Biomedical subjects

F J Rachford

Publications and source records attributed to F J Rachford.

6 recordsLinked to original sources

PubMed2004

Optical properties of an ideal homogeneous causal left-handed material slab.

The optical properties of a homogeneous slab of material characterized by causal permittivity epsilon (f) , and permeability mu (f) are investigated through finite difference time domain simulations. Lorentzian epsilon (f) and mu (f) are used to produce values of interest in the resulting index n (f) , namely, n<0 , 0<n<1 , and n approximately 0 . Negative refraction of both cw and pulsed waves is observed when operated in the left-handed medium (LHM) regime. In particular, we examined materials having both an impedance matched to free space and an impedance not matched to free space, as is the case of a split-ring resonator structure, and find it to exhibit LHM behavior, in agreement with previously published claims. Spherical aberration and channeling are observed as the magnitude of the negative index is increased beyond -1 . Critical angle phenomena are seen for indices in the range 0<n<1 , and a spatial filter effect is identified in the n approximately 0 region. A transmission bandpass in the vicinity of n approximately 0 is identified as a "ferromagnetic antiresonance," where the material acts as both a frequency and a spatial filter.

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PubMed2003

Electromagnetic waves focused by a negative-index planar lens.

We demonstrate that negative refraction occurs for both cw and pulsed electromagnetic waves when traversing from a "right-handed" (index > 0) to a "left-handed" (index < 0) material (LHM) which has causal dispersive intrinsic properties. We also demonstrate that a divergent line source spaced a distance H in front of a planar LHM slab and excited by either an impulse cw or a Gaussian frequency pulse is imaged at a distance H away, inside the LHM, and at H to the other side of the slab. The image size is approximately lambda consistent with limitations dictated by wave optics. We find no evidence of evanescent mode amplification. The studies were performed using numerical experiments with finite difference time domain solutions and incorporating a causal Lorentzian form for the frequency-dependent material properties.

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PubMed2002

Calculations and measurements of wire and/or split-ring negative index media.

Broadband calculations (70 MHz-20 GHz) and measurements (2-15 GHz) were performed on planar stacks of two-dimensional double-split rings arrays interspersed with arrays of thin wires. Recent work on similar composite structures infers a negative index of refraction (n<0) over a narrow frequency range. We have performed finite-difference time-domain (FDTD) calculations on various combinations of ring geometries, wire arrays, and stack spacings. Excellent agreement was obtained between FDTD simulations and our measured transmission spectra. Examining the FDTD time progression of fields we find regions of both negative and positive indices as well as an inverse Doppler effect.

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PubMed2001

Detecting functional relationships between simultaneous time series.

We describe a method to characterize the predictability and functionality between two simultaneously generated time series. This nonlinear method requires minimal assumptions and can be applied to data measured either from coupled systems or from different positions on a spatially extended system. This analysis generates a function statistic, Theta(c(0)), that quantifies the level of predictability between two time series. We illustrate the utility of this procedure by presenting results from a computer simulation and two experimental systems.

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PubMed2001

Functional dependence and quasiperiodicity in the spatiotemporal dynamics of yttrium iron garnet films.

When thin films of yttrium iron garnet (YIG) are placed in a magnetic field and driven at microwave (rf) frequencies, nonlinear interactions within the material cause the normal microwave spin precession to be modulated at lower frequencies. We measure these lower frequency (kHz) signals at two spatially separated locations on the YIG film and use linear and nonlinear analysis to study the functional dependence of the spin dynamics at one location on the spin dynamics at the other location. We see dynamical states where nonlinear analysis can detect a functional dependence that the linear analysis fails to reveal.

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PubMed2001

Spatiotemporal nonlinear dynamics of a magnetoelastic ribbon.

Magnetoelastic materials have a strong coupling between strain and magnetization, so applying a magnetic field to a magnetoelastic material can change its shape. This coupling leads to interesting dynamics. We have studied the dynamics of a wide ribbon of Metglass 2605sc which was driven by a magnetic field. The ribbon was suspended as a pendulum in a set of Helmholtz coils, which provided both dc and ac magnetic fields. Laser light was reflected off the ribbon to measure its angular displacement. Two points on the ribbon could be simultaneously illuminated, and one of the laser beams could be scanned over the ribbon. We observed quasiperiodic bifurcations in the motion of the ribbon, and characterized the spatial aspect of the motion with some recently developed statistics.

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