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

Area of Science:

• Condensed Matter Physics
• Materials Science
• Nanophotonics

Background:

• Polaritonic materials require precise parameter tuning for nanoscale device applications.
• Alpha-molybdenum trioxide (α-MoO₃) is a versatile material with three phonon dispersions supporting hyperbolic phonon polaritons (HPhPs) in the mid-infrared (MIR).
• Isotopic composition is a potential avenue for tuning material properties.

Purpose of the Study:

• To investigate the tunability of optical and thermal responses in isotopically enriched α-MoO₃.
• To explore the impact of dual-element isotope enrichment on HPhP properties.
• To establish isotope enrichment as a strategy for designing polaritonic materials.

Main Methods:

• Synthesis of isotopically enriched α-MoO₃ (⁹⁸MoO₃, Mo¹⁸O₃, and ⁹⁸Mo¹⁸O₃).
• Spectroscopic analysis (Raman and IR) to observe phonon shifts.
• Measurement of in- and out-of-plane thermal conductivities.
• Ab initio calculations for phonon scattering analysis.
• Scanning near-field optical microscopy (s-SNOM) to probe HPhPs.

Main Results:

• A consistent ~5% spectral redshift in Raman- and IR-active TO phonons was observed due to ¹⁸O enrichment.
• Thermal conductivities were measured for all isotopic variations.
• Significant increases in HPhP Q-factor were observed in ⁹⁸Mo¹⁸O₃ (up to ~100% along [001]).
• Higher-order HPhP modes were observed in isotopically enriched α-MoO₃ slabs without external scatterers.

Conclusions:

• Isotopic enrichment of α-MoO₃ offers a robust method for tuning its optical, thermal, and polaritonic characteristics.
• Dual-element isotope enrichment (⁹⁸Mo¹⁸O₃) provides substantial enhancement of HPhP properties, particularly Q-factor.
• This intrinsic tuning strategy is crucial for designing advanced nanoscale polaritonic devices.