  The design of some optical devices such as semiconductor optical amplifiers
for telecommunication applications requires polarization-insensitive optical
emission at the long wavelengths (1300-1550 nm). Self-assembled InAs/GaAs
quantum dots (QDs) typically exhibit ground state optical emission at
wavelengths shorter than 1300 nm with highly polarization-sensitive
characteristics, although this can be modified by using low growth rates, the
incorporation of strain-reducing capping layers or growth of closely-stacked QD
layers. Exploiting the strain interactions between closely stacked QD layers
also allows greater freedom in the choice of growth conditions for the upper
layers, so that both a significant extension in their emission wavelength and
an improved polarization response can be achieved due to modification of the QD
size, strain and composition. In this paper we investigate the polarization
behavior of single and stacked QD layers using room temperature
sub-lasing-threshold electroluminescence and photovoltage measurements as well
as atomistic modeling with the NEMO 3-D simulator. A reduction is observed in
the ratio of the transverse electric (TE) to transverse magnetic (TM) optical
mode response for a GaAs-capped QD stack compared to a single QD layer, but
when the second layer of the two-layer stack is InGaAs-capped an increase in
the TE/TM ratio is observed, in contrast to recent reports for single QD
layers.
