Georgios C. Trichopoulos

A Broadband Focal Plane Array Camera for Real-time THz Imaging Applications

We present a large-format, sub-millimeter-resolution, focal plane array sensor for THz imaging. Each pixel in the sensor array consists of broadband THz antennas monolithically integrated with ultra-fast heterostructure backward diodes for THz sensing. With the aid of in-house hybrid electromagnetic modeling tools, the focal plane array is optimized for diffraction limited image resolution and conjugate impedance matching for highest THz sensitivity. The camera is designed to operate in the 0.6–1.2 THz band with 5 frames-per-second image acquisition speeds, making it ideal for THz imaging applications, such as security screening, non-destructive evaluation and chemical, pharmaceutical, and medical imaging. The simulation results are validated by measurements to demonstrate sub-millimeter resolution with a pixel optical responsivity of 600 V/W at 0.7 THz.

A Novel Approach for Improving Off-Axis Pixel Performance of Terahertz Focal Plane Arrays

We present an antenna engineering approach to significantly improve off-axis detection properties of terahertz focal plane arrays (FPAs) behind an extended-hemispherical lens. In particular, antennas in the off-axis pixels in a large FPA were redesigned to exhibit tilted receiving patterns. The new antenna structure corrects for beam distortions in off-axis sensors due to the small lens size. Specifically, this approach is well suited for monolithic antenna structures integrated with high-sensitivity heterostructure backward diodes. Although the approach is demonstrated for 100- and 500-GHz FPAs, the presented technique is equally applicable to other planar antenna topologies and sensor materials that operate well into the terahertz regime. In addition, standard RF impedance-matching techniques were used to improve responsivity performance of the diode-coupled FPAs. The beam-correction techniques allow for a factor-of-5 larger size FPA for a fixed lens size. We also note that a tradeoff between Gaussicity and directivity becomes necessary for larger lenses. Thus, increasing the size of the FPA, without resorting to increasing the lens size, enables improved coupling efficiencies (Gaussicity) as well as better pixel resolution (directivity).

Non-Contact Probes for On-Wafer Characterization of Sub-Millimeter-Wave Devices and Integrated Circuits

We present a novel non-contact metrology approach for on-wafer characterization of sub-millimeter-wave devices, components, and integrated circuits. Unlike existing contact probes that rely on small metallic tips that make physical contact with the device on the chip, the new non-contact probes are based on electromagnetic coupling of vector network analyzer (VNA) test ports into the coplanar waveguide environment of integrated devices and circuits. Efficient signal coupling is achieved via a quasi-optical link between the VNA ports and planar antennas that are monolithically integrated with the test device. Experimental validation of the non-contact device metrology system is presented for the first time to demonstrate the accuracy and repeatability of proposed approach for the 325-500-GHz (WR2.2) and 500-750-GHz (WR1.5) bands.

Lumped-Element Equivalent-Circuit Modeling of Millimeter-Wave HEMT Parasitics Through Full-Wave Electromagnetic Analysis

We present a broadband lumped-element parasitic equivalent circuit to accurately capture the frequency response of electromagnetic (EM) interactions inside the structure and surrounding environment of high electron-mobility transistors (HEMTs). A new mutual inductance term is included to account for the high-frequency magnetic field coupling between device electrodes. An analytical method is also proposed, for the first time, to extract the gate-to-drain mutual inductance LMGD, which creates an undesirable inductive feedback path from output to input at millimeter wavelengths. Based on the suggested extrinsic equivalent circuit, we propose a novel multistep parameter extraction procedure that utilizes direct analytic extraction and linear regression techniques systematically to determine the parasitic component values. The accuracy and robustness of the presented extraction algorithm are established via comprehensive comparisons between EM simulations, measurements, and frequency responses of the suggested equivalent circuits up to and beyond 300 GHz in the millimeter-wave (mmW) band. The key parasitic elements that are most detrimental to the microwave performance are identified and optimized through subsequent circuit analysis. Design guidelines are provided for optimum device layout selection to achieve the highest frequency performance. It is demonstrated through a full-wave simulation based parametric study that around 20% improvement in maximum oscillation frequency is achievable via optimization of device gate finger number and unit finger width.

An Indirect Impedance Characterization Method for Monolithic THz Antennas Using Coplanar Probe Measurements

We develop a simple and robust impedance characterization method for planar THz antennas with micron- and submicron-size port geometries. Such antennas are often encountered in THz sensing applications where an ultrafast electronic device, such as a Schottky junction or a heterostructure backward diode, is integrated with a planar antenna structure. Standard probe characterization of such antennas at the device port is not currently possible due to the large contact areas required. The proposed method allows for indirect characterization of antenna impedance seen at the device port using measurements collected at a more suitable, remote location on the antenna plane. Three S11 measurements are performed using contact probes at a larger pad on the antenna periphery, using three terminations (short, open, and a resistive load) of the port under consideration. Through a simple relation, the measured data set is used to compute the port-impedance indirectly. Experimental results are presented to illustrate the accuracy of the proposed approach.

Non-contact probes for device and integrated circuit characterization in the THz and mmW bands

We present a novel, non-contact, on-wafer device characterization method covering both THz (300 GHz-3 THz) and mmW bands (60-300 GHz). Unlike existing contact probes which rely on fragile tips and physical contact with the device on the chip, the new non-contact probe setup is based on radiative coupling of vector network analyzer test ports into the coplanar waveguide environment of monolithic devices and integrated circuits via planar, on-chip, broadband antennas. The on-chip antennas act as “virtual” probe-tips on the test wafer and connect to the device through optimized, impedance matched coplanar waveguide (CPW) lines. Proof-of-concept validation is presented for the 325-750 GHz band using WR2.2 and WR1.5 frequency extender modules and a standard vector network analyzer as the backend. Owing to the non-contact nature, these new probes are free from wear/tear and fragility issues. More importantly, they are low cost and can be easily scaled beyond 900GHz where there is no existing solution for on-wafer device and integrated circuit testing.

Imaging performance of a THz focal plane array

A new broadband monolithic THz antenna design that covers the entire 0.6 – 1.2 THz band is developed as the imaging pixel of THz focal plane array (FPA). Heterostructure backward diodes are integrated monolithically with the THz antennas and are used as the detectors to reproduce digital images from the 2-dimensional received power distribution. A new simplified design is developed which is capable of focusing the antenna beams directly onto the image plane using an extended-hemispherical lens. We show that a resolution of 1.5 mm can be achieved with this simpler design. For achieving optimal pixel sensitivity, we use full-wave antenna modeling to optimize the impedance matching between the antennas and the integrated diodes. In addition, quasi-optical propagation analysis is used to evaluate and optimize the overall imaging resolution of the THz imaging system.

Enhancing the focusing properties of an ellipsoidal beamformer based imaging system: a simulation study

Aim of this study is the improvement of the focusing properties of a microwave radiometry tomography system, used for the imaging of the product of temperature and conductivity in biological tissues via contactless measurements. The operation principle of the device in question is based on an ellipsoidal conductive wall cavity, which provides the required beamforming and focusing. The biological tissue under measurement is placed on one of the two focal points whereas on the other one, a dipole antenna measures the black body type radiation emitted from the heads tissue. In the framework of the present research several approaches are followed in order to improve and optimize the systems focusing properties on the tissue area of interest. Extensive simulations using a commercial FEM tool are performed in a wide range of operation frequencies. Dielectric spheres of various electromagnetic characteristics are placed either around the source (human head model) or the receiver (dipole antenna) in order to improve the matching on the head-air interface. The ability of focusing the electromagnetic energy in specific areas inside the human head is herein investigated in detail and further discussed

On-wafer device characterization with non-contact probes in the THz band

We present a novel approach for on-wafer device characterization in the THz band. A non-contact method eliminating the need for physical contact with test wafer is proposed. Non-contact method is based on radiative coupling of Network Analyzers test ports into coplanar environment of monolithic device (DUT) through integrated planar THz antennas. Broadband butterfly-shaped antennas are used to ensure that the characterization setup is not limited by the bandwidth of the non-contact probes. Calibration is carried out using open and short terminations with different lengths. Initial results based on the full wave simulations demonstrate the broadband and efficient coupling capability of non-contact THz probes as well as the functionality of different terminations for calibration purposes.

Investigation of Nonreciprocal Dispersion Phenomena in Anisotropic Periodic Structures Based on a Curvilinear FDFD Method

The aim of this paper is the investigation of nonreciprocal phenomena in anisotropically loaded 2-D periodic structures. For this purpose, our well-established 2-D curvilinear finite difference frequency domain method is combined with periodic boundary conditions and extended toward the eigenanalysis of periodic structures loaded with both isotropic and general anisotropic materials. The periodic structures are simulated in a 2-D domain, while uniformity is considered along the third axis. The propagation constant along the third axis can either be zero (in-plane-propagation) or nonzero (out-of-plane propagation). Particular effort was devoted to the identification of the appropriate irreducible Brillouin zone to be scanned during the eigenanalysis. It was herein realized that similar to geometrically artificial crystal anisotropy, the wave propagation directional asymmetries modify the irreducible Brillouin zone in the microwave regime as well. Both gyrotropic and particularly magnetized ferrite as well as full tensor anisotropic (arbitrarily biased ferrite) material loadings are investigated through the eigenanalysis of different periodic structures, including strip grating. Interesting nonreciprocal backward wave and unidirectional phenomena are justified as expected.