  The concept of ferrovalley materials has been proposed very recently. The
existence of spontaneous valley polarization, resulting from ferromagnetism, in
such hexagonal two-dimensional materials makes nonvolatile valleytronic
applications realizable. Here, we introduce a new member of ferrovalley family
with orthorhombic lattice, i.e. monolayer group-IV monochalcogenides (GIVMs),
in which the intrinsic valley polarization originates from ferroelectricity,
instead of ferromagnetism. Combining the group theory analysis and
first-principles calculations, we demonstrate that, different from the
valley-selective circular dichroism in hexagonal lattice, linearly polarized
optical selectivity for valleys exists in the new type of ferrovalley
materials. On account of the distinctive property, a prototype of electrically
tunable polarizer is realized. In the ferrovalley-based polarizer, a laser beam
can be optionally polarized in x- or y-direction, depending on the ferrovalley
state controlled by external electric fields. Such a device can be further
optimized to emit circularly polarized radiation with specific chirality and to
realize the tunability for operating wavelength. Therefore, we show that
two-dimensional orthorhombic ferrovalley materials are the promising candidates
to provide an advantageous platform to realize the polarizer driven by electric
means, which is of great importance in extending the practical applications of
valleytronics.
