Hao Hong

Precise twist-angle control in van der Waals (vdW) moiré superlattices enables twistronic tuning of interfacial responses and emergent quantum states, but it remains unclear how atomic registry in three-dimensional stacks is linked to interlayer coupling and whether it can produce an internal twist degree of freedom without external torque. Here, we show that, via interlayer engineering, screw-dislocation-driven multilayer spiral WS ₂ exhibits a reversible pressure-induced rotation of second-harmonic-generation polar pattern that saturates near 1 GPa, corresponding to an effective average interlayer twist of 0.2° per layer. This compression-twist coupling arises from moiré defined hierarchy of stacking energies and the associated atomic reconstruction, which together generate a spontaneous rotational torque at a slipped, metastable interface and couple the twist angle to the interlayer spacing. Our findings establish twist-degree-of-freedom manipulation in vdW crystals, opening pathways to reshape correlation-driven behaviors in moiré-of-moiré architectures and to explore chiral elasticity beyond Cauchy continuum mechanics.

Bright entangled photon sources with on-demand quantum state control are essential for advancing quantum science and technologies. However, simultaneously achieving high brightness and state tunability remains challenging, as this necessitates a dual-phase control mechanism that facilitates both longitudinal phase matching and transverse polarization modulation. Here, we report a twin-twisted two-dimensional (2D) van der Waals (vdW) crystal architecture that enables spontaneous parametric down-conversion (SPDC) with both high generation efficiency and continuously tunable quantum states. The designed crystal comprises two sections of twisted 2D GaSe vdW flakes with opposite chiral handedness, in which interflake twist angle induces phase matching in SPDC, and intersection twisting provides an additional degree of freedom on tunability. We demonstrate that the twin-twisted GaSe crystals achieve a record-high photon-pair generation rate exceeding 13 200 Hz and a Bell state fidelity above 99% within 2D material family, and enable unprecedented tunability of quantum states from the maximally polarization entangled states to the fully separable states. This study opens a practical avenue toward twist-engineered quantum states, and paves the way for developing compact quantum photonic systems with tailored properties.

Chiral spin textures, such as spin spirals and skyrmions, are key to advancing spintronics by enabling ultrathin, energy-efficient memory, and high-density data storage and processing. However, their realization remains hindered by the scarcity of suitable host materials and the formidable experimental challenges associated with the characterization of these intricate chiral magnetic states. Here, we report the observation of tunable chiral magnetic textures in van der Waals magnet CrPS ₄ with nonlinear optics. These tunable textures exhibit strong chiral third-order nonlinear optical responses, driven by interlayer and intralayer spin couplings under varying magnetic fields and temperatures. These pronounced chiral nonlinear optical responses highlight the potency and high sensitivity of the nonlinear optical readout for probing non-collinear magnetic orders. Moreover, our findings position van der Waals magnets and their heterostructures as an exceptional platform for reconfigurable spin-photonics and spintronics, unifying optical, electrical, and magnetic properties through unique intralayer and interlayer spin coupling properties and effective spin interaction between photons and electrons.

Highly sensitive acoustic sensing devices are essential for human‒machine interfaces and modern advanced artificial intelligence (AI) technology. Two-dimensional (2D) materials exhibiting atomically thin thickness and superior mechanical properties are, in principle, ideal membranes for ultimate acoustic sensing. In this work, we present an ultrasensitive microphone using large freestanding reduced graphene oxide (rGO) membranes. The membranes were suspended by a designed pressure-assisted double transfer strategy, resulting in a diameter-to-thickness ratio of ~ 10⁶ (diameter of 8 cm, thin thickness of 80 nm). They exhibit a static pressure responsivity of ~ 500 μm/Pa, and a dynamic signal-to-noise ratio up to ~ 115 dB at 1 kHz. Notably, a rGO-based broadband microphone (100 Hz‒50 kHz) demonstrates a superior language recognition accuracy (90% vs. 70%) compared to that of commercial micro-electromechanical system (MEMS) microphones at a distance of 9 m. Our work provides a reliable route for fabricating large freestanding ultrathin membranes and will promote the development of advanced acoustic devices.

High optical nonlinearity can enable classical and quantum functionalities in all-fibre laser systems. However, despite long-standing efforts to exploit second-order optical nonlinearity in conventional all-fibre systems, nonlinear optical conversion efficiencies remain modest. Here we demonstrate all-fibre integration of twist-phase-matched rhombohedral boron nitride (rBN) flakes on the end facet of optical fibres for second-harmonic generation (SHG) and spontaneous parametric downconversion (SPDC). We provide local and global optimization of the interflake twist angles for phase-matching design, achieving an SHG conversion efficiency of ~4.1% and an SPDC coincidence rate of ~90 in van der Waals crystals integrated on optical fibre devices. Finally, we design an all-fibre frequency-doubling ultrafast laser by integrating a multifunctional nonlinear crystal of a graphene/rBN heterostructure to simultaneously generate mode-locked pulses and intracavity SHG emission. This work establishes a route for developing high-efficiency, second-order nonlinear functionalities, such as optical parametric oscillators, optical modulators and entangled photon sources, in all-fibre lasers.

Semiconducting transition metal dichalcogenides are promising platforms for exploring emergent two-dimensional (2D) exciton physics and constructing 2D optoelectronic devices. However, even in mechanically exfoliated and hexagonal boron nitride encapsulated samples that are expected to maintain the most intrinsic properties, shallow defect states are still unavoidable. Here, we reported a method for eliminating the defect states in monolayer WSe_{2} by coupling it with a c-plane sapphire surface. We found that the defect-relevant photoluminescence peaks at cryogenic temperatures can be completely suppressed in WSe_{2} on c-plane sapphire, resulting in intrinsic exciton radiation, prolonged lifetime, and increased diffusion length. Ab initio calculations revealed that the surface of c-plane sapphire is much more active than other planes, where oxygen atoms preferentially transfer to WSe_{2} to repair selenium vacancy defects. Our Letter provides a new platform for defect engineering, which will promote the development of high-performance electronic and optoelectronic devices based on 2D materials.

The fiber-based saturable absorber (SA) that enables mode-locking within a ring cavity serves as the core component of the ultrafast all-fiber lasers. However, the integration of SAs onto fibers with high compactness suffers from imbalanced saturable absorption properties and unstable mode-locking performance. Here, we present a robust mode-locking SA by integrating a nanocavity composed of a two-dimensional graphene heterostructure on the fiber end facet. We demonstrate a significant reduction in the saturation intensity (~65%) and improved soliton dynamic processes through precise modulation of the optical field within the heterostructure. The designed heterostructure facilitates the formation of a stable single-soliton state for robust mode-locking. A high tolerance to intracavity polarization variations is achieved in the heterostructure-SA (~85% compared to 20% for bare graphene). Our designed heterostructure-SA represents an important advancement in the development of ultracompact mode-locked all-fiber lasers, offering enhanced integrability and stability.

A stable and durable ultrafast electron source is highly desirable for sophisticated vacuum electron technologies. However, free-space excitations based on ultrahigh-power or deep-ultraviolet pulsed lasers usually cause cathode material damage and mechanical vibration even under ultrahigh vacuum. In this work, we present a compact ultrafast electron source consisting of graphene integrated on an optical fiber, taking advantage of the ultrafast hot-electron emission from graphene and well-defined single-mode excitation from the optical fiber. With mild excitation (~1 GW/cm², infrared laser), an ultrashort electron pulse (width of ~ 80 fs) with high stability (fluctuation ≤±0.5% in 8 hours) and longevity (T₉₀ > 500 hours) can be generated even under relatively high ambient pressure (up to 100 Pa). This compact source has been facilely integrated into a commercial electron microscope for time-resolved imaging and spectroscopy. Our graphene optical fiber-based ultrafast electron source offers a promising solution to support the development of vacuum electron instruments.