Humidity sensing based on nanoporous polymeric photonic crystals

In this work, an optical humidity sensor based on a nanoporous polymeric photonic crystal (PC) is demonstrated. The PC sensing structure is created by combining a holographic interference patterning technique with a modified holographic polymer-dispersed liquid crystal system. Changes in relative humidity (RH) induce the modification of the refractive index contrast between the nanoporous and nonporous regions, and thus the transmittance and bandgap position, of the PC structure. For a PC structure with 30% porosity and a grating spacing of 220 nm, a change in the RH from 40% to 95% at 34 °C results in a redshift of 43 nm in the central wavelength at the PC bandgap and an increase from 12% to 87% in the relative transmittance at λ = 600 nm. Other performance analyses have shown that the nanoporous polymeric PC-based humidity sensor is highly stable and reproducible, exhibits minimal hysteresis, and responds relatively fast.