  We explore photospheric emissions from stratified two-component jets, wherein
a highly relativistic spine outflow is surrounded by a wider and less
relativistic sheath outflow. Thermal photons are injected in regions of high
optical depth and propagated until they escape at the photosphere. Due to the
presence of shear in velocity (Lorentz factor) at the boundary of the spine and
sheath region, a fraction of the injected photons are accelerated via a
Fermi-like acceleration mechanism such that a high energy power-law tail is
formed in the resultant spectrum. We show, in particular, that if a velocity
shear with a considerable variance in the bulk Lorentz factor is present, the
high energy part of observed Gamma-ray Bursts (GRBs) photon spectrum can be
explained by this photon acceleration mechanism. We also show that the
accelerated photons may also account for the origin of the extra hard power-law
component above the bump of the thermal-like peak seen in some peculiar bursts
(e.g., GRB 090510, 090902B, 090926A). It is demonstrated that time-integrated
spectra can also reproduce the low energy spectrum of GRBs consistently due to
a multi-temperature effect when time evolution of the outflow is considered.
Finally, we show that the empirical Ep-Lp relation can be explained by
differences in the outflow properties of individual sources.
