  Layered materials can possess valleys that are indistinguishable from one
another except for the momentum. These valleys are individually addressable in
momentum space at the K and K' points in the first Brillouin zone. Such valley
addressability opens up the possibility of utilizing the momentum state of
quasi-particles as a completely new paradigm in quantum and classical
information processing. This review focuses on the physics behind valley
polarization and talks about carriers of valley degree of freedom (VDF) in
layered materials. Then we provide a detailed survey of simple spectroscopic
techniques commonly utilized to identify and manipulate valley polarization in
van der Waals layered materials. Finally, we conclude with the recent
developments towards the manipulation of VDF for device application and
associated challenges.

Materials Science;Mesoscale and Nanoscale Physics