Tailoring Phonon‐Driven Responses in α‐MoO ₃ 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‐MoO 3 , 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‐MoO 3 ( 98 MoO 3, Mo 18 O 3 , and 98 Mo 18 O 3 ) is explored. A uniform ∼5% spectral redshift from 18 O 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 98 Mo 18 O 3 of ∼50% along the [100] in the RB 2 and ∼100% along the [001] in the RB 3 with respect to 98 MoO 3 . Observations in both real and Fourier space of higher‐order HPhP modes propagating in slabs of isotopically enriched α‐MoO 3 without the use of a subdiffractional surface scatterer are presented here. This work establishes the dual‐element isotope enrichment of α‐MoO 3 as an intrinsic strategy to design optical, thermal, and polaritonic properties.