A Time Grating Approach to Ultrahigh‐Q Guided Mode Resonance

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–Hänchen shift over 10 3 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.