Ultralow-Loss Phonon Polaritons in the Isotope-Enriched α-MoO 3 .

α-MoO 3 , a natural van der Waals (vdWs) material, has received wide attention in nano-optics for supporting highly confined anisotropic phonon polaritons (PhPs) from the mid-infrared to the terahertz region, which opens a new route for manipulating light at the nanoscale. However, its optical loss hinders light manipulation with high efficiency. This work demonstrates that the isotope-enriched Mo element enables ultralow-loss PhPs in the α-MoO 3 . Raman spectra reveal that the isotope-enriched Mo element in the α-MoO 3 allows different optical phonon frequencies by efficiently altering the Reststrahlen band's dispersion. The Mo isotope-enriched α-MoO 3 significantly reduces the PhPs' optical loss due to efficient optical coherence, which enhances the propagation length revealed by infrared nanoimaging. These findings suggest that the isotope-enriched α-MoO 3 is a new feasible 2D material with an ultralow optical loss for possible high-performance integrated photonics and quantum optics devices.