Organic-inorganic perovskites have attracted great attentions driven by exceptional progress in photovoltaics, photonics and optoelectronics. Different from the corner sharing framework of three-dimensional (3D) perovskite, two-dimensional (2D) organic-inorganic perovskites possess a layered staking structure composed of alternative organic and inorganic components.Due to the inherent multi-quantum-well-like structure, it is intriguing to explore the optical properties of 2D perovskites enabled by spatial and dielectric confinement. Herein, the twophoton absorption (TPA) properties of 2D perovskite phenylethylamine lead iodide ((PEA)2PbI4) are systematically studied. The 2D perovskite exhibits a giant TPA and saturation effect under excitation of 800 nm femtosecond laser. The TPA coefficient of a (PEA)2PbI4 flake is measured to be about 211.5 cm/MW, which is at least one order of magnitude larger than those of 3D perovskite films and some typical semiconductor Submitted to 2 2 nanostructures. The giant TPA can be attributed to the enhanced quantum and dielectric confinement in the organic-inorganic multi-quantum-well structure. In addition, a highly thickness-dependent TPA is observed for the 2D perovskite flakes. The result advocates a great promise of 2D organic-inorganic perovskites for nonlinear optical absorption related optoelectronic devices.

'Blinking', or 'fluorescence intermittency', refers to a random switching between 'ON' (bright) and 'OFF' (dark) states of an emitter; it has been studied widely in zero-dimensional quantum dots and molecules, and scarcely in one-dimensional systems. A generally accepted mechanism for blinking in quantum dots involves random switching between neutral and charged states (or is accompanied by fluctuations in charge-carrier traps), which substantially alters the dynamics of radiative and non-radiative decay. Here, we uncover a new type of blinking effect in vertically stacked, two-dimensional semiconductor heterostructures, which consist of two distinct monolayers of transition metal dichalcogenides (TMDs) that are weakly coupled by van der Waals forces. Unlike zero-dimensional or one-dimensional systems, two-dimensional TMD heterostructures show a correlated blinking effect, comprising randomly switching bright, neutral and dark states. Fluorescence cross-correlation spectroscopy analyses show that a bright state occurring in one monolayer will simultaneously lead to a dark state in the other monolayer, owing to an intermittent interlayer carrier-transfer process. Our findings suggest that bilayer van der Waals heterostructures provide unique platforms for the study of charge-transfer dynamics and non-equilibrium-state physics, and could see application as correlated light emitters in quantum technology.

Arbitrary control of terahertz (THz) waves remains a significant challenge although it promises many important applications. Here, a method to tailor the reflection and scattering of THz waves in an anomalous manner by using 1‐bit coding metamaterials is presented. Specific coding sequences result in various THz far‐field reflection and scattering patterns, ranging from a single beam to two, three, and numerous beams, which depart obviously from the ordinary Snell's law of reflection. By optimizing the coding sequences, a wideband THz thin film metamaterial with extremely low specular reflection, due to the scattering of the incident wave into various directions, is demonstrated. As a result, the reflection from a flat and flexible metamaterial can be nearly uniformly distributed in the half space with small intensity at each specific direction, manifesting a diffuse reflection from a rough surface. Both simulation and experimental results show that a reflectivity less than −10 dB is achieved over a wide frequency range from 0.8 to 1.4 THz, and it is insensitive to the polarization of the incident wave. This work reveals new opportunities arising from coding metamaterials in effective manipulation of THz wave propagation and may offer widespread applications.

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Two-photon-absorption-induced photoluminescence (TPL) from nanostructures is generally inefficient since it is a typical third-order nonlinear optical process. Herein, a hybrid dielectric structure composed of dielectric microspheres (approximately micrometers in diameter) covering a 2D perovskite flake is reported, which provides a straightforward strategy for enhancing the TPL emission. The microspheres in the hybrid dielectric structure not only concentrate the pumping laser but also effectively increase the detection efficiency of the emitted TPL signal. The internal quantum efficiency of the 2D perovskite is also increased in the hybrid dielectric structure due to a reduced nonradiative rate. These effects cooperatively increase the TPL emission by two orders of magnitude in the hybrid dielectric structure. Moreover, the hybrid dielectric structure is proven to be useful for TPL-based superresolution imaging at a relatively low excitation power of 0.05 mW. This work demonstrates great promise for developing lowcost, high-performance nonlinear nanodevices based on hybrid dielectric structures.

The enhanced second-harmonic generation (SHG) from a monolayer WS2 coupled to a plasmonic nanocavity is experimentally and theoretically investigated. The nanocavity is comprised of monodispersed Ag nanocubes separated from an Ag film by a spacer Al2O3, namely, the nanoparticle on mirror (NPoM) system. When the surface plasmon polariton resonance (SPPR) wavelength of NPoM nanocavity overlaps well with the SHG wavelength of the monolayer WS2 (namely, harmonic resonance), a ∼300-fold SHG enhancement is achieved in experiment. For theoretical understanding, the quantum mechanical density matrix method has been used to develop a theory for SHG. It is found that the SHG intensity of nanohybrid is proportional to the square of the local-field intensity in NPoM nanocavity at SHG wavelength, which is ascribed to the dipole–quadrupole interaction between dipole, P SHG, in the monolayer WS2 and quadrupole, Q Ag, in Ag nanocavity. It is significantly different from that in metal nanoparticles under harmonic resonance, which is proportional to the local-field intensity. Therefore, it provides a novel mechanism for enhancing SHG signals from metal–semiconductor nanohybrids, which has potential applications in nonlinear devices and hybrid nonlinear metasurfaces.

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Wind energy, as a vital renewable energy source, also plays a significant role in reducing carbon emissions and mitigating climate change. It is therefore of utmost necessity to evaluate ocean wind energy resources for electricity generation and environmental management. Ocean wind distribution around the globe can be obtained from satellite observations to compensate for limited in situ measurements. However, previous studies have largely ignored uncertainties in ocean wind energy resources assessment with multiple satellite data. It is against this background that the current study compares mean wind speeds (MWS) and wind power densities (WPD) retrieved from scatterometers (QuikSCAT, ASCAT) and radiometers (WindSAT) and their different combinations with National Data Buoy Center (NDBC) buoy measurements at heights of 10 m and 100 m (wind turbine hub height) above sea level. Our results show an improvement in the accuracy of wind resources estimation with the use of multiple satellite observations. This has implications for the acquisition of reliable data on ocean wind energy in support of management policies.