Tunable waveguide mode of hyperbolic phonon polaritons in <i>α</i> -MoO ₃ flake/gold slit composite structure

Hyperbolic phonon polaritons (HPhPs) in van der Waals layered polar crystals exhibit extreme light confinement capability, providing an unprecedented research opportunity to manipulate nanoscale midinfrared photons. Precise tuning of HPhPs propagation characteristics and near-field energy routing is crucial for applying polaritonic devices. Here, we demonstrate a widely tunable waveguide mode of HPhPs in an α- MoO 3 flake/gold slit composite structure. By varying the width of the gold slit from 3 μm to 220 nm, the compression ratio of HPhPs wavelength relative to the incident light wavelength can be adjusted from 35% to 8.6%, respectively. This is attributed to the introduction of the gold slit, which can effectively excite and guide HPhPs within an α- MoO 3 flake, forming a confined waveguide mode. Notably, the excitation efficiency of HPhPs in the nanoscale ultranarrow waveguide can be enhanced by integrating an extended port at the waveguide port. In addition, the routing of polaritons in a Y-shaped waveguide is realized by modulating the frequency of the incident light. This work presents a promising platform for manipulating deep subwavelength polaritons in planar photonic devices for infrared applications.