Observation of naturally canalized phonon polaritons in LiV2O5 thin layers

Abstract

Polariton canalization is characterized by intrinsic collimation of energy flow along a single crystalline axis. This optical phenomenon has been experimentally demonstrated at the nanoscale by stacking and twisting van der Waals (vdW) layers of α-MoO₃, by combining α-MoO₃ and graphene, or by fabricating an h-BN metasurface. However, these material platforms have significant drawbacks, such as complex fabrication and high optical losses in the case of metasurfaces. Ideally, it would be possible to canalize polaritons "naturally" in a single pristine layer. Here, we theoretically predict and experimentally demonstrate naturally canalized phonon polaritons (PhPs) in a single thin layer of the vdW crystal LiV₂O₅. In addition to canalization, PhPs in LiV₂O₅ exhibit strong field confinement ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mrow><mml:mi>λ</mml:mi></mml:mrow> <mml:mrow><mml:mi>p</mml:mi></mml:mrow> </mml:msub> <mml:mo>~</mml:mo> <mml:mfrac> <mml:mrow> <mml:msub><mml:mrow><mml:mi>λ</mml:mi></mml:mrow> <mml:mrow><mml:mi>0</mml:mi></mml:mrow> </mml:msub> </mml:mrow> <mml:mrow><mml:mi>27</mml:mi></mml:mrow> </mml:mfrac> </mml:math> ), slow group velocity (0.0015c), and ultra-low losses (lifetimes of 2 ps). Our findings are promising for the implementation of low-loss optical nanodevices where strongly directional light propagation is needed, such as waveguides or optical routers.