Tailoring phonon-driven responses in {\alpha}-MoO3 through isotopic enrichment

TL;DR

This study demonstrates how isotopic enrichment in {}-MoO3 can tune its phonon, optical, and thermal properties, enhancing its potential for thermal and nanophotonic applications through experimental and theoretical analysis.

Contribution

It introduces isotopic enrichment as a method to tune the phononic, optical, and thermal responses of {}-MoO3, supported by experimental and ab initio calculations.

Findings

Spectral redshift of ~5% in phonons due to isotope enrichment.

Increased Q-factors of HPhPs in enriched samples.

Observation of higher-order HPhP modes without surface scatterers.

Abstract

The implementation of polaritonic materials into nanoscale devices requires selective tuning of parameters to realize desired spectral or thermal responses. One robust material is {\alpha}-MoO3, which as an orthorhombic crystal boasts three distinct phonon dispersions, providing 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 {\alpha}-MoO3 (98MoO3, Mo18O3 and 98Mo18O3) are 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 their Q- factors are reported. A Q-factor maxima increase of ~50 % along the [100] in the RB2 and ~100 % along the [001] in the RB3 are reported for HPhPs supported in 98Mo18O3. Observations in both real and Fourier space of higher-order HPhP modes propagating in single slabs of isotopically enriched {\alpha}-MoO3 without the use of a subdiffractional surface scatterer are presented here. This work illustrates the tunability of {\alpha}-MoO3 for thermal and nanophotonic applications.