Tunable Waveguide Mode of Hyperbolic Phonon Polaritons in α-MoO₃ 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 mid-infrared photons. Precise tuning of HPhPs propagation characteristics and near-field energy routing are crucial for applying polaritonic devices. Here, we demonstrate a widely tunable waveguide mode of HPhPs in an α-MoO₃ 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 that the introduction of the gold slit can effectively excite and guide HPhPs within α-MoO₃, forming a confined waveguide mode. Notably, the excitation efficiency of HPhPs in the nanoscale ultra-narrow 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.