Twist-tunable polaritonic nanoresonators in a van der Waals crystal

Nonlocal Acoustic Moiré Hyperbolic Metasurfaces

The discovery of the topological transition in twisted bilayer (tBL) materials has attracted considerable attention in nano-optics. In the analogue of acoustics, however, no such topological transition has been found due to the inherent nondirectional scalar property of acoustic pressure. In this work, by using a theory-based nonlocal anisotropic design, the in-plane acoustic pressure is transformed into a spatially distributed vector field using twisted multilayer metasurfaces. So-called "acoustic magic angle"-related acoustic phenomena occur, such as nonlocal polariton hybridization and the topological Lifshitz transition. The dispersion becomes flat at the acoustic magic angle, enabling polarized excitations to propagate in a single direction. Moreover, the acoustic topological transition (from hyperbolic to elliptic dispersion) is experimentally observed for the first time as the twist angle continuously changes. This unique characteristic facilitates low-loss tunable polariton hybridization at the subwavelength scale. A twisted trilayer acoustic metasurface is also experimentally demonstrated, and more possibilities for manipulating acoustic waves are found. These discoveries not only enrich the concepts of moiré physics and topological acoustics but also provide a complete framework of theory and methodologies for explaining the phenomena that are observed.

Gate-Tuning Hybrid Polaritons in Twisted α-MoO3/Graphene Heterostructures

Modulating anisotropic phonon polaritons (PhPs) can open new avenues in infrared nanophotonics. Promising PhP dispersion engineering through polariton hybridization has been demonstrated by coupling gated graphene to single-layer α-MoO3. However, the mechanism underlying the gate-dependent modulation of hybridization has remained elusive. Here, using IR nanospectroscopic imaging, we demonstrate active modulation of the optical response function, quantified in measurements of gate dependence of wavelength, amplitude, and dissipation rate of the hybrid plasmon-phonon polaritons (HPPPs) in both single-layer and twisted bilayer α-MoO3/graphene heterostructures. Intriguingly, while graphene doping leads to a monotonic increase in HPPP wavelength, amplitude and dissipation rate show transition from an initially anticorrelated decrease to a correlated increase. We attribute this behavior to the intricate interplay of gate-dependent components of the HPPP complex momentum. Our results provide the foundation for active polariton control of integrated α-MoO3 nanophotonics devices.

In-plane hyperbolic polariton tuners in terahertz and long-wave infrared regimes

One of the main bottlenecks in the development of terahertz (THz) and long-wave infrared (LWIR) technologies is the limited intrinsic response of traditional materials. Hyperbolic phonon polaritons (HPhPs) of van der Waals semiconductors couple strongly with THz and LWIR radiation. However, the mismatch of photon - polariton momentum makes far-field excitation of HPhPs challenging. Here, we propose an In-Plane Hyperbolic Polariton Tuner that is based on patterning van der Waals semiconductors, here α-MoO₃, into ribbon arrays. We demonstrate that such tuners respond directly to far-field excitation and give rise to LWIR and THz resonances with high quality factors up to 300, which are strongly dependent on in-plane hyperbolic polariton of the patterned α-MoO₃. We further show that with this tuner, intensity regulation of reflected and transmitted electromagnetic waves, as well as their wavelength and polarization selection can be achieved. Our results can help the development of THz and LWIR miniaturized devices.