Surface Leaky Transverse-Magnetic (TM)-Mode Resonance and Photonic-Enhanced Photoluminescence in Shallow Subwavelength Silicon Gratings

All-dielectric shallow silicon subwavelength gratings (SWGs) are demonstrated as a powerful platform for overcoming the fundamental waveguiding cutoffs of conventional bound transverse-magnetic (TM) modes, enabling strong light-matter interactions directly at the SWG-ambient interface. By leveraging laser interference lithography to fabricate ultrashallow silicon profiles, this work uncovers a unique regime where angle-dependent resonant anomalies emerge not from core-confined guided modes, but from the deliberate excitation of polarization-sensitive surface leaky modes operating strictly at the Wood–Rayleigh diffraction threshold. Finite Element Method (FEM) simulations validate that this threshold configuration can act as a giant electromagnetic energy concentrator at the interface, driving asymmetric Fano resonances and producing an exceptional enhancement of the local density of optical states along the grating ridges. Under p-polarized ultraviolet excitation, this localized field distribution triggers a dramatic Purcell acceleration of radiative decay pathways, enabling the detection of intense room-temperature photoluminescence from submonolayer organic residues with a giant ∼2 eV Stokes shift, resulting in an emission that is entirely quenched on unpatterned bulk silicon. These findings elucidate the physical mechanism of threshold-driven field localization in open dielectric nanostructures and highlight their transformative potential for ultrasensitive, label-free biomolecular optical detection.