Robert B. Greegor

Ab initio numerical simulation of left-handed metamaterials: Comparison of calculations and experiments

Using numerical simulation techniques, the transmission and reflection coefficients, or S parameters, for left-handed metamaterials are calculated. Metamaterials consist of a lattice of conducting, nonmagnetic elements that can be described by an effective magnetic permeability μeff and an effective electrical permittivity eeff, both of which can exhibit values not found in naturally occurring materials. Because the electromagnetic fields in conducting metamaterials can be localized to regions much smaller than the incident wavelength, it can be difficult to perform accurate numerical simulations. The metamaterials simulated here, for example, are based on arrays of split ring resonators (SRRs), which produce enhanced and highly localized electric fields within the gaps of the elements in response to applied time dependent fields. To obtain greater numerical accuracy we utilize the newly developed commercially available code MICROWAVE STUDIO, which is based on the finite integration technique with the per...

Simulation and testing of a graded negative index of refraction lens

A gradient index (GRIN) lens using a negative index of refraction material (NIM) has been designed and tested. The GRIN lens was fabricated using a NIM slab with a variable index of refraction perpendicular to the propagation direction. Ray tracing calculations based on the isotropic Eikonal equation determined the index of refraction gradient required for a given focal length. An electromagnetic code was then used to design the required ring and wire unit cells. Finally, the index of refraction was approximated using ten discrete steps in an effective medium simulation for the GRIN lens that agreed with the experimental measurements.

Performance of a negative index of refraction lens

A plano-concave lens with negative index of refraction has been designed and fabricated. Such lenses have been postulated for many years, but only recently has their realization been made possible through improved simulation and fabrication procedures. We report here the simulation, fabrication, and performance of such a lens. The lens images the source field and reproduces the results of standard Gaussian optics. The curved lens with negative index of refraction in the microwave frequency region of the electromagnetic spectrum has been compared to a plano-convex Macor positive index of refraction lens having the same radius of curvature.

Free-space focused-beam characterization of left-handed materials

We used a broadband, free-space, focused-beam system to measure the transmission and reflection in left-handed-material (LHM) slabs. The samples were made of alternating patterns of copper wires and split-ring resonators on Rogers 5880 substrates separated by Rohacell™ spacers. The measured data show very good agreement with numerical simulations. The measured insertion loss of this structure was −1.1 dB/cm at the LHM pass band. Simulations suggest that the loss may be attributable to the finite conductivity of the copper patterns.

Experimental determination and numerical simulation of the properties of negative index of refraction materials

Negative index of refraction materials have been postulated for many years but have only recently been realized in practice. In the microwave region these materials are constructed of rings and wires deposited on a dielectric substrate to form a unit cell. We have constructed, experimentally characterized and simulated several of these structures operating in the 10 - 15 GHz range. Our simulations using Maxwells Equations solvers have included wire arrays, ring arrays and assemblies of unit cells comprised of rings and wires. We find good agreement between the numerical simulations and experimental measurements of the scattering parameters and index of refraction. The procedure was to first model ring and wire structures on the unit cell level to obtain scattering parameters from which effective å, ì and n were retrieved. Next an assembled array of unit cells forming a 12 degrees wedge was used for the Snells Law determination of the negative index of refraction. For the structure examined the computed value of n is within 20% of the one experimentally measured in the Snells Law experiment from 13.6 to 14.8 GHz.

Origin of dissipative losses in negative index of refraction materials

Negative index of refraction materials have been postulated for many years but have only recently been realized in practice. In the microwave region these materials are constructed of rings and wires deposited on a dielectric substrate to form a unit cell. We have constructed, experimentally characterized, and simulated several of these structures operating in the 10–16 GHz range. The origin of the dissipative losses has been identified and effective schemes to reduce them devised and implemented. Numerical simulation and experimental verification shows that losses in negative index of refraction materials can be significantly reduced by minimizing the underlying losses of the constituent materials.

Development of Negative Index of Refraction Metamaterials with Split Ring Resonators and Wires for RF Lens Applications

Metamaterials are engineered ring and wire composites whose response to an incident electromagnetic wave can be described by an effective negative dielectric permittivity ε and magnetic permeability μ. Simultaneous negative ε and μ within a given frequency band of a metamaterial gives rise to a negative index of refraction n. This has been demonstrated via a Snell’s law experiment. The electromagnetic properties of many metamaterial structures in the microwave region are investigated through numerical simulations and experiments. A negative index of refraction, n, allows lenses with reduced primary (Seidel) aberrations compared to equivalent positive index lens. This is demonstrated both for cylindrical lenses and spherical lenses, as well as for the gradient index lenses. Detailed field maps of the focal region of the metamaterials lenses are made and compared to a comparable positive index of refraction lens.

Fabrication of nanometer scale gaps for thermo-tunneling devices

We report a fabrication approach for making nanometer wide gaps between two planar metallic electrodes, which can be utilized for the formation of thermo-tunneling devices. The technique is a three dimensional variant of the electromigration techniques used for creating nanometer sized gaps on planar surfaces. The gap is formed by applying a low level voltage between two parallel electrodes, each deposited on separate wafers that have been bonded together. I-V and thermal characterization of the gap show very good agreement with modeling results, indicating a tunneling gap on the order of 0.5-1 nm.

Applications of metamaterials in the GHz frequency domain

This paper describes the applications of metamaterials to lenses with a negative index of refraction. A negative index of refraction, n, allows lenses with reduced primary (Seidel) aberrations then the ones with an equivalent positive index lens. This is demonstrated both for cylindrical lenses and spherical lenses, as well as for the gradient index lenses. Detailed maps of the focal region of the metamaterials lenses are made and compared to a comparable positive index of refraction lens.