  The photoelectric effect consists in the photoexcitation of electrons above a
potential barrier at a material interface and is exploited for photodetection
over a wide frequency range. This three-dimensional process has an inherent
inefficiency: photoexcited electrons gain momenta predominantly parallel to the
interface, while to leave the material they have to move perpendicular to it.
Here, we report on the discovery of an in-plane photoelectric effect occurring
within a two-dimensional electron gas. In this purely quantum-mechanical,
scattering-free process, photo-electron momenta are perfectly aligned with the
desired direction of motion. The "work function" is artificially created and
tunable in-situ. The phenomenon is utilized to build a direct terahertz
detector, which yields a giant zero-bias photoresponse that exceeds the
predictions by known mechanisms by more than 10-fold. This new aspect of
light-matter interaction in two-dimensional systems paves the way towards a new
class of highly efficient photodetectors covering the entire terahertz range.
