Recent discoveries have shown that, when two layers of van der Waals (vdW) materials are superimposed with a relative twist angle between them, the electronic properties of the coupled system can be dramatically altered. Here, we demonstrate that a similar concept can be extended to the optics realm, particularly to propagating phonon polaritons–hybrid light-matter interactions. To do this, we fabricate stacks composed of two twisted slabs of a vdW crystal (α-MoO3) supporting anisotropic phonon polaritons (PhPs), and image the propagation of the latter when launched by localized sources. Our images reveal that, under a critical angle, the PhPs isofrequency curve undergoes a topological transition, in which the propagation of PhPs is strongly guided (canalization regime) along predetermined directions without geometric spreading. These results demonstrate a new degree of freedom (twist angle) for controlling the propagation of polaritons at the nanoscale with potential for nanoimaging, (bio)-sensing, or heat management.

Phonon polaritons (PhPs)lattice vibrations coupled to electromagnetic fieldsin highly anisotropic media display a plethora of intriguing optical phenomena, including ray-like propagation, anomalous refraction, and topological transitions, among others, which have potential for unprecedented manipulation of the flow of light at the nanoscale. However, the properties of these PhPs are intrinsically dictated by the anisotropic crystal structure of the host material. Although in-plane anisotropic PhPs can be steered, and even canalized, by twisting individual crystal slabs in a van der Waals (vdW) stack, active control of their propagation via external stimuli presents a significant challenge. Here, we report on a technology in which anisotropic PhPs supported by biaxial vdW slabs are actively tuned by simply gating an integrated graphene layer. Excitingly, we predict active tuning of optical topological transitions, which enable controlling the canalization of PhPs along different in-plane directions in twisted heterostructures. Apart from their fundamental interest, our findings hold promises for the development of optoelectronic devices (sensors, photodetectors, etc.) based on PhPs with dynamically controllable properties.

Polaritons-hybrid light-mater excitations -are very appealing for the confinement of light at the nanoscale. Recently, different kinds of polaritons have been observed in thin slabs of van der Waals (vdW) materials, with particular interest focused on phonon polaritons (PhPs) -lattice vibrations coupled to electromagnetic fields in the mid-infrared spectral range with -in biaxial crystals, such as e.g. -MoO3. In particular, hyperbolic PhPs -having hyperbola-like shape of their isofrequency curves -in -MoO3 can exhibit ultra-high momenta and strongly directional in-plane propagation, promising novel applications in imaging, sensing or thermal management at the nanoscale and in a planar geometry. However, the excitation and manipulation of in-plane hyperbolic PhPs have not yet been well studied and understood. Here we propose a technological platform for the effective excitation and control of in-plane hyperbolic PhPs based on polaritonic crystals (PCs) -lattices formed by elements separated by distances comparable to the PhPs wavelength -, twisted with respect to the natural vdW crystal axes. In particular, we develop a general analytical theory valid for an arbitrary PC made in a thin biaxial slab. As a practical example, we consider a twisted PC formed by rectangular hole arrays made in MoO3 slab and demonstrate the excitation of Bragg resonances tunable by the twisting angle. Our findings open novel avenues for both fundamental studies of PCs in vdW crystals

Twisted Polaritonic Crystals In article number 2200428, Alexey Y. Nikitin and colleagues introduce a concept of ultracompact polaritonic crystal –deeply subwavelength periodic structure– engineered in an in‐plane anisotropic van der Waals crystal slab. The polaritons inside the crystal can be efficiently controlled by twisting the axes of the periodic structure with respect to the axes of the anisotropic crystal slab. (Image credits: Aitana Tarazaga)

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