Ultimate tuning of hyperbolic phonon polaritons

Volume-confined hyperbolic phonon polaritons (v-HPhPs), which are waveguide modes tightly bounded within anisotropic thin films, represent a distinctive platform for directional manipulations of light-matter interaction at the nanoscale. While the electrically tunable plasmons of atomically thin graphene offers a vital pathway for dynamical control of v-HPhPs in films of hundreds of nanometer thick, achieving the ultimate tuning efficiency is crucial for advancing v-HPhPs related applications. Here, we theoretically establish the fundamental bounds of v-HPhP tuning and experimentally attain this ultimate limit using interlayer-biased double-layer graphene integrated at the v-HPhP film/air interface. This approach is validated through electrical modulation of canalized v-HPhP propagation in α-MoO₃. Furthermore, we demonstrate the dynamic steering of in-plane directional energy flow at deep-subwavelength scales within mid-infrared range. This breakthrough unlocks promising avenues for dynamic on-chip functionalities, including reconfigurable thermal management and ultrasensitive biosensing.