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 (alpha-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 hetero-structuring, and isotopic enrichment are being developed to alter the intrinsic dielectric 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 (MoO3)-Mo-92 ( Mo-92: 99.93 %) and (MoO3)-Mo-100 ( Mo-100: 99.01 %) crystals to tune the properties and dispersion of HPhPs with respect to natural alpha-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 in-plane HPhPs in alpha-MoO3 and isotopically enriched analogs within a reststrahlen band (approximate to 820 cm(-1) to approximate to 972 cm(-1) ). Results show that isotopic enrichment (e.g., (MoO3)-Mo-92 and 100MoO(3)) alters the dielectric function, shifting the HPhP dispersion (HPhP angular wavenumber x thickness vs. IR frequency)