Reduced absorption mechanisms in semiconductor quantum wells

Abstract

Ultrahigh refractive indices without absorption1 and reduced absorption2 have been predicted and observed for atomic systems. We apply these considerations to semiconductor structures with complicated energy bands and strong phonon interactions. Bulk semiconductors offer limited flexibility, while quantum well structures offer the ability to tailor the dielectric response through control of the conduction subband states and the separation between heavy and light hole states. To observe high indices without absorption, Raman pumping from the ground conduction state should result in a coherent state at the bottom of heavy hole and light hole bands. The high index then exists between the first excited conduction subband and the range of separation between the holes. Since this scheme gives a large refractive index change, we can observe results by photo- or electro-reflectance. However, interactions between subbands and the strong phonon scattering may inhibit the coherent state. To observe reduced absorption without changes of refractive index, two conduction subbands should be coupled by a CO2 laser. Reduced absorption should be observable from the bottom of the hole state to the range between the two dressed states of the conduction subbands. This approach is simpler than coherent pumping, and modulated transmission offers the best chance for observation.