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Topological Control of Porous Silicon Photonic Crystals by Microcontact Printing




Free-standing, uniformly wrinkled multilayered photonic crystals are prepared in porous Si by conformal electrochemical etch of a highly doped p++ type Si wafer that is pre-shaped with periodic grooves. The grooves are prepared by first microcontact printing a parallel line of oligomeric polydimethylsiloxane and then electrochemically etching the patterned wafer. The etch proceeds anisotropically, with the resist-covered regions etching more slowly than the resist-free regions. The grooved substrate is then etched using a sinusoidal current density waveform, generating a porosity-modulated photonic crystal (rugate filter) that is conformal with the grooves. This porous multilayer is then removed, resulting in a freestanding nanostructure with a corrugated topological modulation in the x–y plane and a rugated porosity modulation in the z-direction. In addition to the resonant photonic reflectance signature from the porosity-modulated rugate filter (along the z direction), the structures display optical diffraction in transmission from the x–y plane due to the spatially modulated grooves. The silicon wafer that remains after removal of the porous multilayer still contains a rippled surface, allowing the process to be repeated without additional microcontact printing. Six generations of freestanding, three-dimensional diffraction gratings are produced from a single wafer and only one initial patterning step.