High-quality hybrid dielectric materials for 2D electronics

Silicon-based field effect transistors(FETs)are the building blocks of modern electronics,serving as the cornerstone.However,current silicon technology is approaching its performance-downscaling limits[1].This challenge primarily stems from the substantial decline in electron mobility as silicon-based semiconductor thickness decreases,coupled with the pronounced emergence of short-channel effects upon FET miniaturization[1,2].

Reconstructive spectrometers taper down in price

The development of a low-cost compact reconstructive spectrometer paves the way towards portable pm-resolution spectroscopy.

Real- and momentum-indirect neutral and charged excitons in a multi-valley semiconductor

Excitons dominate the photonic and optoelectronic properties of a material.Although significant advancements exist in understanding various types of excitons,progress on excitons that are indirect in both real-and momentum-spaces is still limited.Here,we demonstrate the real-and momentum-indirect neutral and charged excitons(including their phonon replicas)in a multi-valley semiconductor of bilayer MoS_(2),by performing electric-field/doping-density dependent photoluminescence.Together with first-principles calculations,we uncover that the observed real-and momentum-indirect exciton involves electron/hole from K/Γvalley,solving the longstanding controversy of its momentum origin.Remarkably,the binding energy of real-and momentum-indirect charged exciton is extremely large(i.e.,~59 meV),more than twice that of real-and momentum-direct charged exciton(i.e.,~24 meV).The giant binding energy,along with the electrical tunability and long lifetime,endows real-and momentum-indirect excitons an emerging platform to study many-body physics and to illuminate developments in photonics and optoelectronics.

A Stable, Wideband Tunable, Near Transform-Limited, Graphene- Mode-Locked, Ultrafast Laser

We report an ultrafast laser mode-locked with a graphene saturable absorber.The linear dispersions of the Dirac electrons in graphene enable wideband tunability.We get-1 ps pulses,tunable between 1525 and 1559 nm,with stable mode-locking,insensitive to environmental perturbations.

Ultrafast Stretched-Pulse Fiber Laser Mode-Locked by Carbon Nanotubes

Ultrafast fiber sources having short pulses, broad bandwidth, high energy, and low amplitude fluctuations have widespread applications. Stretched-pulse fiber lasers, incorporating segments of normal and anomalous dispersion fibers, are a preferred means to generate such pulses. We realize a stretched-pulse fiber laser based on a nanotube saturable absorber, with 113 fs pulses, 33.5 nm spectral width and ~0.07% amplitude fluctuation, outperforming current nanotube-based designs.

Phase-matching-induced near-chirp-free solitons in normal-dispersion fiber lasers

Direct generation of chirp-free solitons without external compression in normal-dispersion fiber lasers is a long-term challenge in ultrafast optics.We demonstrate near-chirp-free solitons with distinct spectral sidebands in normaldispersion hybrid-structure fiber lasers containing a few meters of polarization-maintaining fiber.The bandwidth and duration of the typical mode-locked pulse are 0.74 nm and 1.95 ps,respectively,giving the time-bandwidth product of 0.41 and confirming the near-chirp-free property.Numerical results and theoretical analyses fully reproduce and interpret the experimental observations,and show that the fiber birefringence,normal-dispersion,and nonlinear effect follow a phase-matching principle,enabling the formation of the near-chirp-free soliton.Specifically,the phasematching effect confines the spectrum broadened by self-phase modulation and the saturable absorption effect slims the pulse stretched by normal dispersion.Such pulse is termed as birefringence-managed soliton because its two orthogonal-polarized components propagate in an unsymmetrical“X”manner inside the polarization-maintaining fiber,partially compensating the group delay difference induced by the chromatic dispersion and resulting in the selfconsistent evolution.The property and formation mechanism of birefringence-managed soliton fundamentally differ from other types of pulses in mode-locked fiber lasers,which will open new research branches in laser physics,soliton mathematics,and their related applications.

Nanowire encapsulation with polymer for electrical isolation and enhanced optical properties

轻管理和电的隔离为 optoelectronic nanowire (NW ) 的多数是必要的设备。这里，我们在场一种划算的技术基于 parylene-C 并且随后的退火的蒸汽阶段免职，那为 GaAs nanowires 提供保角的封装，反反射的涂层，改进光性质，和电的屏蔽。介绍的过程允许对任何 nanowire 结构合适的灵巧的封装和屏蔽。特别地， parylene-C 封装为 nanowires 作为有效 antireflection 涂层工作，与反射率在下面到 <1% 在可见光谱。而且， parylene-C 涂层增加光致发光紧张，建议指导到 NW 的改进的光。基于这进程，最后，带的 NW 被制作，它显示出好二极管性能和一个清楚的 electroluminescence 信号。我们相信这个过程能扩展制造可能性并且改进 optoelectronic nanowire 设备的表演。

Mode couplings of a semiconductor nanowire scanning across a photonic crystal nanocavity

The position-dependent mode couplings between a semiconductor nanowire(NW)and a planar photonic crystal(PPC)nanocavity are studied.By scanning an NW across a PPC nanocavity along the hexagonal lattice’sΓ– M and M– K directions,the variations of resonant wavelengths,quality factors,and mode volumes in both fundamental and second-order resonant modes are calculated,implying optimal configurations for strong mode-NW couplings and light-NW interactions.For the fundamental(second-order)resonant mode,scanning an NW along the M– K(Γ– M)direction is preferred,which supports stronger light-NW interactions with larger NW-position tolerances and higher quality factors simultaneously.The simulation results are confirmed experimentally with good agreements.

Giant Valley Coherence at Room Temperature in 3R WS_(2) with Broken Inversion Symmetry

Breaking the space-time symmetries in materials can markedly influence their electronic and optical properties.In 3R-stacked transition metal dichalcogenides,the explicitly broken inversion symmetry enables valley-contrasting Berry curvature and quantization of electronic angular momentum,providing an unprecedented platform for valleytronics.Here,we study the valley coherence of 3R WS_(2) large single-crystal with thicknesses ranging from monolayer to octalayer at room temperature.Our measurements demonstrate that both A and B excitons possess robust and thickness-independent valley coherence.The valley coherence of direct A(B)excitons can reach 0.742(0.653)with excitation conditions on resonance with it.Such giant and thickness-independent valley coherence of large single-crystal 3R WS_(2) at room temperature would provide a firm foundation for quantum manipulation of the valley degree of freedom and practical application of valleytronics.

Chip-integrated van der Waals PN heterojunction photodetector with low dark current and high responsivity

Two-dimensional materials are attractive for constructing high-performance photonic chip-integrated photodetectors because of their remarkable electronic and optical properties and dangling-bond-free surfaces.However,the reported chip-integrated two-dimensional material photodetectors were mainly implemented with the configuration of metalsemiconductor-metal,suffering from high dark currents and low responsivities at high operation speed.Here,we report a van der Waals PN heterojunction photodetector,composed of p-type black phosphorous and n-type molybdenum telluride,integrated on a silicon nitride waveguide.The built-in electric field of the PN heterojunction significantly suppresses the dark current and improves the responsivity.Under a bias of 1 V pointing from n-type molybdenum telluride to p-type black phosphorous,the dark current is lower than 7 nA,which is more than two orders of magnitude lower than those reported in other waveguide-integrated black phosphorus photodetectors.An intrinsic responsivity up to 577 mA W^(−1) is obtained.Remarkably,the van der Waals PN heterojunction is tunable by the electrostatic doping to further engineer its rectification and improve the photodetection,enabling an increased responsivity of 709 mA W^(−1).Besides,the heterojunction photodetector exhibits a response bandwidth of~1.0 GHz and a uniform photodetection over a wide spectral range,as experimentally measured from 1500 to 1630 nm.The demonstrated chip-integrated van der Waals PN heterojunction photodetector with low dark current,high responsivity and fast response has great potentials to develop high-performance on-chip photodetectors for various photonic integrated circuits based on silicon,lithium niobate,polymer,etc.

Ultrafast transient sub-bandgap absorption of monolayer MoS2

The light-matter interaction in materials is of remarkable interest for various photonic and optoelectronic applications,which is intrinsically determined by the bandgap of the materials involved.To extend the applications beyond the bandgap limit,it is of great significance to study the light-matter interaction below the material bandgap.Here,we report the ultrafast transient absorption of monolayer molybdenum disulfide in its sub-bandgap region from-0.86 pm to 1.4 pm.Even though this spectral range is below the bandgap,we observe a significant absorbance enhancement up to-4.2%in the monolayer molybdenum disulfide(comparable to its absorption within the bandgap region)due to pump-induced absorption by the excited carrier states.The different rise times of the transient absorption at different wavelengths indicate the various contributions of the different carrier states(i.e.,real carrier states in the short-wavelength region of~<1μm,and exciton states in the long wavelength region of~>1μm).Our results elucidate the fundamental understanding regarding the optical properties,excited carrier states,and carrier dynamics in the technologically important near-infrared region,which potentially leads to various photonic and optoelectronic applications(e.g„excited-state-based photodetectors and modulators)of two-dimensional materials and their heterostructures beyond their intrinsic bandgap limitations.

Introduction to two-dimensional layered materials for ultrafast lasers

We introduce the background and motivation of this feature issue of two-dimensional layered materials for ultrafast lasers. A brief summary of the seven collected articles in this feature issue is also given.

Birefringence-Managed Normal-Dispersion Fiber Laser Delivering Energy-Tunable Chirp-Free Solitons

Chirp-free solitons have been mainly achieved with anomalous-dispersion fiber lasers by the balance of dispersive and nonlinear effects,and the single-pulse energy is constrained within a relatively small range.Here,we report a class of chirp-free pulse in normal-dispersion erbium-doped fiber lasers,termed birefringence-managed soliton,in which the birefringence-related phasematching effect dominates the soliton evolution.Controllable harmonic mode locking from 5 order to 85 order is obtained at the same pump level of~10 mW with soliton energy fully tunable beyond ten times,which indicates a new birefringencerelated soliton energy law,which fundamentally differs from the conventional soliton energy theorem.The unique transformation behavior between birefringence-managed solitons and dissipative solitons is directly visualized via the singleshot spectroscopy.The results demonstrate a novel approach of engineering fiber birefringence to create energy-tunable chirpfree solitons in normal-dispersion regime and open new research directions in fields of optical solitons,ultrafast lasers,and theirapplications.

Coherent modulation of chiral nonlinear optics with crystal symmetry

Light modulation is of paramount importance for photonics and optoelectronics. Here we report all-optical coherent modulation of third-harmonic generation (THG) with chiral light via the symmetry enabled polarization selectivity. The concept is experimentally validated in monolayer materials (MoS2) with modulation depth approaching ~100%, ultra-fast modulation speed (<~130 fs), and wavelength-independence features. Moreover, the power and polarization of the incident optical beams can be used to tune the output chirality and modulation performance. Major performance of our demonstration reaches the fundamental limits of optical modulation: near-unity modulation depth, instantaneous speed (ultra-fast coherent interaction), compact footprint (atomic thickness), and unlimited operation bandwidth, which hold an ideal optical modulation solution for emerging and future nonlinear optical applications (e.g., interconnection, imaging, computing, and quantum technologies).

2D materials as a new platform for photonic applications

Two-dimensional(2D)layered materials possess sheet-like structures with single-atom or few-atom thicknesses.They exhibit exceptional electronic and optical properties due to quantum confinement in the direction perpendicular to the 2D plane.Besides being the basis of various modulators and photodetectors,2D materials have realized light sources with femtosecond pulse duration utilizing their ultrahigh optical nonlinearity.Recent developments and explorations of various 2D materials and their heterostructures have prompted intense research on various photonic devices with superior performance and functionalities.These developments can pave the way for realistic applications of 2D materials,and boost the fundamental study of various physical effects and phenomena.

Nanowire-assisted microcavity in a photonic crystal waveguide and the enabled high-efficiency optical frequency conversions

We report an indium phosphide nanowire(NW)-induced cavity in a silicon planar photonic crystal(PPC)waveguide to improve the light–NW coupling.The integration of NW shifts the transmission band of the PPC waveguide into the mode gap of the bare waveguide,which gives rise to a microcavity located on the NW section.Resonant modes with𝑄factors exceeding 103 are obtained.Leveraging on the high density of the electric field in the microcavity,the light–NW interaction is enhanced strongly for efficient nonlinear frequency conversion.Second-harmonic generation and sum-frequency generation in the NW are realized with a continuous-wave pump laser in a power level of tens of microwatts,showing a cavity-enhancement factor of 112.The hybrid integration structure of NW-PPC waveguide and the self-formed microcavity not only opens a simple strategy to effectively enhance light–NW interactions,but also provides a compact platform to construct NW-based on-chip active devices.

Introduction to the Photonics Based on Two-dimensional Materials feature issue

We are pleased to introduce a feature issue on photonics based on two-dimensional(2D)materials.Enlightened by the unique optical and electronic properties of graphene,2D layered materials have been extensively studied in recent years driven by their promising applications for a large