Photonic crystals

Photonic crystals are periodic arrangements of dielectric materials with different refractive indices, whose dimensions in the direction of propagation are comparable to the wavelength of the electromagnetic wave under consideration. Such structures have a complex photonic band diagram that differs for the two orthogonal polarization states TE (transverse electric) and TM (transverse magnetic) and can therefore exhibit a variety of phenomena that are also interesting for applications in optical communications. One of the known effects in photonic crystals is the existence of a band gap, where the electromagnetic wave cannot propagate in a certain frequency range. In addition, photonic crystals can have a negative refractive index under certain conditions. This can be used, for example, for focusing in order to better couple light into a waveguide.

The effect of negative refraction can also be used to achieve physical separation of the two orthogonal polarization states and to build an integrated polarization splitter from this. The parameters of the photonic crystal must then be dimensioned so that one polarization state is refracted negatively and the other polarization state is refracted positively.

At the Institute for Electrical and Optical Communications Engineering, some of these fundamental properties were investigated in the microwave range around 10 GHz. Due to the wavelength, the phenomena can be studied on macroscopic structures that can be manufactured in a precision engineering workshop. With appropriate geometric scaling, the properties of the structures can then be estimated at typical telecom wavelengths (1.55 µm).