Manipulating Confined Infrared Light via Polaritonic Design of Sub-Diffractional α-MoO₃ Wedges

The Mid-infrared (mid-IR) spectrum of light is crucial for various applications, including thermal imaging, molecular sensing, and free-space communication. However, the application of long free-space wavelengths of mid-IR light is limited in chip-scale devices due to the diffraction limit. This problem can be circumvented by using hyperbolic materials where the dielectric permittivities along the principal crystal directions exhibit opposite signs in the mid-IR spectral region. It has been well-demonstrated that hyperbolic materials (e.g. h-BN, α-MoO₃, α-V₂O₅) can confine high-momentum (short wavelength) electromagnetic waves in the form of hyperbolic phonon polaritons (HPhPs)- quasi-particles made from the hybridization of charged dipoles (phonons) and photons (an external light source). The propagation behavior of these HPhPs can be tuned and confined in deep sub-diffraction volumes using sub-wavelength structures, which offer distinct opportunities in the form of chip-scale nano-photonics devices for integrated optics and photonics applications.<br/><br/>In this work, we investigate sub-wavelength wedges of a hyperbolic material (α-MoO₃) to demonstrate in-plane focusing of electromagnetic waves beyond the diffraction limit and a transition of polaritonic propagation in the forbidden direction of the α-MoO₃ crystal. We design our wedges by optimizing lateral dimensions and wedge thickness using 3D numerical simulations via COMSOL Multiphysics. Numerically, we observed transitions from adiabatic compression to standing wave in polaritonic modes through changing wedge vertex angles (θ_v). For 10°&lt;θ_v&lt;30°, we observe a compression factor (i.e. ratio of wavelength in standing wave region to the adiabatically compressed region in the wedge) of 2.89 (at 700 cm-1) and 1.42 (at 900 cm-1) that converge to 1 upon increasing vertex angle. Further, we investigate the effect of geometrical confinement at vertex angles 120°&lt; θ_v &lt;150° on propagation direction and found that a high vertex angle of wedge enables the propagation of HPhPs in the forbidden direction. After establishing these theoretical foundations, we fabricated such wedges and characterized them with scattering-type Scanning Near-field Optical Microscopy (s-SNOM). Through s-SNOM, we found the presence of adiabatically compressed waves confined at the tip of the wedge at lower vertex angles and node formation along the forbidden propagation direction at high vertex angles. This work offers a novel tunable parameter for manipulating mid-IR light propagation in α-MoO₃ sub-wavelength structures. Further, these findings open avenues for chip-scale mid-IR nanophotonic devices and optical components with the ease of van der Waals integration for integrated optics and photonics applications.