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Recently, symmetry-broken polaritons within low-symmetry crystals have triggered extensive research interest since they present enhanced directionality of polariton propagation for nanoscale manipulation and steering of photons. The latest discovery of hyperbolic shear polaritons (HShPs) in low-symmetry Bravais crystals provides great promise for innovating valleytronics. Herein, we theoretically demonstrate the coherent manipulation of the valley degree of freedom in a two-dimensional valleytronic material interfaced with monoclinic Ga₂O₃ and CdWO₄ crystals. Robust and wideband tunable valley interference values are achieved in the mid- to far-infrared wavelengths. By virtue of stronger shear effect in monoclinic Ga₂O₃, the valley quantum interference fringes modulated by Ga₂O₃ crystal are more than those tuned via CdWO₄ crystal. After the monoclinic Ga₂O₃ crystal is doped by free charge carriers, the number of HShP modes gradually decreases accompanied by the blueshifts and broadening of some hyperbolic dispersion bands as the doping concentration increases. In consequence, main fringes of valley quantum interference are broadened and shift toward the short wavelengths. Additionally, the doping increases the optical losses which limit the effective propagation of shear polaritons in monoclinic crystals. Therefore, the valley quantum interference is gradually reduced to a smaller and smaller negative value range as the doping concentration increases in the monoclinic crystal. Finally, the azimuthal dispersion of the HShP propagation direction gives rise to symmetry-broken valley quantum interference patterns when tuning the azimuth and twist angles of monoclinic hybrid structures. The azimuth angles of quantum interference fringes are susceptive to the variation of lattice displacement direction induced via the doping concentration. Thus, the valley quantum interference has great potential in estimating the doping concentration and the propagation directions of HShPs in monoclinic crystals.
Jia et al. (Mon,) studied this question.