Near-Infrared Silicon Photonic Crystals with High-Order Photonic Bandgaps for High-Sensitivity Chemical Analysis of Water-Ethanol Mixtures.

Aqueous solutions of alcohols are used in several applications, from pharmaceutics and biology, to chemical, biofuel, and food industries. Nonetheless, development of a simple, inexpensive, and portable sensing device for the quantification of water in water-ethanol mixtures remains a significant challenge. Photonic crystals (PhCs) operating at very high-order photonic bandgaps (PBGs) offer remarkable opportunities for the realization of chemical sensors with high sensitivity and low detection limit. However, high-order PhC structures have been mostly confined to mere theoretical speculations so far, their effective realization requiring microfabrication tools enabling the control of periodic refractive index modulations at the submicrometric scale with extremely high accuracy and precision. Here, we report both experimental and theoretical results on high-sensitivity chemical analysis using vertical, silicon/air 1D-PhCs with spatial period of 10 and 20 μm (namely, over 10 times the operation wavelength) featuring ultra-high-order PBGs in the near-infrared region (namely, up to 50th at 1.1 μm). Fabrication of high-order 1D-PhCs was carried out by electrochemical micromachining (ECM) of silicon, which allowed both surface roughness and deviation from vertical of etched structures to be controlled below 5 nm and 0.1%, respectively. Optical characterization of ECM-fabricated 1D-PhCs, which was performed by acquiring reflectivity spectra over the wavelength range 1-1.7 μm, highlighted the presence of ultra-high-order PBGs with minor optical losses (i.e., <1 dB in reflectivity) separated by deep reflectivity notches with high Q-factors (i.e., >6000), in good agreement with theoretical calculations. Remarkably, the use of high-order 1D-PhCs as refractometric transducers for the quantitative detection of traces of water in water-ethanol mixtures, allowed high sensitivity (namely, either 1000 nm/RIU or ∼0.4 nm/% of water), good detection limit (namely, 5 × 10 ^-3 RIU or ∼10% water), and excellent resolution (namely, either 6 × 10 ^-4 RIU or 1.6% of water) to be reliably achieved on a detection volume of about 168 fL.