Cool by diamond - New generation of semiconductor lasers
Lasers became popular with movies like „Star Wars“ or „James Bond“. In reality, lasers are incredibly versatile applicable tools – they are used, e.g., for cutting and welding of a variety of materials. Physicists of the University of Stuttgart succeeded with a technological breakthrough.
This will extend the choice of by semiconductor lasers accessible wavelengths and facilitate new applications. How can semiconductor disk lasers further be improved in terms of output power without losing their other wonderful properties? Scientists from the “Institut für Halbleiteroptik und Funktionelle Grenzflächen” examined this issue. The key solution sounds simple and is consequent though challenging as it is in most cases when going into practice.
Semiconductors themselves are poor thermal conductors. So one has just to omit all parts of the semiconductor structure not essentially necessary to build up a whole laser: the substrate on which the semiconductor layers are deposited has no function during laser operation and can be removed. External mirrors can replace the semiconductor mirror, integrated in all semiconductor disk lasers.
The only thing left of the semiconductor component is the several few hundreds of nanometers thick laser active region which gets sandwiched between two diamond disks. Diamond – transparent and the best thermal conductor available – is suitable in an outstanding way as an integrated heat spreader. The PhD-Students Hermann Kahle and Cherry May Mateo started with wet-chemical etching processes to isolate the laser active region from its substrate. The semiconductor membrane, in total thickness only one eightieth of the diameter of a regular human hair, can be stored and best handled when submerged in a liquid only.
The exciting thing at the end was to transfer the membrane onto a diamond disk which is four millimeter in diameter and 0.5 millimeter in thickness. Likewise, it has to keep in one piece, centered to the diamond and totally flat. And, there was never a second chance. Once bonded to the diamond by capillary forces it was impossible to remove the membrane again without destroying it. Laboratory work like this requires calm hands, lots of skill and endurance. The completed diamond-semiconductor-sandwich was then inserted into a laser resonator and characterized in the optics laboratory. After hours of adjustment work it flashed and the membrane laser was operating. A strong light beam in the red spectral range was emitted after a little more fine adjustment. And, it showed all characteristics we had hoped for: the tunability of the laser wavelength during operation, a perfectly shaped beam profile and especially a for semiconductor lasers high output power; and all this at an operating temperature of 10°C. Without the diamond sandwich the semiconductor membrane would rapidly overheat and stop to operate. Before this approach the whole semiconductor disk laser device sometimes had to be cooled down to minus 30°C.