Andrew D. Hellicar

Terahertz imaging at 77 K

Terahertz (THz) technology is receiving increasing attention around the world due to its important potential in many application areas. Novel compact solid-state sources and detectors are being sought for?THz radiation and detection. We report the realization of a?THz imager based on a high- Tc superconducting (HTS) Josephson detector working above liquid nitrogen temperature (77?K). The detector, made of a YBa2Cu3O7?x (YBCO) step-edge Josephson junction, is coupled to a thin-film ring-slot antenna and a hemispheric silicon lens. Images of high visual quality are obtained which demonstrate unique properties of?THz radiation such as the sensitivity to water content and the ability to penetrate packaging materials. The results should stimulate further research leading to the development of a HTS superconducting?THz imaging system.

Terahertz imaging using a high-Tc superconducting Josephson junction detector

A high-Tc superconducting (HTS) detector based on a YBa2Cu3O7−x (YBCO) step-edge Josephson junction has been developed and applied to terahertz (THz) detection. The detector was coupled to a ring-slot antenna designed for operation at 600 GHz, and used for THz imaging. The results suggest that the characteristic voltage and frequency of our HTS step-edge junctions can be readily optimized for the chosen THz frequency range at easily achievable temperatures. The images also clearly demonstrate some of the unique properties of THz radiation, including the sensitivity to water content and the ability to penetrate packaging materials. (Some figures in this article are in colour only in the electronic version)

Phase Retrieval of Scattered Fields

The measurement of phase information at high frequencies (e.g., terahertz) is difficult, particularly if an imaging array of all electronic components is desired. This paper modifies two techniques from the antenna diagnostics community and applies them to reconstructing the phase from amplitude only measurements of a field disturbed by an unknown target.

Efficient Sampling of Electromagnetic Fields via the Adaptive Cross Approximation

The adaptive cross approximation method is applied to the problem of efficiently sampling an electromagnetic field across two dimensions. The algorithm uses previously collected samples to determine which field samples to take next and reduces the number of samples required to accurately represent the field. In this way the field may be sampled faster than it would have been if it had been raster scanned. This is achieved without significant changes to the sensor hardware.

Phase Retrieval for Millimeter- and Submillimeter-Wave Imaging

Phase-retrieval techniques are demonstrated on scattered-field amplitude-only measurements at 186 and 580 GHz. These techniques allow for imaging of a scatterer using a simple amplitude-only measurement system.

Bag of Class Posteriors, a new multivariate time series classifier applied to animal behaviour identification

A new multi-scale time series classifier is proposed using class posterior estimates.The classifier infers a large set of animal behaviour using motion based time series.The proposed classifier outperforms benchmark classifiers by between 43% and 77%.The proposed classifier is found to be more efficient than the Bag of Features model. In this paper, two new multivariate time series classifiers are introduced as the Bag of Class Posteriors (BOCP) and the Bag of Class Posterior with Ordering (BOCPO). The models propose a new multi-scale feature representation where the class posterior estimates of contiguous local patterns are aggregated over longer time scales. The models are employed as part of an animal behaviour monitoring system that are comprised of sensors, which are fitted to the animals, and a classifier that translates sensor data into knowledge of the animals behaviour.Animal monitoring systems are commonly developed to infer a small number of behaviours with relevance to a specific application. To investigate if a standard monitoring system with an Inertial Measurement Unit (IMU) can be reused for different management applications, a set of ten cattle behaviours relevant to different management applications were classified with the proposed models. Results indicate that the multi-scale BOCP and BOCPO models were far more capable of classifying the cow behaviours offering a 43% to 77% improvement over benchmark time interval classifiers with fixed time resolution. In addition, the BOCPO model was shown to offer a far more efficient feature representation than the related multi-scale Bag of Features (BOF) classifier (up to 200 times smaller) making it better suited to deploy upon monitoring devices fitted to animals in the field.

Evolved-Profile Dielectric Rod Antennas

A systematic approach is presented for the design of profiled dielectric rod antennas that satisfy specified radiation pattern objectives. The approach uses a body of revolution method of moments technique to rapidly analyze arbitrarily profiled dielectric rods while a genetic algorithm is used to achieve the design objectives. As examples we present dielectric rod designs optimized for maximum gain and low sidelobes. These designs are compared with a conventional linear profile design. Measured results are presented and these are shown to agree well with the calculated radiation patterns. We show that improved gain and sidelobe performance is achieved using a non-linear rod profile compared to a standard linear profile. The generality of the approach is demonstrated with a shaped beam antenna design that has a cosecant-squared pattern.

Development of a Terahertz Imaging System

This paper introduces work being conducted by the authors towards developing terahertz (THz) imaging technologies. Specifically this paper addresses two topics: the development and implementation of a THz imaging system, and design of a THz detector. The THz imaging system has been implemented to allow exploration across a broad range of applications. An overview and design of this system are presented, along with early images acquired with the system. A high temperature superconducting device capable of detection at THz frequencies is being designed. As development of this detector is at an early stage simulated detector performance results are given; however, it is expected that detector results will be presented at the conference.

Performance Evaluation of a Passive Millimeter-Wave Imager

A cross-correlating 186-GHz passive millimeter-wave imager has been built. The key components in the signal processing hardware are two 186-GHz receivers and a broadband complex correlator. To evaluate the performance of this imager, its point-spread function, beam pattern, baseline vector, and their variations with the scanning direction have been experimentally measured and derived. Some of these results are needed for optimizing the imagers parameter settings. Others are required for implementing the modulated-beam and modulated-scene algorithms proposed in a previous paper dealing with the imagers fringe in its point-spread function. These results will also reveal any problems in the construction process of the imager. The theoretical bases for these measurements are analyzed. Novel algorithms for deriving each antennas point-spread function and beam pattern, as well as the imagers baseline vector from the measurement results of the imagers point-spread function and beam pattern are proposed and successfully applied in the measurements. Experimental results are presented and discussed.

Towards large scale HTS Josephson detector arrays for THz imaging

We present the design and implementation of a high-temperature superconducting (HTS) Josephson junction detector array for terahertz (THz) imaging. The array device is made of ten YBa2Cu3Ox 7 (YBCO) step-edge junctions coupled to gold thin-film ring-slot antennas on a MgO substrate. The design and characterization of the detector array in response to a 0.6 THz signal are presented. The development of multi-channel biasing and read-out electronics and the system integration with a commercial cryocooler are also described. (Some figures may appear in colour only in the online journal)