Dynamic tuning of Bloch modes in anisotropic phonon polaritonic crystals

Polaritons—hybrid light-matter particles—allow light to be squeezed into deep subwavelength scales, holding great promise for ultra-compact photonic devices. By structuring materials into periodic crystals, known as polaritonic crystals (PoCs), researchers can engineer exotic optical modes called Bloch modes for enhanced light control. However, once fabricated, these crystals and their Bloch modes are fixed, lacking the dynamic tunability required for adaptive optical devices. While graphene supports highly tunable plasmon polaritons, their performance is limited by substantial optical losses. In contrast, low-loss materials such as alpha-phase molybdenum trioxide (α-MoO3) can support hyperbolic phonon polaritons (PhPs) with strong field confinement and in-plane anisotropy, yet they inherently lack this dynamic tunability.

In a new paper published in Light: Science & Applications, a team of scientists from Tongji University, Central South University, the City University of New York, and Pohang University of Science and Technology, has overcome this challenge by creating a hybrid PoC. They combined a low-loss, anisotropic α-MoO3 crystal, patterned with a nanoscale hole array, with a layer of electrically tunable graphene. This heterostructure enables the coupling between hyperbolic phonon polaritons in α-MoO3 and surface plasmon polaritons in graphene, forming hybrid phonon-plasmon polaritons (HPPPs). This coupling mechanism effectively merges the low-loss, anisotropic nature of α-MoO3 PhPs with the dynamic electrical tunability of graphene plasmons, thereby overcoming the static nature of conventional low-loss polaritonic crystals.

The key to their achievement is electrostatic gating. By applying a gate voltage, the researchers can tune the graphene's properties (i.e., Fermi level), which in turn dynamically alters the optical response of the entire heterostructure. Using a technique called scattering-type scanning near-field optical microscopy (s-SNOM), they directly observed—at the nanoscale—how the Bloch modes change shape, intensity, and wavelength in real-time as the voltage is varied.

A particularly groundbreaking finding was the electrical control over the crystal's band structure. The team showed that gating can shift special "flat-band" regions to align with the laser excitation frequency. These flat bands possess a high density of states, leading to a dramatic selective enhancement of the Bloch mode resonance. Furthermore, they achieved on-demand switching of the far-field radiation by electrically moving these flat bands in and out of the light cone, a critical boundary for light emission.

"This work establishes a reconfigurable platform for low-loss Bloch modes with electrically switchable far-field leakage in a graphene-gated α-MoO3 phonon polaritonic crystal" the scientists stated.

"This platform paves the way for adaptive nanophotonic systems, including reconfigurable optical devices and on-chip switches, advancing the field of dynamic nanophotonics, " they forecast.

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