  Since its discovery in 1966, the photorefractive effect, i.e. the change of
the refractive index upon illumination with light, has been studied extensively
in various materials and has turned out to play a key role in modern optical
technologies like photonics. This article focuses on substances that change
their refractive index for neutrons when irradiated with light. In analogy to
light optics, we call them photo-neutronrefractive. After a short introduction
to the relevant concepts of neutron optics, two materials exhibiting that
effect, a photopolymer and an electrooptic crystal, are presented. Further, we
discuss the the progress made concerning the development of creating
light-induced gratings for neutron diffraction, which culminated in the setup
of an interferometer for cold neutrons. Experiments performed on
photo-neutronrefractive materials are surveyed and the variety of corresponding
results obtained is presented, including a discussion of their impact on
material science, neutron optics, and the foundations of physics.
