2026/07/17 by Rajveer Fandan, Jorge Pedrós
#cond-mat.mes-hall
Achieving optical isolation requires breaking symmetry between forward- and backward-propagating light, a long-standing challenge at the nanoscale in the absence of magnetic fields. Here we theoretically demonstrate electrically tunable nonreciprocal phonon-plasmon polaritons in a graphene/α-MoO3/SiC heterostructure operating in the mid-infrared. A dc current in graphene induces a wavevector-dependent Doppler shift that breaks reciprocity and generates strong directional asymmetry in hybrid plasmon-phonon propagation. In the reciprocal regime, hybridization between graphene plasmons and hyperbolic phonon polaritons in α-MoO3, further shaped by the SiC substrate, enables gate-controlled transitions of isofrequency contours, including canalization along orthogonal crystal axes. At drift velocities of 5% of the Fermi velocity, the system exhibits pronounced momentum-dependent nonreciprocity with contrast reaching ∼ 0.3, while directions orthogonal to the drift remain unaffected due to symmetry imposed constraints. Real-space calculations confirm that this momentum-space asymmetry translates into directional near-field intensity modulation. These results establish current-biased van der Waals heterostructures as a platform for electrically tunable, magnet-free nonreciprocal nanophotonics in the mid-infrared.