Polarization control of gain of stacked InAs∕InP (100) quantum dots at 1.55μm: Interplay between ground and excited state transitions

The linear polarization of the optical gain of closely stacked $\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{P}$ (100) quantum dots (QDs) grown by metal-organic vapor-phase epitaxy with emission wavelength tuned into the $1.55\mu \mathrm{m}$ region is controlled by the number of stacked QD layers and the injection current. Increasing the number of stacked QD layers to five rotates the linear polarization of the cleaved-side photoluminescence and QD ground state (GS) gain, determined from the amplified spontaneous emission (ASE) of a Fabry–Pérot ridge-waveguide laser, from transverse electric (TE) to transverse magnetic due to vertical electronic coupling. When the QD GS ASE and gain saturate with an increase of the injection current and the excited state ASE and gain become dominant, the linear polarization of ASE and gain changes back to TE. This limits the polarization insensitive operation of QD-based semiconductor optical amplifiers, however, opening routes to novel functionalities.