Resonant Mode Engineering of Photonic Crystal Sensors Clad with Ultralow Refractive Index Porous Silicon Dioxide

Porous SiO₂ (PSiO₂) with ultralow refractive index (n = 1.09) is incorporated as the cladding of a photonic crystal (PC) refractive index sensor with enhanced sensitivity through the establishment of resonant modes that principally reside in the liquid medium covering the PC surface. PSiO₂, obtained by thermal oxidation of porous Si that has been transferred to a transparent substrate, is transparent at visible and near infrared wavelengths with a refractive index determined by its porosity. The PSiO₂ periodic grating structure (Λ = 590 nm) is patterned by nanoimprint lithography and reactive ion etching, then conformally coated by sputtering high refractive index TiO₂ to seal the pores from liquid infiltration. With the refractive index of PSiO₂ much lower than that of water, the resonant mode “flips” its spatial distribution from within the solid dielectric regions of the photonic crystal to reside mainly in the water media covering the PC, resulting in 4× greater resonant wavelength shift for a fixed refractive index change. This study demonstrates design, fabrication, and testing of the sensor as a refractometer, supported by electromagnetic simulations of the resonant mode spatial distribution, in which porous PC sensors are compared to nonporous PC sensors.