Dome A, with an altitude of 4093 m, is presumably the most promising ground-based observing site for THz astronomy. Purple Mountain Observatory is planning to build a 5 m terahertz telescope (DATE5) there, with one important band centered at approximately 1.4 THz which is a frequency band that there is an abundance of molecular spectral lines and fine atomic structure spectral lines (e.g., high-J CO, H2D+ and NII) which are very important tracers for probing the physical and chemical properties and dynamic processes of objects such as stars and planetary systems. The advantages of high sensitivity and low LO power requirement make HEB mixers the best choice for the DATE5 telescope. So we developed a 1.4 THz twin-slot antenna coupled superconducting HEB mixer which consists of a 2 mm wide, 0.2 mm long and 3.5 nm thick NbN microbridge based on a highly resistive and natively oxidized Si substrate, and a twin-slot antenna. The twin-slot antenna has a slot length L of 0.31 λ0, a slot separation S of 0.178 λ0 and a slot width W of 0.023 λ0, with λ0 as the free space wavelength.
Fig1. Measured near-field amplitude (left) and phase (right) distributions at 850 GHz. Note that the signal source is 500 mm away from the HEB mixer
In order to characterize the performance of this superconducting HEB mixer, a series of experiments have been performed in Millimeter and Sub-Millimeter Lab of Purple Mountain Observatory. The double-sideband receiver noise temperature was measured by Kangmin ZHOU and Wei MIAO, and is 600 K at 1.3 THz which can be as low as 300 K with the optical losses corrected. The intrinsic frequency response of the HEB mixer was measured by Kangmin ZHOUand Shaoliang LI, it shows a high coupling efficiency in the frequency range of 0.8~1.5 THz, in good agreement with the simulated coupling efficiency of the twin-slot antenna. The far-field beam pattern of the HEB mixer, measured by Kangmin ZHOU, Zheng LOU and Jie HU, agrees well with the simulated one. The coupling efficiency between the measured beam pattern and a fundamental-mode Gaussian beam is 0.98 and 0.995 in E-plane and H-plane, respectively.
All the results of our measurements have been published by IEEE Transactions on Applied Superconductivity, for a little more insight into this research see：
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6975136&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D6975136
