is applied to structure silicon on large areas in a simple and three-dimensional manner. First, a solution with micrometer-sized spheres of polystyrene is applied to the silicon surface. After drying, these spheres automatically form in a dense monolayer on the silicon. Upon metal coating and the removal of the spheres, a honeycomb etching mask remains on the silicon surface.

Image: Deep below the silicon surface, the SPRIE method produces regular structures in the micrometer range that refract light. © KIT/CFN

“This etching mask is our two-dimensional template for the construction of the three-dimensional structure,” says Frölich. The free areas are removed by etching with a reactive plasma gas. An electric field is applied to make the gas particles etch into the depth only or homogeneously in all directions.” In addition, we can specifically passivate the walls of the hole, which means that it is protected from further etching by a polymer layer.”

Repeated etching and passivation makes the holes of the etching mask grow into the depth. With up to 10µm, their depth exceeds their width by a factor of more than 10. The process steps and the electric field are adjusted precisely 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 1.5µm. With our etching method, we produce a corrugated structure in the micrometer range along the wall.”

Alexandru Vlad, Andreas Frölich, Thomas Zebrowski, Constantin Augustin Dutu, Kurt Busch, Sorin Melinte, Martin Wegener and Isabelle Huynen: Direct Transcription of Two-Dimensional Colloidal Crystal Arrays into Three-Dimensional Photonic Crystals, In: Advanced Functional Materials, Early View, October 08, 2012, DOI:10.1002/adfm.201201138: http://dx.doi.org/10.1002/adfm.201201138

In a new experiment, a silica fibre just 500 nm across has been shown not to obey Planck‘s law of radiation. The researcher investigate the ther- malization via heat radiation of a silica fiber with a diameter smaller than the thermal wavelength. The temperature change of the subwavelength-diameter fiber is determined through a measurement of its optical path length in conjunction with an ab initio thermodynamic model of the fiber structure. The results differ significantly from the predictions of Planck‘s law based on the spectral emissivity of silica. C. Wuttke, A. Rauschenbeutel: Probing Planck‘s Law for an Object Thinner than the Thermal Wavelength, In: ar- Xiv:1209.0536: http://arxiv.org/abs/1209.0536