Light-Wave Engineering for Selective Polarization of a Single <b>Q</b> Valley in Transition Metal Dichalcogenides

The selective control of specific momentum valleys lies at the core of valleytronics, a field that has thus far focused primarily on the <b>K</b> and <b>K'</b> valleys in transition metal dichalcogenides (TMDs). However, direct optical access to other low-lying yet conventionally inaccessible valleys such as the 6-fold degenerate <b>Q</b> valleys has remained an outstanding challenge, fundamentally limiting the exploitation of the full valley degree of freedom for information processing. Here, we theoretically introduce a light-wave valley selection rule that enables deterministic and high-fidelity excitation of any single <b>Q</b> valley in TMDs. By combining a circularly polarized pump pulse with a linearly polarized driver pulse, we engineer distinct quantum pathways that unambiguously excite electrons into a targeted <b>Q</b> valley, completely decoupled from the conventional <b>K</b> and <b>K'</b> valleys. This all-optical scheme achieves near-unity (∼100%) valley polarization across an exceptionally broad ultrafast window, from terahertz (10¹² Hz) to petahertz (10¹⁵ Hz) regimes.