Exceptional Points of Degeneracy Directly Induced by Space-Time Modulation of a Single Transmission Line

Abstract

We demonstrate how exceptional points of degeneracy (EPDs) are induced in a single transmission line (TL) directly by applying periodic space-time modulation to the per-unit-length distributed capacitance. In such space-time modulated (STM) TL, two eigenmodes coalesce into a single degenerate one, in their eigenvalues (wavenumbers), and eigenvectors (voltage-current states) when the system approaches the EPD condition. The EPD condition is achieved by tuning a parameter in the space-time modulation, such as spatial or temporal modulation frequency, or the modulation depth. We unequivocally demonstrate the occurrence of the EPD by showing that the bifurcation of the wavenumber around the EPD is described by the Puiseux fractional power series expansion. We show that the first-order expansion is sufficient to approximate well the dispersion diagram, and how this 'exceptional' sensitivity of the STM-TL's wavenumber to tiny changes of any TL or modulation parameter enables a possible application as a highly sensitive TL sensor when operating at an EPD.