Simultaneous launching of phonon polaritons with two distinct topologies in Lu2SiO5 at a single frequency

Simultaneously exciting phonon polaritons with distinct topologies in a single crystal at a unified excitation frequency offers a route to combine their different propagation characteristics. In this work, by selecting an appropriate crystal plane in Lu2SiO5, we investigated the gold-disk-patterned Lu2SiO5 crystal using scattering-type scanning near-field optical microscopy (s-SNOM) and achieved the simultaneous excitation of two phonon polaritons with hyperbolic and elliptic-like isofrequency contours at a single frequency. The hyperbolic and elliptic-like phonon polaritons are launched by the fabricated gold disks and the SNOM tip, respectively. The propagation wave of the elliptic-like phonon polaritons is reflected by the gold disks and then is detected by the SNOM tip again, resulting in a momentum that is twice the intrinsic momentum, which is further confirmed by attenuated total reflection spectroscopy measurements using a coupled ZnSe prism for excitation. Our finding significantly enriches the anisotropic propagation behavior of phonon polariton in a single crystal.