign values for ε and µ. It is our aim in this paper to show that the previous conjectures were indeed valid: unambiguous values for ε and µ can be applied to electromagnetic metamaterials. Our approach here utilizes the transmission and reflection coefficients (Sparameters, equivalently) calculated for a wave normally incident on a finite slab of metamaterial. We invert the scattering data to determine n and z, for systems of several thicknesses, from which we obtain self-consistent values for ε and µ. While we utilize simulation data in this study, the technique we describe will be readily applicable to the experimental characterization of metamaterial samples whenever the scattering parameters are known.

The collaborative oscillation of conductive electrons in metal nanoparticles results in a surface plasmon resonance that makes them useful for various applications including biolabeling. We investigate the coupling between pairs of elliptical metal particles by simulations and experiments. The results demonstrate that the resonant wavelength peak of two interacting particles is red-shifted from that of a single particle because of near-field coupling. It is also found that the shift decays approximately exponentially with increasing particle spacing and become negligible when the gap between the two particles exceeds about 2.5 times the particle short-axis length.

We present a systematic study of the effect of size and shape on the spectral response of individual silver nanoparticles. An experimental method has been developed that begins with the detection and characterization of isolated nanoparticles in the optical far field. The plasmon resonance optical spectrum of many individual nanoparticles are then correlated to their size and shape using high-resolution transmission electron microscopy. We find that specific geometrical shapes give distinct spectral responses. In addition, inducing subtle changes in the particles’ morphology by heating causes a shift in the individual particle spectrum and provides a simple means of tuning the spectral response to a desired optical wavelength. Improved colloidal preparation methods could potentially lead to homogeneous populations of identical particle shapes and colors. These multicolor colloids could be used as biological labels, surface enhanced Raman scattering substrates, or near field optical microscopy sources covering the full range of wavelengths in the visible spectrum.