Observation of topological polaritons and photonic magic angles in twisted van der Waals bi-layers (2004.14217v1)

Abstract: Twisted two-dimensional bi-layers offer exquisite control on the electronic bandstructure through the interlayer rotation and coupling, enabling magic-angle flat-band superconductivity and moir\'e excitons. Here, we demonstrate how analogous principles, combined with large anisotropy, enable extreme control and manipulation of the photonic dispersion of phonon polaritons (PhPs) in van der Waals (vdW) bi-layers. We experimentally observe tunable topological transitions from open (hyperbolic) to closed (elliptic) dispersion contours in twisted bi-layered {\alpha}-MoO3 at photonic magic angles, induced by polariton hybridization and robustly controlled by a topological quantity. At these transitions the bilayer dispersion flattens, exhibiting low-loss tunable polariton canalization and diffractionless propagation with resolution below {\lambda}0/40. Our findings extend twistronics and moir\'e physics to nanophotonics and polaritonics, with great potential for nano-imaging, nanoscale light propagation, energy transfer and quantum applications.

• The paper experimentally demonstrates tunable topological transitions in polariton dispersion from hyperbolic to elliptic states in twisted vdW bi-layers.
• It identifies photonic magic angles where dispersion flattening enables low-loss, diffractionless polariton canalization with resolution below λ₀/40.
• The study employs advanced infrared nanoimaging and numerical simulations to validate the theoretical predictions and experimental observations.

Topological Polaritons and Photonic Magic Angles in Twisted van der Waals Bi-Layers

This paper investigates the potential of twisted van der Waals (vdW) bi-layers as a platform for controlling photonic dispersion properties via topological transitions. The research expands the conventional twistronics concepts, previously explored within electronic materials, into the domain of nanophotonics and polaritonics. The authors concentrate on α-phase molybdenum trioxide (α-MoO₃) to observe transformative shifts in phonon polaritons (PhPs), the collective oscillations resulting from interactions between photons and lattice vibrations.

The key objective of this work is to experimentally demonstrate tunable topological transitions from hyperbolic to elliptic dispersion contours in twisted bi-layered α-MoO₃ at specific angles, which are termed as photonic magic angles. These polaritonic transitions are governed by a topological quantity, which dictates the dispersion behavior in the bi-layer system. A significant finding is the emergence of dispersion flattening at these topological transition angles, supporting low-loss, diffractionless polariton canalization with resolution subtending below λ₀/40. This precision holds substantial promise for nano-imaging, nanoscale light propagation, energy transfer, and quantum applications.

Key Contributions and Results

1. Topological Transitions in Polaritons: The paper presents experimental confirmation of topological transitions in polariton dispersion in twisted vdW bi-layers. The change in dispersion from hyperbolic to elliptic is quantitatively linked to the number of anti-crossing points (N_ACP), a topological invariant influenced by the angle of rotation between the bi-layers.
2. Magic Angle Dispersion Flattening: At the so-called photonic magic angles, the dispersion flattens, yielding topological transitions and facilitating a canalization regime for polaritons. This enables the unique phenomena of low-loss, collimated, and diffractionless polariton propagation, directly analogous to the flat band conditions responsible for superconductivity in twisted bi-layer graphene.
3. Experimental Setup and Validation: Through meticulous sample preparation and utilizing advanced techniques such as infrared real-space nanoimaging based on scattering-type scanning near-field optical microscopy, the researchers successfully visualize these polaritonic transitions and confirm their theoretical predictions.
4. Numerical and Analytical Corroboration: The experimental results are rigorously supported by numerical simulations and analytical dispersion models, providing robust validation of the proposed concepts.

Implications and Speculation for Future Research

The implications of this research are significant for the field of photonics and polaritonics. By utilizing the natural anisotropy and twist angle degrees of freedom in vdW materials, this work suggests new avenues for achieving subwavelength light manipulation and enhanced photonic dispersion control, potentially paving the way for novel devices in nanoimaging, sensing, and energy transfer. The inherent robustness and tunability of the topological transitions present opportunities for implementing stable and tunable devices across a wide range of operational frequencies.

Looking forward, future research directions could include the exploration of multi-layer systems with more complex twist angles or different material combinations to achieve even finer control over polaritonic properties. Furthermore, integrating this approach with other optical functionalities and materials may lead to innovative applications in quantum information processing and advanced optoelectronic devices. This work underscores the importance of connecting the advancements in twistronics with photonic implementations, opening up a versatile domain for the application of topology in functional material science.