  Comparing with the spin of electron, the electronic orbitals, which have been
long ignored in non-equilibrium transport, are getting more and more
attentions, due to the prediction and experimental verification of orbital Hall
effect (OHE) recently. In this report, the recent advances, including intrinsic
mechanisms, direct and indirect experimental observations of OHE, are reviewed
comprehensively. The orbital current, generated by OHE, shows much more
fundamental features than spin current in explaining Hall conductivity and
manipulating magnetization electronically. Theoretically, in momentum space,
the OHE is proposed to originate from electric-field driven non-equilibrium
orbital texture, based on tight-binding model. However, current experimental
works still stay on the level to reveal how the OHE behaves after orbital
current is generated, and the unambiguous experimental evidence to verify the
generation mechanism of OHE is still missing, limiting the interpretation of
OHE. Angle-resolved photoemission spectroscopy with polarized incident photon,
on the other hand, could be used to identify the orbital textures on energy
band and reveal the non-equilibrium nature of orbital-momentum locking behind
OHE, which may help to provide the direct experimental evidence to examine the
intrinsic mechanism of OHE, and reveal the nature of wave packet with
non-vanishing orbital angular momentum.
