Electronic and photonic band structure engineering of semiconductor lasers

Abstract

In modern semiconductor lasers the electronic band structure is being artificially modified by strain and quantum confinement in order to reduce the valence band effective mass. Likewise, a photonic band structure is being consciously created, with the aim of producing a photonic band gap in which spontaneous emission is eliminated. This paper is a review of both trends, which seem to be converging into an interesting new ultra-low threshold semiconductor laser technology. If this new technology is successful, there will be microampere threshold lasers generating number-state squeezed light. Single-strained quantum-well (SSQW) lasers are rapidly becoming preferred for many applications. They overcome one of the main problems in III-V semiconductors, namely, the heavy valence band. A light hole mass reduces laser threshold requirements, minimizes intervalence band absorption, cuts down Auger recombination, and allows faster direct modulation. In spite of early fears of strained material instability, SSQW lasers are showing themselves to be more reliable than conventional GaAs lasers. At threshold, in a good-quality SSQW laser, spontaneous emission can dominate all other parasitic processes. We are now beginning to learn how to control spontaneous emission. In a three-dimensionally periodic dielectric medium it is possible to create a "photonic band gap" that is a forbidden energy gap for photons. In this energy band optical modes, spontaneous emission, and zero point fluctuations are all absent. This may be called band structure engineering for photons. The combination of all these ideas in a very small SSQW laser will lead to ultra-low microampere thresholds. More importantly, the quantum efficiency into the lasing mode can approach unity. In the absence of parasitic processes and under that conditions just described, we can look forward to the prospect of high-quality photon number-state squeezed light from these lasers.