Superconducting transition-edge sensors (TESs) are highly sensitive detectors and can detect electromagnetic wave radiations from millimeter/submillimeter, optical to 𝑥/γrays, suitable for cosmology, astrophysics, quantum information, and biosensing. In principle, thousands of TESs even more are required to enhance the detection efficiency for large-scale survey. Among other multiplexing schemes, microwave SQUID multiplexer (μMUX), consisting of resonators and RF SQUIDs, has a bandwidth of several GHz, thus multiplexing factor on the order of thousands, more suitable for readout of large TES arrays. We designed and fabricated superconducting coplanar waveguide (CPW) resonators with a high qualityfactor and second-order gradient RF SQUID with two inductive coupling structures respectively. Then, we optimized the critical current density of the Josephson junction and measured the mutual inductance parameters of the second-order gradient structure SQUID, which are consistent with the simulation results. Finally, we fabricated a cryogenic μMUX chip based on RF SQUID and resonator. We discussed the results of the development of μMUX in more detail.
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.
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.
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.
Graphene has an extremely weak coupling of electrons to phonons due to its nonionic character of lattice. This remarkable property makes graphene very attractive for hot electron bolometers (HEBs). In this paper, we present the development of a graphene-based terahertz hot electron bolometer (HEB) with Johnson noise readout. The HEB is essentially a graphene microbridge that is connected to a log spiral antenna by Au contact pads. We study the responsivity, noise equivalent power (NEP) and time constant of the graphene-based HEB in a perpendicular magnetic field. In order to understand the thermal transport inside the graphene microbridge, we also measure the graphene-based HEB at different bath temperatures between 3 K and 10 K. Detailed experimental results and analysis will be presented.
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 spectrum measurement of a terahertz (THz) pulse signal using a Fourier transform spectroscopy (FTS) system. The THz pulse signal is a quantum cascade laser (QCL) at 3.7THz with changeable repeating frequency and duty cycle. With a fixed duty cycle, the repeating frequency is changed to investigate the maximum value that can be measured with an FTS system. The relationship between the spectrum intensity and the pulse width is investigated through the variance of the duty cycle with a given repeating frequency. Detailed experimental results will be presented.
We report on a twin-slot antenna coupled superconducting NbN hot electron bolometer (HEB) mixer designed for 1.6
THz. Terahertz (THz) radiation is quasi-optically coupled to the HEB with an uncoated elliptical Si lens. Measured DSB
receiver noise temperatures are 1500 K at 0.85 THz, 1200 K at 1.27 THz, 1100 K at 1.31 THz, 1100 K at 1.4 THz, and
1000 K at 1.63 THz. This value at 1.63 THz is reduced to 750 K when the hot/cold loads in vacuum are used. The
frequency dependence of the noise temperature is consistent with the measured FTS spectral response. The measured farfield
beam patterns of the lens/antenna combination show nearly collimated beams with the side lobes below -16dB by
adding a 40 μm extension to a standard Si elliptical lens design, which is understood by considering a slightly lower
dielectric constant of Si (εSi) of 11.4 instead of 11.7. The good performance of such NbN HEB mixers makes it suitable
for future high-resolution spectroscopic astronomical applications.
Superconducting Hot Electron Bolometer mixers offer the highest sensitivity for heterodyne detections at frequencies above 1 THz. Important efforts have been made these recent years to further increase the HEB mixers' sensitivity and working frequency and also to design multi-pixel configuration.
We present in this paper the developments of a non-standard quasi-optical membrane based HEB mixer where the commonly used focusing element, the silicon lens, is replaced by a micro-mirror and a membrane-based back-short. This configuration offers many advantages: easier processing for circuits at very high frequencies, better noise temperature brought by lower RF coupling loss and higher gain of the antenna. This design is also considered very attractive for multi-pixel receivers. The devices are made of phonon-cooled NbN HEB mixers processed on 1.4 μm thick stress-less Si3N4/Si02 membrane. Quasi-optical designs have been made for frequencies at 600 GHz and 2.5 THz. The design and the device fabrication process will be discussed and both DC and RF measurements at 600 GHz 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.
We report the sensitivity of a superconducting NbN hot electron bolometer mixer integrated with a tight spiral antenna at
5.3 THz. Using a measurement setup with black body calibration sources and a beam splitter in vacuo, and an
antireflection coated Si lens, we obtained a double sideband receiver noise temperature of 1150 K, which is 4.5 times
hν/kB (quantum limit). Our experimental results in combination with an antenna-to-bolometer coupling simulation
suggest that HEB mixer can work well at least up to 6 THz, suitable for next generation of high-resolution spectroscopic
of the neutral atomic oxygen (OI) line at 4.7 THz.
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