  Steady illumination of a non-centrosymmetric semiconductor results in a bulk
photovoltaic current, which is contributed by real-space displacements
(`shifts') of charged quasiparticles as they transit between Bloch states. The
shift induced by interband excitation via absorption of photons has received
the prevailing attention. However, this excitation-induced shift can be far
outweighed ($\ll$) by the shift induced by intraband relaxation, or by the
shift induced by radiative recombination of electron-hole pairs. This
outweighing ($\ll$) is attributed to (i) time-reversal-symmetric, intraband
Berry curvature, which results in an anomalous shift of quasiparticles as they
scatter with phonons, as well as to (ii) topological singularities in the
interband Berry phase (`optical vortices'), which makes the photovoltaic
current extraordinarily sensitive to the linear polarization vector of the
light source. Both (i-ii) potentially lead to nonlinear conductivities of order
$mAV^{-2}$, without finetuning of the incident radiation frequency, band gap,
or joint density of states.

Mesoscale and Nanoscale Physics;Materials Science