SILICON OPTICAL MICROSYSTEMS FOR LABEL-FREE BIOSENSING: THE CASE OF PHOTONIC CRYSTALS

In the last two decades, one-dimensional (1D) photonic crystals (PhCs), as well as 2D and 3D, have been successfully used for label-free refractometric and biosensing applications [1], due to their high-sensitivity to small changes in both dielectric constant and thickness of the materials assembling the PhC structure. Delivery of liquids either over [1, 2] or through [3-5] the PhC structure has been also reported, the latter envisaging higher sensitivity and lower limit of detection than the former [3]. Pressure-driven operation, through the use of external pumps, has been commonly used for the deliver of liquids in PhCs [2, 4], which limits somehow the applications of PhCs as sensing elements for point-of-care analysis. Nevertheless, capillarity-driven operation, without the use of external pumps, has been also recently demonstrated [5], thus envisaging the development of self-powered drop-and-measure platforms based on PhCs. Among PhC structures, vertical, high-aspect-ratio silicon/air 1D PhCs able to control light propagation in a plane parallel to the silicon substrate represent an stimulating solution for the fabrication of miniaturized platforms to be employed in (bio)sensing [6]. In fact, vertical 1D PhCs intrinsically features a microfluidic path, which is perpendicularly to and independent of the optical path and that can be used to infiltrate the liquids “trhough” the PhC structure. Recently, deep-etching of complex silicon microstructures and microsystems with sub-micrometer accuracy at aspect-ratio values beyond standard dry and wet etching technologies has been demonstrated by electrochemical micromachining technology (ECM) [7] for both biomedical [8, 9] and photonic [10, 11] applications. ECM technology allows silicon microstructuring to overcome limitations of modern dry etching technologies, with the further advantage of the low cost of ECM technology with respect to dry etching technologies, which could make advanced silicon microstructuring available in any lab. In this work 1D PhC all-silicon platforms, fabricated by electrochemical micromachining technology (ECM) and integrating vertical, silicon/air 1D PhCs together with fluidic and optical paths, for refractometric and biosensing applications both under pressure-driven and capillarity-driven operation are reviewed.