Abstract
Advanced Materials 2026, 38, e73629
Tailoring Phonon-Driven Responses in ?-MoO3 through Isotopic Enrichment
T.S. Arnaud, R.W. Spangler, J.D. Georgaras, J.B. Haber, D. Hirt, M. Obst, G. Alonzo-Perez, M. Long III, F.G. Kaps, J. Wetzel, C. Ragle, J.E. Buchner, Y. Kim, A. Senarath, R. Niemann, M. He, G. Carini, U. Arregui-Leon, A.C. Behara, R. Bangari, N. Sahoo, N.C. Brumby, J.M. Klopf, M. Wolf, L.M. Eng, S.C. Kehr, T.G. Folland, A. Paarmann, P.E. Hopkins, F. Jornada, J-P. Maria, J.D. Caldwell
The implementation of polaritonic materials into nanoscale devices requires selective tuning of parameters to realize desired spectral or thermal responses. One robust material, ?-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 ?-MoO3 (98MoO3, 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 ?-MoO3 without the use of a subdiffractional surface scatterer are presented here. This work establishes the dual-element isotope enrichment of ?-MoO3 as an intrinsic strategy to design optical, thermal, and polaritonic properties.