  We report both zinc-blende (ZB) and wurtzite (WZ) crystal phase
self-assembled GaAs quantum dots (QDs) embedding in a single GaAs/AlGaAs
core-shell nanowires (NWs). Optical transitions and single-photon
characteristics of both kinds of QDs have been investigated by measuring
photoluminescence (PL) and time-resolved PL spectra upon application of
hydrostatic pressure. We find that the ZB QDs are of direct band gap transition
with short recombination lifetime (~1 ns) and higher pressure coefficient
(75-100 meV/GPa). On the contrary, the WZ QDs undergo a direct-to-pseudodirect
bandgap transition as a result of quantum confinement effect, with remarkably
longer exciton lifetime (4.5-74.5 ns) and smaller pressure coefficient (28-53
meV/GPa). These fundamentally physical properties are further examined by
performing state-of-the-art atomistic pseudopotential calculations.
