H. V. Nguyen

110 GHz measurement of large-area graphene integrated in low-loss microwave structures

We report high-frequency scattering parameter measurement of large-area monolayer graphene integrated on low-loss quartz substrates. High-quality graphene was grown by chemical vapour deposition on copper, chemically hole doped, and transferred to quartz. Microwave measurements were performed from 0.01 to 110 GHz. Simple microwave models were used to extract graphene impedance parameters. We find that contact resistance is effectively shunted above 3 GHz. Atomically thin large area graphene behaves as a wideband resistor with negligible kinetic inductance and negligible skin effect.

Faraday rotation in magnetically biased graphene at microwave frequencies

Faraday rotation is experimentally observed at microwave frequencies in a large-area graphene sheet biased with a static magnetic field, and interrogated by polarized fields in a hollow circular waveguide. A Faraday rotation of up to 1.5° and an isolation of more than 30u2009dB is observed, suggesting possible applications to graphene based isolators, circulators, and other non-reciprocal devices. An analytic model is developed for the scattering parameters of the measured structure. The model shows excellent agreement with the measurements and is used to extract the graphene conductivity, carrier density, and mobility.

Analog Direction of Arrival Estimation Using an Electronically-Scanned CRLH Leaky-Wave Antenna

A novel direction of arrival (DoA) estimation system based on a leaky-wave antenna (LWA) is presented. Compared to traditional array-based DoA techniques, which require multiple radiating elements, a large feeding network, and expensive phase shifters consuming power, the proposed scheme requires only a simple, compact, and low-cost radiating element consuming no power. The system estimates the DoA by measuring the received power at both ports of the LWA using detectors as the beam is scanned. It is implemented here using a composite right/left-handed (CRLH) leaky-wave antenna (LWA), selected for its efficient electronic full-space scanning capability. A detailed theoretical explanation is presented. Three methods of operation are described: forward scanning, backward scanning, and full-space scanning. Furthermore, full-wave simulation and experimental results both validate the theoretical analysis. To the best of the authors knowledge, this is the first time a LWA is employed in a DoA system.

CRLH–CRLH C-Section Dispersive Delay Structures With Enhanced Group-Delay Swing for Higher Analog Signal Processing Resolution

A novel dispersive delay structure (DDS) based on a composite right/left-handed (CRLH)-CRLH coupler is proposed and demonstrated by both full-wave and experimental results. The experimental prototypes are compact and shielded multilayer DDSs implemented in low-temperature co-fired ceramics technology. Compared to the conventional all-pass C-section DDS, the proposed CRLH C-section DDS exhibit a larger group-delay swing, which leads to higher resolution in analog signal processing applications, as a result of their higher coupling capability. Moreover, they feature a larger bandwidth and a smaller size. A generalized wave-interference analysis is presented to both rigorously derive the transfer functions and group-delay characteristics of C-section DDSs and provide deeper insight into their operating mechanisms in both their left- and right-handed regimes.

A Tapered CRLH Interdigital/Stub Leaky-Wave Antenna With Minimized Sidelobe Levels

A nonuniform (NU) tapered composite right/left-handed (CRLH) leaky-wave antenna (LWA) with minimized sidelobe level (SLL) is proposed. The design of this antenna is performed using an automated cosimulation approach based on a genetic algorithm optimization. Fair agreement is found between the full-wave and the experimental results. The experimental results show an 8-dB SLL reduction compared to a uniform CRLH LWA with the same gain in a 12-unit-cell structure.

Field displacement in a traveling-wave ring resonator meta-structure

Field displacement is demonstrated for the first-time in a non-molecular-scale structure, namely a meta-structure composed of rings equipped with an isolator. The structure is explained in terms of rotating magnetic dipole moments and demonstrated to produce typical gyrotropic field displacement.

Differential Bi-directional CRLH Leaky-wave Antenna in CPS Technology

A differential bi-directional composite right/left- handed (CRLH) transmission line (TL) leaky-wave antenna (LWA) in co-planar strip (CPS) technology is presented. The three distinct advantages of the proposed CPS CRLH LWA compared to previously reported microstrip LWAs are: 1) differential excitation (lower noise and simpler co-design with differential components), 2) bi-directional radiation, and 3) high isolation ratio between co-to-cross polarizations. The transmission and radiation characteristics of the CPS CRLH LWA are theoretically presented and experimentally demonstrated.

Contactless impedance measurement of large-area high-quality graphene

We present experimental work on the contactless measurement of graphene sheet impedance at frequencies up to 110 GHz in different waveguide geometries. Low-loss coplanar waveguides in series and shunt configuration have been demonstrated. A new coaxial waveguide coupled Corbino disk geometry with facile fabrication is introduced. Critical to the success of these measurements is a low contact impedance at high-frequencies, wherein the dc contact resistance is shunted by a contact capacitance that ultimately enables contactless measurement. The quasi-optic nature of waveguide measurements minimizes the effect of inevitable cracks in the graphene sheet, in contrast with typical transport measurements. We have applied our technique to the characterization of the sheet impedance and contact impedance of large-area, high-quality graphene grown by chemical vapour deposition.

Mixed-mode characterization of a circularly-polarized CRLH leaky-wave antenna

In this work, the mixed-mode approach is employed to characterize the modal parameters of a newly-invented circularly-polarized (CP) composite right/left-handed (CRLH) leaky-wave antenna (LWA). The single-ended CP-LWA is comprised of two coupled CRLH LWAs, which operate as a balanced structure supporting simultaneously common and differential mode excitations resulting in orthogonal current distributions, and a balun-like structure which induces the dual excitations with a phase-quadrature between them. The mixed-mode approach is advantageous over traditional approaches using either baluns or external probes due to its simplicity, efficiency, low cost and wide frequency response. Using full-wave simulation results of the CP-LWA, the modal parameters from the mixed-mode approach efficiently identified performance limitations for the CP-LWA.

Automated design of CRLH structures using co-design synthesis computational approach

An automated tool for designing CRLH structures using a co-design synthesis computational approach is proposed and demonstrated experimentally. The co-design approach uses a computation tool and an electromagnetic simulation tool to synthesize, simulate and optimize a balanced CRLH transmission line.