A 1.5D Photonic Crystal Interface for Prism-Free, Near-Normal, Low-Background Photoluminescence

Photonic crystal gratings (PCGs) and photonic crystal Bragg mirrors (PCMs) are widely used to enhance photoluminescence (PL) in fluorescence-based biosensing, yet most implementations either rely on bulky prism coupling or suffer from strong background from uncoupled spontaneous emission, limiting system integration. Here, we experimentally demonstrate an all-dielectric PCG-PCM interface in which a TiO₂ ridge-groove PCG is fabricated atop a TiO₂/SiO₂ Bragg mirror to form a 1.5D photonic crystal (PC). Bound states in the continuum-enabled resonances in the PCG increase PL spectral flux, polarization selectivity, and near-normal outcoupling, while the PCM suppresses broadband background via stopband filtering and improves PL extraction by recycling downward radiation through radiative boundary-condition engineering. Relative to standalone PCGs or PCMs, the interface achieves up to 5.2-fold improvement in signal-to-background ratio, supported by angle-resolved spectroscopy, back-focal-plane imaging, and numerical simulations. This 1.5D PC establishes a scalable, compact route to high-contrast fluorescence readout for next-generation biosensing and bioimaging.