Session M31: Nanostructures and Metamaterials I

Moiré trions in MoSe2/WSe2 heterobilayers

Transition metal dichalcogenide moiré bilayers with spatially periodic potentials have emerged as a highly tunable platform for studying both electronic and excitonic phenomena. The power of these systems lies in the combination of strong Coulomb interactions with the capability of controlling the charge number in a moiré potential trap. Electronically, exotic charge orders at both integer and fractional fillings have been discovered. However, the impact of charging effects on excitons trapped in moiré potentials is poorly understood. In this work, we report the observation of moiré trions and their doping dependent photoluminescence polarization in H-stacked MoSe₂/WSe₂ heterobilayers. We found that as moiré traps are filled with either electrons or holes, new sets of interlayer exciton photoluminescence peaks with narrow linewidths emerge about 7 meV below the energy of the neutral moiré excitons. Circularly polarized photoluminescence reveals switching from co-circular to cross-circular polarizations as moiré excitons go from being negatively charged and neutral to positively charged. This switching results from the competition between valley-flip and spin-flip energy relaxation pathways of photo-excited electrons during interlayer trion formation. Our results offer a starting point for engineering both bosonic and fermionic many-body effects based on moiré excitons.   

Observation of Gigahertz Topological Valley Hall Effect in Nanoelectromechanical Phononic Crystals

Topological phononics offers numerous opportunities in manipulating elastic waves that can propagate in solids without being backscattered. Due to the lack of nanoscale imaging tools that aid the system design, however, acoustic topological metamaterials have been mostly demonstrated in macroscale systems operating at low (kilohertz to megahertz) frequencies. Here, we report the realization of gigahertz topological valley Hall effect in nanoelectromechanical AlN membranes. Propagation of elastic wave through phononic crystals is directly visualized by microwave microscopy with unprecedented sensitivity and spatial resolution. The valley Hall edge states, protected by band topology, are vividly seen in both real- and momentum-space. The robust valley-polarized transport is evident from the wave transmission across local disorder and around sharp corners, as well as the power distribution into multiple edge channels. Our work paves the way to exploit topological physics in integrated acousto-electronic systems for classical and quantum information processing in the microwave regime.   

Stacking and species ordering depended electronic properties of SiC/GeC bilayer heterostructur

Two dimensional polar materials, such as SiC or GeC monolayer, possess an in-plane charge transfer between different elements due to their different electron negativities. It is still not clear how the in-plane charge transfer plays a role when these polar materials are stacked vertically forming 2D polar heterostructures. In this work, we have systematically investigated this issue at the first principle level and found that, in addition to the vdW weak interaction, the electrostatic force triggered by such in-plane charge transfer could induce the π-π orbital hybridization between adjacent layers and forming electrostatic interlayer bonding which strongly depends on the stacking arrangement and the out-of-plane species ordering between layers. This additional interaction is the reason of the existence of tunable electronic properties by altering the stacking and species ordering. More interestingly, we found that the net spatial charge redistribution in the interfacial region leads to a built-in electric field, which could reduce the probability of photo-generated carrier recombination, making such polar heterostructures a promising candidate in the application of nanoelectronics   

Elastic Bilayer Metamaterial

Inspired by twisted bilayer graphene, we present an elastic bilayer metamaterial consisting of a plate with two graphene-like lattices of mechanical resonators on both sides of the plate. Each side can support surface resonant modes (SRM) that can interact depending on the thickness of the plate. Beyond twisting a lattice with respect to the other, the coupling strengths between the two SRM can be controlled via the plate thickness which strongly affects the dispersion of elastic waves in the bilayer structure. We theoretically characterize this dispersion by calculating the band structure for the cases of AA and AB stacking configurations, as well as for special cases of twisting angles that produces sublattices with even and odd symmetries. Furthermore, we explore the topology of the bands and uncover the possibility of creating topological elastic Valley states by breaking the symmetry of the bilayer metamaterial. The proposed bilayer plate could constitute a promising platform for manipulating mechanical waves and exploring quantum analog phenomena which could open routes toward innovative mechanical and optomechanical devices at the microscale for instance.   

Exciton-Polaritons in monolayer molybdenum disulphide coupled to hyperbolic metamaterial

We report the experimental observation of Exciton-Polaritons in monolayer Molybdenum disulphide (1L-MoS2) coupled to hyperbolic metamaterial (HMM). HMM offers unprecedented control in confining light at sub-wavelength scale and is a versatile platform for coupling emitters. Silver nanowires grown in a porous Alumina film have directional hyperbolic modes along the axis of nanowires. 1L-MoS2 is uniquely suited for coupling to the field of HMM modes, due its in-layer oriented excitonic transition dipole moment. The interaction between MoS2 and HMM might reach strong coupling regime, even with the losses inherent in a plasmonic system like HMM. Mechanically exfoliated 1L-MoS2 was transferred onto the fabricated HMM with a spacer of 10 nm. A double peak feature was observed at the position of MoS2 A exciton in both Photoluminescence (PL) and Absorption spectra. This confirms the strong coupling of 1L-MoS2 Excitons and HMM Plasmon Polaritons and formation of hybrid modes. Low temperature PL was measured to study the dispersion of hybrid Exciton-Polariton modes. Due to scalable fabrication of HMM, our study provides an economical pathway for realizing Polaritons at room temperature.   

A topological lattice of plasmonic merons

Topology is an emerging topic in physics, which explores geometrical properties that are impervious to continuous change in shape and size. Topological defects impart captivating emergent phenomena and serve as a bridge between microscopic, atomic, and even subatomic scales. Employing electromagnetic simulations and ultrafast, time-resolved photoemission electron microscopy, we describe the geometric transformation of normally incident circularly polarized optical fields from a square coupling structure, into surface plasmon polariton fields where the optical spin is converted into arrays of plasmonic vortices with orbital angular momentum. By electromagnetic simulation, we analyze how the spin orbit interaction molds the plasmonic orbital to spin angular momentum textures within each vortex domain into a lattice of plasmonic meron spin textures. We experimentally examine dynamics of the meron lattice by recording the ultrafast nanofemto plasmonic field evolution with deep subwavelength resolution and sub-optical cycle time accuracy. From these three-dimensional data we extract the linear polarization L-line singularity distribution with sub-diffraction limited resolution, and thereby define the meron periodic boundaries.   

Metasurface Design for the Generation of an Arbitrary Assembly of Different Polarization States

Manipulation of polarization states with metasurfaces is a compelling approach for on-chip photonics and portable information processing. Here we demonstrate a metasurface made of L-shaped resonators with different geometrical sizes to generate different types of polarization states simultaneously. Each resonator diffracts a right- or left-handed circularly polarized state with an additional geometrical-scaling-induced phase. The type of polarization state of each diffracted beam is determined by the enantiomorphism, size, and spatial sequence of the resonators in the unit cell. The number of the beams is modulated by the geometry and separation of the resonators. We provide examples to illustrate how to achieve the specific number of diffracted beams with the desired polarization states in experiments. We suggest that this strategy can be applied for integrated photonics and portable quantum information processing.   

Tuning of band diagrams via elastic deformation of 2D phononic crystals

Phononic crystals are macroscopic elastic metamaterials that have periodic modulations in material properties such as shear modulus, bulk modulus, and density. Propagation of acoustic waves through phononic crystals depends not just on material properties but also on the symmetry properties of the crystal. In the past, it was demonstrated that buckling of compressed phononic crystals could tune wave propagation properties. To explain these observations, we employ a representation theory-based formalism by drawing an analogy between band theory in electronic systems and that in phononic systems. In particular, we use representation theory to predict the lifting of degeneracies in band diagrams due to changes in phononic crystal symmetries upon elastic deformation. These predictions were confirmed in numerical calculations of band diagrams for compressed 2D phononic crystals. This holds the potential to better inform design strategies for the development of deformable phononic devices.   

Enhancement Goos–Hänchen shift in flatland hyperbolic metamaterials

In recent years experimental measurement of photonic spin Hall effect of metasurfaces has shown significant progress. This study investigates lateral Goos-Hänchen (GH) and transverse Imbert-Fedorov (IF) shifts of differently polarized light beams totally reflected from flatland hyperbolic metasurfaces that support hyperbolic phonon polaritons. Position and width of the isofrequency tunability of the hyperbolic phonon polariton (HPP) modes demonstrate the comparison of rotation of the metamaterial with that of infinite thickness. As a result, the GH-shift and IF-shift both show larger shifts that can be adjusted from the near to mid-infrared spectral region within and out of the natural Reststrahlen band. Further, we demonstrate the mechanism to obtain maximum values of shifts by changing the geometrical shape, filling factor of the flatland metamaterial, and the incidence angle of the light. The theoretical results obtained offer an alternative platform in designing new metasurfaces for nano-optical devices applications.   

Gate tunable topological transitions of biaxial hyperbolic polaritons in van der Waals heterostructures

α-MoO3 has shown great promise for infrared nanophotonics applications due to its biaxial hyperbolicity. However, one critical shortcoming is the lack of active tunability of these biaxial phonon polaritons. In this work, we address this problem by integrating graphene with α-MoO3, yielding a mode hybridization that results in the formation of broadband biaxial hyperbolic plasmon phonon polaritons, whose dispersion can easily be tuned via electrostatic gating of graphene. We also discuss the topological transitions of the isofrequency surfaces of these polaritons as a function of gate voltage and show that this heterostructure can lead to tunable anisotropic spontaneous emission rate, which has important consequences for applications like radiative heat transfer, quantum interference, and Kerr nonlinearity enhancement.   

Nanophotonic metasurface for a novel thermal management system

Electricity is pivotal to modern human civilization, powering different devices used in houses, business and industry. However, the most reliable source of electricity generation has been fossil fuel, which generates greenhouse gases leading to global warming. Indoor temperature regulation alone accounts for half the electricity use in US, making it the largest expense in the country. Therefore, a photonic platform that can regulate household temperature autonomously depending on ambient temperature without the use of electricity is the need of the hour. In this work, we present a passive nanophotonic thermostat based on VO2 thin films, capable of self-adjusting its absorptivity and emissivity to maintain a set temperature approximately locked within the phase transition regions. The experimental results demonstrate that the structured metasurface sample reflects most of the incident IR radiation during high temperature operation and absorbs most of it during low temperature conditions, exhibiting promising characteristics needed to realize a passive temperature regulation. This work would open further avenues for research such as artificial thermal skin for personal temperature regulation, thermal sensing and could solve critical challenges in radiative heat emergent phenomena.