A time grating approach to ultrahigh-Q guided mode resonance

TL;DR

This paper introduces a novel time modulation method for guided mode resonance, enabling tunable, high-Q resonances with enhanced reflection properties without relying on traditional spatial patterning.

Contribution

It presents a homogeneous waveguide with periodic temporal refractive index modulation to achieve tunable GMRs, diverging from traditional patterned structures.

Findings

Q-factors diverge as modulation depth decreases

Resonances exhibit near-unity reflection for fundamental harmonics

Achieves giant Goos-Hänchen shift exceeding 103 wavelengths

Abstract

Guided mode resonance (GMR), the resonant coupling of free-space light into leaky waveguide modes, is traditionally achieved with periodic patterned structures. However, this approach makes its key properties such as quality factor (Q-factor) fabrication-dependent and non-tunable. Here, we introduce a time grating platform, i.e., a homogeneous waveguide whose refractive index is modulated periodically in time, that allows tunable GMRs through temporal modulation engineering rather than spatial structural redesign. We show that the Q-factors of these GMRs diverge as the modulation depth vanishes. Furthermore, unconstrained by energy conservation, the resonances exhibit near-unity reflection for fundamental harmonics and values exceeding 40 for first-order harmonics. Our findings not only apply to yield a giant Goos-H\"anchen shift over 103 times wavelength without sacrificing the reflection magnitude, but also open new avenues for related phenomena such as bound states in the continuum, unidirectional GMRs and beyond.