Multidirectional lateral gradient films with position-dependent photonic signatures made from porous silicon

We describe here the fabrication of laterally graded porous silicon films which display gradients of photonic reflectance peaks spanning the optical spectrum. We demonstrate that up to three of these gradients can be overlayed to produce multidirectional photonic gradients with position-dependent spectral bar-codes. Each gradient is generated by asymmetric anodisation of silicon using temporal variations (sinusoidal or square-wave) in current density affording rugate and Bragg reflectors, respectively. The fabricated optical structures and the quality of the photonic resonances are characterised by optical reflectivity measurements and scanning electron microscopy. We finally remove the pSi gradient layers from the silicon substrate by applying an electropolishing current and embed the free-standing pSi membranes in polydimethylsiloxane to form flexible and foldable photonic films.