Syrus C. Nemat-Nasser

Microwave transmission through a two-dimensional, isotropic, left-handed metamaterial

We present experimental data, numerical simulations, and analytical transfer-matrix calculations for a two-dimensionally isotropic, left-handed metamaterial (LHM) at X-band microwave frequencies. A LHM is one that has a frequency band with simultaneously negative eeff(ω) and μeff(ω), thereby having real values of index of refraction and wave vectors, and exhibiting extended wave propagation over that band. Our physical demonstration of a two-dimensional isotropic LHM will now permit experiments to verify some of the explicit predictions of reversed electromagnetic-wave properties including negative index of refraction as analyzed by Veselago [Usp. Fiz. Nauk 92, 517 (1964), Sov. Phys. Usp. 10, 509 (1968)].

Loop-wire medium for investigating plasmons at microwave frequencies

We present numerical simulations and microwave measurements on a loop-wire structure that acts as an effective medium exhibiting a well-defined bulk plasma frequency in the microwave regime, with an effective negative dielectric function below this plasma frequency. The dependence of this plasmonic response on the self-inductance of the constituent wire elements is made explicit. A finite structure, approximately spherical, composed of this inductive medium is studied, and reveals subwavelength surface plasmon resonances below the bulk plasma frequency.

A Biologically Motivated Solution to the Cocktail Party Problem

We present a new approach to the cocktail party problem that uses a cortronic artificial neural network architecture (Hecht-Nielsen, 1998) as the front end of a speech processing system. Our approach is novel in three important respects. First, our method assumes and exploits detailed knowledge of the signals we wish to attend to in the cocktail party environment. Second, our goal is to provide preprocessing in advance of a pattern recognition system rather than to separate one or more of the mixed sources explicitly. Third, the neural network model we employ is more biologically feasible than are most other approaches to the cocktail party problem. Although the focus here is on the cocktail party problem, the method presented in this study can be applied to other areas of information processing.

Left-Handed Metamaterials

The response of a material to electromagnetic radiation can be entirely characterized by the material parameters: the electrical permittivity, or e, and the magnetic permeability, or μ. The range of possible values for the material parameters, as dictated by fundamental considerations such as causality or thermodynamics, extends beyond that found in naturally occurring materials. We thus seek to extend the material parameter space by creating electromagnetic metamaterials—ordered composite materials that display electromagnetic properties beyond those found in naturally occurring materials. Recently, we have demonstrated a metamaterial made of a repeated lattice of conducting, nonmagnetic elements that exhibits an effective μ and an effective e, both of which are simultaneously negative over a band of frequencies [1]. Such a medium has been termed Left-Handed [2], as the electric field (E), magnetic intensity (H) and propagation vector (k) are related by a left-hand rule. We introduce the reader to the expected properties predicted by Maxwell’s equations for Left-Handed media, and describe our recent numerical and experimental work in developing and analyzing this new metamaterial.

Negative permeability from split ring resonator arrays

Summary form only. The range of values observed for the magnetic permeability, /spl mu/(/spl omega/), appears to be more restricted than the values observed for the electric permittivity, /spl epsi/(/spl omega/), where very large, and even negative values are observed. This is in part due to the simple fact that there are no magnetic monopoles to provide the analogous response to that of electrons. In particular, as one moves away from zero frequency, the magnitude of the magnetic response from most materials, or /spl mu/(/spl omega/), decreases rapidly, and has never been observed to take negative values. While the general lack of magnetic response is observed to be the case, Maxwells equations do not preclude a material having a large /spl mu/(/spl omega/), either positive or negative. The essential requirement on the material constants appears only to be d/d and d/d for frequency-dependent materials. Pendry et al. (1999) have introduced conducting nonmagnetic split ring resonators (SRRs), and predicted that periodic arrays of SRRs can have a resonantly enhanced effective permeability displaying frequency regions with large positive or negative values. Combining numerous SRRs into a lattice forms an effective medium, for which there exists a band of frequencies where the effective permeability is negative. The SRR medium offers the possibility of engineering materials to respond to time-varying magnetic fields as well as time-varying electric fields. Combining such composite media with standard materials offers the potential to yield novel and advantageous electromagnetic devices.