arXiv subjects

Haozhe Tong

Publications and source records attributed to Haozhe Tong.

3 recordsLinked to original sources

arXiv2026

Local control and lateral nanofocusing of hyperbolic phonon polaritons

Phonon polaritons in van der Waals crystals enable exceptional light confinement and control over low-loss nanolight propagation. The polariton wavelength can be controlled by the crystal geometry, isotopic composition, or surrounding environment -- for which substrate engineering is particularly effective. However, existing approaches of substrate nanopatterning are binary and offer limited leverage. Here, we demonstrate local control over the wavelength of phonon polaritons in hexagonal boron nitride by employing a sinusoidally corrugated gold surface to smoothly vary the gap between the van der Waals crystal and metallic substrate. The nonuniform gap provides a continuous and nearly threefold local variation of the polariton wavelength across the structure, verified by near-field optical microscopy. Our platform further enables lateral nanofocusing by gradually compressing and decompressing the wavelength of propagating polaritons by a factor of around 2.5 achieved solely through substrate geometry, consistent with our local control experiments and theoretical calculations. Our results push the boundaries of substrate engineering and showcase a powerful method for precise and local tailoring of polaritonic modes.

physics.optics↗

arXiv2026

Launching of Visible-Range Hyperbolic Polaritons by Gold Nanoantennas in a natural van der Waals crystal

Anisotropic van der Waals materials provide a powerful platform for nanoscale optoelectronics, enabling strong light$-$matter interaction and deep electromagnetic field confinement mediated by polaritons, hybrid light$-$matter excitations with unique dispersion properties. While polaritonic phenomena in van der Waals heterostructures have been extensively explored in the mid-infrared frequency range, their behaviour at the visible frequencies remains largely unexplored, in part due to the lack of knowledge on natural materials supporting anisotropic and highly confined visible-range polaritons. In this context, MoOCl$_2$, an anisotropic van der Waals metal, is particularly interesting, since it supports hyperbolic plasmon polaritons (PPs) that enable directional propagation and subwavelength light compression. Here, we investigate the strategy for launching anisotropic PPs in MoOCl$_2$ in the visible frequency range using gold rod nanoantennas. The nanoantennas, placed on top of the MoOCl$_2$ crystal, excite in-plane anisotropic PP modes, effectively overcoming the momentum mismatch between waves in free-space and nanoscale PPs. We demonstrate a strong electromagnetic field confinement, angle-dependent absorption, and controlled anisotropic PP launching enabled by gold nanoantennas, highlighting the potential of MoOCl$_2$ as a compact platform for nanoscale waveguiding and optical signal processing. By providing a practical antenna-based strategy for exciting visible-range PPs, this work addresses the lack of compact elements for optical signal manipulation and opens new opportunities for optoelectronic devices based on van der Waals polaritonics.

physics.optics↗

arXiv2026

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

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.

physics.optics↗