Emergent resonances in a thin film tailored by optically-induced small permittivity asymmetries
arxiv(2024)
Abstract
Resonances are usually associated with finite systems - the vibrations of
clamped strings in a guitar or the optical modes in a cavity defined by
mirrors. In optics, resonances may be induced in infinite continuous media via
periodic modulations of their optical properties. Here we demonstrate that
periodic modulations of the permittivity of a featureless thin film can also
act as a symmetry breaking mechanism, allowing the excitation of photonic
quasi-bound states in the continuum (qBICs). By
interfering two ultrashort laser pulses in the unbounded film, transient
resonances can be tailored through different parameters of the pump beams. We
show that the system offers resonances tunable in wavelength and
quality-factor, and spectrally selective enhancement of third harmonic
generation. Due to a fast decay of the permittivity asymmetry, we observe
ultrafast dynamics, enabling time-selective near-field enhancement with
picosecond precision. Optically-induced permittivity asymmetries may be
exploited in on-demand weak to ultrastrong light-matter interaction regimes and
light manipulation at dynamically chosen wavelengths in lithography-free
metasurfaces.
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