Karlsruhe, Germany--Silicon photonic crystals for use in telecom devices are being made into 3D structures from 2D templates by researchers at Karlsruhe Institute of Technology, Université Catholique de Louvain (Louvain, Belgium), and Humboldt Universität zu Berlin and Max Born Institut (both in Berlin, Germany). A silicon surface is structured via nanosphere lithography to make a template, then the sequential passivation and reactive ion etching (SPRIE) method combines reactive-plasma gas etching with an applied electric field to direct the plasma either down or in all directions.
Repeated etching and passivation cause the holes of the etching mask to grow deeper. For depths of up to 10 microns, their depth exceeds their width by a factor of more than 10. The process steps and the electric field is adjusted to control the structure of the walls. Instead of a simple hole with vertical smooth walls, every etching step produces a spherical depression with a curved surface. This curvature is the basis for the regular repeating structures of novel waveguides.
“Optical telecommunication takes place at a wavelength of 1.5 µm. With our etching method, we produce a corrugated structure in the micrometer range along the wall,” says Andreas Frölich from Karlsruhe Institute of Technology. The resulting photonic crystals, which have stop bands at telecom wavelengths, can have slab geometries and even contain engineered defects.
The SPRIE method can produce a 3D photonic crystal within a few minutes, as it is based on conventional industrial processes. Different types of photonic crystals can be created, including waveguides with very small radii of curvature and small losses, and extremely narrowband optical filters and multiplexers. The results were described in the Advanced Functional Materials journal.
