Broadband and enhanced nonlinear optical response of MoS2/graphene nanocomposites for ultrafast photonics applications

Recent advances in two-dimensional perovskite materials for light-emitting diodes

Light-emitting diodes (LEDs) are an indispensable part of our daily life. After being studied for a few decades, this field still has some room for improvement. In this regard, perovskite materials may take the leading role. In recent years, LEDs have become a most explored topic, owing to their various applications in photodetectors, solar cells, lasers, and so on. Noticeably, they exhibit significant characteristics in developing LEDs. The luminous efficiency of LEDs can be significantly enhanced by the combination of a poor illumination LED with low-dimensional perovskite. In 2014, the first perovskite-based LED was illuminated at room temperature. Furthermore, two-dimensional (2D) perovskites have enriched this field because of their optical and electronic properties and comparatively high stability in ambient conditions. Recent and relevant advancements in LEDs using low-dimensional perovskites including zero-dimensional to three-dimensional materials is reported. The major focus of this article is based on the 2D perovskites and their heterostructures (i.e., a combination of 2D perovskites with transition metal dichalcogenides, graphene, and hexagonal boron nitride). In comparison to 2D perovskites, heterostructures exhibit more potential for application in LEDs. State-of-the-art perovskite-based LEDs, current challenges, and prospects are also discussed.

Nonlinear Nano-Imaging of Interlayer Coupling in 2D Graphene-Semiconductor Heterostructures

The emergent electronic, spin, and other quantum properties of 2D heterostructures of graphene and transition metal dichalcogenides are controlled by the underlying interlayer coupling and associated charge and energy transfer dynamics. However, these processes are sensitive to interlayer distance and crystallographic orientation, which are in turn affected by defects, grain boundaries, or other nanoscale heterogeneities. This obfuscates the distinction between interlayer charge and energy transfer. Here, nanoscale imaging in coherent four-wave mixing (FWM) and incoherent two-photon photoluminescence (2PPL) is combined with a tip distance-dependent coupled rate equation model to resolve the underlying intra- and inter-layer dynamics while avoiding the influence of structural heterogeneities in mono- to multi-layer graphene/WSe2 heterostructures. With selective insertion of hBN spacer layers, it is shown that energy, as opposed to charge transfer, dominates the interlayer-coupled optical response. From the distinct nano-FWM and -2PPL tip-sample distance-dependent modification of interlayer and intralayer relaxation by tip-induced enhancement and quenching, an interlayer energy transfer time ofτ ET ≈ ( 0 . 35 - 0.15 + 0.65 ) $\tau _{\rm ET} \approx (0.35^{+0.65}_{-0.15})$  ps consistent with recent reports is derived. As a local probe technique, this approach highlights the ability to determine intrinsic sample properties even in the presence of large sample heterogeneity.

Picosecond dissipative soliton generation from an ytterbium-doped fiber laser based on a BP/SnSe2-PVA mixture saturable absorber

Stable picosecond dissipative soliton pulses were observed in an ytterbium-doped fiber laser employing a high-quality mixture of BP/SnSe₂-PVA saturable absorber (SA). The modulation depth, saturation intensity, and non-saturable loss of the mixture of BP/SnSe₂-PVA SA were measured with values of 5.98%, 18.37 MW/cm², and 33%, respectively. Within the pump power range of 150-270 mW, stable dissipative soliton pulses were obtained with an output power of 1.68-4 mW. When the minimum pulse duration is 1.28 ps, a repetition rate of 0.903 MHz, center wavelength of 1064.38 nm and 3 dB bandwidth of 2 nm were obtained. The maximum pulse energy of 4.43 nJ and the signal-to-noise ratio up to 72 dB were achieved at pump power of 270 mW. The results suggest that the BP/SnSe₂-PVA mixture SA has outstanding nonlinear saturable absorption characteristics and broad ultrafast laser applications.

Nonlinear Absorption Response of Zirconium Carbide Films

Zirconium carbide (ZrC), a novel representative of the MXene family, has attracted considerable interest because of its outstanding physicochemical properties and potential applications in optoelectronic devices. For improving its performance as an optical modulator for ultrashort lasers, there is a call to continue studying the nonlinear optical behavior of MXene ZrC. Herein, for the first time, MXene ZrC films were fabricated on fused silica by magnetron sputtering deposition technology and used as a saturable absorber (SA) optical modulator in a passive Q-switched Nd:YAG laser. The saturation absorption behaviors of the prepared ZrC films were characterized by the Z-scan method. Their morphology, band structure, damage threshold, carrier recovery time, and saturation absorption properties were analyzed. The experimental results show that the MXene ZrC SA films exhibit excellent nonlinear optical characteristics, with a saturation intensity of 48.4 MW/cm², a large modulation depth of 6.9%, and an ultrashort recovery time of 2.72 ps. In addition, the damage threshold of MXene ZrC SA films was estimated to be greater than 0.2516 J/cm². By integrating the ZrC SA film optical modulator into the oscillator of the Nd:YAG laser, we achieved stable operation of the Q-switched laser with a central wavelength at 1.06 μm, with the shortest pulse width of 78 ns. The results of this study demonstrate the potential use of MXene ZrC SA films as optical modulators in ultrashort lasers.

Revealing the Modulation Effects on the Electronic Band Structures and Exciton Properties by Stacking Graphene/h-BN/MoS2 Schottky Heterostructures

Stacking two dimensional tunneling heterostructures has always been an important strategy to improve the optoelectronic device performance. However, there are still many disputes about the blocking ability of monolayer (1L-) h-BN on the interlayer coupling. Graphene/h-BN/MoS₂ optoelectronic devices have been reported for superior device results. In this study, starting with graphene/h-BN/MoS₂ heterostructures, we report experimental evidence of 1L-h-BN barrier layer modulation effects about the electronic band structures and exciton properties. We find that 1L-h-BN insertion only partially blocks the interlayer carrier transfer. In the meantime, the 1L-h-BN barrier layer weakens the interlayer coupling effect, by decreasing the efficient dielectric screening and releasing the quantum confinement. Consequently, the optical conductivity and plasmon excitation slightly improve, and the electronic band structures remain unchanged in graphene/h-BN/MoS₂, explaining their fascinating optoelectronic responses. Moreover, the excitonic binding energies of graphene/h-BN/MoS₂ redshift with respect to the graphene/MoS₂ counterparts. Our results, as well as the broadband optical constants, will help better understand the h-BN barrier layers, facilitating the developing progress of h-BN-based tunneling optoelectronic devices.

Dicarbon nitride and Janus transition metal chalcogenides van der Waals heterojunctions for photocatalytic water splitting

Two-dimensional graphene-like dicarbon nitride (C₂N) is a newly synthesized metal-free material, which has attracted significant research interest owing to the direct band gap, high carrier mobility, thermal stability, and great tunable properties. However, their application in photocatalytic water splitting has not been well explored. In this work, the properties of photocatalytic water decomposition in heterojunctions composed of C₂N and transition metal dichalcogenides (TMDs) with Janus structure MoXY (X, Y = S, Se, Te) are systematically studied by the first-principles calculations based on density functional theory. The results show that except for MoTeS/C₂N, the other five heterojunctions have type-Ⅱ band alignment, which causes electrons and holes to gather in the C₂N and MoXY layer separately. Because the coupled built-in electric field at the intra-layer and inter-layer of asymmetric TMDs with Janus structure forms van der Waals heterojunction, the external electric field is an effective means of modulating the electronic properties of the heterojunction. Under the imposition of an external electric field, the MoSeS/C₂N, MoTeSe/C₂N, and MoTeS/C₂N heterojunctions meet the band edge requirements for the photocatalytic decomposition of water. Detailed analysis demonstrates that the MoSeS/C₂N heterojunction could effectively improve the optical absorption properties of monolayer C₂N, making it a potential photocatalytic water decomposition material.

Photonic bandgap engineering in (VO2)n/(WSe2)nphotonic superlattice for versatile near- and mid-infrared phase transition applications

The application of the photonic superlattice in advanced photonics has become a demanding field, especially for two-dimensional and strongly correlated oxides. Because it experiences an abrupt metal-insulator transition near ambient temperature, where the electrical resistivity varies by orders of magnitude, vanadium oxide (VO₂) shows potential as a building block for infrared switching and sensing devices. We reported a first principle study of superlattice structures of VO₂as a strongly correlated phase transition material and tungsten diselenide (WSe₂) as a two-dimensional transition metal dichalcogenide layer. Based on first-principles calculations, we exploit the effect of semiconductor monoclinic and metallic tetragonal state of VO₂with WSe₂in a photonic superlattices structure through the near and mid-infrared (NIR-MIR) thermochromic phase transition regions. By increasing the thickness of the VO₂layer, the photonic bandgap (PhB) gets red-shifted. We observed linear dependence of the PhB width on the VO₂thickness. For the monoclinic case of VO₂, the number of the forbidden bands increase with the number of layers of WSe₂. New forbidden gaps are preferred to appear at a slight angle of incidence, and the wider one can predominate at larger angles. We presented an efficient way to control the flow of the NIR-MIR in both summer and winter environments for phase transition and photonic thermochromic applications. This study's findings may help understand vanadium oxide's role in tunable photonic superlattice for infrared switchable devices and optical filters.

The Third-Order Nonlinear Optical Properties of Sb2S3/RGO Nanocomposites

Antimony sulfide/reduced graphene oxide (Sb2S3/RGO) nanocomposites were synthesized via a facile, one-step solvothermal method. XRD, SEM, FTIR, and Raman spectroscopy were used to characterize the uniform distribution of Sb2S3 nanoparticles on the surface of graphene through partial chemical bonds. The third-order nonlinear optical (NLO) properties of Sb2S3, RGO, and Sb2S3/RGO samples were investigated by using the Z-scan technique under Nd:YAG picosecond pulsed laser at 532 nm. The results showed that pure Sb2S3 particles exhibited two-photon absorption (TPA), while the Sb2S3/RGO composites switched to variable saturated absorption (SA) properties due to the addition of different concentrations of graphene. Moreover, the third-order nonlinear susceptibilities of the composites were also tunable with the concentration of the graphene. The third-order nonlinear susceptibility of the Sb2S3/RGO sample can achieve 8.63 × 10−12 esu. The mechanism for these properties can be attributed to the change of the band gap and the formation of chemical bonds supplying channels for photo-induced charge transfer between Sb2S3 nanoparticles and the graphene. These tunable NLO properties of Sb2S3/RGO composites can be applicable to photonic devices such as Q-switches, mode-locking devices, and optical switches.

Novel nanomaterials based saturable absorbers for passive mode locked fiber laser at 1.5μm

Compared with continuous wave lasers, ultrafast lasers have the advantages of ultra-short pulse width and ultra-high peak power, and have significant applications in optical communications, medical diagnostics, and precision machining. Saturable absorber (SA) technology is the most effective technique for the generation of ultra-fast lasers, which are based on artificial SAs and natural SAs. Among them, the semiconductor saturable absorber mirror has become the most commonly used form at present. Recently, basic research and application of nanomaterials such as carbon nanotubes (CNTs) and graphene have been developed rapidly. Researchers have found that nanomaterials exhibit extraordinary characteristics in ultrafast photonics, such as the low saturation intensity of CNTs, zero-band gap of graphene, and extremely high modulation depth of the topological insulator nano-films. Since graphene was first reported as an SA in 2009, many other nanomaterials have been successively explored, resulting in the rapid development of novel nanomaterial-based SAs. In this paper, we classified the nanomaterials used in SA mode-locking technology at 1.5μm and reviewed their research progress with a particular focus on nonlinear optical properties, integration strategies, and applications in the field of ultrafast photonics.

2D van der Waals materials for ultrafast pulsed fiber lasers: review and prospect

2D van der Waals materials are crystals composed of atomic layers, which have atomic thickness scale layers and rich distinct properties, including ultrafast optical response, surface effects, light-mater interaction, small size effects, quantum effects and macro quantum tunnel effects. With the exploration of saturable absorption characteristic of 2D van der Waals materials, a series of potential applications of 2D van der Waals materials as high threshold, broadband and fast response saturable absorbers (SAs) in ultrafast photonics have been proposed and confirmed. Herein, the photoelectric characteristics, nonlinear characteristic measurement technique of 2D van der Waals materials and the preparation technology of SAs are systematically described. Furthermore, the ultrafast pulsed fiber lasers based on classical 2D van der Waals materials including graphene, transition metal chalcogenides, topological insulators and black phosphorus have been fully summarized and analyzed. On this basis, opportunities and directions in this field, as well as the research results of ultrafast pulsed fiber lasers based on the latest 2D van der Waals materials (such as PbO, FePSe₃, graphdiyne, bismuthene, Ag₂S and MXene etc), are reviewed and summarized.

Recent advances in the development of nanomedicines for the treatment of ischemic stroke

Ischemic stroke is still a serious threat to human life and health, but there are few therapeutic options available to treat stroke because of limited blood-brain penetration. The development of nanotechnology may overcome some of the problems related to traditional drug development. In this review, we focus on the potential applications of nanotechnology in stroke. First, we will discuss the main molecular pathological mechanisms of ischemic stroke to develop a targeted strategy. Second, considering the important role of the blood-brain barrier in stroke treatment, we also delve mechanisms by which the blood-brain barrier protects the brain, and the reasons why the therapeutics must pass through the blood-brain barrier to achieve efficacy. Lastly, we provide a comprehensive review related to the application of nanomaterials to treat stroke, including liposomes, polymers, metal nanoparticles, carbon nanotubes, graphene, black phosphorus, hydrogels and dendrimers. To conclude, we will summarize the challenges and future prospects of nanomedicine-based stroke treatments.

Recent Progress of Two-Dimensional Materials for Ultrafast Photonics

Owing to their extraordinary physical and chemical properties, two-dimensional (2D) materials have aroused extensive attention and have been widely used in photonic and optoelectronic devices, catalytic reactions, and biomedicine. In particular, 2D materials possess a unique bandgap structure and nonlinear optical properties, which can be used as saturable absorbers in ultrafast lasers. Here, we mainly review the top-down and bottom-up methods for preparing 2D materials, such as graphene, topological insulators, transition metal dichalcogenides, black phosphorus, and MXenes. Then, we focus on the ultrafast applications of 2D materials at the typical operating wavelengths of 1, 1.5, 2, and 3 μm. The key parameters and output performance of ultrafast pulsed lasers based on 2D materials are discussed. Furthermore, an outlook regarding the fabrication methods and the development of 2D materials in ultrafast photonics is also presented.

Ultrafast Fiber Lasers with Low-Dimensional Saturable Absorbers: Status and Prospects

Wide-spectral saturable absorption (SA) in low-dimensional (LD) nanomaterials such as zero-, one-, and two-dimensional materials has been proven experimentally with outstanding results, including low saturation intensity, deep modulation depth, and fast carrier recovery time. LD nanomaterials can therefore be used as SAs for mode-locking or Q-switching to generate ultrafast fiber laser pulses with a high repetition rate and short duration in the visible, near-infrared, and mid-infrared wavelength regions. Here, we review the recent development of emerging LD nanomaterials as SAs for ultrafast mode-locked fiber laser applications in different dispersion regimes such as anomalous and normal dispersion regimes of the laser cavity operating in the near-infrared region, especially at ~1550 nm. The preparation methods, nonlinear optical properties of LD SAs, and various integration schemes for incorporating LD SAs into fiber laser systems are introduced. In addition to these, externally (electrically or optically) controlled pulsed fiber laser behavior and other characteristics of various LD SAs are summarized. Finally, the perspectives and challenges facing LD SA-based mode-locked ultrafast fiber lasers are highlighted.

MXenes: Synthesis, Optical Properties, and Applications in Ultrafast Photonics

Recently, 2D materials are in great demand for various applications such as optical devices, supercapacitors, sensors, and biomedicine. MXenes as a kind of novel 2D material have attracted considerable research interest due to their outstanding mechanical, thermal, electrical, and optical properties. Especially, the excellent nonlinear optical response enables them to be potential candidates for the applications in ultrafast photonics. Here, a review of MXenes synthesis, optical properties, and applications in ultrafast lasers is presented. First, aqueous acid etching and chemical vapor deposition methods for preparing MXenes are introduced, in which the storage stability and challenges of the existing synthesis techniques are also discussed. Then, the optical properties of MXenes are discussed specifically, including plasmonic properties, optical detection, photothermal effects, and ultrafast dynamics. Furthermore, the typical ultrafast pulsed lasers enabled by MXene-based saturable absorbers operated at different wavelength regions are summarized. Finally, a summary and outlook on the development of MXenes is presented in the perspectives section.

Highly Sensitive Graphene-Au Coated Plasmon Resonance PCF Sensor

This paper presents a graphene-Au coated photonic crystal fiber (PCF) sensor in the visible regime. Designing a side-polish D-shaped plane over the PCF's defect of the periodic air holes can effectively enhance the evanescent field. Graphene on gold can enhance the sensor's sensitivity because it can stably adsorb biomolecules and increase the propagation constant of the surface plasmon polariton (SPP). Using the finite element method (FEM), we demonstrated that the sensing performance is greatly improved by optimizing the PCF's geometric structural parameter. The proposed PCF sensor exhibited high performance with a maximum wavelength sensitivity of 4200 nm/RIU, maximum amplitude sensitivity of 450 RIU-1, and refractive index resolution of 2.3 × 10-5 RIU in the sensing range 1.32-1.41. This research provides a potential application for the design a new generation of highly sensitive biosensors.

Graphene Oxide Sensitized No-Core Fiber Step-Index Distribution Sucrose Sensor

By coating graphene oxide (GO) onto the surface no-core fiber (NCF), we designed a single-mode no-core single-mode (SNS) fiber Step-Index Distribution sucrose sensor. With wavelength demodulation and the beam propagation method (BPM), the sensor without a GO coating was studied in the low RI range of 1.33~1.389, and the high RI range of 1.389~1.4185. The experiments show that the RI sensitivity of the sensor respectively reaches 132.9 nm/RIU and 292.22 nm/RIU. Both the numerical simulation and the experiments are highly consistent with the theoretical analysis results. Especially, having coated GO on the NCF for sensitization, a high sensitivity was achieved for the response to sucrose concentration solutions. The sensor’s RI sensitivity was increased from 132.9 nm/RIU up to 1348.67 nm/RIU in the ultra-narrow range of 1.33 to 1.3385. This result provides a theoretical and experimental basis for the enrichment and development of sensor detection with a low threshold sucrose concentration.

Recent advances in ultrathin two-dimensional materials and biomedical applications for reactive oxygen species generation and scavenging

Graphene and graphene-like two-dimensional (2D) nanomaterials, such as black phosphorus (BP), transition metal carbides/carbonitrides (MXene) and transition metal dichalcogenides (TMD), have been extensively studied in recent years due to their unique physical and chemical properties. With atomic-scale thickness, these 2D materials and their derivatives can react with ROS and even scavenge ROS in the dark. With excellent biocompatibility and biosafety, they show great application potential in the antioxidant field and ROS detection for diagnosis. They can also generate ROS under light and be applied in antibacterial, photodynamic therapy (PDT), and other biomedical fields. Understanding the degradation mechanism of 2D nanomaterials by ROS generated under ambient conditions is crucial to developing air stable devices and expanding their application ranges. In this review, we summarize recent advances in 2D materials with a focus on the relationship between their intrinsic structure and the ROS scavenging or generating ability. We have also highlighted important guidelines for the design and synthesis of highly efficient ROS scavenging or generating 2D materials along with their biomedical applications.

Revival of Zeolite-Templated Nanocarbon Materials: Recent Advances in Energy Storage and Conversion

Nanocarbon materials represent one of the hottest topics in physics, chemistry, and materials science. Preparation of nanocarbon materials by zeolite templates has been developing for more than 20 years. In recent years, novel structures and properties of zeolite-templated nanocarbons have been evolving and new applications are emerging in the realm of energy storage and conversion. Here, recent progress of zeolite-templated nanocarbons in advanced synthetic techniques, emerging properties, and novel applications is summarized: i) thanks to the diversity of zeolites, the structures of the corresponding nanocarbons are multitudinous; ii) by various synthetic techniques, novel properties of zeolite-templated nanocarbons can be achieved, such as hierarchical porosity, heteroatom doping, and nanoparticle loading capacity; iii) the applications of zeolite-templated nanocarbons are also evolving from traditional gas/vapor adsorption to advanced energy storage techniques including Li-ion batteries, Li-S batteries, fuel cells, metal-O2 batteries, etc. Finally, a perspective is provided to forecast the future development of zeolite-templated nanocarbon materials.

Advancing Applications of Black Phosphorus and BP-Analog Materials in Photo/Electrocatalysis through Structure Engineering and Surface Modulation

Black phosphorus (BP), an emerging 2D material semiconductor material, exhibits unique properties and promising application prospects for photo/electrocatalysis. However, the applications of BP in photo/electrocatalysis are hampered by the instability as well as low catalysis efficiency. Recently, tremendous efforts have been dedicated toward modulating its intrinsic structure, electronic property, and charge separation for enhanced photo/electrocatalytic performance through structure engineering. Simultaneously, the search for new substitute materials that are BP-analogous is ongoing. Herein, the latest theoretical and experimental progress made in the structural/surface engineering strategies and advanced applications of BP and BP-analog materials in relation to photo/electrocatalysis are extensively explored, and a presentation of the future opportunities and challenges of the materials is included at the end.

Role of carrier-transfer in the optical nonlinearity of graphene/Bi2Te3 heterojunctions

Two-dimensional (2D) topological insulators (TIs) have attracted a lot of attention owing to their striking optical nonlinearity. However, the ultra-low saturable intensity (SI) of TIs resulting from the bulk conduction band limits their applications, such as in mode-locking solid-state lasers. In this work, through fabricating a graphene/Bi₂Te₃ heterojunction which combines monolayer graphene and a Bi₂Te₃ nanoplate, the optical nonlinearities are analyzed. Moreover, the thickness-dependent characteristics are also investigated by varying the thickness of the Bi₂Te₃ when synthesizing the heterojunctions. Furthermore, with the aid of the estimated junction electron escape time, a model of the photo-excited carrier-transfer mechanism is proposed and used to describe the phenomena of depression of ultra-low saturable absorption (SA) from the Bi₂Te₃ bulk band. The increased modulation depth of the graphene/Bi₂Te₃ heterojunction can accordingly be realized in more detail. In addition, a Q-switched solid-state laser operating at 1064 nm with heterojunction saturable absorbers is built up and characterized for validating the proposed model. The laser performance with varied Bi₂Te₃ thickness, such as pulse duration and repetition rate, agrees quite well with our proposed model. Our work demonstrates the functionality of optical nonlinear engineering by tuning the thickness of the graphene/Bi₂Te₃ heterojunction and demonstrates its potential for applications.

Recent advances of two-dimensional materials in smart drug delivery nano-systems

Smart drug delivery nano-systems show significant changes in their physical or chemical properties in response to slight change in environmental physical and/or chemical signals, and further releasing drugs adjusted to the progression of the disease at the right target and rate intelligently. Two-dimensional materials possess dramatic status extend all over various scientific and technological disciplines by reason of their exceptional unique properties in application of smart drug delivery nano-systems. In this review, we summarized current progress to highlight various kinds of two-dimensional materials drug carriers which are widely explored in smart drug delivery systems as well as classification of stimuli responsive two-dimensional materials and the advantages and disadvantages of their applications. Consequently, we anticipate that this review might inspire the development of new two-dimensional materials with smart drug delivery systems, and deepen researchers' understanding of smart nano-carries based on two-dimensional materials.

Engineering Mono-Chalcogen Nanomaterials for Omnipotent Anticancer Applications: Progress and Challenges

Belonging to the chalcogen group, the elements selenium (Se) and tellurium (Te) are located in Group VI-A of the periodic table. Zero-valent nanodimensioned Se (nano-Se) and Te (nano-Te) have displayed important biomedical applications in recent years. The past two decades have witnessed an explosion in novel cancer treatment strategies using nano-Se and nano-Te as aggressive weapons against tumors. Indeed, they are both inorganic nanomedicines that suppress tumor cell proliferation, diffusion, and metastasis. Abundant synthesis strategies for rational and precise surface decoration of nano-Se and nano-Te make them significant players in resisting cancers by means of powerful multi-modal treatment methods. This review focuses on the design and engineering of nano-Se- and nano-Te-based nanodelivery systems and their precise uses in cancer treatment. The corresponding anticancer molecular mechanisms of nano-Se and nano-Te are discussed in detail. Given their different photo-induced behaviors, the presence or absence of near infrared illumination is used as a defining characteristic when describing the anticancer applications of nano-Se and nano-Te. Finally, the challenges and future prospects of nano-Se and nano-Te are summarized and highlighted.

Graphene/WS2 heterostructure saturable absorbers for ultrashort pulse generation in L-band passively mode-locked fiber lasers

Graphene/WS₂ (G/WS₂) van der Waals (vdW) heterostructures are utilized as saturable absorbers (SAs) in compact mode-locked fiber lasers operating in the telecommunication L-band for the first time. The interlayer coupling is confirmed by Raman and photoluminescence spectra. In comparison with pure WS₂, the heterostructure exhibits excellent nonlinear optical properties in terms of larger modulation depth and lower saturation intensity due to the strong interlayer coupling. By incorporating the G/WS₂-based SA into an all-anomalous-dispersion fiber laser, stable conventional-soliton pulses with a pulse duration down to 660 fs can be realized at 1601.9 nm, manifesting better output performance compared to pure WS₂. In addition, through shifting the cavity dispersion to the net-normal dispersion, the G/WS₂ SA can also be applied for dissipative-soliton generation. Resultant output pulses feature the central wavelength of 1593.5 nm and the pulse duration of 55.6 ps. Our results indicate that the G/WS₂ vdW heterostructure is a promising candidate as SA for pulsed laser applications, which pave the way for the development of novel ultrafast photonic devices with desirable performance.

Recent advances in two-dimensional-material-based sensing technology toward health and environmental monitoring applications

Monitoring harmful and toxic chemicals, gases, microorganisms, and radiation has been a challenge to the scientific community for the betterment of human health and environment. Two-dimensional (2D)-material-based sensors are highly efficient and compatible with modern fabrication technology, which yield data that can be proficiently used for health and environmental monitoring. Graphene and its oxides, black phosphorus (BP), transition metal dichalcogenides (TMDCs), metal oxides, and other 2D nanomaterials have demonstrated properties that have been alluring for the manufacture of highly sensitive sensors due to their unique material properties arising from their inherent structures. This review summarizes the properties of 2D nanomaterials that can provide a platform to develop high-performance sensors. In this review, we have also discussed the advances made in the field of infrared photodetectors and electrochemical sensors and how the structural properties of 2D nanomaterials affect sensitivity and performance. Further, this review highlights 2D-nanomaterial-based electrochemical sensors that can be used to check for contaminations from heavy metals, organic/inorganic compounds, poisonous gases, pesticides, bacteria, antibiotics, etc., in water or air, which are severe risks to human wellbeing as well as the environment. Moreover, the limitations, future prospects, and challenges for the development of sensors based on 2D materials are also discussed for future advancements.

Recent advances in two-dimensional ferromagnetism: materials synthesis, physical properties and device applications

Two-dimensional (2D) ferromagnetism is critical for both scientific investigation and technological development owing to its low-dimensionality that brings in quantization of electronic states as well as free axes for device modulation. However, the scarcity of high-temperature 2D ferromagnets has been the obstacle of many research studies, such as the quantum anomalous Hall effect (QAHE) and thin-film spintronics. Indeed, in the case of the isotropic Heisenberg model with finite-range exchange interactions as an example, low-dimensionality is shown to be contraindicated with ferromagnetism. However, the advantages of low-dimensionality for micro-scale patterning could enhance the Curie temperature (T_C) of 2D ferromagnets beyond the T_C of bulk materials, opening the door for designing high-temperature ferromagnets in the 2D limit. In this paper, we review the recent advances in the field of 2D ferromagnets, including their material systems, physical properties, and potential device applications.

Recent advances in solution-processed photodetectors based on inorganic and hybrid photo-active materials

Due to their excellent and tailorable optoelectronic performance, low cost, facile fabrication, and compatibility with flexible substrates, solution-processed inorganic and hybrid photo-active materials have attracted extensive interest for next-generation photodetector applications. This review gives a comprehensive compilation of solution-processed photodetectors. The basic structures of the device and important parameters of photodetectors will be firstly summarized. Then the development of various solution processing technologies containing solution synthesis and liquid phase film-forming processes for the preparation of semiconductor films is described. From the materials science point of view, we give a comprehensive overview about the current status of solution processed semiconductor materials including inorganic and hybrid photo-active materials for the application of photodetectors. Moreover, challenges and future trends in the field of solution-processed photodetectors are proposed.

In-plane anisotropic electronics based on low-symmetry 2D materials: progress and prospects

Low-symmetry layered materials such as black phosphorus (BP) have been revived recently due to their high intrinsic mobility and in-plane anisotropic properties, which can be used in anisotropic electronic and optoelectronic devices. Since the anisotropic properties have a close relationship with their anisotropic structural characters, especially for materials with low-symmetry, exploring new low-symmetry layered materials and investigating their anisotropic properties have inspired numerous research efforts. In this paper, we review the recent experimental progresses on low-symmetry layered materials and their corresponding anisotropic electrical transport, magneto-transport, optoelectronic, thermoelectric, ferroelectric, and piezoelectric properties. The boom of new low-symmetry layered materials with high anisotropy could open up considerable possibilities for next-generation anisotropic multifunctional electronic devices.

Monolayer β-tellurene: a promising p-type thermoelectric material via first-principles calculations

Inspired by the experimental growth of two-dimensional (2D) tellurene (Yixin et al., Nat. Electron, 2018, 1, 228-236) and the recent study of unusually low thermal conductivity of atomically thin 2D tellurium (Jie Ren et al., Nanoscale, 2018, 10, 12997), we carried out systematic calculations for monolayer β-tellurene, focusing on its electrical transport and electronic and thermoelectric properties by means of density functional theory (DFT) combined with deformation potential theory and Boltzmann transport theory. We have found that monolayer β-tellurene exhibits a band gap of 1.5 eV. The carrier transport is highly direction-temperature-dependent, with a high room-temperature transport mobility of 1343 cm2 V-1 s-1 and a relaxation time of 283 fs in the armchair direction for hole transport at room-temperature. It is coincidently favourable in the armchair direction for both the Seebeck coefficient and the electrical conductivity, making the p-type monolayer β-tellurene a highly promising thermoelectric candidate. With a low intrinsic lattice thermal conductivity, the maximum figure of merit (ZT) is 2.9 and 0.84 along the armchair and zigzag directions for p-type doping at 700 K, respectively. The predicted properties demonstrate that monolayer β-tellurene can be a prospective material towards thermoelectric applications.

Memristive devices based on emerging two-dimensional materials beyond graphene

With the explosion of data in the information universe and the approaching of fundamental limits in silicon-based flash memories, the exploration of new device architectures and alternative materials is necessary for next-generation memory technology. Accordingly, emerging two-dimensional (2D) material-based memristive devices have attracted increasing attention due to their unique properties and great potential in flexible and wearable devices, and even neuromorphic computing systems. Herein, we provide an overview of the recent progress on memristive devices based on 2D materials beyond graphene. The device structures and choice of active materials and electrodes materials are summarized for various types of 2D material-based memristive devices. Following the discussion and classification on the device performances and mechanisms, the challenges and perspectives on future research based on 2D materials are also presented.

MoS2 with Organic Fragment - a New Hybrid Material for Laser Writing

New nanostructured metasurfaces capable change the composition and physical properties upon pulse laser excitation recently received a marked attention for nanophotonic technologies. In this study, well adherent to the metal substrate and significantly thicker nanoplatelet-shaped MoS2-based arrays were synthesized by one pot hydrothermal way via addition of ethanolamine in the synthesis solution containing ammonium heptamolybdate and thiourea. It was shown that the lightening of this material with green light ns-laser pulses at a suitable fluencies results in the detachment of organic species and compositional transformations to significantly pure MoS2 material. For characterization the synthesized products scanning electron microscopy (SEM), glancing angle X-ray diffraction (GA-XRD), diffuse reflection, Raman, and time-of-flight secondary ion mass spectrometry (ToF-SIMS) methods before and following green light picosecond laser pulse illumination were applied. We envisaged that these films can be successfully used as metamaterial for laser writing.

Organophilic graphene nanosheets as a promising nanofiller for bio-based polyurethane nanocomposites: investigation of the thermal, barrier and mechanical properties

The present study focuses on the design of new nanocomposite films using bio-based thermoplastic polyurethane (TPU) as a polymer matrix and long chain amine functionalized reduced graphene oxide (G-ODA) as a nanofiller.

A flower-structured MoS2-decorated f-MWCNTs/ZnO hybrid nanocomposite-modified sensor for the selective electrochemical detection of vitamin C

We synthesized an MoS₂/f-MWCNTs/ZnO composite and successfully used it to prepare an electrochemical sensor for the selective detection of AA in blood serum samples.

Tunable Ultrafast Nonlinear Optical Properties of Graphene/MoS2 van der Waals Heterostructures and Their Application in Solid-State Bulk Lasers

For van der Waals (vdW) heterostructures, optical and electrical properties ( e.g., saturable absorption and carrier dynamics) are strongly modulated by interlayer coupling, which may be due to effective charge transfer and band structure recombination. General theoretical studies have shown that the complementary properties of graphene and MoS₂ enable the graphene/MoS₂ (G/MoS₂) heterostructure to be used as an important building block for various optoelectronic devices. Here, density functional theory was used to calculate the work function values of G/MoS₂ with different thicknesses of MoS₂, and its relaxation dynamic mechanism was illustrated. The results reveal that the G/MoS₂ heterostructure interlayer coupling can be tuned by changing the thickness of MoS₂, furthering the understanding of the fundamental charge-transfer mechanism in few-layer G/MoS₂ heterostructures. The tunable carrier dynamics and saturable absorption were investigated by pump-probe spectroscopy and open-aperture Z-scan technique, respectively. In the experiments, we compared the performances of Q-switched lasers based on G/MoS₂ heterostructures with different MoS₂ layers. Taking advantage of ultrafast recovery time and good saturable absorption properties, a femtosecond solid-state laser at 1.0 μm with G/MoS₂ heterostructure saturable absorber was successfully achieved. This study on interlayer coupling in G/MoS₂ may allow various vdW heterostructures with controllable stacking to be fabricated and shows the promising applications of vdW heterostructures for ultrafast photonic devices.

A broadband, self-biased photodiode based on antimony telluride (Sb2Te3) nanocrystals/silicon heterostructures

Low bulk band gaps and conductive surface electronic states of tetradymite topological insulators (TTI) make them potential candidates for next generation ultra-broadband photodevices. Here, we demonstrate a broadband and self-biased photodiode based on a Sb2Te3-Si heterostructure. A low-cost thermal evaporation technique was employed to fabricate the photodiode. The self-biased nature of the photodiode was due to the built-in potential at the Sb2Te3-Si interface. Upon characterizing the Sb2Te3 nanocrystalline film via AFM, SEM, EDX, and XPS it was found that the film exhibited p-type behavior due to antimony vacancies or antisites. The fabricated photodiode showed an excellent rectification ratio of 3388 with n-Si confirming a robust Schottky barrier at the interface and a well-defined photocurrent upon illumination. Due to the p-type behavior of the Sb2Te3 nanocrystalline film, a rectification ratio of only 0.38 was observed with p-Si. The barrier at the interface also increases the carrier lifetimes, thereby eliminating one of the biggest drawbacks of ultrafast carrier recombination times in TTI as a photodetection material. Moreover, the photodiode exhibited excellent Ion/Ioff of three orders of magnitude under the self-biased conditions, and photocurrents ranging from 520 nm to 980 nm wavelengths were observed.

Towards spontaneous parametric down conversion from monolayer MoS2

We present a detailed study of the second order nonlinearity of 2D (mono-atomic layer) dichalcogenide MoS₂, both in the visible and in the IR regime, and test its potential for spontaneous parametric down-conversion (SPDC), the amplification of vacuum fluctuations mediated by optical nonlinearity. We develop a model of SPDC from a deeply subwavelength nonlinear medium, where phase matching conditions are completely relaxed, and make predictions about the rate of emitted photons, their momentum, polarisation and spectrum. We show that detection in the visible spectral region is hindered by the strong photoluminescence background. Moving to the IR regime we observe indications of SPDC by performing polarization, power dependence and lifetime measurements around 1560 nm. We show that the signal from a single monolayer is qualitatively different from that generated by multi-layer MoS₂. Finally, we characterize the latter as a new kind of photo-luminescence emission which is enhanced at the edges of multi-layer MoS₂.

Evaluation of Transport Parameters in MoS2/Graphene Junction Devices Fabricated by Chemical Vapor Deposition

We demonstrated imaging of the depletion layer in a MoS₂/graphene heterojunction fabricated by chemical vapor deposition and obtained their transport parameters such as diffusion length, lifetime, and mobility by using scanning photocurrent microscopy (SPCM). The device exhibited a n-type operation, which was determined by the MoS₂ layer with a lower mobility. The SPCM revealed the presence of the depletion layer at the heterojunction, whereas graphene provided an excellent electrical contact for the MoS₂ layer without resulting in a rectifying behavior, even if they were anchored within a very short range. The polarity of the photocurrent signal switched when we applied a drain-source bias voltage, from which we extracted the potential barrier at the junction. More importantly, a bias-dependent SPCM allowed us to simultaneously record the diffusion lengths of both majority and minority carriers for the respective MoS₂ and graphene layers. By combining the diffusion lengths with the lifetimes measured by femtosecond SPCM, we determined the electron and hole mobilities in each layer, from which we found that the electron mobility (160 cm² V^-1 s^-1) was higher than the hole mobility (80 cm² V^-1 s^-1) in MoS₂, whereas the hole mobility (15 000 cm² V^-1 s^-1) was relatively higher in graphene.

Enhanced nonlinear optical response of Se-doped MoS2 nanosheets for passively Q-switched fiber laser application

An enhanced nonlinear optical (NLO) performance was observed in Se-doped MoS₂ nanosheets synthesized through a facile annealing process. Se-doped MoS₂ nanosheets with a large saturable intensity and high modulation depth generated stable passively Q-switched fiber laser pulses at 1559 nm. In comparison with the Q-switched fiber laser utilizing the pristine MoS₂ nanosheets as a saturable absorber, the passive Q-switching operation based on Se-doped MoS₂ nanosheets could be conducted at a lower threshold power of 50 mW, a wider range of repetition rates from 28.97 to 130 kHz, and a higher SNR of 56 dB. More importantly, the shortest pulse duration of 1.502 μs was realized and the output power and pulse energy reached 17.2 mW and 133.07 nJ, respectively. These results indicate that tailoring the chemical composition of optical nanomaterials by introducing a dopant is a feasible method of improving the NLO response of the MoS₂ nanosheets and realizing excellent ultrafast pulse generation.

Emerging Low-Dimensional Materials for Nonlinear Optics and Ultrafast Photonics

Low-dimensional (LD) materials demonstrate intriguing optical properties, which lead to applications in diverse fields, such as photonics, biomedicine and energy. Due to modulation of electronic structure by the reduced structural dimensionality, LD versions of metal, semiconductor and topological insulators (TIs) at the same time bear distinct nonlinear optical (NLO) properties as compared with their bulk counterparts. Their interaction with short pulse laser excitation exhibits a strong nonlinear character manifested by NLO absorption, giving rise to optical limiting or saturated absorption associated with excited state absorption and Pauli blocking in different materials. In particular, the saturable absorption of these emerging LD materials including two-dimensional semiconductors as well as colloidal TI nanoparticles has recently been utilized for Q-switching and mode-locking ultra-short pulse generation across the visible, near infrared and middle infrared wavelength regions. Beside the large operation bandwidth, these ultrafast photonics applications are especially benefit from the high recovery rate as well as the facile processibility of these LD materials. The prominent NLO response of these LD materials have also provided new avenues for the development of novel NLO and photonics devices for all-optical control as well as optical circuits beyond ultrafast lasers.

Wavelength tunable ultrafast fiber laser via reflective mirror with taper structure

Laser sources with a controllable flexible wavelength have found widespread applications in optical fiber communication, optical sensing, and microscopy. Here, we report a tunable mode-locked fiber laser using a graphene-based saturable absorber and a tapered mirror as an end mirror in the cavity. The phase layer in the mirror is precisely etched by focused ion beam (FIB) milling technology, and the resonant wavelength of the mirror shifts correspond to the different etch depths. By scanning the tapered mirror mechanically, the center wavelength of a mode-locked fiber laser can be continuously tuned from 1562 to 1532 nm, with a pulse width in the sub-ps level and repetition rate of 27 MHz.

Visible light nonlinear absorption and optical limiting of ultrathin ZrSe3 nanoflakes

The nonlinear absorption and nonlinear refractive properties of ZrSe₃ nanoflakes were studied with a 6.5 ns pulse laser at 532 nm. Open-aperture Z-scan curves reveal that ZrSe₃ nanoflakes have a strong reverse saturable absorption property, and close-aperture Z-scan curves show that ZrSe₃ dispersions possess a positive nonlinear refractive index caused by self-focusing. The nonlinear absorption coefficient, the nonlinear refraction coefficient, and the figures of merit (FOM) of ZrSe₃ dispersed in water with linear transmittances of 0.86 at input energy of 18 μJ are 6.35 × 10^-10 m W^-1 15.73 × 10^-17 m² W^-1, and 10.09 × 10^-11 esu · cm respectively. In addition, nonlinear optical (NLO) performance of ZrSe₃ nanoflakes depends on organic solvent dispersions. ZrSe₃ nanoflakes in water dispersions have the largest FOM of 10.27 × 10^-11 esu · cm, while the FOM in ethanol dispersions is 5.41 × 10^-11 esu · cm at the same input energy of 26.5 μJ. The optical limiting threshold F_th of ZrSe₃ nanosheet is 2.2 J cm^-2 under picosecond laser pulse. The Results imply that ZrSe₃ nanoflakes are an extraordinarily promising material for novel nanophotonic devices like optical limiters.

Vertically standing layered MoS2 nanosheets on TiO2 nanofibers for enhanced nonlinear optical property

Vertical layered MoS₂ nanosheets rooting into TiO₂ nanofibers were successfully prepared by a facile two-step method: prefabrication of porous TiO₂ nanofibers based on an electrospinning technique, and assembly of MoS₂ ultrathin nanosheets through a simple hydrothermal reaction. Significant enhancement of nonlinear optical response of the MoS₂/TiO₂ nanocomposite was confirmed by an open-aperture z-scan measurement. The nanocomposite displayed strong optical limiting (OL) effects to ultrafast laser pulses with a low OL threshold of ~22.3 mJ/cm², which is lower than that of pristine TiO₂ nanofibers and MoS₂ nanosheets. In addition to the contribution of the strong nonlinear absorption of MoS₂ nanosheets and TiO₂ nanofibers, such phenomenon is also attributed to the unique structure of vertically standing layered MoS₂ nanosheets on TiO₂ nanofibers with a large amount of exposed edge states, large surface areas and fast electron transfer between TiO₂ and MoS₂. This work broadens our vision to engineering novel hierarchical MoS₂-based nanocomposite for efficiently enhanced nonlinear light-matter interaction.

Tunable scalar solitons from a polarization-maintaining mode-locked fiber laser using carbon nanotube and chirped fiber Bragg grating

Generation of tunable scalar solitons from a polarization-maintaining (PM) mode-locked fiber laser is presented. A single-walled carbon nanotube (SWCNT) absorber is used for self-started mode locking. A chirped fiber Bragg grating (CFBG) mounted on a cantilever is employed as a tunable all-fiber filter. Mode-locked solitons are obtained with typical pulse duration of ~6.94 ps and repetition rate of 28.94 MHz. Linearly polarized laser emission is characterized with degree of polarization (DOP) of ~99.5%. The wavelength of the emitted scalar soliton can be continuously tuned through adjusting the CFBG, while maintaining the polarization stability.

Reverse saturable absorption and nonlinear refraction of ultrathin ZrS3 nanobelts

The nonlinear optical (NLO) properties of a ZrS3 nanobelt were measured with a 6.5 ns pulse laser at 532 nm. Its optical response to the incident light exhibits good optical absorptive and refractive effects, with the nonlinear absorption coefficient β = 4.42 × 10(-10) m W(-1) and the nonlinear refraction coefficient γ = 5.86 × 10(-17) m(2) W(-1) for the ZrS3 nanobelt in ethanol dispersions at an input energy of 34.25 μJ. In addition, the β values and γ values have dependence on input energy. Results show that the ZrS3 nanobelts have an excellent reverse saturable absorption (RSA) performance in nanosecond pulses, demonstrating that ZrS3 nanobelts are an extraordinarily promising novel optical power limiting material. Meanwhile, compared to the pure ZrS3, graphene oxide (GO) and reduced graphene oxide (RGO), composites (ZrS3/GRO) exhibit an enhanced nonlinear absorption response at the same input energy.