Truong Khang Nguyen

Four-leaf-clover-shaped antenna for a THz photomixer

To improve the output power of a photomixer used as a THz source, we propose a four-leaf-clover-shaped antenna structure composed of a highly resonant radiation element and a stable DC feed element. The resonance characteristics of the proposed structure were first investigated on a half-infinite substrate as a simplified radiation environment to reduce the computation time. Based on the antenna characteristics on that half-infinite substrate, the antenna structure was designed to have a maximum total efficiency and a maximum directivity on an extended hemispherical lens. The input resistance of this structure was six times that of a full-wavelength dipole, significantly improving the mismatch efficiency between a photomixer and the antenna. The terahertz output power from this structure is expected to be 2.7 times that of a full-wavelength dipole.

Multiband Dual Spiral Stripline-Loaded Monopole Antenna

We propose a multiband antenna that exhibits a dual coupling characteristic and dual frequency operation using a dual spiral stripline-loaded monopole in conjunction with L-shaped slots in a ground plane. The slots allow the upper part of the ground plane to function as an additional monopole that resonates in concordance with the existing resonances of the upper and lower spiral stripline monopoles. As a result, the impedance band- widths are greatly enhanced while maintaining relatively good omnidirectional radiation characteristics. The proposed antenna occupies a volume of 36 times 7 times 102 mm3 including the ground plane, and the measured impedance bandwidths with VSWR < 2 simultaneously cover CDMA, GSM900, DCS1800, PCS1900, UMTS, IMT-2000, and WiMAX2350 bands. We describe the detailed antenna structure and present measurement results.

Effects of Antenna Design Parameters on the Characteristics of a Terahertz Coplanar Stripline Dipole Antenna

This paper presents the antenna design parameter dependency on the impedance and radiation characteristics of a terahertz coplanar stripline dipole antenna. The antenna response is numerically investigated by applying a semi-inflnite substrate and by generating a constant voltage source to drive a signal on the antenna. In this way, we can analyze the antenna characteristics without the photoconductive material response and the substrate lens geometrical efiects. Further, we explain the mechanism underlying the preferable uses of several millimeter length DC bias striplines in a typical THz coplanar stripline dipole antenna design. The antenna, consisting of a center dipole connected to long bias striplines, has a traveling wave characteristic supporting an attenuated current, rather than a resonant characteristic supporting a standing wave of current. The traveling wave behavior produces stable antenna input impedances and minimal changes in the antenna radiation patterns. We also found that the length of the center dipole has a prominent efiect on the antenna gain response.

Full-Wavelength Dipole Antenna on a GaAs Membrane Covered by a Frequency Selective Surface for a Terahertz Photomixer

An antenna consisting of a full-wavelength dipole on a GaAs membrane covered by a frequency selective surface is proposed for improved output power from photomixers used as terahertz (THz) sources. The antenna structure reduces the vertical dimension of a typical photomixer antenna using a substrate lens while still exhibiting high total e-ciency and high directivity. The resulting antenna after optimization produced an input resistance of 3870› and a radiation e-ciency of 60%, corresponding to a total e-ciency, i.e., the e-ciency of the antenna in THz wave radiation, of approximately 48.3% and a maximum directivity of 20.2dBi at the resonance frequency of 1.03THz. The proposed antenna is expected to be a promising alternativeTHz photomixer design.

Entropy Generation in Thermal Radiative Loading of Structures with Distinct Heaters

Thermal loading by radiant heaters is used in building heating and hot structure design applications. In this research, characteristics of the thermal radiative heating of an enclosure by a distinct heater are investigated from the second law of thermodynamics point of view. The governing equations of conservation of mass, momentum, and energy (fluid and solid) are solved by the finite volume method and the semi-implicit method for pressure linked equations (SIMPLE) algorithm. Radiant heaters are modeled by constant heat flux elements, and the lower wall is held at a constant temperature while the other boundaries are adiabatic. The thermal conductivity and viscosity of the fluid are temperature-dependent, which leads to complex partial differential equations with nonlinear coefficients. The parameter study is done based on the amount of thermal load (presented by heating number) as well as geometrical configuration parameters, such as the aspect ratio of the enclosure and the radiant heater number. The results present the effect of thermal and geometrical parameters on entropy generation and the distribution field. Furthermore, the effect of thermal radiative heating on both of the components of entropy generation (viscous dissipation and heat dissipation) is investigated.

Resonant Antennas on Semi-Infinite and Lens Substrates at Terahertz Frequency

This part is concerned with four different types of resonant antennas on semi-infinite and lens substrates made of high-permittivity dielectric materials at terahertz frequency. Full-wavelength single-dipole, full-wavelength dual-dipole, half-wavelength single-slot, and full-wavelength four-leaf-clover-shaped dipole antennas are studied at a frequency of around 1.0 THz. The resonance characteristics of the four antennas are first investigated on a semi-infinite substrate, using the method of moments simulator FEKO. Based on these characteristics, the antennas are placed on an extended hemispherical lens to examine and compare the radiation effects of a substrate lens on each antenna, using the finite-integration time-domain simulator CST Microwave Studio. The shape of an extended hemispherical lens with optimum thickness and quarter-wavelength matching layer plays an important role in maximizing the directivity, while the radiation efficiency is generally affected by the antenna structure.

Impact of varying the DC bias stripline connection angle on terahertz coplanar stripline dipole antenna characteristics

We investigated the dependence of the input impedance and radiation characteristics on the connection angle between the DC bias stripline and the center dipole of a terahertz coplanar stripline dipole antenna in a semi-infinite substrate. Three connection angles, namely 0° (typical horizontal connection), 45° (diagonal connection), and 90° (vertical connection), were examined and an overall performance comparison was provided. The antenna with a sufficiently long bias stripline exhibited a traveling-wave behavior with stable impedance variation, regardless of the connection angle between the DC bias stripline and the center dipole. The antenna with diagonal and vertical bias connections performed much better than the typical H-shaped electrode with a horizontal bias connection in terms of radiation characteristics exhibiting flat gain and low frequency cut-off of the radiation efficiency. In addition, the antenna with a diagonal bias connection exhibited a better radiation pattern and more efficient use of a given substrate area than that with a vertical bias connection. The result indicates that in terms of the overall aspect of the terahertz stripline dipole antenna designs on a high-dielectric-constant substrate, the antenna with a diagonal bias connection is the best configuration.

Nanostructured Metal–Insulator–Metal Metamaterials for Refractive Index Biosensing Applications: Design, Fabrication, and Characterization

Nanofabrication of nanostructured metal-insulator-metal metamaterial (NMIM2) by the electron beam lithography and the liftoff technique is reported in this paper. The NMIM2 consists of periodic arrays of gold (Au) nanotriangles deposited on a gold layer (acting as a mirror) separated by a thin dielectric insulator. Such an NMIM2 stack was fabricated on a silicon wafer. Reflection of the NMIM2 was measured by the Fourier transform infrared spectroscope. The experimental measurement for reflection coefficients are in good agreement with the full-wave finite-different time-domain simulation results. At resonance, we numerically observed a strong field localized inside the gap between the Au nanostructures and the Au mirror, resulting in a strong confinement of incoming light within the insulator spacer and, thus, a pronounced absorption at telecommunication wavelengths. This resonant spectral response is investigated for sensitive label-free refractive index biosensing applications.

Design of a Substrate-Integrated Fabry-Pérot Cavity Antenna for K-Band Applications

This paper presents the design of a planar, low-profile, high-gain, substrate-integrated Fabry-Perot cavity antenna for K-band applications. The antenna consists of a frequency selective surface (FSS) and a planar feeding structure, which are both lithographically patterned on a high-permittivity substrate. The FSS is made of a circular hole array that acts as a partially reflecting mirror. The planar feeding structure is a wideband leaky-wave slit dipole fed by a coplanar waveguide whose ground plane acts as a perfect reflective mirror. The measured results show that the proposed antenna has an impedance bandwidth of more than 8% (VSWR ≤ 2), a maximum gain of 13.1 dBi, and a 3 dB gain bandwidth of approximately 1.3% at a resonance frequency of around 21.6 GHz. The proposed antenna features low-profile, easy integration into circuit boards, mechanical robustness, and excellent cost-effective mass production suitability.

Asymmetrically engineered metallic nanodisk clusters for plasmonic Fano resonance generation

In this paper, we numerically introduce a planar metamolecule that generates plasmonic Fano resonance. The engineered molecule consists of closely packed asymmetric gold nanodisks deposited on a glass substrate operating at visible and near-infrared wavelengths. The asymmetric arrangement of nanodisks plays a key role in Fano resonance generation. The induced extinction cross-section spectroscopy has a Fano-like shape owing to interference between bright and dark plasmonic modes sustained by the asymmetric nanodisk clusters. The Fano dips are shown to be highly sensitive to the interdisk gaps as well as to the surrounding environment. As a result, we introduce a potential refractive index nanosensor having a sensitivity of 660 nm/RIU and a figure of merit of 4.75. The proposed metamolecule holds potential for various applications, such as Fano-induced enhancement of solar energy harvesting, molecular fluorescence, and photo upconversion.