MoOCl$_2$ as a Hyperbolic Planar Platform for Nanooptics at Telecom Frequencies

Abstract

On-chip optoelectronics is fundamental to modern telecommunication, yet the diffraction limit of light remains a major obstacle to the extreme miniaturization of photonic integrated circuits (PICs). Hyperbolic polaritons (HPs) $-$ hybrid light-matter excitations in materials with opposite-signed dielectric permittivity tensor components $-$ offer a solution through their ability to support deep sub-wavelength confinement and unique optical phenomena such as canalization and negative refraction. To date, however, the most widely studied hyperbolic van der Waals (vdW) crystals, including hBN and $α$-MoO$_3$, operate mainly in the mid-infrared, leaving the telecommunication bands (1260$-$1675 nm) largely uncovered. Here, we predict HPs operating directly in the telecommunication window in the vdW crystal molybdenum oxychloride (MoOCl$_2$). Building on recent evidence that MoOCl$_2$ can support plasmon polaritons in the visible, we theoretically investigate its optical response at telecom wavelengths and identify the conditions under which strongly confined, canalized HPs modes emerge. Beyond establishing a telecom platform, we outline device-level opportunities enabled by these modes, including diffraction-free waveguides based on canalization, tunable polaritonic crystals, and high-efficiency spontaneous emission-enhancement platforms. These paradigms cover the essential pillars of on-chip information processing: emission, propagation, modulation and detection. Our results establish MoOCl$_2$ as a potentially transformative material that bridges physics of hyperbolic PPs with potential practical implementations, opening avenues for ultra-compact, high-density, and low-power photonic components.