Ermolaev, Georgy A.Voronin, Kirill V.Toksumakov, Adilet N.Grudinin, Dmitriy V.Fradkin, Ilia M.Mazitov, ArslanSlavich, Aleksandr S.Tatmyshevskiy, Mikhail K.Yakubovsky, Dmitry I.Solovey, Valentin R.Kirtaev, Roman V.Novikov, Sergey M.Zhukova, Elena S.Kruglov, IvanVyshnevyy, Andrey A.Baranov, Denis G.Ghazaryan, Davit A.Arsenin, Aleksey V.Martin-Moreno, LuisVolkov, Valentyn S.Novoselov, Kostya S.2024-04-172024-04-172024-04-172024-03-0610.1038/s41467-024-45266-3https://infoscience.epfl.ch/handle/20.500.14299/207191WOS:001180826600026Nature is abundant in material platforms with anisotropic permittivities arising from symmetry reduction that feature a variety of extraordinary optical effects. Principal optical axes are essential characteristics for these effects that define light-matter interaction. Their orientation - an orthogonal Cartesian basis that diagonalizes the permittivity tensor, is often assumed stationary. Here, we show that the low-symmetry triclinic crystalline structure of van der Waals rhenium disulfide and rhenium diselenide is characterized by wandering principal optical axes in the space-wavelength domain with above pi/2 degree of rotation for in-plane components. In turn, this leads to wavelength-switchable propagation directions of their waveguide modes. The physical origin of wandering principal optical axes is explained using a multi-exciton phenomenological model and ab initio calculations. We envision that the wandering principal optical axes of the investigated low-symmetry triclinic van der Waals crystals offer a platform for unexplored anisotropic phenomena and nanophotonic applications.|Principal optical axes define light-matter interactions in crystals and they are usually assumed to be stationary. Here, the authors report the observation of wavelength-dependent principal optical axes in ternary van der Waals crystals (ReS2 and ReSe2), leading to wavelength-switchable propagation directions of their waveguide modes.Hyperbolic Surface-PolaritonsAtomically ThinNegative RefractionPhotonic CrystalsSymmetryRes2AnisotropyExcitonstext::journal::journal article::research article