Photonic Crystals

Scanning electron micrographs of 3D Silicon woodpile photonic crystal. The structure was produced via direct laser writing followed by a silicon-double-inversion procedure.
3D photonic quasicrystal
Setup for 3D direct laser writing applications

What is the excitement about?

To appreciate the excitement about the concept of photonic crystals, it is helpful to stress the analogy to semiconductors for electrons. In a semiconductor, such as, e.g., silicon, the electrons travel as waves through the periodic potential of the nuclei. This leads to Bloch waves with a special dispersion relation, simply the band structure of the solid. For the case of silicon, the band structure has a band gap – the starting point of doping, transistors, computer chips, etc. In analogy, light waves moving in a periodic dielectric material can also be described by their band structure. If the parameters are just “right”, one also gets a band gap, i.e., a range of frequencies for which propagation is forbidden irrespective of direction, and the photonic crystal becomes a so-called photonic-band-gap material. This concept was independently introduced by Yablonovitch and John in 1987. Broadly speaking, these “semiconductors for light” smell like fun on the one hand and offer novel options for tailoring optical materials of the 21st century on the other hand.

High refractive index

 

To fabricate such structures, we have developed and optimized a technique called direct laser writing (DLW) enabling the fabrication of complex three-dimensional polymer structures. However, to achieve complete three-dimensional photonic band gaps, the refractive index of the photoresist is not sufficiently large. Using a silicon-double-inversion procedure developed in in our group in collaboration with chemists (based on silica atomic-layer deposition and silicon chemical-vapor deposition), the polymer structures can be replicated into high-index silicon, leading to complete three-dimensional photonic band gaps at infrared wavelengths.

On this basis, for example, ultra-compact optically driven particle accelerators become possible by incorporating special hollow wave guides. One future challenge is the realization of complete three-dimensional photonic band gaps at visible frequencies.  Here, silicon is replaced by titania.

 

A complete list of publications can be found here.