Clara Fausta Moldovan

Near optimal graphene terahertz non-reciprocal isolator.

Isolators, or optical diodes, are devices enabling unidirectional light propagation by using non-reciprocal optical materials, namely materials able to break Lorentz reciprocity. The realization of isolators at terahertz frequencies is a very important open challenge made difficult by the intrinsically lossy propagation of terahertz radiation in current non-reciprocal materials. Here we report the design, fabrication and measurement of a terahertz non-reciprocal isolator for circularly polarized waves based on magnetostatically biased monolayer graphene, operating in reflection. The device exploits the non-reciprocal optical conductivity of graphene and, in spite of its simple design, it exhibits almost 20 dB of isolation and only 7.5 dB of insertion loss at 2.9 THz. Operation with linearly polarized light can be achieved using quarter-wave plates as polarization converters. These results demonstrate the superiority of graphene with respect to currently used terahertz non-reciprocal materials and pave the way to a novel class of optimal non-reciprocal devices.

Electromagnetic Performance of RF NEMS Graphene Capacitive Switches

The RF performance of a nanoelectromechanical systems (NEMS) capacitive switch based on graphene is evaluated. Our results show that graphene can be a good candidate for the membrane of RF NEMS switches in applications where low actuation voltage and fast switching are required. The conductivity of the membrane is accurately modeled in the up- and down-state positions of the switch by considering the field effect of graphene. Rigorous full-wave simulations are then performed to obtain the scattering parameters of the switch. It is shown that graphenes conductivity variation due to electric field effect has a limited yet beneficial impact on the performance of the switch. It is also demonstrated that while monolayer graphene results in quite high switch losses at high frequency, the use of multilayer graphene, can considerably reduce the switch losses and improve the RF performance. Finally, an equivalent circuit model for the graphene-based RF NEMS switch is extracted and the results are compared with the full-wave 3-D electromagnetic simulation. These results motivate further efforts in the fabrication and characterization of graphene RF NEMS.

Steep-slope Metal-Insulator-Transition VO2 Switches with Temperature-Stable High ION

This letter reports a detailed experimental investigation of the slope of the current switching between OFF and ON states exploiting the metal-insulator-transition (MIT) in vanadium dioxide devices. The reported devices are CMOS compatible two-terminal switches. We experimentally demonstrate for the first time the very little dependence on temperature of the steep slope of these switches, ranging from 0.24 mV/decade at room temperature, to 0.38 mV/decade at 50 °C. The fabricated devices show excellent ON-state conduction, with ION > 1.8 mA/μm or RON <; 3 mΩ/μm, for the whole range of investigated temperatures (from room temperature to the MIT transition temperature), which recommends them as future candidates for steep-slope, highly conductive, and temperature-stable switches.

Tri-Band, Polarization-Independent Reflectarray at Terahertz Frequencies: Design, Fabrication, and Measurement

In this paper, two THz reflectarray surfaces have been designed and fabricated in order to deflect a plane wave with any polarization and with a specific incident angle to three different specific directions each at distinct three frequencies of 0.7, 1.0 and 1.5 THz. The surface is composed of an array of 100 ×100 cells, each comprised of gold crosses and parasitic dipoles printed on thin grounded high resistivity silicon. Finite-element method (FEM) simulations are in line with the measurement results obtained using THz time-domain spectroscopy (THz TDS) showing the intended deflections for the two fabricated samples each with an arbitrary frequency-vs-deflection angle relationship. In addition, the use of silicon as the substrate paves the way for the integration of reconfigurable technologies which enhances the reflectarray versatility.

Tunable capacitors and microwave filters based on vanadium dioxide metal-insulator transition

We report the fabrication, modeling and characterization of novel microwave tunable capacitors based on the metal-insulator transition (MIT) of Vanadium Dioxide (VO2). We present the advantages of VO2-based capacitors over alternative technologies for microwave reconfigurable electronics in terms of ease of integration, design and performance at high frequency. We show the potential of the proposed devices for RF reconfigurable electronics by fabricating a tunable bandstop filter (22.5-19.8 GHz) with insertion loss <; 2 dB up to 40 GHz.

Graphene Quantum Capacitors for High Frequency Tunable Analog Applications

Graphene quantum capacitors (GQC) are demonstrated to be enablers of radio-frequency (RF) functions through voltage-tuning of their capacitance. We show that GQC complements MEMS and MOSFETs in terms of performance for high frequency analog applications and tunability. We propose a CMOS compatible fabrication process and report the first experimental assessment of their performance at microwaves frequencies (up to 10 GHz), demonstrating experimental GQCs in the pF range with a tuning ratio of 1.34:1 within 1.25 V, and Q-factors up to 12 at 1 GHz. The figures of merit of graphene variable capacitors are studied in detail from 150 to 350 K. Furthermore, we describe a systematic, graphene specific approach to optimize their performance and predict the figures of merit achieved if such a methodology is applied.

CMOS-compatible abrupt switches based on VO 2 metal-insulator transition

We report an extensive experimental study of CMOS-compatible switches based on vanadium dioxide abrupt metal-insulator transition. We perform scaling studies to provide guidelines for optimization of the geometry of the VO2 device for integration on CMOS circuits, discussing the trade-off between the design parameters and the effect on performance for DC and RF applications. A VO2 thin film with resistivity varying from 5·10-2 Ω·m in the insulating state to 1.5-10-5 Ω·m in the conductive state allowed us to fabricate devices with actuation current ranging from 1.43 to 2.46 mA and resistance in the insulating state ranging from 5.8 kΩ to 35.9 kΩ. We identified a minimum actuation current of 5.75 μA for our technology, in a device miniaturized up to the limits of standard optical lithography. We show through RF measurements and simulations that the performance at high frequency of the switch can be improved using VO2 thin films with higher conductivity values, even if the contrast between the two states is lower.

Investigation of the metal-insulator transition in VO 2 for electronic switches with sub-1mV/decade steep subthreshold slope

We report a thorough investigation of the electrically-induced metal-insulator transition in vanadium dioxide (VO2) for abrupt switching in 2-terminal devices (0.24 mV/dec at 25 °C, 0.38 mV/dec at 50 °C). We exploit the electrothermal actuation model based on Joule heating to model and predict the low temperature dependence of the slope in VO2 switches.

Field-enhanced design of steep-slope VO 2 switches for low actuation voltage

The abrupt metal-insulator transition in vanadium dioxide (VO2) offers novel performance and functionality for beyond CMOS switches, enabling simultaneous high ON current and ultra-steep subthreshold slope with low temperature dependence. We developed a field-enhanced design of 2-terminal VO2 switches that allows decreasing their actuation voltage without affecting their performance and reliability. Exploiting this design, we characterized VO2 switches with extremely abrupt transitions (<; 1 mV/dec) until 60°C and a reduction in actuation voltage up to 38.3% with respect to conventional devices.

Graphene quantum capacitors for high-Q tunable LC-tanks for RF ICs

We propose and characterize graphene quantum capacitors, tunable with voltage by the control of their charge density, for tunable LC tanks as essential building blocks for Radio-Frequency (RF) functions in densely integrated circuits. We fabricate and investigate their performance in RF, and we demonstrate quantum capacitances, Cq, in the range of pF with a tuning range of >1.3:1 within 1.25 V, with Q-factors up to 14.5 at 0.4 GHz. Our capacitors have a high capacitance density, up to 2.65 fF/μm2, which is 100× higher than the one of RF MEMS capacitors. Based on calibrated models and simulations, we demonstrate the potential to replace their semiconductor counterparts and RF MEMS capacitors in LC tanks, for key RF analog application.