Universal radiation dynamics by temporal transitions in optical waveguides

• Main Authors: Shlivinski, Amir, Hadad, Yakir
• Format: Journal Article
• Language: English
• Published: 17-09-2024
• Subjects: Physics - Optics
• DOI: 10.48550/arxiv.2409.11526

When an excited electromagnetically open optical waveguide goes through a
temporal transition of its material properties, it radiates to the ambient
surroundings. In this letter, we explore this radiation and reveal, using
asymptotic evaluation of path integral in the complex frequency (Laplace)
plane, a peculiar space-time dependence of its frequency. Specifically, we
derive an exact formula (Eq. (11)) for the instantaneous radiation frequency,
which exhibits a chirp behavior with respect to time. This simple formula
depends on the ambient properties and on the longitudinal wavenumber \beta of
the guided mode before the temporal transition but not on the specific
waveguide structure or materials. In addition, we derive a t^(-3/2) decay rate
of the radiative field on time. We verify our analytic results using full-wave
simulations of a dispersive and lossy Indium Tin Oxide waveguide that undergoes
smooth temporal long transitions over up to ~200 cycles at the initially guided
mode frequency. Thus, these theoretical findings offer valuable insights into
the behavior of general optical waveguides experiencing temporal transitions
and provide a powerful tool for analyzing and designing such THz and optical
setups, with potential use in sensing and imaging.