Carl Pfeiffer

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.

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°.

A Power Link Study of Wireless Non-Radiative Power Transfer Systems Using Resonant Shielded Loops

This paper discusses the use of magnetically coupled resonators for midrange wireless non-radiative power transfer (WNPT). A quasi-static (circuit) model is developed to establish key measures of performance and to aid in design. The use of directly fed, resonant shielded loops for WNPT is also proposed for the first time. Two experimental WNPT systems employing shielded loops are reported. A comprehensive experimental study is performed, and the performance of the WNPT systems shows close agreement with analytical predictions and developed circuit models. With a single-turn system of loop radius 10.7 cm, power transfer efficiency of 41.8% is achieved at a loop separation of 35 cm (3.3 loop radii). When the number of turns is increased to ten, a power transfer efficiency of 36.5% is achieved at a loop separation of 56 cm (5.3 loop radii). Measured magnetic field levels in the vicinity of the WNPT systems are shown to closely agree with analytical field values.

Roadmap on optical metamaterials

Optical metamaterials have redefined how we understand light in notable ways: from strong response to optical magnetic fields, negative refraction, fast and slow light propagation in zero index and trapping structures, to flat, thin and perfect lenses. Many rules of thumb regarding optics, such as mu = 1, now have an exception, and basic formulas, such as the Fresnel equations, have been expanded. The field of metamaterials has developed strongly over the past two decades. Leveraging structured materials systems to generate tailored response to a stimulus, it has grown to encompass research in optics, electromagnetics, acoustics and, increasingly, novel hybrid materials responses. This roadmap is an effort to present emerging fronts in areas of optical metamaterials that could contribute and apply to other research communities. By anchoring each contribution in current work and prospectively discussing future potential and directions, the authors are translating the work of the field in selected areas to a wider community and offering an incentive for outside researchers to engage our community where solid links do not already exist.

Planar Lens Antennas of Subwavelength Thickness: Collimating Leaky-Waves With Metasurfaces

High-gain lens antennas with a subwavelength thickness that radiate linearly and circularly polarized waves are reported. The lens antennas are fed with a planar, leaky radial waveguide that generates a TM-polarized Bessel beam (Bessel beam launcher). Two different metasurface lenses are placed a subwavelength distance from the Bessel beam launcher to collimate the radiation with minimal reflection loss. The unit cells of the two metasurface lenses are designed to act as inhomogenous wave plates to convert the polarization from radial to linear and circular, respectively. The lens antennas are fabricated using standard printed circuit board processes, and their performance is experimentally characterized. The antennas achieve an order of magnitude thickness reduction over previously reported lens antennas since the metasurface lenses are directly integrated with the antenna feed.

Direct Transfer Patterning of Electrically Small Antennas onto Three‐Dimensionally Contoured Substrates

A direct transfer patterning process is presented that allows metallic patterns to be stamped onto a contoured substrate. This process was used to make some of the most efficient electrically small antennas to date, while maintaining bandwidths approaching the physical limit.

Design and Free-Space Measurements of Broadband, Low-Loss Negative-Permeability and Negative-Index Media

We present the design and measurement of broadband, volumetric negative-permeability and negative-refractive-index (NRI) media. Both of these media are fabricated using standard printed-circuit-board techniques and operate at X-band frequencies. The S-parameters of four-cell slabs of the negative-permeability and NRI media are measured, and the material parameters of the NRI lens are extracted. The four-cell-thick (λ0/3) NRI lens exhibits a backward-wave bandwidth of 41.2% and a total loss of 0.67 dB at the operating frequency (where μr ≈ -1). Super-resolved focusing in free space is also demonstrated, and spatial frequencies beyond the free-space wavenumber are recovered over a bandwidth of 7.4%. A focus with a half-power beamwidth of 0.27λ0 is achieved at 10.435 GHz.

A Circuit Model for Electrically Small Antennas

A circuit model for electrically small antennas is introduced that is based on their frequency-dependent polarizabilities. This model is useful for straightforwardly analyzing several different small antenna geometries. A negative permittivity sphere, shell, and spheroid are all analyzed. An inductively loaded dipole, a top-hat loaded dipole, and a spherical sheet impedance are also analyzed. The circuit model provides the antennas radiation quality factor (Q), radiation efficiency (ηrad), and bandwidth. It also offers insight into the operation of the antenna which can aid and simplify design.

Energy-Autonomous Wireless Communication for Millimeter-Scale Internet-of-Things Sensor Nodes

This paper presents an energy-autonomous wireless communication system for ultra-small Internet-of-Things (IoT) platforms. In the proposed system, all necessary components, including the battery, energy-harvesting solar cells, and the RF antenna, are fully integrated within a millimeter-scale form factor. Designing an energy-optimized wireless communication system for such a miniaturized platform is challenging because of unique system constraints imposed by the ultra-small system dimension. The proposed system targets orders of magnitude improvement in wireless communication energy efficiency through a comprehensive system-level analysis that jointly optimizes various system parameters, such as node dimension, modulation scheme, synchronization protocol, RF/analog/digital circuit specifications, carrier frequency, and a miniaturized 3-D antenna. We propose a new protocol and modulation schemes that are specifically designed for energy-scarce ultra-small IoT nodes. These new schemes exploit abundant signal processing resources on gateway devices to simplify design for energy-scarce ultra-small sensor nodes. The proposed dynamic link adaptation guarantees that the ultra-small IoT node always operates in the most energy efficient mode for a given operating scenario. The outcome is a truly energy-optimized wireless communication system to enable various classes of new applications, such as implanted smart-dust devices.