"Generation and receiving of mm and submm waves by multijunction Josephson structures"

The goal of the project

The goals of the project are development of design theoretical basis, fabrication and experimental investigation of multyjunction Josephson structures suitable for generation and receiving of weak electromagnet radiation of millimeter and submillimeter wave length. Mulyjunction Josephson structure topology will be designed and dynamic properties of the structure (microwave impedance, spectral characteristics, electrical response on external millimeter or submillimeter wave irradiation, Josephson self-generation) will be tested.

Background

The multielement josephson structures are known to allow implementation of the unique properties of the Josephson junctions as electromagnetic waves sources with high change rate of generation frequency, working in the millimeter and submillimeter wavelength diapasones. Especial unterest presents the application of the Josephson generation for the self-pumpingin the Josephson structures operating in the mode of supersencitive detection of external radiation. Both these application demand substantial decrease of the bandwidth of the intrinsic Josephson generation and can be achieved by mutual sincronisation of the Josephson oscillations of junctions in a multielement structure. An optimal connection of the Josephson junctions in the generation mode allows bandwidth as low as 1 MHz with a generated power of 1 mW in the millimeter and submillimeter wavelength diapasones. For the self-pumping mode of a receiver the noise temperature T_N less than the quant energy: kT_N<hf. The results close to this theoretical prediction were obtained for the arrays of 2000 Josephson junctions, formed in a Nb films.

Still the problem of fabrication of the syncronised multielement Josephson structures is far from its solution. Even the optimal type of the junction electrodynamic interconnection isn't chosen. The experimental studies are limited to the 1D arrays of junctions connected in series with the distance between the junctions equal to the half of the wvaelength of the first harmonics of the Josephson generation, and to the simple 2D arrays of Josephson junctions. Implementation of the HTSC Josephson junctions is even more complicated due to low reproducibility of the junction parameters along the substrate, resulting mainly from underdeveloped junction preparation technology.

Substantial progress in understanding of the syncronisation processes in the multielement Josephson structures was achieved in last three years. Modification of the PSCAN superconducting circuit modelling program complex allowed systematic study of the syncronisation processes in an elemental cell of a multielement structure for different frequencies, cell and junction parameters. These studies allowed determination of the optimal configurations of an elemental cell. First one consists of two junctions and interconnects, allowing both current flow both shunting the junction and coupling the two junctions. The second optimal configuration is the four-junction interferometer (superconducting circle with four junctions in it), especially important for the @D arrays of Josephson junctions. Both optimal configurations provide syncronisation of all junctions in the multielement structure for parameter (junction critical current) spread up to 60% at frequencies close to the characteristic of the josephson junction. Estimations of dynamics of the multielement structure made of Nb junctions with two-junction cells show possibility of the tunable generators for the 0.5...1.0 THz diapasone with the minimal generation frequency 3-4 times lower than the maximal generation frequency. Application of the HTSC junctions in structures of four-junction interferometers allow preparation of generators for frequencies higher than 1 THz due to higher I_cR_N product compared with than of Nb junctions.

Experimental studies were mainly performed using the step-edge Josephson junctions, allowing fabrication of the multijunction circuits in a single superconducting layer. The experiments showed, though, that even the single Josepson junction dynamics substantially differs from the simple resistively-shunted junction (RSJ) model, that is valid usually for the Nb junctions. In particular, the external irradiation provides subharmonic steps on the IV curves, and the detector responce contains subharmonic peculiarities. Such HTSC junction behavior can be explained by high inhomogeneity of the junction area on the scale much lower than the junction geometrical width. As a result the junction can be modelled as an array of parallel-connected junctions, each of which operates in accordance with the RSJ model.

Project objectives

The possibility of fabrication of the syncronised multielement Josephson structures, operating as generators and supersencitive detectors at 1 THz and higher frequencies, will be studied theoretically and experimentally:

• the theoretical study will optimize the syncronised multielement Josephson structures, and will study the possibility of decrease of the bandwidth of Josephson generation due to increase of number of elements in the multielement structure;

• the theoretical studies will be performed both using the analytical methodics, and by process dynamics modelling using the PSCAN program complex. Additional improvements, simulating noise effect on the superconducting circuit, will be programmed. Microscopic junction model by Werthammer will be applied instead of the simple RSJ macroscopic model;

• the goal of the experimental investigation will be fabrication of the most promicing multielement structures and measurement of their main properties. Both Nb-based structures and structures of HTSC will be fabricated;

• technological studies will be concentrated on preparation of the HTSC multielement structures on the sapphire substrate, most suitable for the high-frequency applications, using the CeO₂ buffer layer.