Abstract
The steep slope of the asymmetric Fano resonance offers potential for enhancing signal readout in compact photonic sensors across gas and liquid environments. However, achieving and controlling Fano resonance shapes on ultra-compact, fabrication-constrained platforms, particularly across variable claddings, remains challenging. We demonstrate a CMOS-compatible Si(3)N(4) photonic platform based on a photonic crystal nanobeam (PhCN) side-coupled to a racetrack microring resonator (MRR), enabling engineered Fano resonances through passive geometric control. By varying the PhCN length and coupling gap, we systematically modulate the interference conditions that define resonance asymmetry and slope. Numerical and experimental results under both air and aqueous claddings show that the cladding-dependent modal transition, from leaky (air) to guided (liquid) backgrounds, enables robust, geometry-driven Fano behavior. A temporal coupled-mode theory model supports the results. The fabricated devices show steep asymmetric lineshapes, with [Formula: see text]>5[Formula: see text]10(3), ER > 14dB (up to 20 dB maximum across all devices), [Formula: see text] > 0.4, and slope responsivity >5 nm(–1)(or 40–50 dB/nm), all within a compact footprint of ~40 × 34 µm(2). The performance is comparable to similar MRR-based Fano implementations. This work provides a reproducible strategy for slope-optimized, passive Fano devices suitable for intensity-based refractive index sensing in lab-on-chip systems operating under variable cladding conditions without requiring ultra-high [Formula: see text] or extreme ER. Thus, it serves as a design framework for future implementations. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1038/s41598-026-35490-w.