Superconducting integrated circuits (SICs) represent a natural step forward for devices operating at frequencies from microwave up to sub-millimeter wavelengths. They offer massive miniaturization via compact design based on low-loss superconducting transmission lines. At sub-millimeter wavelengths, the development of SICs is driven by astronomical instruments where it could allow the realization of an imaging spectrometer, combining simultaneous imaging and spectroscopy capabilities into a single instrument analogous to integral field units in the infrared and optical regimes. Such an imaging spectrometer can be achieved with SICs by integrating the required elements, such as spectral filters and polarizers, with the detectors onto a single chip. Without this integration, the dispersive system for even a single spatial pixel at these wavelengths would be prohibitively large and could not be realistically scaled up to allow imaging. Astronomical signals are exceedingly weak, typically requiring many nights of exposure to get a good signal to noise ratio. It is therefore imperative that the instrument has minimal losses before its detectors. As a consequence, the losses of each element in the SIC needs to be minimized, which requires careful characterization of the individual elements, including antenna, filters, detectors and connecting transmission lines. The primary focus of this thesis lies on the experimental characterization of the wideband antenna and the low-loss superconducting transmission lines.
Superconducting integrated circuits (SICs) represent a natural step forward for devices operating at frequencies from microwave up to sub-millimeter wavelengths. They offer massive miniaturization via compact design based on low-loss superconducting transmission lines. At sub-millimeter wavelengths, the development of SICs is driven by astronomical instruments where it could allow the realization of an imaging spectrometer, combining simultaneous imaging and spectroscopy capabilities into a single instrument analogous to integral field units in the infrared and optical regimes. Such an imaging spectrometer can be achieved with SICs by integrating the required elements, such as spectral filters and polarizers, with the detectors onto a single chip. Without this integration, the dispersive system for even a single spatial pixel at these wavelengths would be prohibitively large and could not be realistically scaled up to allow imaging. Astronomical signals are exceedingly weak, typically requiring many nights of exposure to get a good signal to noise ratio. It is therefore imperative that the instrument has minimal losses before its detectors. As a consequence, the losses of each element in the SIC needs to be minimized, which requires careful characterization of the individual elements, including antenna, filters, detectors and connecting transmission lines. The primary focus of this thesis lies on the experimental characterization of the wideband antenna and the low-loss superconducting transmission lines.
Cosmic dust, cold matter and many other objects at far space have many spectral lines at the THz frequency range, leading to a rich information spectrum for astronomical observations. Ordinarily, these detected sub-mm signals from space are weak, and consequently very sensitive detectors are required to reduce the minimal time of the measurements with the desired accuracy. The main focus of this work is the design and development of superconducting high-frequency waveguide receivers at 211-275 GHz, 600-720 GHz and 780-950 GHz frequency ranges, in addition to superconducting planar integrated circuits matched with a planar Flux-Flow-Oscillator. This work is bridging the calculation methods applied for the design on one hand and the development of experimental instruments on the other.
Superconducting microwave resonators have the potential to revolutionize submillimeter and far-infrared astronomy, and with it our understanding of the universe. The field of low-temperature detector technology has reached a point where extremely sensitive devices like transition-edge sensors are now capable of detecting radiation limited by the background noise of the universe. However, the size of these detector arrays are limited to only a few thousand pixels. This is because of the cost and complexity of fabricating large-scale arrays of these detectors that can reach up to 10 lithographic levels on chip, and the complicated SQUID-based multiplexing circuitry and wiring for readout of each detector. In order to make substantial progress, next- generation ground-based telescopes such as CCAT or future space telescopes require focal planes with large-scale detector arrays of 104–106 pixels. Arrays using microwave kinetic inductance detectors (MKID) are a potential solution. These arrays can be easily made with a single layer of superconducting metal film deposited on a silicon substrate and pattered using conventional optical lithography. Furthermore, MKIDs are inherently multiplexable in the frequency domain, allowing ∼ 103 detectors to be read out using a single coaxial transmission line and cryogenic amplifier, drastically reducing cost and complexity. An MKID uses the change in the microwave surface impedance of a superconducting thin-film microresonator to detect photons. Absorption of photons in the superconductor breaks Cooper pairs into quasiparticles, changing the complex surface impedance, which results in a perturbation of resonator frequency and quality factor. For excitation and readout, the resonator is weakly coupled to a transmission line. The complex amplitude of a microwave probe signal tuned on-resonance and transmitted on the feedline past the resonator is perturbed as photons are absorbed in the superconductor. The perturbation can be detected using a cryogenic amplifier and subsequent homodyne mixing at room temperature. In an array of MKIDs, all the resonators are coupled to a shared feedline and are tuned to slightly different frequencies. They can be read out simultaneously using a comb of frequencies generated and measured using digital techniques. This thesis documents an effort to demonstrate the basic operation of ∼ 256 pixel arrays of lumped-element MKIDs made from superconducting TiNx on silicon. The resonators are designed and simulated for optimum operation. Various properties of the resonators and arrays are measured and compared to theoretical expectations. A particularly exciting observation is the extremely high quality factors (∼ 3 × 107) of our TiNx resonators which is essential for ultra-high sensitivity. The arrays are tightly packed both in space and in frequency which is desirable for larger full-size arrays. However, this can cause a serious problem in terms of microwave crosstalk between neighboring pixels. We show that by properly designing the resonator geometry, crosstalk can be eliminated; this is supported by our measurement results. We also tackle the problem of excess frequency noise in MKIDs. Intrinsic noise in the form of an excess resonance frequency jitter exists in planar superconducting resonators that are made on dielectric substrates. We conclusively show that this noise is due to fluctuations of the resonator capacitance. In turn, the capacitance fluctuations are thought to be driven by two-level system (TLS) fluctuators in a thin layer on the surface of the device. With a modified resonator design we demonstrate with measurements that this noise can be substantially reduced. An optimized version of this resonator was designed for the multiwavelength submillimeter kinetic inductance camera (MUSIC) instrument for the Caltech Submillimeter Observatory.
Low-loss deposited dielectrics will benefit superconducting devices such as integrated superconducting spectrometers, superconducting qubits, and kinetic inductance parametric amplifiers. Compared with planar structures, multilayer structures such as microstrips are more compact and eliminate radiation loss at high frequencies. Multilayer structures are most easily fabricated with deposited dielectrics, which typically exhibit higher dielectric loss than crystalline dielectrics. We measure the subkelvin and low-power microwave and millimeter-submillimeter-wave dielectric loss of hydrogenated amorphous silicon carbide (a-SiC:H), using superconducting chips with Nb-Ti-N/a-SiC:H/Nb-Ti-N microstrip resonators. We deposit the a-SiC:H by plasma-enhanced chemical vapor deposition at a substrate temperature of $400{\phantom{\rule{0.1em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The a-SiC:H has a millimeter-submillimeter loss tangent ranging from $0.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ at 270 GHz to $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ at 385 GHz. The microwave loss tangent is $3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. These are the lowest low-power subkelvin loss tangents that have been reported for microstrip resonators at millimeter-submillimeter and microwave frequencies. The a-SiC:H films are free of blisters and have low stress: $\ensuremath{-}20$ MPa compressive at 200-nm thickness to 60 MPa tensile at 1000-nm thickness.
Since the birth of astrophysics, astronomers have been using free-space optics to analyze light falling on Earth. In the future however, thanks to the advances in photonics and nanoscience/nanotechnology, much of the manipulation of light might be carried out using not optics but confined waveguides, or circuits, on a chip. This new generation of instruments will be not only extremely compact, but also powerful in performance because the integration enables a greater degree of multiplexing. The benefit is especially profound for space- or air-borne observatories, where size, weight, and mechanical reliability are of top priority. Recently, several groups around the world are trying to integrate ultra-wideband (UWB), low-resolution spectrometers for millimeter-submillimeter waves onto microchips, using superconducting microelectronics. The scope of this Paper is to provide a general introduction and a review of the state-of-the-art of this rapidly advancing field.
The terahertz regime, as a last radio window, remains to be fully explored, and astronomical and atmospheric observations in this regime are scientifically important. Like other frequency regimes, developing high-sensitivity detectors (coherent and incoherent) is of particular significance for both ground-based and space-borne facilities. As the coherent detector of choice below 1.4 THz, superconductor-insulator-superconductor (SIS) heterodyne mixers have achieved as high a sensitivity as five times the quantum limit around 1.4 THz. It is, however, still a challenge to developing SIS mixers at frequencies beyond 1.4 THz with considerable transmission loss in superconducting circuits due to the Cooper-pair breaking by energetic photons and increased many difficulties in designing and fabricating. So far, superconducting hot electron bolometer (HEB) mixers have been the most sensitive heterodyne detectors at frequencies above 1.5 THz, and successfully used to detect molecular spectral lines up to 2.5 THz from ground-based and space telescopes. Although spiral-antenna coupled NbN HEB mixers show a good sensitivity in the whole THz frequency range, the directly measured spectral response with Fourier transform spectrometer falls quickly as frequency increases, especially above 3 THz. The terahertz band is also of particular importance to observe astronomical objects such as cosmic microwave background, early distant objects, cold objects and dusty objects. Aiming at such objects, we develop a terahertz imaging array system by combining advanced superconducting detectors such as transition edge sensor (TES) and microwave kinetic inductance detectors (MKIDs), thus the system has a frequency band centred at 350 m, an operational temperature of 0.3 K, and a sensitivity reaching background limit performance for ground-based applications. In addition, it is expected to have some breakthroughs in ultra-sensitive superconducting TES and MKID, low noise multi-channel readout and multiplexing, efficient terahertz-wave coupling technology, and large-scale array system integration. The developed terahertz imaging array system will serve as the next-generation instrument of Dome A 5 m terahertz telescope, conducting a 350 m-band legacy survey for studying the planets, stars, galaxies and cosmology. Besides the application in astronomy, the developed terahertz imaging array system can also be applied to some areas requiring rapid detection such as security, deep space exploration, and biomedical imaging. In this paper, we mainly introduce the superconducting detectors developed at Purple Mountain Observatory and those for international collaborative projects.
In millimeter and submillimeter superconducting receivers, which are widely applied in radio astronomical observations, waveguide-to-planar transmission line transitions are key components connecting waveguide medium and the superconducting planar circuit. Although conventional quartz substrate-based transitions perform well for single-pixel receivers, these waveguide probes are not suitable for imaging arrays because their narrow chip space prohibits a higher degree integration of superconducting circuits. For multi-beam receiving applications, we have demonstrated fully functional monolithic microwave integrated circuit superconductor-insulator-superconductor (SIS) mixers with on-chip silicon membrane-based transitions at 2 mm wavelengths. In the present work, we aim to assess the transition as a stand-alone component and investigate its bandwidth potential. We designed a full-height waveguide transition which covers 125–211 GHz, more than 50% fractional bandwidth. The experimental study was carried out by performing cryogenic measurements of the transmission of a back-to-back probe pair with a mm-wave network analyzer. The measured transmission is well consistent with the simulated one in the frequency range of 125–170 GHz, which is covered by the network analyzer in this measurement.