Qinghui Yang

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.

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.

Graphene based all-optical spatial terahertz modulator

We demonstrate an all-optical terahertz modulator based on single-layer graphene on germanium (GOG), which can be driven by a 1.55 μm CW laser with a low-level photodoping power. Both the static and dynamic THz transmission modulation experiments were carried out. A spectrally wide-band modulation of the THz transmission is obtained in a frequency range from 0.25 to 1 THz, and a modulation depth of 94% can be achieved if proper pump power is applied. The modulation speed of the modulator was measured to be ~200 KHz using a 340 GHz carrier. A theoretical model is proposed for the modulator and the calculation results indicate that the enhanced THz modulation is mainly due to the third order nonlinear effect in the optical conductivity of the graphene monolayer.

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.

Effects of off-stoichiometry and density on the magnetic and magneto-optical properties of yttrium iron garnet films by magnetron sputtering method

Yttrium iron garnet films were deposited on Si and Si/CeO2 substrates by magnetron sputtering method followed by postannealing. By varying the fabrication parameters such as sputtering atmosphere, sputtering power, and annealing atmosphere, single phase garnet films were obtained with different off-stoichiometry and film density. The dependence of cation ratio, magnetic and magneto-optical characteristics, and absorption coefficient were systemically investigated. The results reveal that a proper oxygen pressure in both sputtering and annealing process give rise to a small cation ratio of (Fe2+ or Fe4+)/Fe3+, thus is beneficial to obtain large saturation magnetization, large Faraday rotation, and small optical absorption. The sputtering power can also affect the properties of the film through changing the film density. Our results indicate that the properties of sputtering deposited yttrium iron garnet (YIG) film can be easily tuned and optimized by modifying the off-stoichiometry and density of the film,...

Correlation between sputtering parameters and composition of SmCo-based films for microelectromechanical system applications

About 3.0 μm thick SmCo-based films with additives of Fe, Cu, and Zr were deposited on Si substrates. Based on a developed semiempirical theoretical model, the dependence of the film composition on the sputtering parameters was discussed. The experimental results show that the Sm concentration increases with decreasing sputtering power or increasing Ar gas pressure, which are in good agreement with the calculated results when the preferential sputtering effect is disregarded. The effect of the sputtering parameters on the film composition provides an opportunity for the same composite target to fabricate films with Sm concentration varying from 13.8 to 17.3 at. %, which is reasonable for the permanent magnetic phase transformation (Sm2Co17→SmCo7→SmCo5). Furthermore, the observed TbCu7-type film shows a better crystal texture with a low remanence ratio for the hysteresis loops measured out plane to in plane of 0.08.

Tuning the phase transitions of VO2 thin films on silicon substrates using ultrathin Al2O3 as buffer layers

High quality VO2 thin films have been fabricated on silicon substrates using magnetron sputtering by introducing Al2O3 thin films as a buffer. The ultrathin Al2O3 deposited by plasma-assisted atomic layer deposition leads to a greatly improved crystallinity and textures in VO2 films. Dramatic change in electrical resistivity (4 orders of magnitude) and a small thermal hysteresis loop (~4 K) are obtained across the metal–insulator phase transition (MIT). Remarkably, by applying perpendicular voltage to a VO2/Al2O3 based metal/VO2/semiconductor device, electrically driven MIT switching characteristics have been observed with a tiny tunneling leakage current of ~10 μA. These results show that an electric field alone is sufficient to trigger the MIT, and the realization of VO2 based ultrafast electrical switching devices on a silicon substrate is possible.

Magnetic Characteristics of Carbon-Doped Nanocrystalline TiO

Carbon doped TiO2 nanocrystal powder samples with various C contents were synthesized by a low-temperature sol-gel method, and a large lattice C contents of 12.7% was obtained. We demonstrated that the C ions were homogenously doped into the TiO2 matrix at the form of substitutional C (Cs-o) and interstitial C (CI), without precipitation of TiC and other impurity phase. Our results show that C-doped TiO2 is ferromagnetic with the Curie temperature well above room temperature. The magnetization is mainly related to the content of Cs-o, suggesting that the intrinsic ferromagnetism originates from the Ti-C system in the TiO2 environment.