Torsten Geppert

Photonic crystal gas sensors

The bandstructure of photonic crystals offers intriguing possibilities for the manipulation of electromagnetic waves. During the last years, research has mainly focussed on the application of these photonic crystal properties in the telecom area. We suggest utilization of photonic crystals for sensor applications such as qualitative and quantitative gas and liquid analysis. Taking advantage of the low group velocity and certain mode distributions for some k-points in the bandstructure of a photonic crystal should enable the realization of very compact sensor devices. We show different device configurations of a photonic crystal based on macroporous silicon that fulfill the demands to serve as a compact gas sensor.

Silicon-based low-loss photonic crystal waveguides

We present a new high-index-contrast material system to realize ridge waveguides and PhC waveguides made of a thin silicon slab embedded in two silica layers. Hence fully symmetrical structures can be etched and two important conditions for low-loss guiding of light can be matched: The symmetry properties of the material avoid polarization mixing and the high index contrast leads to strong confinement of light. Because of operating completely below the lightcone the PhC waveguides allow theoretically lossless guiding of light.

Low Group Velocity Devices in Silicon Photonics

Two possible concepts to slow down the light are discussed: (coupled) cavities in comparison to the concept of low group velocities at flat bands in photonic crystals. Two devices using the second concept are presented.

Photonic bandgap waveguide structures

Passive optical components based on high-index materials like silicon are becoming increasingly attractive because they allow dense integration. Photonic crystal waveguides support modes due to Bragg reflections as well as modes due to total internal reflection if the waveguide material is made out of the higher refractive medium. We have studied, based on macroporous silicon, the transmission through a 2D-photonic crystal waveguide. These structures are really two-dimensional since they exhibit aspect ratios of more than 100. Their transmission has strong spectral features, resulting from the coupling of index-guided modes with Bragg-guided modes. If both modes have the same symmetry, they interfere with each other and open up spectral gaps (Ministop bands). Moreover, the Bragg-guided modes can exhibit very small group velocities enabling their use for dispersion compensation or gas sensing devices.

Design of a New Taper for Light Coupling Between a Ridge Waveguide and a Photonic Crystal Waveguide

We propose the design of a new taper to improve light coupling between a photonic-crystalbased W1 waveguide and a ridge waveguide of similar width. The taper design is directly deduced from band structure calculations and allows an adiabatic mode conversion. The comparison between light propagation from the ridge waveguide through the W1 waveguide and through the taper, respectively, shows good improvement of the coupling efficiency.