Effect of Substrate Dielectric Properties on Hyperbolic Phonon Polaritons in Van Der Waals α‐MoO ₃

Hyperbolic phonon polaritons (HPhPs) in van der Waals (vdW) α‐MoO 3 crystals enable deep subwavelength confinement of infrared (IR) and terahertz (THz) waves, making them promising for nanophotonic and optoelectronic applications. HPhPs propagation and loss are strongly influenced by the dielectric properties of the supporting substrate, necessitating careful substrate selection. Here, we systematically investigate HPhPs in α‐MoO 3 on substrates with varying dielectric constants, including insulators, semiconductors, and suspended (air) configurations. Using an analytical model supported by numerical simulations and experimental validation via scattering‐type scanning near‐field optical microscopy (s‐SNOM), we demonstrate that substrate permittivity significantly modulates HPhP behavior. Suspended α‐MoO 3 exhibits elongated polariton wavelengths, longer propagation lengths, and a figure of merit (FOM) improved by up to 93%, 50%, and 6%, respectively, compared to supported counterparts. These results offer critical insight into substrate‐mediated polaritonic tuning and pave the way for design of high‐performance, low‐loss polaritonic devices.