The electronic interaction of two different atomic sheets stacked together with a relative twist leads to a spatially periodic potential-energy landscape: the moiré superlattice. Here we will present magneto-optical spectroscopy of MoSe2/WSe2 heterobilayer devices with a small relative twist. We will discuss moiré-trapped inter-layer excitons, which can emit quantum light, and intra-layer excitons, which are sensitive to a large number of strongly correlated electron and hole states as a function of fractional filling.

The two-dimensional (2D) moiré superlattices of transition metal dichalcogenides (TMDCs) have emerged as a promising platform to study correlated physics and excitons in 2D. The moiré potential strongly modulates the valley contrasting excitons resulting from enhanced Coulomb interaction. The moiré flatband further reduces the kinetic energy and leads to strongly correlated electrons. In this talk, we will discuss how to use optical spectroscopy to investigate the excitonic physics and strongly correlated physics in TMDC moiré superlattices, as well as strong interactions between excitons and correlated electrons.

We show that dark excitons in transition metal dichalcogenides (TMDs) can be controlled for optoelectronic applications. We report on the experimental observation of intervalley momentum-forbidden KΛ excitons. We show evidence that their formation and emission is related to compressive strain that activates a phonon-assisted intervalley scattering process that can be used as an ultrasensitive optical strain sensing mechanism. We also report on the repulsion-driven propagation of dark spin-forbidden excitons that, due to their permanent dipole, can travel for several microns. The unusual large distance covered by these exotic excitonic states can be further used for propagating valley and spin information across TMD samples, enabling several optoelectronic applications.

We explore possibilities to electrically control optical bistability by interfacing a two-level atomic system with an extended graphene sheet. Our theory incorporates the self-interaction of the optically-excited atom and its coupling to electromagnetic vacuum modes, both of which are sensitive to the actively-tunable interband transition threshold in graphene, thus enabling electrical tuning between bistable configurations. We show that bistability and hysteresis can manifest in average radiation power and resonance fluorescence spectrum of the atom, with the latter exhibiting a transition between a single Rayleigh peak and Mollow triplet by tuning the Fermi energy of the graphene sheet. The optically-driven atom-graphene system thus constitutes a platform for active control over driven quantum optical systems for explorations in coherent quantum control and atomic physics.

Here, I discuss the potential of developing exquisite quantum simulators composed of van der Waals heterostructures and the moiré superlattices that emerge in such materials. I outline several practical steps needed to realize such exciton quantum simulators: (1) tunable trapping potentials, (2) precise initialization of single excitations per trap, (3) tunable interaction parameters, (4) single excitation readout. Notably, these steps parallel the early stages of development in several existing quantum simulator technologies. In discussing this outlook, I simultaneously highlight our own experimental results that aim to demonstrate some of these foundational steps. While our measurements indicate several promising avenues for future technologies, I speculate that - perhaps most importantly - our results establish moiré superlattices as a promising next generation quantum simulator architecture.

The recent emergence of low dimensional quantum materials provides an excellent platform to investigate various elementary excitations for next generation devices. In some materials, the Coulombic attraction between 2D electrons and holes binds to form hydrogen-like quasiparticles known as excitons. The large binding energy together with the unprecedented light-matter interactions provides a unique platform for future classical and quantum devices serving various applications spanning from communication to sensing. In this talk, I will discuss our results on manipulating exciton dynamics including energy transport in 2D material system of transition metal dichalcogenides that leverage the complex band system, mechanical flexibility as well as dielectric screening. In particular, I will discuss the interaction of the 2D excitons with surface acoustic waves as a viable platform for hybrid quantum optoexcitonic devices.

The fundamental opto-electronic properties of two-dimensional (2D) materials can be tailored based on their nanoscale charge environment. Charge transfer at the interface of two atomically-thin layers offers a route to nanoscale charge modulation at the smallest possible length scales. In our study, we exploit this behavior to achieve nanoscale control of charge-transfer plasmon-polaritons (CPPs) and phonon-polaritons (PhPs) in graphene/α-RuCl3 and hBN/α-RuCl3 heterostructures, respectively. Using infrared near-field optical microscopy, we directly observe CPPs and PhPs, revealing emergent charge doping and optical conductivity at these novel 2D interfaces. Our results validate charge-transfer interfaces as tunable platforms for confined light.

Van der Waals materials can be placed on a variety of substrates with minimal degradation of their optical properties. They therefore open the opportunity to create polaritons in nearly arbitrary photonic structures. We will discuss a few different types of cavity systems integrated with van der Waals materials to realize polaritons with unusual properties, potentially enabling novel manybody phenomena.

Spin defects in foils of hexagonal boron nitride have potential in quantum sensing applications. In this contribution we discuss recent optically detected magnetic resonance experiments with ensembles of negatively charged boron vacancies. Time resolved detection is used to determine the spin-dependent intersystem crossing rates and to measure the zero-field splitting of the optically excited state. Furthermore, a continuous dynamic decoupling protocol is used to stabilize Rabi-oscillations of the ground state spin, extending the coherence time up to 4 µs, an improvement of ~150 times.

Wafer-scale epitaxial growth of semiconducting transition metal dichalcogenide (TMD) monolayers such as MoS2, WS2 and WSe2 is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Epitaxy is required to achieve single crystal films over large areas via coalescence of TMD domains. Our research has focused on epitaxial growth of 2D semiconducting TMDs on sapphire substrates using metalorganic chemical vapor deposition (MOCVD). Steps on the miscut sapphire surface serve as preferential sites for nucleation and can be used to induce a preferred crystallographic direction to the TMD domains which enables a reduction in inversion domain boundaries in coalesced films. The step-directed growth is dependent on the surface termination of the sapphire which can be altered through pre-growth annealing in H2 and chalcogen-rich environments. Uniform growth of TMD monolayers with significantly reduced inversion domains is

I will discuss our latest advances on the epitaxial growth of 2D van der Waals magnets and their integration with topological insulators (TI). These may produce novel topological states and highly efficient spin-orbit torque. Our initial studies of MnSe2 growth on Bi2Se3 showed a tendency for the interdiffusion of Mn into the Bi2Se3, ultimately leading to the synthesis of MnBi2Se4, a new magnetic TI. For bilayers of 2D magnets and TIs, we first optimized Fe3GeTe2 by studying its growth on Ge(111) and subsequently integrated with Bi2Te3. Interestingly, we observe room temperature ferromagnetism in Fe3GeTe2/Bi2Te3 heterostructures by varying the growth conditions.

2D transition metal dichalcogenides have two degenerate valleys which can be used to store and process information for quantum computing and communications. Here we report robust valley polarization in monolayer TMDs/chiral perovskite heterostructures at room temperature. We control valley index in monolayer TMDs via spin selective charge extraction with chiral perovskite and obtain 7% degree of polarization. We further investigate the charge transfer dynamics in the heterostructures by measuring transient absorption using pump-probe spectroscopy and show that charge transfer from monolayer TMD to chiral perovskite occurs within sub-picosecond timescales. Our results pave the way for practical valleytronics devices based on TMD/perovskite heterostructures.

The materials that have enabled the information technology revolution over the past decades will soon reach their physical limits. Novel nanomaterials and technologies have therefore become a major focus of current solid-state device research, with two-dimensional (2D) materials being one of the most promising candidates. While much progress has been made on an individual device level, only a few studies have looked into more complex systems. In this talk I will present realizations of optoelectronic and photonic systems, comprising a large number of 2D material-based devices. In particular, I will discuss photosensors that are able to simultaneously sense and process optical information without latency. We achieved both supervised and unsupervised learning and successfully trained the sensors to classify and encode images, that are optically projected onto the chip. I will discuss investigations of 2D device variability, its physical origins, and how it affects system performance.

I will present two experimental studies that leverage the optically-addressable valley degree of freedom in transition metal dichalcogenide monolayers (TMDs) to locally manipulate the symmetry of optical and magnetic processes. I will discuss our realization of an electrically-tunable chiral nanophotonic interface with a TMD. We fabricate optical waveguides directly on the surface of tungsten diselenide (WSe2) and demonstrate electrically-switchable chiral scattering into the waveguide. We also show that the waveguide acts as a local source for diffusive, spin-polarized excitonic fluxes. Second, I will show that ferromagnetic order in electrostatically-doped TMDs can be controlled by local optical pumping. We observe that interactions between the electrons can effectively amplify an input spin-imbalance. The local control of optical and magnetic symmetry in TMDs can unlock new spin-photonic technologies.

While high index dielectrics and plasmonics offer many opportunities for research and techonology in the field of nanophotonics, 2D materials can expand this potential in the visible and near-infrared due to high refractive indices, a large range of transparency windows, and new fabrication possibilities due to van der Waals adhesion to any substrate. We extract dielectric constants of 11 layered materials including TMDs, III-VI semiconductors, and magnetics. We fabricate nanoantennas and observe Mie resonances as well as strong coupling of TMD excitons and anapole modes with Rabi splittings of 140 meV. We also observe room temperature Purcell enhancement of WSe2 monolayer emission and low temperature formation of single photon emitters with enhanced quantum efficiencies. Due to weak adhesion to the substrate, we employed an AFM tip in the repositioning of dimer nanoantennas to form ultra-small hotspots enabling optical trapping of quantum emitters with Purcell factors above 150.

Transition metal dichalcogenide(TMD) monolayers and their heterostructures host promising nanoscale light emitters such as single defects and intra/interlayer-excitons. By resonantly coupling these emitters to optical nanocavities, various light-matter interaction phenomena including Purcell enhancement and strong coupling emerge. However, achieving exact matching between cavity and excitonic resonances is difficult mainly because of the inability to precisely control the resonant wavelength of fabricated optical structures. In this work, we demonstrate the use of a cryogenic strain cell to continuously and reversibly tune the optical resonance of GaP nanobeam cavities over a large range. We discuss the merits and challenges of this tuning technique and report our progress in using it to study cavity coupled phenomenon of emitters in monolayer TMD.

Strong light-matter interaction results in the formation of polaritons, quasiparticles that take on the properties of both light and material excitations. In this talk we will discuss our recent work on coupling of magnetically correlated excitons in van der Waals magnets with cavity photons. The prospects for modifying magneto-optical response and realizing opto-magnetic devices based on these magneto-polaritons will also be discussed.

Nonlinear optics plays an essential role in various photonic and optoelectronic applications, such as wavelength conversion and information processing. Recently, the extraordinarily large nonlinear optical properties of two-dimensional layered materials have attracted significant attention. However, the conversion efficiency of two-dimensional layered materials is typically limited by the atomically thick light-matter interaction length. Here, I will discuss the strategies to enhance optical nonlinearities of two-dimensional layered materials (e.g., graphene and transition metal dichalcogenides) for various integrated photonic and optoelectronic applications, such as high-purity quantum emitters, wavelength converters, and ultrafast lasers. I will also present our recent results of employing hybrid structures, such as mixed-dimensional heterostructures, plasmonic structures, and silicon/fibre waveguides integrated structures.

Strong coupling between light and excitations of a two-dimensional electron gas (2DEG) are important to both pure physics and to the development of future photonic nanotechnologies. Studying the relationship between spin polarisation of a 2DEG in monolayer semiconductor MoSe2, and resultant light-matter interactions modified by a zero-dimensional optical microcavity, finds the robust spin-susceptibility of the 2DEG simultaneously enhances/supresses trion-polariton formation in opposite photon helicities. This leads to optical non-linearities arising from the highly non-linear behaviour of the valley-specific strong light-matter coupling regime and allowing all-optical tuning of the enhanced polaritonic Zeeman splitting from 4 to more than 10 meV. https://www.nature.com/articles/s41566-022-01025-8

We propose a graphene-based metamaterial for achieving tunable spectral absorptivity in the infrared. The metamaterial is modeled as a periodic array of electrically-tunable, coupled resonators. The structural parameters of the resonators are chosen to implement a dark-bright mode coupling scheme in the context of temporal coupled-mode theory. The spectral response of the metamaterial can be tuned from single-peaked to double-peaked absorption by tuning the resonance wavelengths of the constituent resonators relative to each other. Our results thus suggest the possibility of achieving tunable multi-band absorption using metamaterials composed of multiple coupled resonators.

2D materials have many fascinating features, and here we report on the potential levitation of an ultra-thin magnetic material above a planar Au plate facilitated by repulsive Casimir forces. We considered a system consisting of an electric plate facing a magnetic plate. Through an investigation of the magnetic plate parameters, we show enhancement of the repulsion in a configuration where the plate has nm-scale thickness and a large permeability near ω=0. Additionally, high system temperatures can be used to enhance the effect. Magnetic 2D materials are thus excellent candidates for large-Casimir-repulsion systems and could be configured for levitation.

Excitons in 2D materials like transitional metal dichalcogenides (TMDCs) are important platforms for quantum information science (QIS). However, their short exciton lifetime and spin coherence time prevents them from being employed as on-chip devices in QIS. A dielectric metasurface that localizes light both temporally and spatially offers a new platform to enhance valley-polarized emission from TMDCs. We design and experimentally demonstrate a high-quality-factor Si metasurface with chiral “meta-atoms” that can strongly suppress intervalley scattering and enhance valley-polarized emission, potentially enabling room-temperature operation in strong-coupling regime.