The National Institute of Information and Communications Technology (NICT, President: TOKUDA Hideyuki, Ph.D.) has developed a unique superconducting hot electron bolometer mixer (HEBM) using magnetic materials. As a result, the noise of the 2 THz band heterodyne receiver has been reduced and the wide IF band has been achieved. The 2 THz band HEBM produced this time has a low noise performance of about 570 K (DSB), which is about 6 times the quantum noise limit, and a wide IF band characteristic of about 6.9 GHz, which is about 3 GHz larger than the conventional structure HEBM. Both of these are world-class performance.
This technology is expected to contribute to the development of new frequency resources such as high-speed wireless communication, non-destructive inspection, global environment measurement, and radio astronomy as basic technology in the THz frequency domain, which is an undeveloped frequency domain.


Figure 1
Figure 1 HEBM structure and a micrograph
The terahertz frequency range is an unexplored region and is expected to be applied to high-speed wireless communication, nondestructive inspection, security, medical care, global environment measurement and radio astronomy. However, in order to realize them, it is important to first develop the fundamental technology of oscillation and detection technology.
So far, superconducting SIS mixers have reported excellent heterodyne receiver performance in the lowest noise and wide IF band in the frequency domain up to 1 THz. However, the upper limit frequency of its operation is considered to be about 1.5 THz, and a superconducting hot electron bolometer mixer (HEBM) is currently under research and development as a low-noise mixer element in the frequency region exceeding 1.5 THz.
It has already been reported that HEBM exhibits low-noise receiver operation below 10 times the quantum noise limit in the frequency region above 1.5 THz. However, HEBM had a problem to be solved for its application, because the IF bandwidth, which means the amount of information that can be processed at one time, is narrow. Compared to a superconducting SIS mixer that can secure an IF bandwidth of 20 GHz or more, HEBM was less than a quarter of that, at 3 to 5 GHz. The expansion of the IF bandwidth has great application merits, and there has been a demand for a wider IF bandwidth of HEBM.


Figure 2
Figure 2 Schematics of a conventional and a new HEBM structure
NICT has developed a new HEBM structure using magnetic materials as a detection technology, which is a basic technology for terahertz waves, in collaboration with the Advanced ICT Research Institute and the Applied Electromagnetic Research Institute under research collaboration at the Terahertz Technology Research Center. The new HEBM offers low noise performance and wide IF bandwidth at 2 THz.
HEBM has a structure in which a small superconducting thin film piece (superconducting strip) is placed between two metal electrodes and is a mixer that utilizes the strong impedance nonlinearity generated between the superconducting-normal transition (See Figure 1 (a)). This time, we have developed a new HEBM structure unique to NICT that inserts a nickel (Ni) thin film, which is a magnetic material, between the superconducting thin film and metal electrode to leave superconductivity only in the superconducting strip between the electrodes (See Figure 2 (b)). This structure allows HEBM to be further miniaturized and has realized a wider IF band as well as lower detector noise.
Figure 3
Figure 3 IF bandwidth of the HEBM with new structure
Therefore, this time, we prepared a miniaturized HEBM with a superconducting strip length of 0.1 μm and achieved Trx = 570 K (DSB) as the mixer noise temperature corrected for the loss of the input optical system at the measurement frequency of 2 THz. This is an extremely low noise operation that is about 6 times the quantum noise limit. In addition, the IF bandwidth of about 6.9 GHz, which is about 3 GHz larger than that of the conventional HEBM, was obtained, and it was confirmed that the new HEBM structure using magnetic materials is effective in improving the receiver performance (See Figure 3). These results are the results of evaluation at the actual operating temperature of 4 K, and we believe that they have the world's top-level performance as a terahertz band HEBM.

Future Prospects

NICT is working on the development of waveguide HEBM with the aim of commercializing 2 THz band HEBM. We aim to apply it to remote sensing technology such as global environment measurement and radio astronomy.

Paper details

Journal: IEEE Transactions on Terahertz Science and Technology, Vol. 8, No. 6, November 2018
DOI: 10.1109/TTHZ.2018.2874355
Title: Broadening the IF Band of a THz Hot-Electron Bolometer Mixer by Using a Magnetic Thin Film
Authors: Akira Kawakami, Yoshihisa Irimajiri, Taro Yamashita, Satoshi Ochiai, Yoshinori Uzawa
Part of this research is supported by a fund-accepting joint research contract with Inter-University Research Institute Corporation, National Institutes of Natural Sciences, National Astronomical Observatory of Japan.


A bolometer is an electromagnetic wave detector that receives an electromagnetic wave incident power with a material that has a high temperature dependence of electrical resistance, the incident power raises the material temperature, and measures the power as a change in resistance of the material. Miniaturization of the bolometer directly reduces the heat capacity and leads to an improvement in response speed (wider IF band). At the same time, it also works to increase the sensitivity of the detector by reducing the energy required for temperature changes.
heterodyne receiver
Operation overview of heterodyne receiver
[Click picture to enlarge]
An element (mixer) with non-linear characteristics such as a diode is simultaneously irradiated with a signal electromagnetic wave (Sig) to be measured and a stable reference electromagnetic wave (LO) with a known frequency, and an intermediate frequency (IF) that is the frequency of the difference between Sig and LO. Refers to a receiver that converts the frequency into an electric signal (IF signal). Information such as the amplitude and phase of the Sig is held in this IF signal. The receiver, which converts signal information that has a high frequency such as the THz frequency region and is difficult to handle directly such as amplification into a signal in the frequency band that is easy to handle such as the microwave band, is an important technology for utilizing THz band electromagnetic waves.
IF band
In a heterodyne receiver, when a high frequency signal electromagnetic wave is converted into a low frequency IF signal, the frequency domain of the IF signal that retains the original signal information is called the IF band. The IF bandwidth means the amount of information that can be observed and processed at one time. Therefore, the expansion of the IF bandwidth has a great merit in application.
quantum noise limit
The heterodyne receiver simultaneously measures the phase and amplitude of the signal electromagnetic wave. However, due to the limitation of the uncertainty principle, the measurement accuracy of each cannot be increased as much as possible, and noise corresponding to the fluctuation (hf) of about one photon of the signal electromagnetic wave is inevitable (Here, h is Planck's constant, f is signal frequency). This is called the quantum noise limit, and its noise temperature is expressed by T = hf / k (k is Boltzmann's constant). When the noise temperature of the heterodyne receiver is about several times the quantum noise limit, the receiver is considered to be extremely low noise.
undeveloped frequency domain
Utilization of electromagnetic waves and undeveloped frequency domain
[Click picture to enlarge]
Compared to other frequency domain, the terahertz frequency domain of 0.1 to 10 THz, which is located at the boundary region between light and radio waves, was difficult to develop basic technologies such as oscillators (light sources) and detectors. It is an unused frequency domain. Therefore, it was called the "undeveloped frequency domain". In the future, this frequency domain is being actively researched and developed as a new frequency resource that realizes various industrial applications such as ultra-high-speed communication and remote sensing.
superconducting SIS mixer
A two-terminal element having a superconducting / insulator / superconducting laminated structure in which an ultrathin insulator is sandwiched between two superconducting electrodes is called a superconducting SIS junction. It is known that the current flowing through the superconducting SIS junction exhibits extremely strong non-linearity, and superconducting SIS mixers take advantage of this non-linearity. Currently, the mixer reports excellent characteristics in the lowest noise and wide IF band in the frequency domain up to 1 THz. However, due to the relatively large junction capacitance which came from the sandwich structure, a tuning circuit in the operating frequency band is indispensable. This tuning circuit requires ultra-low loss conductor characteristics, and superconducting thin film materials have been mainly used. However, since superconductivity has a gap frequency that determines the upper limit of ultra-low loss characteristics, this gap frequency determines the operation upper limit frequency of the superconducting SIS mixer. Currently, it is considered difficult to realize a superconducting SIS mixer over 1.5 THz.
superconducting proximity effect (see Figure 2 (a))
When a superconductor and a normal conductor such as a metal are connected, a superconducting electron pair, which is a carrier of superconducting current, seeps out to the normal conductor side, and the normal conductor exhibits superconductivity. This effect is called “proximity effect”. When the HEBM is irradiated with electromagnetic waves, the electron temperature in the superconducting strip rises due to the irradiation power, and the region exceeding the superconducting transition temperature (Tc) becomes the normal conduction region (hot spot) (see Figure 1 (a)).
This hotspot can be considered a "normal conductor" even within the superconducting strip. Therefore, the hot spot undergoes leaching of superconducting electron pairs due to the superconducting proximity effect from the surrounding superconducting region in contact. Here, the superconducting region existing in the overlapping region under the metal electrodes is not directly suppressed by electromagnetic wave irradiation due to the shielding effect of the metal electrodes, the superconducting proximity effect from these regions act stably to suppress hotspot formation. This leads to a decrease in mixer sensitivity and It is expected to inhibit the miniaturization of HEBM.

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