  The optical selection rules obeyed by two-dimensional materials with
spin-valley coupling enable the selective excitation of carriers. We show that
six members of the monolayer MA2Z4 (M = Mo and W; A = C, Si, and Ge; Z = N, P,
and As) family are direct band-gap semiconductors with protected valley states
and that circularly polarized infrared light can induce valley-selective
inter-band transitions. Our optovalleytronic device demonstrates a close to
100% valley- and spin-polarized current under in-plane bias and circularly
polarized infrared light, which can be exploited to encode, process, and store
information.
