Metasurface-Controlled Photonic Rashba Effect for Upconversion Photoluminescence.

We demonstrate the effect of spin-momentum locking of upconversion photoluminescence emitted from rare-earth doped nanocrystals coupled to a phase-gradient dielectric metasurface. We observe different directionalities for left and right circular polarized light and associate this experimental observation with the photonic Rashba effect realized for upconverted photoluminescence that is manifested in the spin-dependent splitting of emitted light in the momentum space.

Hyperspectral study of the coupling between trions in WSe2 monolayers to a circular Bragg grating cavity.

Circular Bragg gratings compose a very appealing photonic platform and nanophotonic interface for the controlled light-matter coupling of emitters in nanomaterials. Here, we discuss the integration of exfoliated monolayers of WSe2 with GaInP Bragg gratings. We apply hyperspectral imaging to our coupled system, and explore the spatio-spectral characteristics of our coupled monolayer-cavity system. Our work represents a valuable step towards the integration of atomically thin quantum emitters in semiconductor nanophotonic cavities.

Shaping and spatiotemporal characterization of sub-10-fs pulses focused by a high-NA objective.

We describe a setup consisting of a 4f pulse shaper and a microscope with a high-NA objective lens and discuss the aspects most relevant for an undistorted spatiotemporal profile of the focused beam. We demonstrate shaper-assisted pulse compression in focus to a sub-10-fs duration using phase-resolved interferometric spectral modulation (PRISM). We introduce a nanostructure-based method for sub-diffraction spatiotemporal characterization of strongly focused pulses. The distortions caused by optical aberrations and space-time coupling from the shaper can be reduced by careful setup design and alignment to about 10 nm in space and 1 fs in time.

Interactions between Fermi polarons in monolayer WS2.

Interactions between quasiparticles are of fundamental importance and ultimately determine the macroscopic properties of quantum matter. A famous example is the phenomenon of superconductivity, which arises from attractive electron-electron interactions that are mediated by phonons or even other more exotic fluctuations in the material. Here we introduce mobile exciton impurities into a two-dimensional electron gas and investigate the interactions between the resulting Fermi polaron quasiparticles. We employ multi-dimensional coherent spectroscopy on monolayer WS2, which provides an ideal platform for determining the nature of polaron-polaron interactions due to the underlying trion fine structure and the valley specific optical selection rules. At low electron doping densities, we find that the dominant interactions are between polaron states that are dressed by the same Fermi sea. In the absence of bound polaron pairs (bipolarons), we show using a minimal microscopic model that these interactions originate from a phase-space filling effect, where excitons compete for the same electrons. We furthermore reveal the existence of a bipolaron bound state with remarkably large binding energy, involving excitons in different valleys cooperatively bound to the same electron. Our work lays the foundation for probing and understanding strong electron correlation effects in two-dimensional layered structures such as moiré superlattices.

Large Area Ultrathin InN and Tin Doped InN Nanosheets Featuring 2D Electron Gases.

Indium nitride (InN) has been of significant interest for creating and studying two-dimensional electron gases (2DEG). Herein we demonstrate the formation of 2DEGs in ultrathin doped and undoped 2D InN nanosheets featuring high carrier mobilities at room temperature. The synthesis is carried out via a two-step liquid metal-based printing method followed by a microwave plasma-enhanced nitridation reaction. Ultrathin InN nanosheets with a thickness of ∼2 ± 0.2 nm were isolated over large areas with lateral dimensions exceeding centimeter scale. Room temperature Hall effect measurements reveal carrier mobilities of ∼216 and ∼148 cm2 V-1 s-1 for undoped and doped InN, respectively. Further analysis suggests the presence of defined quantized states in these ultrathin nitride nanosheets that can be attributed to a 2D electron gas forming due to strong out-of-plane confinement. Overall, the combination of electronic and plasmonic features in undoped and doped ultrathin 2D InN holds promise for creating advanced optoelectronic devices and functional 2D heterostructures.

Direct measurement of a non-Hermitian topological invariant in a hybrid light-matter system.

Topology is central to understanding and engineering materials that display robust physical phenomena immune to imperfections. Different topological phases of matter are characterized by topological invariants. In energy-conserving (Hermitian) systems, these invariants are determined by the winding of eigenstates in momentum space. In non-Hermitian systems, a topological invariant is predicted to emerge from the winding of the complex eigenenergies. Here, we directly measure the non-Hermitian topological invariant arising from exceptional points in the momentum-resolved spectrum of exciton polaritons. These are hybrid light-matter quasiparticles formed by photons strongly coupled to electron-hole pairs (excitons) in a halide perovskite semiconductor at room temperature. We experimentally map out both the real (energy) and imaginary (linewidth) parts of the spectrum near the exceptional points and extract the novel topological invariant­fractional spectral winding. Our work represents an essential step toward realization of non-Hermitian topological phases in a condensed matter system.

Ultrathin Ga2 O3 Glass: A Large-Scale Passivation and Protection Material for Monolayer WS2.

Atomically thin transition metal dichalcogenide crystals (TMDCs) have extraordinary optical properties that make them attractive for future optoelectronic applications. Integration of TMDCs into practical all-dielectric heterostructures hinges on the ability to passivate and protect them against necessary fabrication steps on large scales. Despite its limited scalability, encapsulation of TMDCs in hexagonal boron nitride (hBN) currently has no viable alternative for achieving high performance of the final device. Here, it is shown that the novel, ultrathin Ga2 O3 glass is an ideal centimeter-scale coating material that enhances optical performance of the monolayers and protects them against further material deposition. In particular, Ga2 O3 capping of monolayer WS2 outperforms commercial-grade hBN in both scalability and optical performance at room temperature. These properties make Ga2 O3 highly suitable for large-scale passivation and protection of monolayer TMDCs in functional heterostructures.

Observation of bosonic condensation in a hybrid monolayer MoSe2-GaAs microcavity.

Bosonic condensation belongs to the most intriguing phenomena in physics, and was mostly reserved for experiments with ultra-cold quantum gases. More recently, it became accessible in exciton-based solid-state systems at elevated temperatures. Here, we demonstrate bosonic condensation driven by excitons hosted in an atomically thin layer of MoSe2, strongly coupled to light in a solid-state resonator. The structure is operated in the regime of collective strong coupling between a Tamm-plasmon resonance, GaAs quantum well excitons, and two-dimensional excitons confined in the monolayer crystal. Polariton condensation in a monolayer crystal manifests by a superlinear increase of emission intensity from the hybrid polariton mode, its density-dependent blueshift, and a dramatic collapse of the emission linewidth, a hallmark of temporal coherence. Importantly, we observe a significant spin-polarization in the injected polariton condensate, a fingerprint for spin-valley locking in monolayer excitons. Our results pave the way towards highly nonlinear, coherent valleytronic devices and light sources.

Observation of hybrid Tamm-plasmon exciton- polaritons with GaAs quantum wells and a MoSe2 monolayer.

Strong light matter coupling between excitons and microcavity photons, as described in the framework of cavity quantum electrodynamics, leads to the hybridization of light and matter excitations. The regime of collective strong coupling arises, when various excitations from different host media are strongly coupled to the same optical resonance. This leads to a well-controllable admixture of various matter components in three hybrid polariton modes. Here, we study a cavity device with four embedded GaAs quantum wells hosting excitons that are spectrally matched to the A-valley exciton resonance of a MoSe2 monolayer. The formation of hybrid polariton modes is evidenced in momentum resolved photoluminescence and reflectivity studies. We describe the energy and k-vector distribution of exciton-polaritons along the hybrid modes by a thermodynamic model, which yields a very good agreement with the experiment.Light and matter excitations from host media can hybridize in the strong coupling regime, resulting in the formation of hybrid polariton modes. Here, the authors demonstrate hybridization between tightly bound excitons in a MoSe2 monolayer and excitons in GaAs quantum wells via coupling to a cavity resonance.