Emergent resonances in a thin film tailored by optically-induced small permittivity asymmetries

TL;DR

This paper demonstrates that periodic optical modulations in a featureless thin film can induce and control resonances, including quasi-bound states in the continuum, enabling ultrafast, tunable light-matter interactions and enhanced nonlinear effects.

Contribution

It introduces a novel method of inducing and tuning resonances in a continuous thin film via optically-generated permittivity asymmetries, enabling dynamic control of photonic states.

Findings

Resonances can be optically tailored in wavelength and quality-factor.

Spectral enhancement of third harmonic generation observed.

Ultrafast, picosecond-scale near-field enhancement achieved.

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 $\textit{quasi}$-bound states in the continuum ($\textit{q}$BICs). 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.