Helmut Kanter

High-frequency response of Josephson point contacts

For a current‐driven Josephson junction shunted by an Ohmic resistance, the dc voltage response and impedance to external high‐frequency currents is calculated with a second‐order perturbation method based on the unperturbed solution for the time evolution of the voltage as given by Aslamazov and Larkin. The response is proportional to signal power and has three characteristic‐frequency regions depending on whether the internal self‐generated Josephson frequency, ω0, is larger than, equal to, or smaller than the signal frequency, ω. For ω0 ω the response could be expressed by easily observable parameters of the dc voltage‐current characteristics. This permits comparison of the predictions with experimental results obtained on point‐contact junctions whose dc characteristics were only approximately represented by the simple model chosen for analysis. For ω0≪ω the response is found to be proportional to (i) the slope of the dc characteristic, (ii) the inverse square of the applied frequency, (iii) the ...

High‐detectivity GaAs quantum well infrared detectors with peak responsivity at 8.2 μm

GaAs quantum well infrared detectors with peak responsivity at 8.2 μm and significant response beyond 10 μm have been demonstrated with detectivities of 4×1011 cm (Hz)1/2 /W at 6 K; this detectivity is the highest reported for a quantum well detector. The detectors comprised 50 GaAs quantum wells of width 40 A with an average Si doping density of 1×1018 cm−3 separated by 280‐A barriers of Al0.28Ga0.72As. In this design, the state to which electrons are excited by infrared absorption and from which they are subsequently collected lies in the continuum above the energy of the Al0.28Ga0.72As conduction‐band minimum. The maximum detector responsivity was mesured to be 0.34 A/W. The device dark current density is 5.5×10−6 A/cm2 with the detector biased for maximum detectivity (3.5 V), and the dark current remains constant with increasing temperature up to 50 K. The detector noise current was observed to be a constant fraction (70%) of the shot noise down to noise currents of 10−14 A/(Hz)1/2. A theoretical mode...

Self‐Pumped Josephson Parametric Amplification

Parametric gain has been demonstrated at 30 MHz using the nonlinear inductance of Josephson point‐contact devices. SQUID‐type configurations were used to achieve degenerate‐mode negative‐resistance amplification with self‐pumping achieved via the internal Josephson oscillations. Gain as large as 11 dB was measured. The small‐signal analysis predicts parametric amplification and delineates the importance of the critical current, circuit loss, and circuit coupling efficiency.

A novel parametric negative‐resistance effect in Josephson junctions

The negative‐resistance effect recently predicted for current‐controlled highly damped Josephson junctions is experimentally demonstrated at a frequency of 9 GHz. Characteristically, the pump (or Josephson) frequency is smaller than the amplified one, and the pump mechanism is effective over a broad range of frequencies in contrast to the conventional parametric negative‐resistance effect where the pump frequency exceeds that of signal and idler and where the frequency of operation is limited to a narrow range. The broad‐band feature renders operation uncritical to tuning and bias voltage fluctuations. The fact that the junction is the element of lowest impedance in the circuit alleviates to a degree the matching problem generally encountered with Josephson contacts.

Parametric amplification with self-pumped Josephson junctions

The parametric nature of supercurrent in Josephson junctions may be exploited for amplification of high frequency signals in several modes of operation, where in each case the reactance variation is provided by the internal oscillations due to the average contact potential, a)One mode is negative resistance amplification with a single idler. This mode is entirely equivalent to that generally employed in conventional parametric amplifiers with varactor diodes. b)Negative resistance amplification with several idlers at further combination frequencies. This mode of operation is typical for the resistively shunted junction model, c)Amplification by upconversion, which is conceptually employed in the rf SQUID used in magnetometry. Experimental verifications of these various modes are described and their suitability for low noise amplification discussed. The upconverter experiment permitted the measurement of device noise by variation of the temperature of the input termination. An upper noise limit of 15°K is established. This experiment demonstrates that in high frequency application of active self-oscillating Josephson junctions the fundamental noise achievable is not greatly in excess of the thermal limit.

Two‐idler parametric amplification with Josephson junctions

A self‐pumped Josephson junction can be operated in a mode which is described in first order by a two‐idler parametric amplifier analysis for a sinusoidal elastance variation. Thus the analysis and operation follows the two‐idler negative resistance behavior predicted by Penfield and Rafuse for varactor diodes. Experimental observations on parametric negative conductance excitations at 9 GHz by a Josephson point contact are readily explained by the two‐idler model. A total signal power gain of 12 dB was achieved with an experimental device consisting of a niobium titanium point contact in a shallow rectangular cavity coupled through a room‐temperature circulator.

SHOT NOISE IN JOSEPHSON POINT CONTACTS

Experimental evidence is presented showing that the noise in Josephson point contacts is shot noise limited. Comparison with theory indicates that the pair current contribution is not as large as predicted.

SLOW‐ELECTRON BEAM ATTENUATION BY GOLD FILMS

An attenuation length of 34 A for electrons with 6‐eV energy above the Fermi level was found by penetration of slow‐energy electron beams through gold films about 200‐A thick. The collector current was determined for various primary energies as a function of film thickness, which was increased in situ by vapor deposition. The attenuation length, representative of electron‐electron collisions, decreased to 29 A, for an increase of energy of 1 eV.

Millimeter wave behavior of superconducting point contact SQUID

A superconducting point contact waveguide SQUID has been operated at 89 GHz. The performance is qualitatively the same as at lower frequencies in agreement with the SQUID model. With the Nb point located across a 0.15 mm high E band waveguide, reflection coefficient measurements were made as a function of both supercurrent phase as controlled by an externally applied magnetic field and millimeter wave power. Large parametric reactance variations were produced by changing the supercurrent phase. As the millimeter wave power is increased flux cycling was observed which involved up to four steps. This behavior corresponds to a flux transition time less than 10-12sec. Implications of the \cos \phi term and negative resistance effects in the observed behavior of the reflection coefficient will be discussed.

Response of superconducting point contacts to high frequency radiation

Abstract The high frequency response predicted by a perturbation approach for a highly damped current driven Josephson junction agrees in its parameter dependence remarkably well with observations on point contacts used as video type detectors of microwave radiation.