  Valleytronics is one of the breaking-through to the technology of
electronics, which provides a new degree of freedom to manipulate the
properties of electrons. Combining DFT calculations, optical absorption
analysis and the linear polarization-resolved transmission measurement
together, we report that three pairs of valleys, which feature opposite optical
absorption, existing in the 3-dimensional (3D) group-IV monochalcogenids. By
applying the linearly-polarized light, valley polarization is successfully
generated for the first time in a 3D system, which opens a new direction for
the exploration of the valley materials and provides a good platform for the
photodetector and valleytronic devices. Valley modulation versus the in-plane
strain in GeSe is also studied, suggesting an effective way to get the
optimized valleytronic properties.
