Superconducting kinetic inductance bolometer focal plane array for passive terahertz imaging system

Abstract

The demands for high performance concealed object detection in mass transit, public events or industrial facilities have been increasing in light of increased risks. Concealed object detection at high throughput rates in places where lives can be jeopardized or where there is a risk of smuggling prohibited objects is a major challenge using conventional methods. Terahertz radiation has attracted attention for utilization in security imaging applications, due to its ability to penetrate dielectric materials, such as common clothing and packaging materials, while being strongly reflected from metals and strongly absorbed by water. Terahertz radiation also strongly interacts with organic matter, enabling spectroscopic techniques for the detection of explosives and narcotics, for example. The utilization of terahertz imaging helps to solve the problems of conventional physical security requiring straight body human contact. Terahertz radiation is also inherently safe when compared to alternative technologies such as X-ray imaging, due to its non-ionising nature. The diffraction-limited spatial resolution of sub-millimeter wave imaging (in the order of 15 millimeters) does not reveal anatomical details, guaranteeing the privacy of the scanned people.Meanwhile, ever cheaper, more compact and easier to use cryocoolers capable of reaching temperatures down to less than 10 Kelvin (-263 °C) enable the utilization of superconducting electronics and detectors. These low temperatures allow for much lower thermal noise in detectors when compared to their room temperature equivalents, enabling much higher sensitivity. These high sensitivity imaging detectors are able to operate passively, without the need for bulky, intrusive and expensive high-power coherent terahertz sources, detecting the natural thermal blackbody terahertz radiation. The low temperatures also allow exploiting superconducting materials and their phenomena, such as exceptionally low conduction losses and high kinetic inductance.Enabled by modern microfabrication technologies, VTT's (Technical Research Centre of Finland) superconducting kinetic inductance bolometers provide a solution for implementing a large focal plane array of detector pixels, enabling a high-performance passive terahertz imaging system for security imaging of human-sized objects at a close range and video frame rates. The high responsivity and inherent multiplexability negate the need for expensive and complex cryogenic signal amplification or high-bandwidth RF components in the system.In this thesis, the superconducting properties of these kinetic inductance bolometers were characterized as a function of their temperature and geographical location on the wafer and assessed considering the theoretical predictions of both the two-fluid and the Mattis-Bardeen models. Further measurements were made in an attempt to characterize the noise and thermal properties of the bolometers and to determine the superconducting parameters of the thin superconducting films. The obtained data provides the basis for understanding the performance of the detectors and improving the design and fabrication process for bolometers in the future