Twisted polaritonic crystals in thin van der Waals slabs

TL;DR

This paper introduces a theoretical framework for twisted polaritonic crystals in thin van der Waals slabs, enabling controlled excitation of hyperbolic phonon polaritons with potential applications in nanoscale imaging and sensing.

Contribution

It develops a general analytical theory for twisted polaritonic crystals in biaxial vdW slabs and demonstrates tunable Bragg resonances in MoO3-based structures.

Findings

Analytical model for twisted polaritonic crystals in biaxial slabs

Demonstration of tunable Bragg resonances via twisting angle

Potential for enhanced mid-infrared sensing and photodetection

Abstract

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 and the development of mid-infrared sensing and photodetection applications