Anthony Grbic

Experimental verification of backward-wave radiation from a negative refractive index metamaterial

A composite medium consisting of an array of fine wires and split-ring resonators has been previously used to experimentally verify a negative index of refraction. We present a negative refractive index (NRI) metamaterial that goes beyond the original split-ring resonator/wire medium and is capable of supporting a backward cone of radiation. We report experimental results at microwave frequencies that demonstrate backward-wave radiation from a NRI metamaterial—a characteristic analogous to reversed Cherenkov radiation. The conception of this metamaterial is based on a fresh perspective regarding the operation of NRI metamaterials.

Periodic analysis of a 2-D negative refractive index transmission line structure

The propagation characteristics of a two-dimensional (2-D) negative refractive index (NRI) transmission line (TL) structure are explained using Bloch theory. Bloch analysis of a generalized 2-D periodic electrical network is performed and the results are applied to the NRI TL structure. A 2-D Brillouin diagram of the NRI TL metamaterial is presented and its band structure is intuitively explained. Voltage and current relationships, Bloch impedance expressions and dispersion equations which aid in the design, proper excitation and termination of such structures are derived. Effective material parameters for regions of isotropic and homogeneous operation are also derived, providing a simplified understanding of the NRI TL metamaterials 2-D band structure. Finally, simulations of negative refraction for a relative refractive index of n=-1 are shown. The simulation results verify the analytic expressions presented in this paper and demonstrate the proper termination and excitation of finite size structures.

Near-Field Plates: Subdiffraction Focusing with Patterned Surfaces

Using a patterned, grating-like plate to control the electromagnetic near field, we demonstrate focusing well beyond the diffraction limit at ∼ 1 gigahertz. The near-field plate consists of only capacitive elements and focuses microwaves emanating from a cylindrical source to a spot of size ≈ λ/20 (half-power beamwidth), where λ is the free-space wavelength. These plates will find application in antennas, beam-shaping devices, nonradiative wireless power-transfer systems, microscopy, and lithography.

A Printed Leaky-Wave Antenna Based on a Sinusoidally-Modulated Reactance Surface

A simple procedure for designing a sinusoidally-modulated reactance surface (SMRS) that radiates at an arbitrary off-broadside angle is outlined. The procedure allows for nearly independent control of the leakage and phase constants along the surface. Printing an array of metallic strips over a grounded dielectric substrate is discussed as a way to practically implement the theoretical SMRS. A method of mapping the gaps between metallic strips to a desired surface impedance is presented as an efficient alternative to mapping methods used in the past. A printed leaky-wave antenna with a sinusoidally-modulated surface reactance is designed using the procedure mentioned above. The TM-polarized antenna radiates at 30° from broadside at 10 GHz, and exhibits an experimental gain of 18.4 dB. Theoretical, simulated, and experimental results are presented.

Leaky CPW-based slot antenna arrays for millimeter-wave applications

A uniplanar leaky-wave antenna (LWA) suitable for operation at millimeter-wave frequencies is introduced. Both unidirectional and bidirectional versions of the antenna are presented. The proposed structure consists of a coplanar waveguide fed linear array of closely spaced capacitive transverse slots. This configuration results in a fast-wave structure in which the n=0 spatial harmonic radiates in the forward direction. Since the distance, d, between adjacent elements of the array is small d/spl Lt//spl lambda//sub o/, the slot array essentially becomes a uniform LWA. A comprehensive transmission line model is developed based upon the theory of truncated periodic transmission lines to explain the operation of the antenna and provide a tool for its design. Measured and simulated radiation patterns, directivity, gain, and an associated loss budget are presented for a 32-element antenna operating at 30 GHz. The uniplanar nature of the structure makes the antenna appropriate for integration of shunt variable capacitors such as diode or micro-electromechanical system varactors for fixed frequency beam steering at low-bias voltages.

Cascaded metasurfaces for complete phase and polarization control

A metasurface lens that focuses light and controls its polarization at a wavelength of 2 μm is presented. This lens demonstrates high transmission and complete phase control within a subwavelength thickness at near-infrared frequencies. By cascading four patterned sheets, the efficiency is dramatically improved over more common single sheet designs. In addition, by utilizing anisotropic sheets, arbitrary birefringence can be achieved. A planar lens that both focuses light and converts its polarization from linear to circular is analyzed.

Millimeter-Wave Transmitarrays for Wavefront and Polarization Control

Two separate transmitarrays that operate at 77 GHz are designed and fabricated. The first transmitarray acts as a quarter-wave plate that transforms a linearly polarized incident wave into a circularly polarized transmitted wave. The second transmitarray acts as both a quarter-wave plate and a beam refracting surface to provide polarization and wavefront control. When the second transmittarray is illuminated with a normally incident, linearly polarized beam, the transmitted field is efficiently refracted to 45 °, and the polarization is converted to circular. The half-power bandwidth was measured to be 17%, and the axial ratio of the transmitted field remained below 2.5 dB over the entire bandwidth. Both designs have a subwavelength thickness of 0.4 mm (λ°/9.7). The developed structures are fabricated with low-cost printed-circuit-board processes on flexible substrates. The transmitarrays are realized by cascading three patterned metallic surfaces (sheet admittances) to achieve complete phase control, while maintaining high transmission. Polarization conversion is accomplished with anisotropic sheets that independently control the field polarized along the two orthogonal axes. The structures are analyzed with both circuit- and fields-based approaches.

Growing evanescent waves in negative-refractive-index transmission-line media

We show the enhancement of evanescent waves by a realizable negative-refractive-index (NRI) medium consisting of a periodic 2-D L,C loaded transmission-line (TL) network. This network is referred to as a dual TL structure. Growing evanescent waves within the dual TL structure are predicted analytically and demonstrated through simulation. These findings confirm that the dual TL structure is not simply a phase compensator that corrects the phase of propagating waves, but is in fact a NRI medium, since it also enhances the amplitudes of evanescent waves. This structure is a likely candidate for microwave subwavelength focusing and imaging applications.

An isotropic three-dimensional negative-refractive-index transmission-line metamaterial

We propose an isotropic three-dimensional (3D) negative-refractive-index medium using transmission lines loaded with reactive elements. The medium is referred to as the 3D dual transmission line (TL). Analytical expressions are derived which provide insight into the backward-wave propagation supported by the proposed medium. The dispersion characteristics of a physically realizable 3D dual TL are computed using a full-wave eigenmode solution of Maxwell’s equations based on the finite-element method. In addition, scattering parameters for a slab made of the 3D dual TL are presented. The described approach can be generalized for synthesizing a wide range of 3D metamaterials with tailored material parameters including both isotropic and anisotropic designs.

A Printed, Broadband Luneburg Lens Antenna

The design of a 2D broadband, Luneburg lens antenna implemented using printed circuit board techniques is detailed. The refractive index of the lens is controlled through a combination of meandering crossed microstrip lines and varying their widths. The 12.4λ° diameter lens is designed to operate in the transverse electromagnetic (TEM) mode at 13 GHz. The lens antenna was designed, fabricated, and measured. The measured half power beamwidth of the experimental antenna is 4.34°.