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.
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.
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.
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.
For sensitive wideband spectroscopy at TeraHertz frequencies one needs a wide-range electrically tunable THz source and a sensitive detector. In this paper a superconducting normal metal cold electron bolometer (CEB) was used as a broadband sensor. Bolometers were integrated with broadband log-periodic antenna designed for 0.2-2 THz frequency range and double-dipole antennas designed for 300 and 600 GHz central frequency. A Josephson junction was used as a wide band electrically tuned terahertz cryogenic oscillator. Bicrystal YBaCuO Josephson junctions demonstrated a characteristic voltage IcRn of over 4 mV that corresponds to characteristic frequency about 2 THz. The bolometer chip is attached to a Si substrate lens at 260 mK and the oscillator chip is attached to the sapphire substrate lens at 1.8 K, with lenses facing each other at the distance of few centimeters. High signal was measured in the whole frequency range up to 1.7 THz by simple changing the bias voltage of Josephson junction from zero to 3.5 mV. A voltage response of the bolometer up to 4*108 V/W corresponds to an amplifier-limited technical noise equivalent power of the bolometer NEP=1.25*10-17 W/Hz1/2. Combining a Terahertz band Josephson junction, a high-sensitive hot electron bolometer, and a sample under test in between, makes it possible to develop a cryogenic compact Terahertz-band transmission spectrometer with a resolution below 1 GHz corresponding to the linewidth of Josephson oscillations. For frequencies below 600 GHz a conventional Nb shunted SIS junction can be used as Josephson oscillator.
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.
Extensive measurements of radiation emitted from long Josephson junctions d.c.-current biased on zero-field and Fiske-steps are reported. The measurements were made in the band 8-10 GHz on long Nb-Nb oxide-Pb overlap tunnel junctions with the lengths 0.5 mm and 1.0 mm having fundamental frequencies of 10 GHz and 5 GHz, respectively. Depending on the fundamental frequency of the junction 1st or 2nd harmonic detection was employed. In the fundamental scheme the frequency f n vs. the d.c.-voltage V n along the steps follows a modified Josephson voltage/frequency relation f n / V n = 2 e/nh , where n is the step-number n = 1, 2, 3,... for Fiske-steps and n = 2, 4, 6,... for zero-field steps. On a given step the frequency could be tuned (200-500 MHz) by varying the bias point. The linewidth was measured to be less than 5 kHz. All the observed phenomena are consistent with a novel unified dynamic bunched fluxon model applicable to both zero-field and Fiske-steps and are in good agreement with recent numerical solutions of the perturbed sine-Gordon equation with proper boundary conditions. Taking advantage of operation on higher order steps and strong harmonic generation the present upper frequency limit for the Josephson fluxon oscillator is above 200 GHz. The maximum power observed so far is only 5 pW, but the estimated available power is of the order 1 μW, which is quite sufficient to pump other Josephson devices.
We report heterodyne measurements of Josephson microwave radiation emitted by a parallel array of small superconductor-insulator-superconductor (SIS) junctions at submillimeter-wave frequencies. The array consists of five Nb/Al–AlOx/Nb junctions nonevenly distributed in a niobium superconducting stripline, and is optimized for rf coupling in the 450–640 GHz range. We observed Fiske-like resonant steps in its I-V curve in the presence of magnetic field. The device was placed in a waveguide mount, and its radiation was quasioptically coupled out of the cryostat, to a SIS-mixer spectrometer in the same frequency range, with a 4–8 GHz band for spectral analysis. We detected a coherent signal in the spectra when the array was biased on the first and third steps, respectively, at the first harmonic frequency of 242 GHz and at the fundamental frequency of 493 GHz, both being the Josephson frequencies associated with their dc voltages. This strongly suggests that this type of parallel arrays optimized for wideband rf coupling, though strongly discretized, hosts dynamic fluxon regimes similar to long Josephson junctions, which could find applications in superconducting digital electronics or integrated heterodyne submillimeter-wave receivers.