Multi-mode Photonic Time Crystals Based on Time-Modulated Metasurface Waveguides

TL;DR

This paper introduces a multimode photonic time crystal platform using time-modulated metasurface waveguides, enabling novel intermodal band gaps and directional wave amplification.

Contribution

It demonstrates a versatile, experimentally accessible multimode platform supporting both guided surface and volume modes with novel intermodal band gaps.

Findings

Intermodal band gaps arise from coupling between different guided modes.

Tilted intermodal band gaps are not restricted to half the modulation frequency.

Phase control enables selective suppression or enhancement of band gaps.

Abstract

Photonic time crystals are electromagnetic media with periodically time-varying parameters, enabling momentum band gaps, parametric amplification, and frequency conversion beyond what is possible in time-invariant systems. So far, they have been explored mainly in single-mode systems, which limits the range of accessible physical phenomena. Here, we introduce an impenetrable metasurface waveguide as a multimode time-varying platform supporting both guided surface modes and higher-order guided volume modes. We show that temporal modulation in this platform gives rise not only to conventional intramodal band gaps associated with same-branch coupling, but also to tilted intermodal band gaps originating from coupling between different guided-mode branches. Unlike intramodal band gaps, these intermodal band gaps are not restricted to half the modulation frequency and can enable directional wave amplification, where the amplified field carries energy along the waveguide even inside the band gap. We further show that the modulation phase difference provides an effective symmetry-control parameter: by exploiting temporal glide symmetry, one can selectively suppress or enhance gap opening for interactions between modes of the same or different symmetry. These results establish a versatile multimode platform for photonic time crystals, offering one of the simplest and most experimentally accessible routes to tilted band gaps compared with volumetric dispersive PTC implementations and, more broadly, opening new opportunities for time-varying electromagnetic systems.