Twistable Polaritonics with In-Operando Rotatable van der Waals Bilayers

As shown in previous chapters, in structures assembled from two-dimensional hyperbolic materials, like \(\alpha \text{-MoO}_{3}\) , the characteristics of propagating polaritons vary strongly with the rotation angle between the crystal lattices. These variations give rise to phenomena such as topological transitions and polariton canalization. However, usually the twist angle between layers is fixed after assembly. While the reconfiguration of layers has been successfully demonstrated in twisted \(\alpha \text{-MoO}_{3}\) trilayers in Chap. 8 , this methodology does not allow dynamically adjusting the twist angle during operation, and can also result in sample degradation. In this chapter, we develop several strategies aimed at achieving mechanically in-operando rotatable bilayers. Firstly, we unsuccessfully try to exert pressure with atomic force microscope (AFM) tip on the top slab of a bilayer to induce rotation and vary the twist angle. Then, we employ a methodology wherein synthesized microribbons of \(\alpha \text{-MoO}_{3}\) are transferred onto larger exfoliated \(\alpha \text{-MoO}_{3}\) flakes, and, again, we push the microribbon with the AFM tip to vary the twist angle. Through this approach, we observe variations with twist angle in properties such as the polariton wavelength within the same sample, which would be difficult to tune in static rotated structures. However, a clear elucidation of these effects is hindered due to the small microribbons’ dimensions, which function as microresonators, thereby introducing complexity to the result interpretation. Consequently, as a third avenue of exploration, we assemble a twisted bilayer from two exfoliated \(\alpha \text{-MoO}_{3}\) flakes, and we shape the upper \(\alpha \text{-MoO}_{3}\) layer via focused ion beam (FIB) to facilitate pushing with the AFM tip. Nonetheless, complications stemming from friction and dangling bonds between the \(\alpha \text{-MoO}_{3}\) slabs come into play into this approach, which remains a work in progress. The chapter culminates with a discussion on the conclusions and prospects of this research project. The experiments discussed in this chapter were carried out in Prof. Basov’s laboratory during a Fulbright stay at Columbia University (USA) in 2022, in collaboration with Dr. Maëlle Kapfer, Francesco L. Ruta and Samuel L. Moore. Sample fabrication was performed in collaboration with Ana Isabel Fernández Tresguerres-Mata. Focused Ion Beam was carried out by Dr. Lucía Herrer and Prof. José María de Teresa (Instituto de Nanociencia y Materiales de Aragón, Spain). The synthesized \(\alpha \text{-MoO}_{3}\) microribbons were provided by Shang-Jie Yu and Jonathan A. Fan (Stanford University, USA).