  Following how the electrical transistor revolutionized the field of
electronics,the realization of an optical transistor in which the flow of light
is controlled optically should open the long-sought era of optical computing
and new data processing possibilities. However, such function requires photons
to influence each other, an effect which is unnatural in free space. Here it is
shown that a ferroelectric and photovoltaic crystal gated optically at the
onset of its bandgap energy can act as a photonic transistor. The light-induced
charge generation and distribution processes alter the internal electric field
and therefore impact the optical transmission with a memory effect and
pronounced nonlinearity. The latter results in an optical computing
possibility, which does not need to operate coherently. These findings advance
efficient room temperature optical transistors, memristors, modulators and
all-optical logic circuits.
