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IEEE Transactions on Applied Superconductivity

THz Microscopy of Quantum Cascade Laser Radiation Using Josephson Junctions

Julius Mumme, Paul Julius Ritter, Sebastian Gloor, Max Pröpper, Valerio Digiorgio, Marc-André Tucholke, Mattias Beck, Giacomo Scalari, Meinhard Schilling, Benedikt Hampel

Published Jan 12, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2026.3652510

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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.
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.
Superconducting Josephson junctions have major advantages as detectors of millimeter wave radiation. Frequency of the radiation can be easily derived from the Shapiro steps of the current-voltage characteristics. However, system performance is highly sensitive to impedance mismatch between the antenna and the junction; therefore, optimization is essential. We analyzed and implemented an improved antenna structure, in which the junction is displaced from the antenna center and placed between the ends of two matching strips. Based on theoretical analysis and advanced electromagnetic simulations, we optimized strip dimensions, which affect both the detection magnitude and the frequency of the reflection coefficient dip. Accordingly, two Au bow-tie antennas with different matching strip widths were fabricated. Superconducting Yttrium Barium Copper Oxide (YBCO) thin films were deposited exactly at the bicrystal substrate misorientation points, forming Josephson junctions at the ends of two matching strips. We found a very high correlation between the simulations and the response to Radio Frequency (RF) radiation in the range of 145–165 GHz. Experimental results agree extremely well with the design, showing best performance of both antennas around the frequency for which impedance matching was derived.
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.
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 have investigated the Josephson radiation from different types of YBa 2 Cu 3 O 7 (YBCO) thin-film Josephson junctions: step-edge junctions (SEJs), biepitaxial junctions and SNS junctions with N=Au. The radiation was detected using a non-resonant radiometer system with a receiving frequency of 11-12 GHz. The current-voltage characteristics were measured simultaneously with the radiation spectra in the temperature range 4.2-90 K. All junctions exhibited a large emission peak at a voltage which was related to the frequency through the second Josephson relation. Typically, for high temperatures, and therefore small critical currents, the measured radiation linewidth agreed well with the theoretical predictions. At lower temperatures the experimental linewidths deviated from the theoretical values due to additional noise sources in the junctions. Some of the SEJs showed a non-monotonic dependence of the linewidth on temperature. It is suggested that such SEJ data may be discussed in terms of a model which treats the SEJ as an interferometer consisting of a parallel array of random Josephson junctions.
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.