Lens integrated twin slot antennas have been widely used in superconducting transition edge sensor (TES) detectors due to its high directivity and low cross-polarization. In this paper, we present the design and simulation of a 210 GHz dual-polarized twin slot antenna for TES detectors. We used Ansoft HFSS to simulate the return loss and isolation of the dual-polarized twin slot antenna. The results show that the return loss of the dual-polarized twin slot antenna is less than -15 dB and the isolation is great than 10 dB in the frequency range from 170 GHz to 230 GHz. We also used FEKO to simulate the beam pattern of the dual-polarized twin slot antenna integrated with a silicon lens with a diameter of 5 mm. After optimizing the extended length of the silicon lens, a near Gaussian beam with a half power beam width (HPBW) of 19.4 degrees and a side lobe level of 17.3 dB was obtained. In addition, we designed and simulated an air bridge that is used to transmit the signals received by the twin slot antennas in the orthogonal directions. We find that the transmission coefficient of the air bridge is close to 0 dB and the isolation in the orthogonal directions exceeds 35 dB.
Microwave kinetic inductance detectors (MKIDs) can detect photons ranging from terahertz, infrared, optical to X-ray frequencies. And it has the advantages of high sensitivity, easy frequency-domain multiplexing (FDM) readout, and simple structure for large-format arrays. With these advantages MKIDs are becoming the choice of incoherent detectors in astronomy such as imaging cameras and broadband imaging spectrometers. In this paper, we introduce the design and fabrication of a 5-THz 10×10 MKIDs array on a single chip from an 80-nm-thick aluminum (Al) superconducting film based on a high resistance silicon substrate. Each pixel is a quarter-wavelength resonator with its short-circuited end integrated with a 5-THz dipole antenna. The noise performance of the MKIDs is measured at 200-mK using both phase readout and amplitude readout, giving an electrical NEP ~10−17 W/Hz0.5. Detailed results will be presented.
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.
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.
Fourier phase gratings play a vital role in the multi-beam heterodyne receiver in sub-millimeter astronomical instruments. In this study, a 1×4 beam grating at 660 GHz is developed, by which the surface structure is generated with an iterative algorithm. Far-field beam pattern is simulated with FEKO, where a relative high efficiency of 91% as well as a uniformity of power distribution among 4 beams of less than 1% are obtained. The grating was manufactured in aluminum material by a micro-milling machine. A PC-controlled scanning stage is employed for the beam pattern measurement. Despite the discrepancy from the manufacture of less than 6 μm, measurement results exhibit a good agreement with simulation in both power efficiency and far-field spatial distribution.
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.
A novel wavefront-based algorithm for the beam simulation of both reflective and refractive optics in a complicated quasi-optical system is proposed. The algorithm can be regarded as the extension to the conventional Physical Optics algorithm to handle dielectrics. Internal reflections are modeled in an accurate fashion, and coating and flossy materials can be treated in a straightforward manner. A parallel implementation of the algorithm has been developed and numerical examples show that the algorithm yields sufficient accuracy by comparing with experimental results, while the computational complexity is much less than the full-wave methods. The algorithm offers an alternative approach to the modeling of quasi-optical systems in addition to the Geometrical Optics modeling and full-wave methods.
Terahertz band, which is roughly defined as 0.1 THz to 10 THz, is an interesting frequency region of the electromagnetic spectrum to be fully explored in astronomy. THz observations play key roles in astrophysics and cosmology. High sensitive heterodyne and direct detectors are the main tools for the detection of molecular spectral lines and fine atomic structure spectral lines, which are very important tracers for probing the physical and chemical properties and dynamic processes of objects such as star and planetary systems. China is planning to build an THz telescope at Dome A, Antarctica, a unique site for ground-based THz observations. We are developing THz superconducting hot electron bolometer (HEB) mixers and transition edge sensors (TES), which are quantum limited and back-ground limited detectors, respectively. Here we first introduce the working principles of superconducting HEB and TES, and then mainly present the results achieved at Purple mountain Observatory.
In this paper, we report on the measured and simulated far-field beam-patterns of a quasi-optical NbN superconducting hot electron bolometer (HEB) mixer at 600GHz. This superconducting HEB mixer consists of an extended hemispherical lens with a diameter of 12.7mm and an extension length of 2.45mm, a twin-slot planar antenna (two slots measuring 148.5μm × 10.4μm with a separation of 78.98μm) and a 5.5-nm thick NbN thin-film micro-bridge with an area of 2μm × 0.2μm . The far-field beam pattern of this mixer is measured by a direct-detection technique with a dynamic range of nearly 25dB, showing an FWHM beam angle of 2.7° and -18dB level of the first side-lobe. The measured beam of the quasi-optical mixer is nearly collimated and has good Gaussian beam efficiency. In addition, the far-field beam-pattern is measured at different DC bias voltages of the superconducting HEB mixer and at different bath temperatures. The measured results are compared with the ones simulated by two different methods. Detailed measurement and simulation results will be presented.
Fourier transform spectroscopy (FTS) is a measurement technique widely used in characterizing the spectrum of light sources and the frequency response of detectors. Some “ghost” spectral lines, however, are often observed in measured Fourier transform spectra, such as high-frequency harmonics of the light source due to multiple reflections in the measurement system and unexpected high frequency lines owing to low-frequency interferences in the data acquisition. Here we study the effects of multiple reflections and low-frequency interferences on the THz spectra measured by a Fourier transform spectrometer for different THz sources and detectors. Experimental and simulation results will be presented.
In this paper, the direct detection behaviors of a superconducting hot electron bolometer integrated with a log spiral
antenna are investigated by using Fourier Transform Spectrometer (FTS). We find the response of the bolometer to a
modulated signal can be detected by a lock-in amplifier not only from the DC bias current, but also from the output noise
power at the IF port of the HEB. We attribute the response in output noise power to Johnson noise and thermal
fluctuation noise. Both the current response and the output noise power response measured at different bias voltages can
be explained by one dimensional distributed hot spot model. In addition, the frequency response of the hot electron
bolometer measured from the response in DC bias current is in good agreement with that in IF output noise power.
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