Tuning polariton hybridization in hyperbolic hetero-bicrystals by twist angle engineering

Phonon polaritons are hybrid light-matter quasiparticles that enable confinement and control of electromagnetic modes at the nanoscale. Particular interest has been paid to hetero-bicrystals composed of molybdenum oxide (α-MoO₃) and isotopically pure hexagonal boron nitride (h¹¹BN), which feature polariton dispersion tailorable via the spectral gap originating from polariton hybridization. In this work, we propose unexplored hetero-crystals assembled from Ca-intercalated metal oxide α'-(Ca)V₂O₅ and α-MoO₃, allowing the polaritons to travel along closed trajectories inside the bicrystal within the spectral gap. We systematically study the dependence of the spectral gap on the twist angle and thickness ratio of constituting layers in α'-(Ca)V₂O₅/α-MoO₃ and the initial h¹¹BN/α-MoO₃. We show that on-off switching and strong tuning of the spectral gap in polariton dispersion can be realized by varying the twist angle in both structures. In particular, the spectral gap in α'-(Ca)V₂O₅/α-MoO₃ can exist over a wide range of twist angles, three times as broad as that in h¹¹BN/α-MoO₃. Moreover, the spectral gap can also be significantly tuned by altering the thickness ratio. The spectral gap in α'-(Ca)V₂O₅/α-MoO₃ emerges in a higher frequency range that is not achievable in h¹¹BN/α-MoO₃. Our results demonstrate that α'-(Ca)V₂O₅/α-MoO₃ and h¹¹BN/α-MoO₃ provide two powerful bicrystal systems for polariton engineering via tuning the twist angle and thickness ratio.