In situ optical spectroscopy during liquid imbibition into a nanoporous photonic crystal resolves precursor film spreading

Abstract

A photonic crystal (PC) can be integrated into nanoporous silicon by electrochemical etching. The inset in the upper left of the main image displays a SEM side view of the PC. Its resonance wavelength is determined by the size of a high porosity central defect layer, which is the biggest of the horizontally aligned dark regions highlighted by red edges. It is surrounded by smaller regions of alternating porosities acting as mirror layers. The main image displays the time evolution of the visible-to-near-infrared spectrum during the imbibition with polydimethylsiloxane (PDMS). The PC light reflection has a prominent local minimum at 775 nm indicated by a red arrow. Once liquid substitutes air in the central cavity, the resonance wavelength shifts proportionally. Insets show the color change of a similar sample tuned in the visible region in direct optical imaging. The technique allows high precision assessment of the fluid front’s shape, including even a sub-nanometer sized precursor flim.