Session U67: Nano-Plasmonics in Van Der Waals Materials and Beyond

Nanoscale optics of 2D material superlattices

Moiré patterns are well-known phenomena in art, textiles and mathematics, which originate from the overlay of two periodic patterns. Intriguingly, atomically thin materials can be stacked on top of each other such that a new periodic pattern can emerge: the moiré superlattice. This can result in a dramatic modification of the electronic and optical properties of twisted 2D materials, compared to those of a single layer. The moiré superlattice can give rise to a plethora of interesting phenomena such as topological bands [1] and many-body phases like superconductivity and magnetism [2].
In this talk, we give an overview of the nano-optical properties of 2D material superlattices, obtained by twisting or nanostructuring. Nanoscale optical techniques such as near-field optical microscopy reveal unique observations of topological domain wall boundaries, hyperbolic phononic lattices, and interband collective modes in charge neutral TBG near the magic angle [3]. The freedom to engineer these so-called optical and electronic quantum metamaterials [4] is expected to expose a myriad of unexpected phenomena.
References
[1] Ju et al., Nature (2015)
[2] Y. Cao et al. Nature 556, 80 (2018), Cao et al. Nature 556, 43 (2018)
[3] Hesp et al., Arxiv 1910.07893
[4] Song, Gabor et. al., Nature Nanotechnology (2019)
  

Collective modes of twisted van der Waals materials

Rotationally misalignment profoundly modifies electronic and lattice properties of van der Waals materials. If the twist angle between adjacent layers is small, a periodic network of dislocations (solitons) delineating large commensurate domains appears. We show that the solitons can have a surprisingly strong influence on collective modes of the system, e.g., plasmon- and phonon-polaritons. In particular, a twisted bilayer graphene can act as a photonic crystal for either two-dimensional or one-dimensional plasmon modes. On the other hand, in twisted boron nitride, the solitons can induce shift and splitting of the phonon-polaritons. Our theory provides explanation of recent nano-optical experiments done with twisted van der Waals heterostructures.
  

New quantum materials: Sculpting electromagnetic fields on the atomic scale

Tailoring light–matter interactions in monolayer MoS₂ is critical for its use in optoelectronic and nanophotonic devices. Optical cavities with multiple tunable resonances have the potential to provide unique electromagnetic environments at two or more distinct wavelengths—critical for control of optical processes such as nonlinear generation, entangled photon generation, or photoluminescence (PL) enhancement. Here, we show a plasmonic nanocavity based on a nanopatch antenna design that has two tunable resonant modes in the visible spectrum. The importance of utilizing two resonances simultaneously is demonstrated by integrating monolayer MoS₂, a two-dimensional semiconductor, into the colloidally synthesized nanocavities. We observe a 2000-fold enhancement in the PL intensity of MoS₂—which has intrinsically low absorption and small quantum yield—at room temperature, enabled by the combination of tailored absorption enhancement at the first harmonic and PL quantum-yield enhancement at the fundamental resonance [1]. Next, we explore how the emission spectrum can be tailored, including complex excitonic states. We demonstrate that the peak emission wavelengths of the A and B excitons can be tuned up to 40 and 25 nm, respectively, by integrating monolayer MoS₂ into a plasmonic nanocavity with tunable plasmon resonances [2]. Contrary to the intrinsic photoluminescence spectrum of monolayer MoS₂, we are also able to create a dominant B exciton peak when the nanocavity is resonant with its emission. Additionally, we observe a 1200-fold enhancement of the A exciton emission and a 6100-fold enhancement of the B exciton emission when normalized to the area under a single nanocavity and compared to a control sample on thermal oxide.
[1] G. Akselrod, T. Ming, C. Argyropoulos, T. Hoang, Y. Lin, X. Ling, D. R. Smith, J. Kong & M. H. Mikkelsen, Nano Letters 15, 3578 (2015)
[2] J. Huang, G. M. Akselrod, T. Ming, J. Kong & M. H. Mikkelsen, ACS Photonics, 5 (2), 552 (2017)

  

Collective mode spontaneous symmetry breaking: out-of-equilibrium plasmonic magnetism

Spontaneous symmetry breaking lies at the heart of the description of interacting phases of matter. We will argue that a driven interacting system subject to a linearly polarized (achiral) driving field can spontaneously magnetize (acquire chirality). In particular, we find when a metal is driven close to its plasmon resonance, it hosts strong internal ac fields that enable Berryogenesis: the spontaneous generation of a self-induced Bloch band Berry flux, which supports and is sustained by a circulating plasmonic motion, even for a linear polarized driving field. This non-equilibrium phase transition occurs above a critical driving amplitude, and depending on system parameters, can enter the spontaneously magnetized state in either a discontinuous or continuous fashion. Berryogenesis relies on nontrivial interband coherences for electronic states near the Fermi energy generated by ac fields readily found in a wide variety of multiband systems. We anticipate that graphene devices, in particular, which can host high quality plasmons, provide a natural and easily available platform to achieve Berryogenesis and spontaneous non-equilibrium (plasmon-mediated) magnetization in present-day devices, e.g., those based on graphene plasmonics.   

Nano-polaritonics in twisted van der Waals heterostructures

Interlayer coupling in atomic van der Waals heterostructures plays a rather unique role in controlling their optical and electronic properties. The character of the interlayer coupling can be manipulated by a particular stacking arrangement of the proximal layers and by adjusting the orientation of the neighboring planes. The latter method is known to trigger the long-range periodic modulations referred to as twisted moiré superlattices. The presence of periodic moiré patterns enables further fine tuning of the electronic band structure and yielding rich insights into the electronic phenomenon. This has been manifested in graphene/hexagonal boron nitride (G/hBN) moiré patterns, twisted bilayer graphene structures as well as twisted hBN crystals. Using nano-infrared optical microscopy we have experimentally studied the collective excitations in these twisted moiré structures. We analyzed these soliton networks and obtained the local electrodynamical characters based on the infrared active polaritonics at the nanoscale.

Reference:
G. X. Ni et al. Nature 505, 190 (2018).
S. S. Sunku et al. Science 362, 1153 (2018).
G. X. Ni et al. Nature Communications 10, 4360 (2019).
G. X. Ni et al. Nature Materials. 14, 1217 (2015).