Modulation of polaritons in chemically doped graphene ribbon/α-MoO ₃ heterostructures

Optical modulation is fundamental to photonics and optoelectronics, yet conventional modulators are often constrained by their physical size and operational efficiency. Two-dimensional (2D) materials, owing to their exceptional properties, offer compelling prospects for developing compact and efficient optical modulators. This study introduces a nanoscale polariton phase modulator utilizing a graphene/α-MoO₃ van der Waals heterostructure. By hybridizing surface plasmon polaritons in graphene with phonon polaritons in α-MoO₃, we realize a hybridized plasmon-phonon polariton (HPPP) mode. Chemical doping is employed to modulate the carrier concentration in graphene, which in turn induces a continuous topological transformation of the HPPP isofrequency contours in momentum space from hyperbolic to elliptical. This transformation directly alters the HPPP transmission wave vectors, enabling effective phase modulation within the heterostructure. Scanning near-field optical microscopy measurements reveal that the phase shift of the HPPP mode can be precisely controlled from 0 to π by varying the Fermi energy of graphene between 0.2 and 0.7 eV. Furthermore, the phase modulation effect is frequency-dependent, exhibiting robust controllability across the lower Reststrahlen band of α-MoO₃. This HPPP modulation scheme based on the graphene/α-MoO₃ heterostructure presents a novel technological pathway for creating ultra-compact optical modulators, holding significant potential for subwavelength-scale optical manipulation.