The niobium oxide dihalides have recently been identified as a new class of van der Waals materials exhibiting exceptionally large second-order nonlinear optical responses and robust in-plane ferroelectricity. In contrast to second-order nonlinear processes, third-order optical nonlinearities can arise irrespective of whether a crystal lattice is centrosymmetric. Here, we report third harmonic generation (THG) in two-dimensional (2D) transition metal oxide iodides, namely NbOI₂ and TaOI₂. We observe a comparable THG intensity from both materials. By benchmarking against THG from monolayer WS₂, we deduce that the third-order susceptibility is approximately on the same order. THG resonances are revealed at different excitation wavelengths, likely due to enhancement by excitonic states and band edge resonances. The THG intensity increases for material thicknesses up to 30 nm, owing to weak interlayer coupling. After this threshold, it shows saturation or a decrease, due to optical interference effects. Our results establish niobium and tantalum oxide iodides as promising 2D materials for third-order nonlinear optics, with intrinsic in-plane ferroelectricity and thickness-tunable nonlinear efficiency.

Controlling the interlayer coupling by tuning lattice parameters through pressure engineering is an important route for tailoring the optoelectronic properties of two-dimensional materials. In this work, we report a pressure-dependent study on the exciton transitions of bilayer MoS₂ exfoliated on a diamond anvil surface. The applied hydrostatic pressure changes from ambient pressure up to 11.05 GPa using a diamond anvil cell device. Raman, photoluminescence, and reflectivity spectra at room temperature are analyzed to characterize the interlayer coupling of this bilayer system. With the increase of pressure, the indirect exciton emission disappears completely at about 5 GPa. Importantly, we clearly observed the interlayer exciton from the reflectivity spectra, which becomes invisible at a low pressure around 1.26 GPa. This indicates that the interlayer exciton is very sensitive to the hydrostatic pressure due to the oscillator strength transfer from the direct transition to the indirect one.

The van der Waals crystal NbOX2 (X = Cl, Br, I) has recently attracted much attention due to its remarkable in-plane anisotropy and substantial second-order nonlinear optical response. Moreover, the importance of modulating its physical properties through strain has become increasingly prominent. Herein, we investigate the anisotropic phonon response of NbOX2 along various crystallographic directions under uniaxial tensile strain by Raman spectroscopy. The results show that opposing frequency shifts manifest in the Raman modes when uniaxial tensile strain is applied either parallel or perpendicular to the polar axis, and the highest frequency peak, P5, shows significant blue or red shifts. Density functional theory calculation results are consistent with the observed Raman shifts under strain. The change in Nb–O bond length is the main reason for the significant blue shift of P5. The anisotropic and pronounced frequency shifts of NbOX2 under strain can serve as sensitive indicators for strain modulation, offering insights for potential applications such as flexible optoelectronic devices and strain sensors.

Band structures of the photonic crystal slabs play a significant role in manipulating the flow of light and predicting exotic physics in photonics. In this Letter, we show that the key features of photonic band structures can be achieved experimentally by the polarization- and momentum-resolved photoluminescence spectroscopy utilizing the light emission properties of SiN_x. The two-dimensional spectra clearly reveal the energy-momentum dispersion of band structures, which is in perfect agreement with the simulation results. The isofrequency contours can be measured easily by adding a bandpass filter with a desired photon energy. Furthermore, it is convenient to observe clearly and directly the optical singularity—the optical bound states in the continuum featured by dark point in three-dimensional photoluminescence spectra. The polarization-resolved isofrequency contours clearly show that this dark point is the center of an azimuthally polarized vortex. Finally, the helical topological edge states can be easily observed in photonic topological insulators with deformed hexagonal lattices. Our work provides a simple and effective approach for exploring topological photonics and other intriguing phenomena hidden in the photonic crystal slabs.

In 2H stacking bilayer MoS₂, the exciton with an interlayer nature has been evidenced due to the hybridization of hole states among both layers. The transition energy of this interlayer exciton is located between the A and B excitons. In this work, we investigate the evolution of optical properties in stacking MoS₂ bilayers with the twisted angles ranging from 0° to 60°, especially focusing on the interlayer exciton. The clear modulations of the exciton responses are observed by the room-temperature reflectance. The interlayer exciton transition is observed in the artificial stacking bilayer MoS₂ with the twisted angle around 60°. We found that the interlayer exciton is very sensitive to the twisted angle. Once the stacking angle deviates the 2H stacking by a few degrees, the interlayer transition is quenched. This is due to the bilayer symmetry and interlayer coupling of this unique system.

Reservoir computing, a recurrent neural network paradigm, shows potential in tracing chaotic dynamics in, e.g., motion tracking, spatiotemporal pattern recognition, and anomaly detection. However, the iterative nonlinear mapping required for reservoir activation poses challenges for digital computing. Realizing physical nonlinear systems from low-dimensional materials as the reservoir for performing analog nonlinear mapping emerges as a promising solution. Though promising, current advances remain largely at conceptual explorations via simulations, limited by the practical circuit design and fabrication challenges, and there has been a lack of hardware-algorithm co-design studies. In this work, we investigate hardware-algorithm co-design in analog reservoir activation with the nonlinearity derived from solution-processed two-dimensional (2D) materials. We show that the nonlinearity can be fitted as analog activation functions in implementing a reservoir computing model and, by co-design optimizations, the device parameterized model can achieve long-term synchronization and robust generalization in regression of chaotic systems, with resilience to noise. Given this performance, and the scalability of solution-processed 2D materials, the co-design scheme manifests the potential for the design and implementation of scalable, lightweight analog reservoir computing systems with solution-processed 2D materials for widespread applications in, e.g., IoTs, wearables, and robotics.