K. M. Zhou

A quasi-optical vector near-field measurement system at terahertz band

This paper describes a vector near-field measurement system at terahertz band based on a high sensitivity superconducting receiver equipped with a quasi-optical probe for high resolution near-field sensing. A novel single-receiver rather than commonly used dual-receiver configuration is adopted for vector measurement. Performances of the measurement system including stability and dynamic range are studied. Vector near-field measurement of a diagonal feedhorn at 850 GHz is presented and shows good agreement with simulation and direct far-field measurement.

High sensitive THz superconducting hot electron bolometer mixers and transition edge sensors

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.

Demonstration of a fully integrated superconducting receiver with a 2.7 THz quantum cascade laser

We demonstrate for the first time the integration of a superconducting hot electron bolometer (HEB) mixer and a quantum cascade laser (QCL) on the same 4-K stage of a single cryostat, which is of particular interest for terahertz (THz) HEB/QCL integrated heterodyne receivers for practical applications. Two key issues are addressed. Firstly, a low power consumption QCL is adopted for preventing its heat dissipation from destroying the HEBs superconductivity. Secondly, a simple spherical lens located on the same 4-K stage is introduced to optimize the coupling between the HEB and the QCL, which has relatively limited output power owing to low input direct current (DC) power. Note that simulation techniques are used to design the HEB/QCL integrated heterodyne receiver to avoid the need for mechanical tuning. The integrated HEB/QCL receiver shows an uncorrected noise temperature of 1500 K at 2.7 THz, which is better than the performance of the same receiver with all the components not integrated.

Effects of multiple reflections and low-frequency interferences on measured terahertz Fourier transform spectra

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.

Noise temperature and IF bandwidth of a 1.4 THz superconducting HEB mixer

In this paper, the noise temperature and IF bandwidth of a 1.4 THz twin-slot antenna coupled NbN superconducting HEB mixer are thoroughly investigated. The RF noise and conversion gain of the HEB mixer have been measured and analyzed. An anti-reflection coating has been applied on the elliptical lens to reduce the RF noise, the measured lowest noise temperature (450 K) achieves state-of-the-art sensitivity at 1.3 THz. The measured IF noise bandwidth is about 3 GHz, which is sufficiently large for some astronomical applications.

A novel wavefront-based algorithm for numerical simulation of quasi-optical systems

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.

Performance of a 1.3-THz twin-slot antenna superconducting HEB mixer integrated with different elliptical lenses

The performance of a 1.3 THz twin-slot antenna coupled superconducting hot-electron-bolometer (HEB) mixer integrated with elliptical lenses of different extension lengths are investigated. The elliptical lenses have the same radius but different extension lengths: 1.109 mm, 1.149 mm, 1.189 mm and 1.229 mm. The measured noise temperature and frequency response appears insensitive to the lens extension length. The far-field beam-patterns, directivity and Gaussicity are clearly related to the lens extension length.

Characterization of superconducting FeSe 0.5 Te 0.5 hot electron bolometer

Characteristics of superconducting FeSe_0.5Te_0.5 hot electron bolometer are reported. The superconducting FeSe_0.5Te_0.5 film is grown by pulsed laser deposition on MgO substrate, and its transition temperature and complex conductivity indicate that the film quality is good enough for hot electron bolometer. We then fabricate hot electron bolometer based on the superconducting FeSe₀.₅Te₀.₅ film, and study the resistive transition curve and its current-voltage characteristics.

Twin slot antenna patterns of a quasi-optical HEB mixer at THz frequencies

Results of far-field beam pattern measurements of the 1.25 THz superconducting Hot Electron Bolometer (HEB) mixer on a quasi-optical integrated lens antenna are presented. The integrated antenna consists of a planar twin slot antenna mounted on the flat surface of an extended hemispherical lens. The beam patterns have been measured at three frequencies: 1.35 THz, 1.395 THz and 1.47 THz with a new setup which has a focus lens between source and HEB mixer. The first order side lobe level equals -13 dB. The dynamic range is nearly 30 dB. The simulated result is also presented, it agrees with measured one well.

Direct detection behavior of a superconducting hot electron bolometer measured by Fourier transform spectrometer

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.