Tailoring Phonon-Driven Responses in α-MoO₃ through Isotopic Enrichment

The implementation of polaritonic materials into nanoscale devices requires selective tuning of parameters to realize desired spectral or thermal responses. One robust material, α-MoO₃, an orthorhombic crystal boasting three distinct phonon dispersions, provides three polaritonic dispersions of hyperbolic phonon polaritons (HPhPs) across the mid-infrared (MIR). Here, the tunability of both optical and thermal responses in isotopically enriched α-MoO₃ (⁹⁸MoO_3, Mo¹⁸O₃, and ⁹⁸Mo¹⁸O₃) is explored. A uniform ∼5% spectral redshift from ¹⁸O enrichment is observed in both Raman- and IR-active TO phonons. Both the in- and out-of-plane thermal conductivities for the isotopic variations are reported. Ab initio calculations both replicate experimental findings and analyze the select-mode three-phonon scattering contributions. The HPhPs from each isotopic variation are probed with s-SNOM, and we report an HPhP Q-factor maxima increase in ⁹⁸Mo¹⁸O₃ of ∼50% along the [100] in the RB₂ and ∼100% along the [001] in the RB₃ with respect to ⁹⁸MoO₃. Observations in both real and Fourier space of higher-order HPhP modes propagating in slabs of isotopically enriched α-MoO₃ without the use of a subdiffractional surface scatterer are presented here. This work establishes the dual-element isotope enrichment of α-MoO₃ as an intrinsic strategy to design optical, thermal, and polaritonic properties.