Device and Circuit Technologies for Advanced Terahertz Receiver Systems

In this work, novel device and circuit technologies for realizing advanced terahertz (THz) receiver systems will be explored. THz detection techniques have shown great promises in astronomy research, remote sensing, security screening, spectroscopy and medical diagnostics. Although various THz devices and circuits have been developed in recent years in response to the needs of the above applications, the performance of current THz receiver systems still remains unsatisfactory. Mixers or receivers based on superconducting Hot-Electron Bolometers (HEBs) have been widely used for THz sensing and detection owing to their advantages of high sensitivity, low noise, and low local oscillator (LO) power requirement. However, single-element HEB mixers may suffer from noise introduced by LO injection. Balanced HEB mixers are superior to single-element ones since the thermal noise and AM noise from the LO injection can be effectively suppressed. Although a 1.3 THz balanced waveguide HEB mixer has been reported, waveguide mixer configurations offer relatively narrow RF bandwidths. In this work, we report on the development of HEB devices, broadband balanced mixer circuit and novel tunable/reconfigurable components for realizing advanced THz receivers that can potentially achieve both ultra-high sensitivity and multiband/broadband operation. In the balanced mixer configuration, a lens-coupled dual-polarization sinuous antenna (four arms) was designed for operation from ~0.2 - 1.0 THz with a nearly frequency-independent embedding impedance of ~105 Ohm. Two identical superconducting niobium HEB devices were integrated at the antenna feedpoints, connecting each opposing pair of antenna arms to form a balanced mixer configuration. The balanced mixer circuit has been fabricated, and the sinuous antenna has been characterized at multiple frequency ranges demonstrating the broadband capability of the proposed balanced receiver circuit design. In addition to the development of a quasi-optical balanced mixer architecture, unique device processing technologies have been developed that permit the direct integration of two HEBs and an airbridge into planar antennas for realizing ultra-sensitive mixers. The DC testing results for the fabricated HEB devices including resistance vs. temperature (R-T) and current vs. voltage (I-V) characteristics indicate successful fabrication of superconducting HEB devices with dimensions below 400 nm. To fully characterize the performance of the balanced HEB mixers, a cryostat RF testing system has been designed and constructed. The noise performance of the proposed balanced HEB receiver has been initially simulated at 585 GHz, and compared with that of a single-ended mixer, showing that improvement on noise performance can be achieved using a balanced configuration.

In order to achieve multiband receiver operation, a tunable THz mesh filter has been proposed. At THz frequencies, mesh filters are routinely used in low noise receiver front ends to block thermal radiation and filter out undesired signals. However, traditional THz mesh filters have fixed center frequencies, so they are not suitable for multiband receiver operation. In this work, we report a novel electronically-tunable THz mesh-filter based on a two-dimensional array of frequency tunable annular-slot antennas (ASAs). For initial demonstration, a single tunable ASA loaded with a high speed Schottky varactor diode as a tuning element has been designed, fabricated and characterized at WR-5.1 band (140-220 GHz), showing nearly 50 GHz tuning range. A THz mesh filter has been designed and simulated for achieving multiband receiver operation. This approach enables the development of a variety of tunable/reconfigurable THz circuits and components including tunable/reconfigurable mesh filters and tunable detectors.

As discussed above, balanced configuration can improve the noise performance of a receiver by suppressing the noise introduced by LO injection. For optimum balanced receiver performance, it is required that the two mixers are fed with equal LO power. However, this is very challenging to achieve in a practical system, and a high-performance variable attenuator can be used to solve this problem by dynamically controlling the LO power for the two mixers. In this thesis, a novel optically-controlled attenuation approach has been investigated for realizing high-performance variable attenuators. For prototype demonstration, a compact WR-4.3 (170-260 GHz) optically-controlled waveguide attenuator has been developed and characterized, showing that high performances including a 0.6 dB insertion loss, greater than 10 dB return loss, and an average of ~25 dB tuning range have been achieved over most of the WR-4.3 band, with decent long-term stability in a regular lab environment. The demonstrated optically-controlled attenuation approach can also be applied to develop a variety of important components, such as modulators, switches and reconfigurable filters, and will find promising applications in advanced THz receiver/imaging systems and future high-speed THz/millimeter wave communication systems.

To the best of the author's knowledge, the quasi-optical balanced receiver proposed in this thesis is the first broadband balanced mixer design reported at THz frequencies. The electronically tunable WR-5.1 antenna and optically-controlled WR-4.3 variable waveguide attenuator are also unique demonstrations and contribute significantly to the development of reconfigurable THz/millimeter wave components. The accomplishments achieved in this work not only provide novel devices and circuits which are essential for realizing advanced THz receiver systems, but also a comprehensive set of techniques that can be extended to multiple directions and applied to broader applications.