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Register a new userStrongly Coupled Photonic Moire Superlattices
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The moire superlattice of misaligned atomic bilayers paves the way for designing a new class of materials with wide tunability. In this work, we propose a photonic analog of the moire superlattice based on dielectric resonator quasi-atoms. In sharp contrast to van der Waals materials with weak interlayer coupling, we realize the strong coupling regime in a moire superlattice, characterized by cascades of robust flat bands at large twist angles. Surprisingly, we find that these flat bands are characterized by a non-trivial band topology, the origin of which is the moire pattern of the resonator arrangement. The physical manifestation of the flat band topology is a robust one-dimensional conducting channel on the edge, protected by the reflection symmetry of the moire superlattice. By explicitly breaking the underlying reflection symmetry on the boundary terminations, we show that the first-order topological edge modes naturally deform into higher-order topological corner modes. Our work pioneers the physics of the moire superlattice beyond the weakly coupled regime and introduces a designable platform to control photonic topological insulator phases using moire patterns.
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Two-dimensional photonic crystal membranes provide a versatile planar architecture for integrated photonics to control the propagation of light on a chip employing high quality optical cavities, waveguides, beamsplitters or dispersive elements. When combined with highly non-linear quantum emitters, quantum photonic networks operating at the single photon level come within reach. Towards large-scale quantum photonic networks, selective dynamic control of individual components and deterministic interactions between different constituents are of paramount importance. This indeed calls for switching speeds ultimately on the systems native timescales. For example, manipulation via electric fields or all-optical means have been employed for switching in nanophotonic circuits and cavity quantum electrodynamics studies. Here, we demonstrate dynamic control of the coherent interaction between two coupled photonic crystal nanocavities forming a photonic molecule. By using an electrically generated radio frequency surface acoustic wave we achieve optomechanical tuning, demonstrate operating speeds more than three orders of magnitude faster than resonant mechanical approaches. Moreover, the tuning range is large enough to compensate for the inherent fabrication-related cavity mode detuning. Our findings open a route towards nanomechanically gated protocols, which hitherto have inhibited the realization in all-optical schemes.
Moire superlattices in van der Waals (vdW) heterostructures have given rise to a number of emergent electronic phenomena due to the interplay between atomic structure and electron correlations. A lack of a simple way to characterize moire superlattices has impeded progress in the field. In this work we outline a simple, room-temperature, ambient method to visualize real-space moire superlattices with sub-5 nm spatial resolution in a variety of twisted vdW heterostructures including but not limited to conducting graphene, insulating boron nitride and semiconducting transition metal dichalcogenides. Our method utilizes piezoresponse force microscopy, an atomic force microscope modality which locally measures electromechanical surface deformation. We find that all moire superlattices, regardless of whether the constituent layers have inversion symmetry, exhibit a mechanical response to out-of-plane electric fields. This response is closely tied to flexoelectricity wherein electric polarization and electromechanical response is induced through strain gradients present within moire superlattices. Moire superlattices of 2D materials thus represent an interlinked network of polarized domain walls in a non-polar background matrix.
We demonstrate reversible strain-tuning of a quantum dot strongly coupled to a photonic crystal cavity. We observe an average redshift of 0.45 nm for quantum dots located inside the cavity membrane, achieved with an electric field of 15 kV/cm applied to a piezo-electric actuator. Using this technique, we demonstrate the ability to tune a quantum dot into resonance with a photonic crystal cavity in the strong coupling regime, resulting in a clear anti-crossing. The bare cavity resonance is less sensitive to strain than the quantum dot and shifts by only 0.078 nm at the maximum applied electric field.
Atomically-thin layers of two-dimensional materials can be assembled in vertical stacks held together by relatively weak van der Waals forces, allowing for coupling between monolayer crystals with incommensurate lattices and arbitrary mutual rotation. A profound consequence of using these degrees of freedom is the emergence of an overarching periodicity in the local atomic registry of the constituent crystal structures, known as a moire superlattice. Its presence in graphene/hexagonal boron nitride (hBN) structures led to the observation of electronic minibands, whereas its effect enhanced by interlayer resonant conditions in twisted graphene bilayers culminated in the observation of the superconductor-insulator transition at magic twist angles. Here, we demonstrate that, in semiconducting heterostructures built of incommensurate MoSe2 and WS2 monolayers, excitonic bands can hybridise, resulting in the resonant enhancement of the moire superlattice effects. MoSe2 and WS2 are specifically chosen for the near degeneracy of their conduction band edges to promote the hybridisation of intra- and interlayer excitons, which manifests itself through a pronounced exciton energy shift as a periodic function of the interlayer rotation angle. This occurs as hybridised excitons (hX) are formed by holes residing in MoSe2 bound to a twist-dependent superposition of electron states in the adjacent monolayers. For heterostructures with almost aligned pairs of monolayer crystals, resonant mixing of the electron states leads to pronounced effects of the heterostructures geometrical moire pattern on the hX dispersion and optical spectrum. Our findings underpin novel strategies for band-structure engineering in semiconductor devices based on van der Waals heterostructures.
We theoretically demonstrate that moire phonons at the lowest-energy bands can become chiral. A general symmetry analysis reveals that they originate from stacking configurations leading to an asymmetric interlayer binding energy that breaks the $C_{2z}$ symmetry on the moire length scale. Within elastic theory, we provide a complete classification of van der Waals heterostructures in respect to hosting moire chiral phonons and discuss their emergence in twisted bilayer MoS$_2$ as an example. The formation of the chiral phonons can be qualitatively understood using an effective model, which emphasizes their origin in the energy difference between stacking domains. Since moire chiral phonons are highly tunable, with excitation energies in only a few meV, and moire scale wavelengths, they might find potential applications in phononic twistronic devices.
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