Twisted Polaritonic Crystals in Thin van der Waals Slabs

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

Hyperbolic polaritonic crystals with configurable low-symmetry Bloch modes

“…A peak at 823 cm –1 can be found, corresponding to the TO of α-MoO 3 with a slight red shift caused by the SiO 2 substrate. Besides, more resonant peaks at higher frequencies appear, due to the scattering of wavefronts at holes and subsequent Bragg resonances 38 .…”

Tunable Optical Forces Enabled by Bilayer van der Waals Materials

“…In this paper, we propose and numerically demonstrate in situ tuning of optical forces using 𝛼-phase molybdenum trioxide (𝛼-MoO 3 ), a natural biaxial hyperbolic material that can provide highly anisotropic in-plane permittivities without the need of demanding fabrication process. 𝛼-MoO 3 has attracted wide attention due to its distinct in-plane hyperbolic phonon polaritons in the mid-infrared spectral range, which can exhibit unusual directionality and light confinement in twisted 𝛼-MoO 3 bilayer/trilayers, [25][26][27][28][29][30][31][32][33][34] negative refraction, [35][36][37] as well as dynamic planar focusing. [38][39][40] Although people have demonstrated twisted 𝛼-MoO 3 bilayer using dry transfer method to tailor the isofrequency contours, the necessity to redo the stacking process to change the twist angle makes dynamical tuning very challenging.…”