In order to develop these lasers it is necessary to adjust the effective bandgap of the control regions such that they are less absorbing than the as-grown laser structure. The technique we are employing to carry this out involves an impurity-free interdiffusion method. This requires covering the region on the wafer that needs to be modified with a thin cap of SiO 2 . Then, when the wafer is subjected to a rapid thermal anneal treatment the effective bandgap of the laser active region lying beneath the SiO 2 is increased due to intermixing between the quantum wells and adjacent barrier regions. We have a significant programme on both employing this technology in laser fabrication and in attempting to understand the mechanism by which the bandgap modification occurs.
Current explanations given in the literature are inadequate to account for all our observations.
A collaboration is under way with Dr. D.T. Cassidy to investigate laser structures with active regions consisting of quantum wells of different dimensions within a single laser structure. The latter property can be employed to produce broadly tunable lasers and we are interested in systems that can be used in pollution sensing. Feel free to contact us .