<p>In this study, an all-dielectric etalon-based metasurface composed of TiO₂ nanopillars embedded in polydimethylsiloxane is proposed and numerically optimized for high-performance refractive index sensing. The proposed etalon configuration efficiently confines the electromagnetic field within the cavity formed by two nanopillar arrays, resulting in a remarkable enhancement of both the Q-factor and the figure of merit (FOM) compared to one-sided structures. Simulation results reveal that the proposed etalon-based metasurface achieves a Q-factor of 5513.5 and a FOM of 1237 RIU⁻¹, outperforming previously reported dielectric metasurface sensors. The analysis of the electromagnetic field distribution indicates that the strong confinement within the etalon cavity is responsible for the significant enhancement of the Q-factor and figure of merit (FOM). The integration of an etalon architecture with all-dielectric TiO₂–PDMS materials provides an optimal balance of sensitivity, Q-factor, and FOM, offering a promising platform for next-generation refractive index sensors with high spectral resolution and detection accuracy.</p>