  Breaking the reciprocity of electromagnetic interactions is of paramount
importance in photonic and microwave technologies, as it enables unidirectional
power flows and other unique electromagnetic phenomena. Here we explore a
method to break the reciprocity of electromagnetic guided waves utilizing an
electron beam with a constant velocity. By introducing an effective dynamic
conductivity for the beam, we theoretically demonstrate how nonreciprocal
guided waves and a one-way propagating regime can be achieved through the
interaction of swift electrons with electromagnetic waves in two-dimensional
(2D) parallel-plate and three-dimensional (3D) circular-cylindrical waveguides.
Unlike the conventional electron beam structures such as traveling wave tubes
and electron accelerators, here the goal is neither to generate and/or amplify
the wave nor to accelerate electrons. Instead, we study the salient features of
nonreciprocity and unidirectionality of guided waves in such structures. The
relevant electromagnetic properties such as the modal dispersion, the field
distributions, the operating frequency range, and the nonreciprocity strength
and its dependence on the electron velocity and number density are presented
and discussed. Moreover, we compare the dispersion characteristics of waves in
such structures with some electric-current-based scenarios in materials
reported earlier. This broadband tunable magnet-free method offers a unique
opportunity to have a switchable strong nonreciprocal response in
optoelectronics, nanophotonics, and THz systems.
