Valley polarization driven by two-color circular fields: a rotating frame and strong-field perspective
Valley polarization in hexagonal materials driven by the form of the lightwave and its orientation relative to the lattice, rather than solely by the helicity of the driver, has recently been demonstrated both theoretically and experimentally. This has extended valley control to the non-resonant, sub-cycle regime and to inversion-symmetric materials. One interpretation of this field-orientation-dependent valley polarization is that the laser-dressed, cycle-averaged band structure obtained from a Floquet-type approach is modified so as to lift the degeneracy between the valleys. Here, we derive a complementary explanation based on strong-field tunneling dynamics. We show that the effective valley gap at the dominant injection times depends on the orientation of the field relative to the lattice through the trigonal-warping term of the low-energy dispersion. We derive general expressions for co- and counter-rotating $\omega+N\omega$ fields and show that the leading orientation-dependent contribution survives the cycle average only for field configurations compatible with the threefold lattice symmetry, most notably counter-rotating $\omega+2\omega$ and co-rotating $\omega+4\omega$ fields. As a complementary weak-field result, we show that, for counter-rotating bicircular fields, the one-photon valley selection rules can be represented as valley-dependent energy detunings in a rotating frame where both colors acquire the same frequency.