Spectral tuning of hyperbolic shear polaritons in monoclinic gallium oxide via isotopic substitution

TL;DR

This study demonstrates how isotopic substitution in monoclinic gallium oxide can effectively tune hyperbolic shear polaritons' spectral properties, expanding their accessible frequency range for advanced nanophotonic applications.

Contribution

It introduces a novel spectral tuning method for hyperbolic shear polaritons using isotopic substitution, validated by near-field imaging and ab initio calculations.

Findings

Spectral redshift of ~40 cm$^{-1}$ observed with $^{18}$O substitution.

Near-field microscopy confirms the spectral shift without needing dielectric tensor knowledge.

Ab initio calculations agree with experimental data, validating the tuning method.

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 $\beta$-Ga$_2$O$_3$ (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 $^{18}$O bGO films homo-epitaxially grown on a $^{16}$O bGO substrate, we demonstrate a spectral redshift of $\sim~40~$cm$^{-1}$ for the $^{18}$O bGO, compared to $^{16}$O 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.