  We identify a strong photovoltaic response due to the non-linear optical
transition between heavy and light hole sub-bands enabled by T_d-symmetry in a
quantum well, which we term the quadrupolar photovoltaic effect (QPE). The
photovoltaic current exhibits a strong resonance in the vicinity of the heavy
hole-light hole splitting, with a magnitude governed by the momentum relaxation
time, which can reach nanoseconds in GaAs holes. Since the heavy hole-light
hole splitting can be tuned from a few meV to nearly 100 meV the QPE could
serve as the basis for a terahertz photo-detector. We discuss strategies for
experimental observation and device applications.
