Nanophotonics

The area of nanophotonics deals with the generation, propagation and detection of light on scales comparable or even below the wavelength of light. The prospects to create optical materials with novel properties or the possibility to fabricate densely integrated optical circuits has lead to the investigation of various concepts for nanophotonic structures.

One possibility to control light on very small length scale is the use of artificial dielectrics call ‘Photonic Crystals’, where the refractive index is modulated on a scale comparable to the wavelength. For systems with a sufficiently strong contrast of the refractive indices, optical bandgaps are formed for certain frequency ranges. The photonic crystal effectively acts as an optical insulator.

We mainly concentrate of the study of two-dimensional photonic crystals. In this case, the refractive index is modulated in a plane by etching a hexagonal array of air holes into a semiconductor waveguide, which provides light confinement in the third dimension. The waveguide can either be realized as a classical semiconductor optical waveguide where a high index core is sandwiched between two low index cladding layers, or as a semiconductor membrane suspended in air.

The existence of an optical bandgap in photonic crystals allows the realization of optical resonators which localizes light in a very small volume, typically on the order of a cubic wavelength. The quality factors of the resonators can be up to several 10⁵. We use these unique properties of PhC resonators for the following research directions:

• Cavity quantumelectrodynamics: the radiative properties of a quantum dot inside a high quality cavity are strongly modified, leading to a modification of the spontaneous emission rate or strong coupling of the dot to the cavity field.
   
• Nonlinear optics: the high quality factor leads to a strong build up of electromagnetic energy in the cavity. This enables the study of nonlinear effects at small input powers.
   
• Dispersive properties: due to the long photon lifetime of the cavity, the transmitted light is significantly delayed, leading to a strong dispersion at the wavelength of resonance.

Our second research in the field of nanophotonics is the integration of photonic crystals in semiconductor lasers. Photonic crystals can be used in various places in semiconductor lasers, for example as highly reflecting mirrors, low loss waveguides or frequency selective elements. The compact size of photonic crystal devices allows a dense integration of optical functionality and reduces the complexity of the fabrication process. Since the PhC structures are so small, absorption of unpumped active sections is not an issue, which allows the fabrication of these devices on all-active layer structures without the need for complicated regrowth processes.

Currently, we investigate the following devices:

• Tunable photonic crystal laser with wavelength monitor: this device is investigated in the framework of the European Project FUNFOX. Classical tunable lasers require a look-up table for operation. With an integrated wavelength monitor, any deviation from the target wavelength can be detected and the laser wavelength can be tuned accordingly. The wavelength monitor is based on the wavelength selective transmission of a photonic crystal waveguide, combined with integrated photodiodes to record the transmitted light.
   
• Photonic crystal distributed feedback laser : in this device, the photonic crystal is used to ensure longitudinal and lateral coherence of the laser mode across a broad area laser structure. Due to the diffraction of the light at the photonic crystal, only one mode fullfills the round-trip condition in the laser, resulting in a single wavelength and almost diffraction limited output beam.