Qi-Ye Wen

Dual band terahertz metamaterial absorber: Design, fabrication, and characterization

We report the design, simulation, and measurement of a dual-band metamaterial absorber in the terahertz region. Theoretical and experimental results show that the absorber has two distinct and strong absorption points near 0.45 and 0.92 THz, both which are related to the LC resonance of the metamaterial. The distributions of the power flow and the power loss indicate that the absorber is an excellent electromagnetic wave collector: the wave is first trapped and reinforced in certain specific locations and then completely consumed. This dual-band absorber has applications in many scientific and technological areas.

Terahertz metamaterials with VO2 cut-wires for thermal tunability

An active terahertz (THz) metamaterial with vanadium dioxide (VO2) cut-wire resonators fabricated on glass substrate was proposed, and THz time-domain spectroscopy was used to probe the temperature-tuned electromagnetic properties. By thermal-triggering the insulator-metal phase transition of VO2, THz transmission signals through the metamaterial exhibit a significant decline with amplitude over 65%. Numerical simulations confirm the observations are due to the metallization of the VO2 film with increasing temperature.

Microwave-assisted magnetization switching of Ni80Fe20 in magnetic tunnel junctions

Microwave-assisted magnetization switching was investigated using Fe30Co70∕AlOx∕Ni80Fe20 magnetic tunnel junctions incorporated with a coplanar waveguide. Coercivity field of Ni80Fe20 layer was dramatically reduced in a small amplitude microwave. The authors eliminated the thermal effect in coercivity reduction by comparing two types of measurements which are with and without spin precession in the presence of microwave. It was found that the coercivity reduction depends on both frequency and power of the microwave. The numerical simulation based on Landau-Lifshitz-Gilbert equation reproduced the trend of the experimental data. The results indicate that microwave can be an efficient means to switch the magnetization of a thin film.

Transmission line model and fields analysis of metamaterial absorber in the terahertz band

Metamaterial (MM) absorber is a novel device to provide near-unity absorption to electromagnetic wave, which is especially important in the terahertz (THz) band. However, the principal physics of MM absorber is still far from being understood. In this work, a transmission line (TL) model for MM absorber was proposed, and with this model the S-parameters, energy consumption, and the power loss density of the absorber were calculated. By this TL model, the asymmetric phenomenon of THz absorption in MM absorber is unambiguously demonstrated, and it clarifies that strong absorption of this absorber under studied is mainly related to the LC resonance of the split-ring-resonator structure. The distribution of power loss density in the absorber indicates that the electromagnetic wave is firstly concentrated into some specific locations of the absorber and then be strongly consumed. This feature as electromagnetic wave trapper renders MM absorber a potential energy converter. Based on TL model, some design strategies to widen the absorption band were also proposed for the purposes to extend its application areas.

A tunable hybrid metamaterial absorber based on vanadium oxide films

A tunable hybrid metamaterial absorber (MA) in the microwave band was designed, fabricated and characterized. The hybrid MA was realized by incorporating a VO2 film into the conventional resonant MA. By thermally triggering the insulator–metal phase transition of the VO2 film, the impedance match condition was broken and a deep amplitude modulation of about 63.3% to the electromagnetic wave absorption was achieved. A moderate blue-shift of the resonance frequency was observed which is promising for practical applications. This VO2-based MA exhibits many advantages such as strong tunability, frequency agility, simple fabrication and ease of scaling to the terahertz band.

High-speed and broadband terahertz wave modulators based on large-area graphene field-effect transistors

We present a broadband terahertz wave modulator with improved modulation depth and switch speed by cautiously selecting the gate dielectric materials in a large-area graphene-based field-effect transistor (GFET). An ultrathin Al2O3 film (∼60  nm) is deposited by an atomic-layer-deposition technique as a high-k gate dielectric layer, which reduces the Coulomb impurity scattering and cavity effect, and thus greatly improves the modulation performance. Our modulator has achieved a modulation depth of 22% and modulation speed of 170 kHz in a frequency range from 0.4 to 1.5 THz, which is a large improvement in comparison to its predecessor of SiO2-based GFET.

Effect of Zn interstitials on the magnetic and transport properties of bulk Co-doped ZnO

We have demonstrated that the bound magnetic polaron model is responsible for ferromagnetism in Co?ZnO semiconductors, where the carriers are provided by the interstitial zinc (Zni). Our experiment is unique since by changing the temperature, we are able to cross the carrier concentration threshold above which a long-range ferromagnetic order is established. Consequently, the ferromagnetic order is observed at room temperature but is weakened at temperatures below 100?K. To support our conclusion we have performed a systematic investigation on the structural, magnetic and transport properties which all give consistent results in the context of our proposed two-region model, i.e. (a) a Zni layer where carriers are sufficient to couple Co ions ferromagnetically and (b) a region with little carriers that remain in a paramagnetic state.

Co doping effect on the magnetic properties of CeO2 films on Si(111) substrates

Ce1−xCoxO2−δ films with the stoichiometry of x=0,0.03,0.06,0.1,0.125 were fabricated on Si(111) substrates using O2 assisted pulse laser deposition method. While pure CeO2 film is weak paramagnetism, integration of low Co content of 3at.% introduces ferromagnetim with a giant saturation moment (Ms) of 5μB∕Co at room temperature. Based on the first principle calculation, we attribute the giant magnetic moments to the combined contributions of spin polarized Co, Ce, and O atom with the enhancement of O vacancies. Higher Co content will depress the ferromagnetism, i.e., inverse correlation between Ms and Co contents, which is qualitatively validated by the calculated magnetic moments of Ce1−xCoxO2−δ with different Co content.

Epitaxial growth and metal-insulator transition of vanadium oxide thin films with controllable phases

Vanadium oxide thin films with well controlled phases such as rhombohedra V2O3 and monoclinic VO2 were synthesized on Al2O3 (0001) substrates by optimizing the processing parameters of a polymer assisted deposition technique. X-ray diffraction and high-resolution transmission electron microscopy studies revealed that both V2O3 and VO2 films can be well controlled with good epitaxial quality. The temperature dependency of electrical resistivity demonstrated sharp metal-insulator transitions (MITs) for V2O3 and VO2 films. The crystallinity and the strains in the films are believed to play critical roles in determining the MIT properties.

Real-time evolution of tunneling magnetoresistance during annealing in CoFeB∕MgO∕CoFeB magnetic tunnel junctions

We report the study of the real-time evolution of tunneling magnetoresistance (TMR) in CoFeB∕MgO∕CoFeB junctions during annealing at 380°C. The TMR quickly developed at the early stage of the annealing, with 200% magnetoresistance observed in less than 10min, followed by a slow approach to saturation. This evolution of TMR was correlated with the structural changes, including crystallization of amorphous CoFeB electrodes and improvement of barrier quality during the annealing.