Altering the reflection phase for nano-polaritons: a case study of hyperbolic surface polaritons in Hexagonal Boron Nitride

Polaritons – confined light-matter waves – in van der Waals (vdW) materials has been a research frontier in light-matter interactions with demonstrated nanophotonic advances that are unprecedented in conventional systems. Reflection, as a fundamental property of waves, is of particular importance for vdW polaritons, predominantly because it enables the imaging and investigations of polariton standing waves using the scanning probe technique. While all previous works demonstrate a rigid phase ~ π/4 for the polariton reflection, we report altering of polariton reflection phase by varying the geometry of polaritonic microstructures in the case study of hyperbolic surface polaritons (HSPs) in hexagonal boron nitride (hBN). Our combined s-SNOM experiments and electromagnetic simulations show the polariton reflection phase can be altered from the conventional value ~ 0.25π, increase continuously to ~ 0.5π and even jump to ~ -0.5π by varying the geometry of hBN microstructures. These results correspond to a fundamental mathematic axiom that should govern other polaritonic waves and even all complex physical parameters. The geometric engineering to alter fundamental properties of polariton waves, especially the reflection phase demonstrated here, can be further developed for applications in polaritonic circuits, transformation nano-light, beaming, biosensing, and optical elements.