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Description
Scattering by dielectric resonant nanoparticles has been widely explored as a route to tailoring electromagnetic responses, yet most studies focus on steady-state behaviour. Here, we investigate the transient scattering response of Mie-resonant dielectric metastructures driven by short sub-picosecond pulses, extending resonant metaphotonics into the time domain.
We develop an analytical framework based on resonant state expansion of the time-dependent scattering Mie coefficients into exact contributions from the quasi-normal modes of the resonant structure, using recently established normalization, orthogonality, and completeness relations. The theory is validated against full-wave FDTD simulations in Ansys Optics and further applied to a CMOS-compatible metastructure design.
For a silicon sphere, we show that resonant excitation produces scattered-power dynamics dominated by a single quasi-normal mode, with exponential energy loading and release during the transient response. In contrast, in the anapole regime, sharp scattering bursts appear at pulse switch-on and switch-off due to energy trapping and release through a dynamic scattering dark state, analogous to metamaterial-induced transparency. These peaks arise from interference between a single quasi-normal mode and a broadband non-resonant background, and their temporal bandwidth is set by the quasi-normal-mode lifetime, making it much shorter than the driving pulse. Similar resonant and anapole dynamics are found in forward and backward radar scattering and are reproduced in a practical silicon cylinder on glass. These results reveal unconventional transient regimes in dielectric metastructures with potential applications in ultrafast photonics, pulse compression and shaping, and nanoparticle sorting.