The ability to manipulate light by squeezing it into smaller and smaller dimensions and steering it in the desired direction has given rise to a new field of study known as nanophotonics. One of the goals of nanophotonics is to create all-optical circuits that would replace electrons with photons in telecommunications technology. Over the past decade, hyperbolic phonon polaritons have played a key role in confining light to the nanoscale with low losses and high directionality. These hybridized modes arise from the ultrastrong coupling of infrared light to anisotropic lattice vibrations in uniaxial or biaxial polar crystals. In materials with lower symmetry, such as monoclinic beta-phase gallium oxide (bGO), hyperbolic shear polaritons (HShPs) emerge and their directionality is further enhanced.  However, HShPs have the disadvantage of only existing within a limited spectral range.

This paper of the Lattice Dynamics group reports spectral tuning of HShPs in bGO via isotopic substitution. Employing near-field optical microscopy to image HShPs in 18O bGO films homoepitaxially grown on a 16O bGO substrate at Paul Drude Institute, the publication demonstrates a spectral redshift of approximately 40 cm−1 for the 18O bGO, compared to 16O bGO isotope. 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, resulting from a collaboration with the University of Iowa and with the Theory Department of FHI, demonstrates isotopic substitution to enable significant spectral tuning of HShPs into a previously inaccessible frequency range, creating new avenues for technological applications of such highly directional polaritons. Reference: Carini et al., Adv. Mater. 38, e14561 (2026)