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IEEE Transactions on Applied Superconductivity

Josephson Junction Radiators as Spectral Calibrators of Millimeter-Wave Low-Temperature Detectors

Johannes Hubmayr, Shannon M. Duff, Robert McDermott, Corwin Shiu, Gabriel Spahn, David Harrison, Joel N. Ullom, Anna K. Vaskuri

Published Apr 3, 2026• 0 citations
Publisher

Abstract

The Josephson junction radiator (JJR) is a thin-film superconducting device that converts DC voltage into coherent millimeter-wave (mm-wave) radiation. The source is frequency-tunable, instantaneously narrow-band, compact, lightweight, has no moving parts, and may share the same cryogenic and vacuum space as the device under test. With these favorable characteristics, the JJR may find use as a general purpose calibration tool for low temperature mm-wave detectors. We show that the JJR unambiguously emits mm-wave radiation and couples to a mm-wave detector. We use the JJR to determine the spectral response of a superconducting mm-wave detector that operates in two large frequency passbands centered at 150 GHz and 220 GHz. A comparison to measurements with a conventional Fourier Transform Spectrometer is presented, and the use of the JJR as a precision spectral calibration tool is discussed.

DOI
10.1109/tasc.2026.3680387

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IEEE Transactions on Magnetics•Mar 1, 1991•4 citations
The Josephson junction radiator (JJR) is a thin-film superconducting device that converts DC voltage into coherent millimeter-wave (mm-wave) radiation. The source is frequency-tunable, instantaneously narrow-band, compact, lightweight, has no moving parts, and may share the same cryogenic and vacuum space as the device under test. With these favorable characteristics, the JJR may find use as a general purpose calibration tool for low temperature mm-wave detectors. We show that the JJR unambiguously emits mm-wave radiation and couples to a mm-wave detector. We use the JJR to determine the spectral response of a superconducting mm-wave detector that operates in two large frequency passbands centered at 150 GHz and 220 GHz. A comparison to measurements with a conventional Fourier Transform Spectrometer is presented, and the use of the JJR as a precision spectral calibration tool is discussed.
Although a number of investigators have generated microwave radiation by means of properly biased Josephson junctions, little work has been reported on similar applications to the submillimeter wave region where tunable coherent oscillators are not yet available. We have generated continuously tunable millimeter and submillimeter wave radiation with a point contact Josephson junction in a superconducting transmission line. This transmission line is centered in an oversized multimode brass cavity. With a broad band detector placed in the brass cavity near the transmission line radiation is detected for all bias voltages below 2 mV. The maximum power actually absorbed by the detector is about 2 × 10-10 W. In order to analyze the frequency content of the radiation we have used a lamellar interferometer together with Fourier transform spectroscopic techniques. The amount of power detected in the remote detector cryostat is two orders of magnitude smaller than that detected adjacent to the junction. Only at bias voltages where a particularly large amount of radiation is emitted are spectroscopic measurements possible. For these cases the frequency of the radiation is that predicted from the bias voltage and the Josephson relation. The linewidth is less than the resolution of the interferometer. To see if the radiation is continuously tunable a low temperature spectrometer has been constructed. The instrument consists of a germanium bolometer cooled to 0.3 K by a He3 charcoal adsorption pump, a four position sample rotator at 1.2 K and a Josephson junction transmission line source. By sweeping the bias voltage from zero to 2 mV absorption line spectra have been recorded in three different crystals, they are : 1. the antiferromagnetic resonance at 8.7 cm-1 in MnF2 ; 2. the impurity mode absorption at 14.3 cm-1 in CsI : Tl and 3. the impurity mode absorption at 18.0 cm-1 in KBr : Li. A low temperature double beam spectrometer is now under construction.
The intrinsic response of Josephson junctions to microwave and far-infrared radiation is studied. Junctions are formed by pressing a Nb or Nb-Ti pointed wire onto a Nb thin film which in turn is pressed against the open end of a waveguide. Electromagnetic radiation is applied through the back of the film so that only the wave magnetic field is coupled to the junction. The change of the dc Josephson current is monitored at different levels of microwave power over a frequency range from 22 to 105 GHz. The dependence of the responsivity of these currents on film thickness over a range from 50 to 500 Å is studied. The observed responsivity agrees very well with that predicted by theory. Higher responsivity is observed for thinner films and lower microwave frequencies. The observed responsivity of a 50-Å film junction at 22 GHz is (6±3) ×10−2 V/W. To facilitate comparison of our results with other experiments, a more conventional coupling configuration in which the wave electric field rather than the wave magnetic field is coupled to the junction is studied. The results are comparable with other experiments with the responsivity in the E field coupling case 106 times larger than in the B field coupling configuration. We demonstrate that for successful use of simple Josephson junction detectors in the microwave and millimeter region it seems crucial to impedance match the junctions to the radiation source, probably by making them a part of a resonant structure.
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.
A millimeter and submillimeter microwave source is described in which a point-contact Josephson junction is used as both the emitter and as a homodyne detector of the microwave radiation. The microwave radiation is conveyed from the Josephson junction to the room-temperature environment outside the Dewar of liquid helium by an oversize waveguide. A room-temperature Fabry-Perot resonator refocuses the radiation on the oversize waveguide which returns the radiation to the emitting junction which also serves as a coherent detector with sensitivity 10-15W/√Hz. The detector is sufficiently sensitive that the emitted power of 10-12W can be detected with high signal-to-noise ratio. Power required by the junction is of the order 10-6W from the bias supply. For the experiments reported, the wavelength of the emission could be varied over discrete wavelengths between 1.1 and 2.6 mm by varying the voltage bias across the junction. These wavelengths corresponded to the resonant frequencies of a cavity tightly coupled to the Josephson junction, and the frequencies can be changed by modifying the geometry of the cavity.
We have demonstrated the first applications of Josephson-effect-based Hilbert-transform spectral analysis using high-T/sub c/ Josephson junctions. YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary Josephson junctions have been fabricated on [110] NdGaO/sub 3/ bicrystal substrates for these measurements. At first, spectra of Josephson radiation have been measured by the Hilbert-transform technique in the range 60-800 GHz. A Lorentzian shape of the Josephson radiation and the possibility to obtain a spectral resolution as low as 0.1 cm/sup -1/ in the submillimeter-wave range has been demonstrated at liquid nitrogen temperature. Moreover, spectra of high-harmonic content of the radiation coming from commercial millimeter-wave oscillators have been measured in the spectral range from 60 to 450 GHz. A comparison of Hilbert-transform spectral analysis with conventional techniques is given.
The coupling of millimeter-wave radiation from a long Josephson junction to a small-area junction is discussed. The coupling is obtained by means of thin-film technology and the design parameters allowing radiation coupling are discussed. Since the frequency of the oscillator radiation is smaller than the plasma frequency of the detector junction, the latter shows evidence of chaotic dynamics in the current-voltage characteristic. However, very stable zero-crossing bias steps appear for high values of the applied radiation power. The tunability of the oscillator power allows measuring the dependence of the detector critical current on the external radiation to be measured; from this measured dependence and from the observed amplitude of the photon-assisted tunneling steps it is estimated that maximum coupled power is the range of tens of nanowatts.