Dynamic tuning of Bloch modes in anisotropic phonon polaritonic crystals.

Phonon polaritons, arising from the coupling of photons with lattice vibrations, enable light confinement on deeply subwavelength scales. Phonon polaritonic crystals (PoCs), leveraging these inherently low-dissipation excitations, have further shown exceptional potential for nanoscale light manipulation through engineered Bloch modes. Yet, their static nature has so far hindered dynamic modulation, thus limiting their adaptability for real-time applications. Here, we demonstrate in situ electrostatic control of low-loss anisotropic phonon-polaritonic Bloch modes in α-MoO3 patterned into a periodic hole array with a graphene gate. Through theoretical calculation and real-space nano-imaging, we show that electrostatic gating dynamically modulates key characteristics of Bloch modes in hybrid α-MoO3/graphene PoCs. Critically, gating reshapes the PoC band structure, spectrally aligning high-density-of-states flat-band regions with the excitation laser frequency, thereby selectively amplifying Bloch mode resonances. We further achieve on-demand switching over far-field leakage of Bloch modes by electrostatically steering these flat bands across the light cone. Our work establishes a platform for adaptive nanostructured phonon polaritonic devices. This advancement not only facilitates directional control of low-loss anisotropic phonon-polaritonic Bloch modes, but also paves the way for their practical application in nanophotonics.