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IEEE International Superconductive Electronics Conference (ISEC)

Characterization of 40 GHz and 762 GHz sources with superconducting Josephson cantilevers in a THz microscope

Marco Tollkühn, Ilya Elenskiy, Benedikt Hampel, Meinhard Schilling

Published Jul 1, 2019• 0 citations
Publisher

Abstract

Josephson cantilevers, based on high temperature superconducting Josephson junctions, are versatile detectors for the measurement of frequency and power of microwave and terahertz radiation. A Josephson cantilever consists of at least one Josephson junction, which is equipped with an antenna structure for better coupling. The Josephson junctions, made from the high-temperature superconductor YBa2Cu3O7, are deployed on LaAIO3or MgO bicrystal substrates. Four feedlines are used to carry out four-terminal sensing. The THz microscope was developed, to allow three dimensional measurements [1]. In this measurement setup, the Josephson cantilever is mounted on a 15 × 15 × 15 mm3positioning system inside a vacuum chamber. The Josephson cantilever is cooled by a cryocooler that allows operating temperatures down to 30 K. A triangular current is injected into the junction and the current-voltage characteristic is recorded at each spatial point. The frequency and the power of the external radiation can be determined from the recorded data by evaluating the occurring Shapiro steps using Hilbert spectroscopy [2]. When the Josephson cantilever is irradiated with more than one frequency at a time, it behaves as a frequency mixer, due to its nonlinear current-voltage characteristic. In this work, we investigated the mixing of 40 GHz and 762 GHz signals with the THz microscope.

DOI
10.1109/isec46533.2019.8990940

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•Jan 1, 2004•0 citations
Josephson cantilevers, based on high temperature superconducting Josephson junctions, are versatile detectors for the measurement of frequency and power of microwave and terahertz radiation. A Josephson cantilever consists of at least one Josephson junction, which is equipped with an antenna structure for better coupling. The Josephson junctions, made from the high-temperature superconductor YBa2Cu3O7, are deployed on LaAIO3or MgO bicrystal substrates. Four feedlines are used to carry out four-terminal sensing. The THz microscope was developed, to allow three dimensional measurements [1]. In this measurement setup, the Josephson cantilever is mounted on a 15 × 15 × 15 mm3positioning system inside a vacuum chamber. The Josephson cantilever is cooled by a cryocooler that allows operating temperatures down to 30 K. A triangular current is injected into the junction and the current-voltage characteristic is recorded at each spatial point. The frequency and the power of the external radiation can be determined from the recorded data by evaluating the occurring Shapiro steps using Hilbert spectroscopy [2]. When the Josephson cantilever is irradiated with more than one frequency at a time, it behaves as a frequency mixer, due to its nonlinear current-voltage characteristic. In this work, we investigated the mixing of 40 GHz and 762 GHz signals with the THz microscope.
Josephson cantilevers based on superconducting Josephson junctions can be used for the measurement of frequency and power of microwave and terahertz radiation. For this, Josephson junctions made from the high-temperature superconductor YBa2Cu3O7are deployed on MgO bicrystal substrates. These Josephson cantilevers are utilized as detectors in our THz microscope. An adjustable current is injected into the junction and the current-voltage characteristic is recorded to derive the frequency and the power of an external radiation from the characteristic. To achieve spatial resolution with the THz microscope, the Josephson cantilever is mounted on a three-dimensional positioning system.
We prepared Josephson junctions made of YBa2Cu3O7 on symmetric 24° LaAlO3 bicrystal substrates. For better coupling of high-frequency radiation, we structured the contacts with logarithmic-periodic antennas made of YBa2Cu3O7. Characterization was done by measuring the voltage–current characteristics to determine the critical current IC, the normal-state resistance RN, the noise parameter Γ, and the Stewart–McCumber parameter βC as predicted by the resistively and capactively shunted junction model (RCSJ). The junctions showed excellent RCSJ-like behavior with ICRN products of up to 2.0 mV at 10 K and 220 μV at 77 K. To demonstrate the possibility to detect high-frequency radiation up to the THz regime, we used the Josephson junction as a detector for radiation from a far-infrared laser and we reconstructed the spectra by Hilbert spectroscopy.
Using high-Tc Josephson junctions made of YBa2Cu3O7−δ deposited across MgO bicrystal boundary, at terahertz (THz) frequency band, we demonstrated both fundamental and harmonic mixing. Radiation from a far-infrared laser was coupled to the junction, which was integrated with a planar bow-tie antenna, via an extended hyperhemispherical silicon lens. Fundamental mixing manifested itself in the junction’s dc current–voltage (I–V) curve as a third Shapiro step in addition to those two induced by the THz laser lines from a slightly misaligned resonator. In harmonic mixing between a THz laser line and a microwave local oscillator, the highest harmonic number we could get was 490 with a signal-to-noise ratio of 9 dB at the intermediate frequency.
Abstract This paper presents a wideband terahertz (THz) mixer based on a thin-film antenna-coupled high-temperature superconducting (HTS) YBa 2 Cu 3 O 7−x (YBCO) step-edge Josephson junction. The HTS mixer enables the flexible harmonic mixing operation at multiple THz bands with the same microwave local oscillator (LO) source, and features very wide intermediate-frequency or instantaneous bandwidth. In order to optimize the frequency down-conversion performance of the mixer, systematic electromagnetic design and analysis have been carried out to improve the power coupling of THz radiation as well as wideband transmission of microwave signals. Experimental characterization of a fabricated device prototype has demonstrated that the mixer exhibits good performance at both the 200 GHz and 600 GHz bands. Detailed measurement results including the DC characteristics, LO pumping requirement, frequency response, mixing linearity and conversion gain are presented in this paper.
Josephson junctions (JJs) embedded into the feed point of a thin film antenna made from yttrium barium copper oxide (YBCO) are well suited as a miniature sensor for GHzand THz-frequency fields. A spatially and frequency-resolved image of electromagnetic field intensity is obtained by scanning the region of interest. The sensor is read out by measuring the dc current-voltage (IV) characteristic of the JJ. Discrete frequencies together with their corresponding intensities can be extracted from measurements by evaluating the Shapiro steps in the IV domain. To extract a continuous spectrum from the IV characteristic, the numerical method of Hilbert transform spectroscopy is used and its applicability to our JJs is evaluated. As a benchmark of the setup's capabilities, radiation from a quantum cascade laser (QCL) THz source is characterized both spectrally and spatially. A frequency comb spectrum consistent with the QCL's cavity is revealed by Hilbert transform spectroscopy. The far field radiation patterns of two strong QCL modes are resolved individually and show different characteristics. Optical QCL feedback is observed when the sensor is brought in close proximity.
For sensitive wideband spectroscopy at THz frequencies with performance better than for Fourier Transform Spectrometer, one needs a wide-range electrically tunable, narrow linewidth THz source, and a sensitive broadband detector. To obtain a low noise level of sensor we use a superconducting normal metal cold electron bolometer (CEB). A voltage response of the bolometer is 4.108 V/W and an amplifier-limited technical noise equivalent power of the bolometer is 1.25.10-17 W/Hz1/2. The wide band electrically tuned terahertz band cryogenic oscillator is a Josephson junction. Contrary to conventional Josephson junctions based on niobium, and naturally limited in frequency by energy gap of about 700 GHz, the high critical temperature YBaCuO junctions can extend this limit by an order of magnitude. At 4 K such oscillators can radiate at frequencies exceeding 2 THz. Signals received by bolometers integrated with double-dipole and log-periodic antennas and irradiated by the Josephson oscillator show in our experiments the high frequency response corresponding to an oscillation frequency of up to 1.75 THz. Such cryogenic terahertz network analyzer is suitable for low-signal spectral evaluation of cryogenic detectors, quasioptical sub-mm cryogenic filters, planar antennas with planar microwave filters, integrated structures comprising such elements, and arrays of cryogenic receivers.