Imaging of Hyperbolic Plasmon-Phonon Polaritons in Twisted Double Bilayer Graphene

<br/>Complex van der Waals heterostructures can be assembled, layer-by-layer, via the deterministic transfer of selected two-dimensional layered materials. The transfer method used offers a significant degree of control and precision enabling assembly of new composite materials with engineered optical properties. Here, we have fabricated twisted double bilayer graphene using a new transfer system with far greater spatial resolution (20 nm) than the previous systems at Harvard. We transferred mechanically exfoliated graphene and hexagonal boron nitride (hBN) with this transfer system (which uses a polymer stamp consisting of Polydimethylsiloxane (PDMS) and polycarbonate (PC)) to form hBN encapsulated, twisted double bilayer graphene at very small (~0.03 degrees) twist angles. The top capping layers of hBN was between 5–10nm thick, as verified using atomic force microscopy, and the bottom layers ranged from 10–20nm thick. These stacks were subsequently imaged using scanning near field microscopy in the mid-infrared range to observe the interference fringes from the system's “mixed” hyperbolic plasmon-phonon polaritons. Excited with a tunable QCL laser, we were able to study the dispersion relation and propagation behavior of the hybridized polaritons in twisted double bilayer graphene.