Daniel M. Hailu

Fast Analysis of Terahertz Integrated Lens Antennas Employing the Spectral Domain Ray Tracing Method

We present a computationally efficient spectral ray tracing (SRT) method that requires one spectral domain integration step for each observation point. We compare the SRT method with the finite element method (FEM) and geometrical optics (GO) via simulation of terahertz Gaussian beam propagation through a hemispherical lens. The SRT was able to accurately solve the problem 30 times faster than FEM. The matrix formulation of SRT is promising for solving complex electrically large problems with high accuracy and 2 orders of magnitude reduction in computation time through parallel processing.

Gain Measurement of Embedded On-Chip Antennas in mmW/THz Range

A method for gain measurement of embedded on-chip antennas in mmW/THz range is presented. In this method, the radiation pattern is first measured in a quasi-optical configuration using a power detector. Subsequently, the radiated power is estimated from the integration over the radiation pattern. Finally, the antenna gain is obtained from measurement of a two-antenna system. The experimental results of applying this method on an on-chip planar dipole with substrate lens are demonstrated, and verified against simulation. A radiated power of around 40 μW and measured gain of 17 dB are reported at 180 GHz for this antenna structure.

Hybrid Spectral-Domain Ray Tracing Method for Fast Analysis of Millimeter-Wave and Terahertz-Integrated Antennas

In this paper, we present a computationally efficient hybrid spectral ray tracing (HSRT) method that requires only one spectral domain integration step for each observation point. The HSRT method is capable of modeling arbitrary three-dimensional dielectric and metallic structures. We compare and validate various versions of the HSRT method including MoM-SRT, and MLFMM-SRT, with measurements and commercial software FEKO for method of moments (MoM), multi-level fast multipole method (MLFMM) and physical optics (PO) via simulation of a bow-tie terahertz antenna backed by hyper-hemispherical silicon lens and an on-chip dipole antenna attached to lens. It is shown that the MoM-SRT is more accurate than MoM-PO and comparable in speed. The HSRT algorithm is applied to simulation of on-chip dipole antenna backed by Silicon lens and integrated with a 180-GHz VCO and compared with measurements. In addition, it is shown that the matrix formulation of SRT and HSRT is a promising approach for solving complex electrically large problems with high accuracy.

Simulation of bow-tie THz antenna using hybrid finite element method and spectral ray tracing technique

In this paper, we propose a hybrid method that uses commercially available Finite Element Method (FEM) software and our in-house developed Spectral Ray Tracing (SRT) algorithm. The method is used for calculating the radiation patterns of complex lens antenna such as a bow-tie antenna backed by silicon lens. A traveling-wave terahertz photomixer integrated with a coplanar stripline (CPS) waveguide is used to generate the radiation that couples to the antenna. We employ HFSS, a FEM solver, to model THz propagation along the CPS line and planar bow-tie. Once the near-field distribution is found, the SRT method is then applied to find the radiation patterns and gain of the bow-tie antenna backed by a silicon hyper-hemispherical lens. The results are compared with full FEM simulation of the antenna-lens structure.

Terahertz imaging of biological samples

The terahertz (THz) region of the electromagnetic (EM) spectrum has several advantages for skin cancer detection, dental imaging, and pharmaceutical applications. Continuous-wave (CW) and pulsed THz transmission-mode imaging with nonionizing and noninvasive properties are considered an alternative modality to X-ray imaging. Dental imaging application of THz imaging exploits the change in the refractive index between dental caries and enamel and/or dentine tissues. Due to the curvature of biological samples such as tooth and other electrically-large samples, THz propagation through quasi-optical system with multi-resolution problems need to be studied to improve THz imaging. Spectral Ray Tracing (SRT) [1],[2] can be employed as a reliable alternative for computation of EM field in the near-field and far-field to solve electrically large structure using less computational resources. In this paper, we conduct experimental measurements using a backward-wave oscillator (BWO) THz source for studying a cylindrical sample and cross-section of biological tooth scatterers of THz CW waves. EM measurements of THz propagation through a cylindrical polyethylene sample shows beam profile changes.

Narrow focus ultra-wideband antenna for breast cancer detection

A narrow focus ultra-wideband dielectric-filled antenna has been designed for the purpose of near-field breast cancer detection without the use of coupling media. Instead of immersing the antenna in a lossy liquid coupling medium, direct matching of the substrate and antenna to the biological tissue is proposed here. The proposed feed structure was designed on low-cost printed-circuit-board (PCB) with alumina substrate and the antenna radiating section on HIK dielectric material. A design procedure for the antenna is outlined. The aperture size was 0.1 cm by 6.35 cm. A return loss less than 5 dB from 2 to 10.6 GHz and a minimum return loss of 10 dB was obtained from 4.6 to 10.6 GHz. The antenna had simulated near-field narrow beam with half-power beamwidth of 10.3°.

Simulation of twin slot and spiral THz lens antenna system using hybrid method of moment and spectral ray tracing technique

In this paper a computationally efficient hybrid method, based on Spectral Ray Tracing (SRT) technique, for calculating the radiation patterns of complex lens antennas, is presented. The effect of internal reflection inside the closed volume of the lens is taken into account. SRT has been successfully applied to different kind of lens-antenna combinations and the results have good agreement with the measurement. In this paper we are comparing the results of the proposed method, which combines SRT with Method of Moments, with existing commercial software.

Compact and reconfigurable fiber-based terahertz spectrometer at 1550 nm

In this paper, we present a new compact and versatile spectrometer system operating at 1.55 μm for research and industrial application. The system is capable of testing solid, powder, thin film, gas, and liquid samples for material sensing and characterization applications. A high efficient system with bandwidth up to 1.2 THz is realized by using a fiber coupled terahertz chip packaging technology. The key components are the fiber-coupled THz transmitter and receiver modules, where the laser beam is directly coupled to the THz chip using optical fibers to provide stable and movable transmitter and receiver heads. The antennas are excited by 100 fs optical pulses at 1550 nm telecom wavelength and average power of 10mW. As femtosecond pulses are required on the antenna, the linear dispersion and nonlinear effect resulting from the propagation of the high power optical pulse along the fiber are taken into account and compensated using dispersion compensation fiber. A fast scan optical delay module is employed to realize real-time THz signal and spectrum measurement. The optical delay module also has a long delay scan unit to allow the user to adjust the distance between the transmitter and receiver heads by up to 1m to use the system for characterization of materials in different industrial applications.

Terahertz surface plasmon resonance sensor for material sensing

This paper presents the use of Terahertz (THz) SPR near-field sensor to characterize materials such as PMMA and those used in organic light emitting diode (OLED). The SPR device contains 2D periodic circular or square hole array in 500 nm Al on an 5 mm-thick intrinsic silicon, and was fabricated by photolithography and wet etching. For THz spectrum measurement, the SPR device with and without thin (PMMA) film on it is placed at the focus of the THz beam in transmission THz Time Domain Spectroscopy (TDS), where the spectrum is obtained from the Fourier-transformed sample and reference THz pulses. The transmission is obtained from the ratio between the sample spectrum and reference spectrum, whereas the phase change is the phase difference between the two spectra. To avoid overlap with water absorption lines, the optimal SPR device design has a period of 320 μm and square holes of 150 μm side length. The theoretical SPR frequencies in the THz range are determined for the metal-silicon modes and metal-air modes (0.9375 THz for mode (0, 1) at the metal-air interface). The measurement results confirmed the theoretical SPR frequencies for metal-silicon mode and demonstrate a shift to 0.9211 THz due to 2 μm of PMMA layer on the surface.

Hybrid spectral domain ray tracing algorithm for fast analysis of on-chip terahertz integrated antenna

We present a computationally efficient general Hybrid Spectral Ray Tracing (HSRT) method that requires one spectral domain integration step for each observation point. The HSRT method is capable of modeling arbitrary three-dimensional dielectric and metallic structures. As an example, the HSRT method is applied to on-chip dipole antenna backed by Silicon lens and integrated with a 180-GHz VCO in SiGe:C BiCMOS technology. We compare and validate the HSRT method for different reflection orders with measurements.