Components and metrology for terahertz imaging

Abstract

This thesis addresses detectors, components, and calibration tools for both passive and active imaging systems and radiometers in the millimeter-wave and terahertz frequency range from approximately f = (75-110) GHz to f = (2-3) THz. The components developed in this work combine techniques from both the microwave and infrared ranges of the electromagnetic spectrum.

To date, there have been no standardized methods for measuring detector responsivity in the millimeter-wave/terahertz frequency range. This thesis describes a new water-based calibration source that uses the unique properties of water in this frequency range combined with "optical trap" methods from the infrared and visible regions of the spectrum. The calibration source has an uncertainty of less than ±200 mK in the range of f = (75-450) GHz.

Antenna-coupled microbolometers, a specific form of direct thermal detectors, are characterized in this thesis; device responsivity and antenna patterns are measured; Nb and NbN are compared side-by-side. Using these detectors, measurements of expanded polystyrene foam are performed. Low-level forward scattering (grating lobes) are observed from this nominally random matrix of expanded polystyrene spheres. Additionally, images are acquired with these passive detectors. In this frequency range, clothing and other materials are quite transparent, making the detectors highly useful for concealed threat detection when integrated with a suitable imaging system.

In addition to broadband passive imaging, a fundamental component for monostatic active imaging systems was investigated. Specifically, a quasi-optical linear-to-circular polarizer at f = 95 GHz is developed in this thesis, with a loss of 0.4 dB and axial ratio of 0.23 dB.

In summary, the contributions of this thesis pave the way for standardized measurements for a variety of components useful in passive and active direct-detection imaging systems at millimeter-wave and terahertz frequencies.