Geometrical control of transverse electromagnetic wave propagation in nonuniform microwave superconducting transmission lines

Abstract

It is well known that photonic structures with subwavelength features can be homogenized and thus be accurately approximated by homogeneous yet spatially dispersive structures. This idea is here applied to nonuniform superconducting transmission lines with subwavelength nonuniformities, i.e. subcentimeter features in the microwave regime. A closed form expression is found for the equivalent characteristic impedance and propagation constant of a uniform transmission line that can accurately model the transverse electromagnetic (TEM) propagation within nonuniform superconducting transmission lines with subwavelength inhomogeneities. It is shown that electromagnetic wave propagation, in particular the group velocity of propagating TEM waves, can be well controlled by introduction of subwavelength (subcentimeter) geometrical variations within the line. This ability is of great importance in design of matching stages. In our numerical examples, quasi-TEM superconducting microstrip lines with a superconductive strip of varying width are considered and the group velocity of the quasi-TEM modes is controlled by duly varying the line width. The obtained results are validated by being compared against the well-known technique of stair-case approximation whereby the considered nonuniform line is sliced into several sections each being small enough to be considered uniform.