Hirohiko M. Shimizu

Characterization of an STJ-based direct detector of submillimeter waves

We have developed submillimeter-wave direct detectors employing niobium-based superconducting tunnel junctions (STJs), with broadband spectral response, high sensitivity and imaging capability. Spectral response peaked at 650 GHz with a fractional bandwidth of 14 percent, which fits one of the important atmospheric windows for an astronomical project. We also confirmed the linearity of the detector response in the dynamic range greater than 10/sup 6/. The measured coupling efficiency was about 0.2, which was lower than the expected value of 0.5. The measured noise current of the detector was 10 fA//spl radic/Hz, which agreed with the shot noise from the residual leakage current of 100 pA at 0.3 K. Resultant noise equivalent power (NEP) is 1.6/spl times/10/sup -16/ W//spl radic/Hz, that is less than the background photon fluctuation limit for ground-based submillimeter-wave observations.

A broad-band THz radiation detector using a Nb-based superconducting tunnel junction

We have proposed and been developing a new broad-band terahertz (THz) radiation detector using a Nb-based superconducting tunnel junction (STJ). The STJs were fabricated on LiNbO/sub 3/ and LiTaO/sub 3/ mono-crystal substrates. We radiated monochromatic THz pulses in the range 1-2 THz with the pulse repetition rate of 49 Hz, and successfully detected the corresponding periodic signals via a charge-sensitive preamplifier. The current signal obtained by the derivative of the output signal gave the time response of about 15-20 microseconds. The frequency response was compared with that of a conventional pyro-electric sensor. We found their frequency response showed the similar shape in frequency, demonstrating the flat response of the STJ detector in 1-2 THz. These results show that the detector can be used as a broad-band THz and far-infrared radiation detector above the frequency corresponding to the energy gap of the superconductor used for the base electrode.

Development of X-ray detectors based on Nb/Al/AlOx/Al/Nb superconducting tunnel junctions

Abstract X-ray detectors based on Nb/Al/AlO x /Al/Nb superconducting tunnel junctions (STJ) have been fabricated. X-ray responses of the fabricated STJs were measured by illumination with X-rays from a 55 Fe electron capture source. The energy resolution of 41, 58 and 65xa0eV (FWHM) at 5.9xa0keV were obtained for 20×20, 100×100, 200×200xa0μm 2 , square-shaped devices, respectively. The X-ray measurements were performed at 0.35xa0K using a charge-sensitive preamplifier at room temperature. We also discussed the capability to suppress the substrate phonon events using Al 2 O 3 insulation layer. An advantage for the combined use of the Al 2 O 3 insulation layer and the sapphire substrate was demonstrated.

Wide-q observation from 10−4 to 2.0 Å−1 using a focusing and polarized neutron small-angle scattering spectrometer, SANS-J-II

In order to extend an upper q-limit [q is the magnitude of the scattering vector q, defined by q = (4π/λ)sinθ, where λ and 2θ are the wavelength and the scattering angle, respectively], high-angle 3He sub-detectors were installed on a focusing and polarized neutron small-angle scattering spectrometer (SANS-J-II) at JRR-3, Tokai, Japan. Consequently, the upper q-limit was improved from 0.2 to 2.0u2005A−1. To quantitatively discriminate spin incoherent scattering from hydrogen or to perform nuclear spin polarization contrast variation, a remanent supermirror analyser is also available on the high-angle sub-detector. By combining a focusing ultra-small-angle scattering, realised by compound and/or magnetic lens and high-resolution area detector, SANS-J-II is able to cover from 3 × 10−4 to 2.0u2005A−1 (four orders of magnitude of q), which benefits investigation of hierarchically ordered systems, found widely in hard, soft and bio-materials.

Properties of Substrate Phonon Events in Superconducting Tunnel Junctions Induced by X-Ray Absorption.

The spectra of substrate phonon events induced by illuminating with X-rays obtained with Nb-based superconducting tunnel junctions (STJs) fabricated on a sapphire substrate were examined. We constructed a geometrical model in which the collection efficiency of phonons is proportional to the solid angle subtended to the STJ from the position of the absorption. By using the Monte-Carlo simulation, we succeeded in fairly reproducing the features of the spectra obtained by our measurements. A comparison of the model spectra with the measured ones revealed that a mismatch of the acoustic impedance at the junction-substrate interface and the spatial spread of a phonon cloud play an important role in the determination of the spectra. By including these effects, we show that the measured spectra can be well reproduced by adjusting only one free parameter, the radius of the phonon cloud. We estimated this radius in a sapphire substrate to be ~20 µm under 5.9 keV X-ray illumination.

Development of superconducting tunnel junctions for ultra soft X-ray detectors

Nb-based superconducting tunnel junctions (STJs) with Al trapping layers have been developed for X-ray detectors. One applications for STJs is a low energy radiation detector having energy resolution and a high photon counting rate. STJs have good potential in the band of energies from 40 eV up to 1 keV, because STJs materials have good absorbing properties in this region. The energy resolution was measured by using the Photon Factory beamline BL-12A of High Energy Accelerator Research Organization (KEK) in Japan, and for 200 eV photon, 11 eV Full Width Half at Half Maximum (FWHM) with a 20/spl times/20 /spl mu/m/sup 2/ junction were achieved. In this article, the fabrication technique and preliminary results of measuring the response for this band of energies are reported.

Nb-based superconducting tunnel junctions as submillimeter-wave direct detectors

Employing an anodization method into our fabrication process, we have successfully fabricated STJs with size of 4 /spl mu/m in diameter, Josephson critical current density of 1000 A/cm/sup 2/, low leakage current of less than 10 pA at 0.3 K and high fabrication yield of more than 90 percent. These STJs were applied to fabricate submillimeter-wave direct detectors using 12 linearly distributed junctions, each circular junction having a current density of 600 A/cm/sup 2/ and size of 4 /spl mu/m in diameter. Direct detection of submillimeter-waves was measured using a Fourier transform spectrometer, which shows a maximum sensitivity at 440 GHz and a fractional bandwidth of 10 percent.

Terahertz direct detector using superconducting tunnel junctions

High-sensitivity terahertz direct detectors using superconducting tunnel junctions were fabricated. They were designed for detecting terahertz radiation in the frequency range of 0.4 and 0.65 THz with the fractional bandwidth of above 10 percent. The results of their performance evaluation of five detector elements are presented. We show the results of the frequency response as well as that the absolute efficiency ranged from 10 to 30 percent and that the the sensitivity was 1.9 x 10-16 W Hz-0.5 in noise equivalent power.

Development of superconducting tunnel junctions as an energy-dispersive detector for UV and visible photons

Abstract We are developing an energy-dispersive detector for UV and visible photons that uses superconducting tunnel junctions (STJs) with Al trapping layers. As a first step to achieve this, we evaluated the performance of the detector for extreme ultraviolet (EUV) photons by using a synchrotron radiation facility. We achieved an energy resolution of FWHM=17.8±0.6 eV (including electrical noise of 17.6±0.2 eV) for 55 eV EUV photons with a 100×100 μm2 STJ. The electrical noise was the main component of the energy resolution. We achieved the electrical noise of FWHM=2.1±0.1 eV for a 20×20 μm2 STJ by replacing the cables connecting the STJ to a preamplifier operated at room temperature. A stronger signal resulting from a thinner tunnel barrier will lead to better energy resolution for detecting UV and visible photons.

Direct and Indirect Detection of Terahertz Waves using a Nb-based Superconducting Tunnel Junction

We are developing a broadband THz detectors using Nb-based superconducting tunnel junctions. We found two components in a current signal for 1-2 THz pulse input. From the comparison of results obtained for the signals, the frequency response for the different size of STJs and the STJs fabricated on different substrate materials, we demonstrate that they correspond to photons absorbed with the superconductor electrode (referred as direct component) and phonons generated by photons absorbed with the substrate (referred as phononmediated component).