Periodic Report Summary 3 - OPUS (Optical Ultra-Sensor)
Remarkably, a laser can emit light with an ultra-narrow spectral linewidth, particularly when distributed Bragg mirrors with a very narrow reflection band are utilized. The narrower the laser linewidth, the easier changes in the spectral position of the line can be detected, e.g. by interference with another, similarly narrow laser line. We generate both laser lines by two resonances inscribed into a single optical waveguide on a silicon chip. Disturbance and a corresponding shift in spectral position of one of these lines, e.g. by interaction with a nano-sized bio-particle that appears close to the optical waveguide, thus becomes detectable with ultra-high sensitivity. In the first half of this project, we have investigated the laser linewidth theoretically and experimentally. Despite claims that the laser linewidth can only be understood quantum-optically, we provide a classical explanation that also sheds new light on the underlying processes of spontaneous and stimulated emission. Currently we compare our theory with the linewidths measured in our samples.