# Spectral Tuning of Hyperbolic Shear Polaritons in Monoclinic Gallium Oxide via Isotopic Substitution

## TL;DR

Scientists tuned the frequency of highly directional light waves in gallium oxide by replacing oxygen isotopes, enabling new applications.

## Contribution

A new method for spectral tuning of hyperbolic shear polaritons via isotopic substitution is demonstrated.

## Key findings

- Isotopic substitution of 16O to 18O in gallium oxide caused a redshift of ~40 cm−1 in hyperbolic shear polaritons.
- Near-field imaging confirmed the spectral shift without needing dielectric tensor knowledge.
- Far-field measurements and ab initio calculations validated the results.

## Abstract

Hyperbolic phonon polaritons ‐ hybridized modes arising from the ultrastrong coupling of infrared light to strongly anisotropic lattice vibrations in uniaxial or biaxial polar crystals ‐ enable to confine light to the nanoscale with low losses and high directionality. In even lower symmetry materials, such as monoclinic β‐Ga2O3 (bGO), hyperbolic shear polaritons (HShPs) further enhance the directionality. Yet, HShPs are intrinsically supported only within narrow frequency ranges defined by the phonon frequencies of the host material. Here, we report spectral tuning of HShPs in bGO by isotopic substitution. Employing near‐field optical microscopy to image HShPs in 18O bGO films homoepitaxially grown on a 16O bGO substrate, we demonstrate a spectral redshift of ∼40 cm−1 for the 18O bGO, compared to 16O bGO. The technique allows for direct observation and a model‐free estimation of the spectral shift driven by isotopic substitution without the need for knowledge of the dielectric tensor. Complementary far‐field measurements and ab initio calculations ‐ in good agreement with the near‐field data ‐ confirm the effectiveness of this estimation. This multifaceted study demonstrates a significant isotopic substitution induced spectral tuning of HShPs into a previously inaccessible frequency range, creating new avenues for technological applications of such highly directional polaritons.

Spectral tuning of highly directional hyperbolic shear polaritons is realized via isotopic substitution of 16O to 18O in monoclinic β‐phase gallium oxide. A red‐shift of almost 40 cm−1 is experimentally demonstrated with near‐field imaging, corroborated by the permittivity change extracted from far‐field experiments and density functional theory.