Superconducting on-chip spectrometers have both imaging and spectroscopic capabilities. In general, the broadband signal coupled from an antenna goes through frequency dispersion via a series of filters that are connected with superconducting detectors like kinetic inductance detectors or bolometers. The filters have the same relative bandwidth, which determines the frequency resolution of the spectrometer. We here present the design and simulations on the twin-slot antenna, CPW-to-microstrip transition, and a ten-channel filter-bank of a verification-stage terahertz spectrometer chip at 350 GHz. The simulation results of the antenna and transition showed low return loss, and the simulation results of the ten-channel filter-bank show that each channel has good readout independence and coupling strength. These designs and simulations can provide assistance for the future development of terahertz on-chip spectrometer.
In this paper, we present a wideband antireflection coating designed for high-resistivity silicon and alumina lenses used in cryogenic terahertz detectors. A dual-layer coating structure based on diamond and polytetrafluoroethylene (PTFE) films is employed to achieve high transmittance in a wide frequency range, the film thickness of diamond and PTFE has been precisely controlled by mature microwave plasma chemical vapor deposition (MPCVD) and thermal spraying technology. The transmittances of coated silicon sample was measured within the frequency ranges of 0.3-0.5 THz using the Quasi Optical Vector Network Analyzers (QO-VNA). The measured transmittance of the one side coated silicon sample accords well with the simulation results, which demonstrates the accuracy of the coating process. A remarkable transmittance level of up to 99% can be achieved by applying the AR coating to silicon lens according to the simulations. This wideband antireflection coating can be applied to cryogenic terahertz detectors, such as superconducting hot electron bolometer (HEB) detectors and superconducting kinetic inductance (KIDs) detectors.
Conventional coherent and non-coherent techniques such as quasi-optical vector network analyze (VNA) and Fourier transform spectroscopy (FTS) can be employed to measure the exhaustive properties of dielectrics in the terahertz band. However, the VNA can only cover a narrow frequency range, and the FTS takes a relatively long period of time for measurement. By contrast, the terahertz time domain spectroscopy (TDS) allows the measurement of material properties such as dielectric constant and loss tangent in a wide frequency range and in a short period of time. Using a terahertz TDS, we characterize the complex properties of some materials commonly used in terahertz superconducting receivers, including high density polyethylene (HDPE), single crystal magnesium oxide (MgO), single crystal quartz, single crystal sapphire, single crystal silicon (S.C. silicon), high resistance silicon (H.R. silicon), and ultra-high molecular weight polyethylene (UHMWPE). The measurements at room temperature have finished yet. The measurements at cryostat temperature are in progress and will be published later.
Phased array feeds (PAF) are playing an increasingly important role in low-frequency radio astronomy, but less explored at THz wavelengths. Here we present the design of a 0.35-THz planar array antenna, which consists of an array of twinslot antennas, a coplanar waveguide (CPW) feed network and airbridges. The planar array antenna is based on a silicon substrate without any lens as well as anti-reflection coating. The airbridges are used to improve the performance of the CPW T-junctions and the transmission efficiency of the feed network. Simulation results show that the substrate thickness can change the power distribution on both sides of the substrate. With an appropriate substrate thickness and some airbridges, the total gain of a 16-element array can reach 15.3 dB and the main-beam efficiency is about 54%. This kind of planar array antenna can be easily integrated with a terahertz detector such as hot electron bolometer (HEB) and kinetic inductance detector (KID), based on photolithographic fabrication.
Microwave Kinetic Inductance Detectors (MKID) are a promising low temperature superconducting detector because of high sensitivity, easy frequency-domain multiplexing and simple structure for large-format arrays. To develop large-format THz detectors for China’s Antarctic THz telescope, we have preliminarily designed an aluminum 64-pixel MKID array operating at the 350 GHz band. In this paper, the characteristics of the MKID array are thoroughly measured.
Superconductor-insulator-superconductor (SIS) mixers, with nearly quantum-limited sensitivity, have been playing an important role in Terahertz astronomy. For practical THz SIS receivers, however, the measured noise temperatures are sometimes higher than the expected value. The extra noise is mainly due to considerable RF noise contribution from the receiver components such as beam splitter, Dewar window, and infrared filter. In this paper, we mainly present the simulation and measurement results of the three components with different materials and thicknesses. Their noise contributions are also analyzed.
High-density Polyethylene (HDPE), with a density above 0.95 g/cm3, has been widely used in terahertz systems. The advantages of low absorption loss, low refractive index and high rigidity make HDPE an ideal material for cryostat window, focus lens and substrate. HDPE can be machined easily and be used as a substrate material for components such as metal mesh filters and polarizers. What’s more, it is quite inert and can be used at cryogenic temperatures. On account of these applications, we need to characterize the dielectric property of HDPE precisely in a wide frequency range. In this paper, we present the transmittance measurements of a 2 mm thick HDPE sheet from 0.1 THz to 15 THz. Three kinds of measurement methods are employed to cover the whole frequency range. A vector network analyzer (VNA) combined with a quasi-optical transmissometer has been used to measure the transmittance and dielectric constant of HDPE from 0.16 THz to 0.18 THz at 300 K and 4 K. A Time Domain Spectrometer (TDS) is employed to cover the frequency range from 0.2 THz to 3 THz since the VNA can’t work upon 1 THz. A Fourier Transform Spectroscopy (FTS) has been used for the measurement from 3 THz to 15 THz since the TDS can’t achieve broad band and fast scan speed. The measured transmittance of HDPE is nearly 0.93 below 1 THz and decrease to 0.3 when the frequency increase to 15 THz. A rather elusive absorption band at 2.2 THz has also been observed. The dielectric constant of HDPE has been measured by VNA and TDS, showing a frequency independency from 0.1 THz to 3 THz.
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