Semiconductor Lasers

Abstract

Semiconductor lasers play an important role in technical applications like data recording, data reading, laser printing, optical devices like pointers and microscopes, and all kind of fabrication processes like polymer hardening, 3d-printing, structuring, and many others. Another important field of application is medicine (surgery, in particular ophthalmology). Laser action sets in a situation far away from thermal equilibrium, characterized by different Fermi energies of valence band and conduction band. This difference should be larger than the band gap in order to fulfill the “laser condition”, a situation which can be achieved in a pn-junction formed by highly doped p- and n-type material and operated at sufficiently high forward current. At a certain forward current, the “laser threshold current”, laser action sets in. Beyond the laser threshold, the emitted light is characterized by extreme monochromacity and directionality as a consequence of the fact that the photons essentially originate from stimulated emission. The “threshold current” can be considerably lowered in more sophisticated arrangements like double heterojunctions, due to “charge confinement” and “light confinement” in the active region, or multi-quantumwell lasers. Blue lasers based on GaInN are at the basis of “Blue ray” data storage. Long distance optical data transmission via optical fibers makes use of IR-lasers operating at 1.54 µm on the basis of the quaternary compound GaInAsP.