The theory of photon-assisted tunneling is outlined. The I–U characteristic with and without radiation and the junction conductance are evaluated for the temperature of 4.2 K. From these results the current responsivity ηc and the voltage responsivity ηv are derived. At a frequency of 72 GHz the theory yields the maximum values ηc = 3.3 × 103 A/W and for a 100Ω junction ηv = 2.5 × 108 V/W. Leakage current and other shortcomings of the real junctions reduce these values to ηc ≈ 3 × 103 A/W and ηv ≈ 106 V/W.



Die Theorie des Photonen-unterstutzten Tunnelprozesses wird skizziert. Die I–U-Kennlinie mit und ohne Einstrahlung sowie die Leitfahigkeit werden fur die Temperatur von 4,2 K berechnet. Aus diesen Resultaten lassen sich Stromempfindlichkeit ηc und Spannungsempfindlichkeit ηv ableiten. Fur die Frequenz von 72 GHz ergeben sich folgende Bestwerte: ηc = 3,3 × 103 A/W und fur eine 100Ω SIS-Struktur ηv = 2,5 × 108 V/W. Leckstrom und andere Mangel der Strukturen reduzieren diese Werte auf ηc ≈ 3 × 103 A/W und ηv ≈ 106 V/W.

Responsivity of SIS tunnel junctions to mm‐wave radiation

In this chapter a few selected application possibilities of Josephson junctions in microwave engineering will be described. Attention will be focussed on Josephson voltage standards, since they have already found a secure place in the technique, namely in precision measurement technique. Josephson junctions as components in microwave receiver devices indeed reveal very interesting possibilities and have often already been successfully tested in the laboratory, but they are not yet in regular use. They are accordingly treated here in somewhat less detail.

Applications of Josephson Junctions in Microwave Engineering

The fabrication of a planar fin-line circuit for rf-coupling to Josephson tunnel junctions and SIS devices is reported. E-field coupling is achieved by means of tapered transitions from a rectangular waveguide to a fin-line and from this line to the active device. Its low impedance can be matched by an antipodal fin-line with a fin-separation in the μm-range. Circuit parameters have been determined for Nb-Pb-devices mounted in a split Ka-band waveguide. Measurements were performed in the frequency range from 33 GHz and 38 GHz. Mismatches as low as 10 dB have been observed.

Broadband RF-coupling to thin-film Josephson and SIS devices

A study was made on Nb based Josephson junctions with evaporated Te barriers. As the first step, a Nb-Te-Pb configuration was tested. The smooth surfaced Nb electrode enabled the formation of a pinhole free barrier. Supercurrent density was found to be large compared to that for surface oxidized junctions. As the second step, a Nb-Te-Nb structure was tested. Although the junction uniformity was not as good as that of the Nb-Te-Pb junctions, the aging property was drastically improved. Considerations were made on the origin of the remaining aging. The junctions showed excellent response to 50 GHz signals.

Nb based Te barrier Josephson junctions

An experiment in video signal detection at 70 GHz was conducted with a Pb micro-bridge prepared by electron-beam exposure and the lift-off method. It was found that for the dc Josephson effect the external noise is dominant over the internal noise of the specimen and that the equivalent noise becomes large near the critical temperature. The response to 100 kHz amplitude-modulated signals was found to be ten times more rapid than that of the superconductor bolometer. Theoretically the maximum responsivity is obtained near the first Shapiro step, which, however, does not give a stable bias; maximum responsivity was actually observed near the zeroth Shapiro step, where it exceeded 6.3 × 104 V/W. The responsivity was found to increase with increasing IcRn.

K.H. Gundlach

Papers

The optimization of a Josephson video-detector for millimetre wavelengths