Since the birth of astrophysics, astronomers have been using free-space optics to analyze light falling on Earth. In the future however, thanks to the advances in photonics and nanoscience/nanotechnology, much of the manipulation of light might be carried out using not optics but confined waveguides, or circuits, on a chip. This new generation of instruments will be not only extremely compact, but also powerful in performance because the integration enables a greater degree of multiplexing. The benefit is especially profound for space- or air-borne observatories, where size, weight, and mechanical reliability are of top priority. Recently, several groups around the world are trying to integrate ultra-wideband (UWB), low-resolution spectrometers for millimeter-submillimeter waves onto microchips, using superconducting microelectronics. The scope of this Paper is to provide a general introduction and a review of the state-of-the-art of this rapidly advancing field.

We have developed a process for the fabrication of high-quality Nb/AlOx/Nb tunnel junctions with small area and high current densities for the heterodyne mixers at millimeter and submillimeter wavelengths. Their dc I-V curves are numerically studied, including the broadening of quasiparticle density of states resulting from the existence of an imaginary part of the gap energy of Nb. We have found both experimentally and numerically that the subgap current is strongly dependent on bias voltage at temperatures below 4.2 K unlike the prediction of the BCS tunneling theory. It is shown that calculated dc I-V curves taking into account the complex number of the gap energy agree well with those of Nb/AlOx/Nb junctions measured at temperatures from 0.4 to 4.2 K. We have successfully built receivers at millimeter and submillimeter wavelengths with the noise temperature as low as 4 times the quantum photon noise, employing those high-quality Nb/AlOx/Nb junctions. Those low-noise receivers are to be installed in the ALMA (Atacama Large Millimeter/Submillimeter Array) telescope and they are going into series production now.

A phase-retrieval method is applied to the quasioptical feed system of the offset Cassegrain antenna of the Superconducting Submillimeter-Wave Limb-Emission Sounder (JEM/SMILES) to be aboard the International Space Station for evaluating the beam alignment by estimating the phase pattern from the beam amplitude pattern measurements. As the result, the application of the phase retrieval method is demonstrated to be effective for measuring and evaluating the quasioptical antenna feed system. It is also demonstrated that the far-field radiation pattern of the antenna main reflector can be estimated from the phase-retrieved beam pattern of the feed system.

This study is intended to realize an in-situ gas sensor based upon the principle of millimeter/submillimeter wave spectroscopy. In-situ gas sensor will be attractive because of gas selectivity, multiple parametric measurement such as gas temperature, pressure and density, and of the in-situ measurement capability. One of the major technical problem to be solved is to develop an instrument accurate enough to discern the spectrum change due to the variation of parameters such as temperature. In this paper a proposed gas absorption measurement system is investigated, which schematically consists of Fabry-Perot type gas cell for effective long path length, and vector signal processing to reject leak signal coupled between resonator input and output ports so as to achieve precise absorption measurement. Also included is an parametric study of oxygen absorption characteristics, which is served as the predicted value in the evaluation of the instrument. The experiment at 60 GHz and 120 GHz bands using oxygen demonstrates the effectiveness of the current system configuration.

The reflection signal in the inverse synthetic aperture radar is measured in the polar coordinate defined by the object rotation angle and the frequency. The reconstruction of fixed scene images requires the coordinate transformation of the polar format data into the rectangular spatial frequency domain, which is then processed by the inverse Fourier transform. In this paper a fast and flexible method of coordinate transformation based on the nearest neighbor interpolation utilizing the Delauney triangulation is at first presented. Then, the induced errors in the transformed rectangular spatial frequency data and the resultant fixed scene images are investigated by simulation under the uniform plane wave transmit-receive mode over the swept frequency 120-160 GHz, and the results which demonstrate the validity of the current coordinate transformation are presented.

Thin-film slot-array receiving antennas fed by coplanar waveguide (CPW) were fabricated on fused quartz substrates, and the antenna properties were investigated at 700 GHz. It was confirmed that the transmission efficiency of CPW was 0.83/λm, and the rate of radiated power from a slot antenna was 0.5 at 700 GHz. The fabricated antennas worked as expected from the theory based on the transmission line model, and the two-dimensional 83 slot-array antenna fed by CPW increased the power gain by 11 dB over a single-slot antenna. The power gain of the antenna was 13 dBi and the aperture efficiency was 40% when the 700 GHz-submillimeter wave was irradiated through the substrate.

A quasi-optical Superconductor-Insulator-Superconductor (SIS) mixer has been designed and tested in the 270-GHz band. The mixer used a substrate-lens-coupled log-periodic antenna and a tuning circuit for RF matching. The antenna is planar and self-complementary, and has a frequency-independent impedance of around 114 Ω over several octaves. The tuning circuit consists of two Nb/AIOx/Nb tunnel junctions separated by inductance for tuning out the junction capacitances and a λ/4 impedance transformer for matching the resistance of the two-junction circuit to the antenna impedance. The IF output from the mixer is brought out in a balanced method at each edge of the antenna, and is coupled to a low noise amplifier through a balun transformer using a 180-degree hybrid coupler for broadband IF matching. Double sideband receiver noise temperatures, determined from experimental Y-factor measurements, are about 150 K across the majority of the desired operating frequency band. The minimum receiver noise temperature of 120 K was measured at 263 GHz, which is as low as that of waveguide receivers. At this frequency, measurement of the noise contribution to the receiver results in input losses of 90 K, mixer noise of 17 K, and multiplied IF noise of 13 K. We found that the major sources of noise in our quasi-optical receiver were the optical losses.

This paper presents an experimental study on the penetration characteristics of submillimeter waves in biological tissues and material. The measured values of the penetration depth in excised natural muscle, fat, and aqueous solution of protein, bovine serum albumin (BSA), over the wavelengths of 281 through 496µm are presented. Penetration depths at these wavelengths are 0.11-0.17mm in the natural pork muscle, and 0.69-0.98mm in the natural pork fat, and are the larger at the longer wavelengths. The values vary considerably from sample to sample. Since the measurement of the penetration depth in this study is shown sufficiently reproducible, the variation of the measured penetration depth is attributed to the variation of natural tissues such as that in water content. It is found that the penetration depth of submillimeter waves in aqueous solution of BSA depends almost linearly on the amount of protein content in the solution, and that the typical values of the penetration depth in the natural muscle roughly agree with that in the 35% aqueous solution of BSA in the submillimeter-wave region.