Tunneling injection quantum-dot lasers

We investigate tunneling injection quantum-dot (QD) lasers both theoretically and experimentally. Our laser structure consists of two tensile-strained quantum wells (QWs) coupled to a compressive-strained QD layer. The QWs serve as efficient carrier collectors and as a medium to inject electrons into the QDs by tunelling. Polarization-resolved amplified spontaneous emission (ASE) spectroscopy is used to extract the transverse-electric (TE) and transverse-magnetic (TM) polarized optical gain spectra at very low to near threshold injection currents. At a low bias current, the TE polarized ASE from the ground state of the QD layer is observed. At an intermediate current level, the coupling of the QW ground state to the QD excited state becomes important and an increase of the TM polarized emission from the tensile-strained QWs at a higher energy level becomes significant. Near threshold current, we observe TE gain narrowing due to the QD excited-state activation and the pinning of TM gain with subsequent TE lasing above threshold. We explain the physics of tunneling injection from the QWs into the QDs and how the tunneling injection affects the polarization resolved optical gain spectra as the injection current level increases.