We study the momentum alignment phenomenon and the optical control of valley
population in gapless and gapped graphene-like materials. We show that the
trigonal warping effect allows for the spatial separation of carriers belonging
to different valleys via the application of linearly polarized light. Valley
separation in gapped materials can be detected by measuring the degree of
circular polarization of band-edge photoluminescence at different sides of the
sample or light spot (optical valley Hall effect). We also show that the
momentum alignment phenomenon leads to the giant enhancement of near-band-edge
interband optical transitions in narrow-gap carbon nanotubes and graphene
nanoribbons independent of the mechanism of the gap formation. A detection
scheme to observe these giant interband transitions is proposed which opens a
route for creating novel terahertz radiation emitters.Comment: 28 pages, 9 figure
