Isotopic effects on in-plane hyperbolic phonon polaritons in MoO₃

Hyperbolic phonon polaritons (HPhPs), hybrids of light and lattice vibrations in polar dielectric crystals, empower nanophotonic applications by enabling the confinement and manipulation of light at the nanoscale. Molybdenum trioxide (α-MoO₃) is a naturally hyperbolic material, meaning that its dielectric function deterministically controls the directional propagation of in-plane HPhPs within its reststrahlen bands. Strategies such as substrate engineering, nano- and heterostructuring, and isotopic enrichment are being developed to alter the intrinsic die ectric functions of natural hyperbolic materials and to control the confinement and propagation of HPhPs. Since isotopic disorder can limit phonon-based processes such as HPhPs, here we synthesize isotopically enriched ⁹²MoO₃ (⁹²Mo: 99.93 %) and ¹⁰⁰MoO₃ (¹⁰⁰Mo: 99.01 %) crystals to tune the properties and dispersion of HPhPs with respect to natural α-MoO₃, which is composed of seven stable Mo isotopes. Real-space, near-field maps measured with the photothermal induced resonance (PTIR) technique enable comparisons of inplane HPhPs in α-MoO₃ and isotopically enriched analogues within a reststrahlen band (≈820 cm^-1 to ≈ 972 cm^-1). Results show that isotopic enrichment (e.g., ⁹²MoO₃ and ¹⁰⁰MoO₃) alters the dielectric function, shifting the HPhP dispersion (HPhP angular wavenumber × thickness vs IR frequency) by ≈-7% and ≈ +9 %, respectively, and changes the HPhP group velocities by ≈ ±12 %, while the lifetimes (≈ 3 ps) in ⁹²MoO₃ were found to be slightly improved (≈ 20 %). The latter improvement is attributed to a decrease in isotopic disorder. Altogether, isotopic enrichment was found to offer fine control over the properties that determine the anisotropic in-plane propagation of HPhPs in α-MoO₃, which is essential to its implementation in nanophotonic applications.