The gain bandwidth of a quantum-cascade (QC) laser is important
for determining the magnitude of the optical gain, the refractive
index change, and the linewidth enhancement factor. We investigate
the effects of the scattering mechanisms on the gain linewidth of
a type-I QC laser and compare our theoretical results with
experimental data. The bandwidth of the gain spectrum of a QC
laser is related to the electron relaxation rate, which is
determined by scatterings that change the electron momentum or
energy in the same subband (intrasubband) or different subbands
(intersubband). Polar optical phonon scattering, impurity
scattering, and electron-electron scattering are the important
mechanisms. In this paper, we investigate the magnitude of the
linewidth of the optical gain spectrum due to these scattering
mechanisms in type-I mid-IR QC laser structures. In particular,
the dependence of the scattering rate on the doping position will
be shown in the case of the impurity scattering. We also present
calculated optical gain, refractive index change, and linewidth
enhancement factor spectra. Our theoretical results agree well
with the experimental data.
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.
We proposed a compact variable all-optical buffer using slow-light in semiconductor nanostructures. We discuss the general design principle via dispersion engineering. The buffering effect is achieved by slowing down the optical signal using an external control light source to vary the dispersion characteristic of the medium via electromagnetically induced transparency effect. We demonstrate that the semiconductor quantum dot structures can be used as a slow-light medium. In such structure, the total buffering time is variable and controlled by an external pump laser. We present a theoretical investigation of the criteria for achieving slow light in semiconductor quantum dots. New pump scheme is proposed to overcome the sample nonuniformity. Finally, optical signal propagation through the semiconductor optical buffer is presented to demonstrate the feasibility for practical applications.