Rapid and Large-Area Characterization of Exfoliated Black Phosphorus Using Third-Harmonic Generation Microscopy

A clean transfer approach to prepare centimetre-scale black phosphorus crystalline multilayers on silicon substrates for field-effect transistors

Recently reported direct growth of highly crystalline centimetre-sized black phosphorus (BP) thin films on mica substrates by pulsed laser deposition (PLD) has attracted considerable research interest. However, an effective and general transfer method to incorporate them into (opto-)electronic devices is still missing. Here, we show a wet transfer method utilizing ethylene-vinyl acetate (EVA) and an ethylene glycol (EG) solution to transfer high-crystalline large-area PLD-BP films onto SiO2/Si substrates. The transferred films were used to fabricate BP-based bottom-gate field-effect transistor (FET) arrays exhibiting good uniformity and continuity, with carrier mobility and current switching ratios comparable to those obtained in as-grown BP films on mica substrates. Our work presents a promising transfer strategy for scalable integration of on-substrate grown 2D BP into devices with more complex structures and further investigation of material properties.

Nonlinear Optical Properties of 2D Materials and their Applications

2D materials are a subject of intense research in recent years owing to their exclusive photoelectric properties. With giant nonlinear susceptibility and perfect phase matching, 2D materials have marvelous nonlinear light-matter interactions. The nonlinear optical properties of 2D materials are of great significance to the design and analysis of applied materials and functional devices. Here, the fundamental of nonlinear optics (NLO) for 2D materials is introduced, and the methods for characterizing and measuring second-order and third-order nonlinear susceptibility of 2D materials are reviewed. Furthermore, the theoretical and experimental values of second-order susceptibility χ(2) and third-order susceptibility χ(3) are tabulated. Several applications and possible future research directions of second-harmonic generation (SHG) and third-harmonic generation (THG) for 2D materials are presented.

Strong anisotropic third-harmonic generation in layered violet phosphorus

We demonstrate that layered violet phosphorus, an emerging 2D semiconductor, undergoes strong anisotropic third-harmonic generation (THG). Polarization dependence of in-plane THG presents a cruciate-flower-shaped curve. Through theoretical modeling of the in-plane THG response, done by considering crystalline symmetry of violet phosphorus, we successfully quantify four non-zero third-order nonlinear optical susceptibility tensor elements. From control experiments, the magnitude of third-order nonlinear optical susceptibility |χ3| is calculated to be about 4.0 × 10-19 m2 V-2, which is comparable to those of conventional 2D layered semiconductors. These results indicate that the layered violet phosphorus can serve as an ideal building block for nonlinear optical applications.

Can 2D Semiconductors Be Game-Changers for Nanoelectronics and Photonics?

2D semiconductors have interesting physical and chemical attributes that have led them to become one of the most intensely investigated semiconductor families in recent history. They may play a crucial role in the next technological revolution in electronics as well as optoelectronics or photonics. In this Perspective, we explore the fundamental principles and significant advancements in electronic and photonic devices comprising 2D semiconductors. We focus on strategies aimed at enhancing the performance of conventional devices and exploiting important properties of 2D semiconductors that allow fundamentally interesting device functionalities for future applications. Approaches for the realization of emerging logic transistors and memory devices as well as photovoltaics, photodetectors, electro-optical modulators, and nonlinear optics based on 2D semiconductors are discussed. We also provide a forward-looking perspective on critical remaining challenges and opportunities for basic science and technology level applications of 2D semiconductors.

Nonlinear Plasmonics in Nanostructured Phosphorene

Phosphorene has emerged as an atomically thin platform for optoelectronics and nanophotonics due to its excellent optical properties and the possibility of actively tuning light-matter interactions through electrical doping. While phosphorene is a two-dimensional semiconductor, plasmon resonances characterized by pronounced anisotropy and strong optical confinement are anticipated to emerge in highly doped samples. Here we show that the localized plasmons supported by phosphorene nanoribbons (PNRs) exhibit high tunability in relation to both edge termination and doping charge polarity and can trigger an intense nonlinear optical response at moderate doping levels. Our explorations are based on a second-principles theoretical framework, employing maximally localized Wannier functions constructed from ab initio electronic structure calculations, which we introduce here to describe the linear and nonlinear optical response of PNRs on mesoscopic length scales. Atomistic simulations reveal the high tunability of plasmons in doped PNRs at near-infrared frequencies, which can facilitate the synergy between the electronic band structure and plasmonic field confinement to drive efficient high-harmonic generation. Our findings establish nanostructured phosphorene as a versatile atomically thin material candidate for nonlinear plasmonics.

Ultrastrong Optical Harmonic Generations in Layered Platinum Disulfide in the Mid-Infrared

Nonlinear optical activities (e.g., harmonic generations) in two-dimensional (2D) layered materials have attracted much attention due to the great promise in diverse optoelectronic applications such as nonlinear optical modulators, nonreciprocal optical device, and nonlinear optical imaging. Exploration of nonlinear optical response (e.g., frequency conversion) in the infrared, especially the mid-infrared (MIR) region, is highly desirable for ultrafast MIR laser applications ranging from tunable MIR coherent sources, MIR supercontinuum generation, and MIR frequency-comb-based spectroscopy to high harmonic generation. However, nonlinear optical effects in 2D layered materials under MIR pump are rarely reported, mainly due to the lack of suitable 2D layered materials. Van der Waals layered platinum disulfide (PtS₂) with a sizable bandgap from the visible to the infrared region is a promising candidate for realizing MIR nonlinear optical devices. In this work, we investigate the nonlinear optical properties including third-and fifth-harmonic generation (THG and FHG) in thin layered PtS₂ under infrared pump (1550-2510 nm). Strikingly, the ultrastrong third-order nonlinear susceptibility χ⁽³⁾(-3ω;ω,ω,ω) of thin layered PtS₂ in the MIR region was estimated to be over 10^-18 m²/V², which is about one order of that in traditional transition metal chalcogenides. Such excellent performance makes air-stable PtS₂ a potential candidate for developing next-generation MIR nonlinear photonic devices.

Third-Order Optical Nonlinearities of 2D Materials at Telecommunications Wavelengths

All-optical signal processing based on nonlinear optical devices is promising for ultrafast information processing in optical communication systems. Recent advances in two-dimensional (2D) layered materials with unique structures and distinctive properties have opened up new avenues for nonlinear optics and the fabrication of related devices with high performance. This paper reviews the recent advances in research on third-order optical nonlinearities of 2D materials, focusing on all-optical processing applications in the optical telecommunications band near 1550 nm. First, we provide an overview of the material properties of different 2D materials. Next, we review different methods for characterizing the third-order optical nonlinearities of 2D materials, including the Z-scan technique, third-harmonic generation (THG) measurement, and hybrid device characterization, together with a summary of the measured n₂ values in the telecommunications band. Finally, the current challenges and future perspectives are discussed.

Engineering the Dipole Orientation and Symmetry Breaking with Mixed-Dimensional Heterostructures

Engineering of the dipole and the symmetry of materials plays an important role in fundamental research and technical applications. Here, a novel morphological manipulation strategy to engineer the dipole orientation and symmetry of 2D layered materials by integrating them with 1D nanowires (NWs) is reported. This 2D InSe -1D AlGaAs NW heterostructure example shows that the in-plane dipole moments in InSe can be engineered in the mixed-dimensional heterostructure to significantly enhance linear and nonlinear optical responses (e.g., photoluminescence, Raman, and second harmonic generation) with an enhancement factor of up to ≈12. Further, the 1D NW can break the threefold rotational symmetry of 2D InSe, leading to a strong optical anisotropy of up to ≈65%. These results of engineering dipole orientation and symmetry breaking with the mixed-dimensional heterostructures open a new path for photonic and optoelectronic applications.

Anisotropic third-harmonic generation of exfoliated As2S3 thin flakes

Van der Waals (vdW) materials have recently attracted significant interest in the context of orientation-dependent linear and nonlinear optical properties. Recently, arsenic trisulfide (As₂S₃) or orpiment is identified as a new vdW layered material having anisotropic vibrational and optomechanical responses due to the reduced in-plane crystal symmetry, but its nonlinear optical response is still not well understood yet. Herein, the anisotropic third-harmonic generation (THG) response of mechanically exfoliated As₂S₃ thin flakes is reported. The polarization-dependent evolution of THG emission from butterfly-shaped pattern to four-lobe pattern is comprehensively explored. Moreover, the third-order nonlinear susceptibility of As₂S₃ crystal is extracted by analyzing the thickness-dependent THG emission. We anticipate that the discussed results will not only update the existing understanding on the nonlinear light-matter interaction in anisotropic vdW materials, but also promote future applications in integrated photonic circuits, on-chip nonlinear signal processing, and polarization-sensitive optical devices.

Natural layered mercury antimony sulfosalt livingstonite with anisotropic optical properties

Naturally occurring layered mineral livingstonite is identified as a new type of van der Waals (vdW) heterostructure based 2D material, consisting of two commensurately modulated alternating layers of HgSb₂S₄ and Sb₂S₄. The heterostructures of livingstonite crystal are prepared as thin flakes via mechanical exfoliation method. The prepared livingstonite crystals are further investigated in the context of vibrational, linear, and nonlinear optical properties, including anisotropic Raman scattering, wavelength-dependent linear dichroism (LD) transition effect, birefringence, and anisotropic third-harmonic generation (THG). Owing to the monoclinic crystal structure, livingstonite crystals exhibit strong anisotropic vibrational and optical responses. In contrast to conventional vdW heterostructures, the anomalous LD transition effect and the evolution of butterfly-shaped THG emission pattern in livingstonite crystals are demonstrated. Furthermore, the third-order nonlinear susceptibility is estimated for livingstonite crystal using the thickness-dependent THG emission response. Overall, the discussed outcomes establish livingstonite as a new type of naturally grown vdW heterostructure based 2D material and offer insights in tailoring linear and nonlinear light-matter interactions in such vdW heterostructures, which may find further relevance in polarized optical applications and on-chip integrated photonic circuits.

Strategies to Improve Photodynamic Therapy Efficacy of Metal-Free Semiconducting Conjugated Polymers

Photodynamic therapy (PDT) is a noninvasive therapy for cancer and bacterial infection. Metal-free semiconducting conjugated polymers (SCPS) with good stability and optical and electrical properties are promising photosensitizers (PSs) for PDT compared with traditional small-molecule PSs. This review analyzes the latest progress of strategies to improve PDT effect of linear, planar, and three-dimensional SCPS, including improving solubility, adjusting conjugated structure, enhancing PS-doped SCPs, and combining therapies. Moreover, the current issues, such as hypoxia, low penetration, targeting and biosafety of SCPS, and corresponding strategies, are discussed. Furthermore, the challenges and potential opportunities on further improvement of PDT for SCPs are presented.

Probing Electronic States in Monolayer Semiconductors through Static and Transient Third-Harmonic Spectroscopies

Electronic states and their dynamics are of critical importance for electronic and optoelectronic applications. Here, various relevant electronic states in monolayer MoS₂ , such as multiple excitonic Rydberg states and free-particle energy bands are probed with a high relative contrast of up to ≥200 via broadband (from ≈1.79 to 3.10 eV) static third-harmonic spectroscopy (THS), which is further supported by theoretical calculations. Moreover, transient THS is introduced to demonstrate that third-harmonic generation can be all-optically modulated with a modulation depth exceeding ≈94% at ≈2.18 eV, providing direct evidence of dominant carrier relaxation processes associated with carrier-exciton and carrier-phonon interactions. The results indicate that static and transient THS are not only promising techniques for the characterization of monolayer semiconductors and their heterostructures, but also a potential platform for disruptive photonic and optoelectronic applications, including all-optical modulation and imaging.

Silica optical fiber integrated with two-dimensional materials: towards opto-electro-mechanical technology

In recent years, the integration of graphene and related two-dimensional (2D) materials in optical fibers have stimulated significant advances in all-fiber photonics and optoelectronics. The conventional passive silica fiber devices with 2D materials are empowered for enhancing light-matter interactions and are applied for manipulating light beams in respect of their polarization, phase, intensity and frequency, and even realizing the active photo-electric conversion and electro-optic modulation, which paves a new route to the integrated multifunctional all-fiber optoelectronic system. This article reviews the fast-progress field of hybrid 2D-materials-optical-fiber for the opto-electro-mechanical devices. The challenges and opportunities in this field for future development are discussed.

2D layered SiP as anisotropic nonlinear optical material

Two-dimensional (2D) material of silicon phosphide (SiP) has recently been shown as a promising optical material with large band gap, fast photoresponse and strong anisotropy. However, the nonlinear optical properties of 2D SiP have not been investigated yet. Here, the thickness-dependent in-plane anisotropic third-harmonic generation (THG) from the mechanically exfoliated 2D layered SiP flakes is reported. The crystal orientation of the SiP flake is determined by the angle-resolved polarized Raman spectroscopy. The angular dependence of the THG emission with respect to the incident linear polarization is found to be strongly anisotropic with the two-fold polarization dependence pattern. Furthermore, the effect of the SiP flake thickness on the THG power is analyzed.

Nonlinear Optical Characterization of 2D Materials

Characterizing the physical and chemical properties of two-dimensional (2D) materials is of great significance for performance analysis and functional device applications. As a powerful characterization method, nonlinear optics (NLO) spectroscopy has been widely used in the characterization of 2D materials. Here, we summarize the research progress of NLO in 2D materials characterization. First, we introduce the principles of NLO and common detection methods. Second, we introduce the recent research progress on the NLO characterization of several important properties of 2D materials, including the number of layers, crystal orientation, crystal phase, defects, chemical specificity, strain, chemical dynamics, and ultrafast dynamics of excitons and phonons, aiming to provide a comprehensive review on laser-based characterization for exploring 2D material properties. Finally, the future development trends, challenges of advanced equipment construction, and issues of signal modulation are discussed. In particular, we also discuss the machine learning and stimulated Raman scattering (SRS) technologies which are expected to provide promising opportunities for 2D material characterization.

In-plane anisotropic third-harmonic generation from germanium arsenide thin flakes

A newly introduced two-dimensional (2D) layered germanium arsenide (GeAs) has attracted growing interest due to its promising highly in-plane anisotropic crystal structure and electronic properties for photonic and optoelectronic applications. The potential of 2D layered GeAs for many applications such as anisotropic photodetection, electronics, superconductivity and thermoelectricity is being investigated in recent studies. However, the intrinsic nonlinear optical properties of 2D layered GeAs have not been explored yet. Here, thickness- and incident polarization-dependent in-plane anisotropic third-harmonic generation (THG) from the mechanically exfoliated thin GeAs flakes is reported. Furthermore, the effect of the flake thickness on the THG conversion efficiency is shown to find the optimal thickness range for high conversion efficiency. The polarization state of the emitted THG signal is also analyzed by measuring the Stokes parameters with different polarization states of the pump beam to demonstrate the capability of controlling the intensity and polarization of TH emission. Our results will create new opportunities for advancing anisotropic optical devices used for future photonic integration, optical communication and optical information processing.

An overview of the optical properties and applications of black phosphorus

Since the year 2014, when scientists first obtained black phosphorus using a sticky tape to peel the layers off, it has attracted tremendous interest as a novel two-dimensional material. After it was successfully produced, its outstanding optical properties have been unveiled. Various applications based on these properties have been reported. This study mainly reviews the unique optical properties and potential applications of black phosphorus. The optical performances of black phosphorus mainly include linear optical properties and nonlinear optical properties. Some examples include the anisotropic optical response, saturable absorption effect and Kerr effect. The researchers found that the nonlinear saturable absorption coefficients of black phosphorus are better than that of MoS₂ and WS₂ from the visible region to the near-infrared region. Compared with graphene, black phosphorus has a better nonlinear saturable absorption performance. After passivation or surface modification, black phosphorus is stable when exposed to oxygen and water. Herein, black phosphorus has the potential to be used in detector/sensors, solar energy harvesting, photocatalysts, optical saturable absorbers in ultrafast lasers, all optical switches, optical modulation, nanomedicine and some others in the near future.

Non-aqueous solution-processed phosphorene by controlled low-potential electrochemical exfoliation and thin film preparation

Black phosphorus (BP) in its monolayer form called phosphorene is thought of as a successor of graphene and is of great interest for (opto)electronic applications. A quantitative and scalable method for the synthesis of (mono-)few-layer phosphorene has been an outstanding challenge due to the process irreproducibility and environmental degradation capability of the BP. Here, we report a facile controlled electrochemical exfoliation method for the preparation of a few-layer phosphorene (FP) with nearly 100% yield. Our approach relies on the low-potential influence in anhydrous and oxygen-free low-boiling acetonitrile (AN) and N,N-dimethylformamide (DMF) using alkylammonium ions. Herein, intercalation of positive ions into BP interlayers occurred with a minimum potential of -2.95 V in DMF and -2.85 V in AN and the non-damaging and highly accurate electrochemical exfoliation lasted at -3.8 V. A variety of analytical methods have revealed that in particular DMF-based exfoliation results in high-quality phosphorene of 1-5 layers with good crystallinity and lateral sizes up to tens of micrometers. Moreover, assurance of the oxygen- and water-free environment allowed us to minimize the surface oxidation of BP and, consequently, exfoliated phosphorene. We pioneer an effective and reproducible printing transfer of electrochemically exfoliated phosphorene films onto various flexible and rigid substrates. The surfactant-free process of exfoliation allowed assembly and transfer of thin films based on FP. The phosphorene-based films characterized as direct gap semiconductors have a layer-number-dependent bandgap with a tuning range larger than that of other 2D materials. We show that on varying the films' thickness, it is possible to modify their optical properties, which is a significant advantage for compact and switchable optoelectronic components.

Anisotropic Enhancement of Second-Harmonic Generation in Monolayer and Bilayer MoS2 by Integrating with TiO2 Nanowires

The ability to design and enhance the nonlinear optical responses in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) is both of fundamental interest and highly desirable for developing TMDC-based nonlinear optical applications, such as nonlinear convertors and optical modulators. Here, we report for the first time a strong anisotropic enhancement of optical second-harmonic generation (SHG) in monolayer molybdenum disulfide (MoS₂) by integrating with one-dimensional (1D) titanium dioxide nanowires (NWs). The SHG signal from the MoS₂/NW hybrid structures is over 2 orders of magnitude stronger than that in the bare monolayer MoS₂. Polarized SHG measurements revealed a giant anisotropy in SHG response of the MoS₂/NW hybrid. The pattern of the anisotropic SHG depends highly on the stacking angle between the nanowire direction and the MoS₂ crystal orientation, which is attributed to the 1D NW-induced directional strain fields in the layered MoS₂. A similar effect has also been observed in bilayer MoS₂/NW hybrid structure, further proving the proposed scenario. This work provides an effective approach to selectively and directionally designing the nonlinear optical response of layered TMDCs, paving the way for developing high-performance, anisotropic nonlinear photonic nanodevices.

High photoresponsivity and broadband photodetection with a band-engineered WSe2/SnSe2 heterostructure

van der Waals (vdW) heterostructures formed by stacking different two-dimensional layered materials have been demonstrated as a promising platform for next-generation photonic and optoelectronic devices due to their tailorable band-engineering properties. Here, we report a high photoresponsivity and broadband photodetector based on a WSe2/SnSe2 heterostructure. By properly biasing the heterostructure, its band structure changes from near-broken band alignment to type-III band alignment which enables high photoresponsivity from visible to telecommunication wavelengths. The highest photoresponsivity and detectivity at 532 nm are ∼588 A W-1 and 4.4 × 1010 Jones and those at 1550 nm are ∼80 A W-1 and 1.4 × 1010 Jones, which are superior to those of the current state-of-the-art layered transition metal dichalcogenides based photodetectors under similar measurement conditions. Our work not only provides a new method for designing high-performance broadband photodetectors but also enables a deep understanding of the band engineering technology in the vdW heterostructures possible for other applications, such as modulators and lasers.

Black phosphorus, a prospective graphene substitute for biomedical applications

2D materials have gained spectacular status across various scientific and technological disciplines owing to their exceptional unique properties. The very recent member of 2D family, Black Phosphorus monolayers, known as Phosphorene have attracted recent scientific attention since its first exfoliation and appreciable rediscovery in 2014. Compared to other 2D materials and graphene analogs, it has outstanding properties like tunable band gap, good carrier mobility, excellent ON-OFF current ratio, potent in vivo biocompatibility and non-toxic biodegradability. Although the outlook of this material seems to be a promising candidate for future biomedical technology, its practical applications are still highly challenging. Unveiling those challenges by proper characterization and functionalization makes this material a mile stone for future theranostic and biomedicine scenario. This review has given precise attention to familiarize with the unique fundamental properties of black phosphorus, which makes it an excellent platform for future biomedical applications. Also underlines various synthesis procedures applicable for BP nanosheets and quantum dot synthesis. Its various biomedical applications including biosensors, cancer therapy, imaging and photothermal/photo acoustic/photodynamic therapy, drug delivery, neuronal regeneration, 3D printing scaffold etc., are subsequently reviewed. Furthermore this review briefly focused on the toxicity of this emerging material.

Modifying the sensibility of nonmetal-doped phosphorene by local or global properties

The dopant atom X can modify the sensibility of X-doped phosphorene by tuning the electronic properties of X-doped phosphorene surfaces effectively. According to the adsorption strength and the amount of charge transfer between the adsorption species and X-doped phosphorene surfaces, the adsorption species can be roughly divided into three types.

Layer-Coupled States Facilitate Ultrafast Charge Transfer in a Transition Metal Dichalcogenide Trilayer Heterostructure

Forming van der Waals multilayer structures with two-dimensional materials is a promising new method for material discovery. The weak van der Waals interlayer interaction without atomic correspondence relaxes lattice matching requirement and allows formation of high-quality interfaces with virtually any combination of two-dimensional materials. However, the weak nature of the van der Waals interaction also makes it challenging to harness emergent properties of such multilayer materials. Previous studies have indicated that in transition metal dichalcogenide bilayer heterostructures, the interlayer charge and energy transfer is highly efficient. Therefore, it is important to understand interlayer coupling in these materials and its role on charge and energy transfer. Here we show that in a MoSe₂/WSe₂/WS₂ trilayer, the interlayer coupling is strong enough to form layer-coupled states in the conduction band with the electron wave function extends to all three layers. Density functional theory calculations reveal that the layer-coupled states in Q valley are about 0.1 eV below the individual monolayer states in K valley, which is consistent with photoluminescence measurements. Transient absorption measurements show that these layer-coupled states provide a channel for ultrafast interlayer charge transfer between the top WS₂ and the bottom MoSe₂ layers. In this process, electrons from the K valley of the individual monolayers are scattered to the layer-coupled states in Q valley. Such a partial charge transfer allows formation of partial-indirect excitons with the holes in one monolayer while electrons shared by three layers. The formation of layer-coupled states is promising for harnessing emergent properties of transition metal dichalcogenide multilayer heterostructures. Our findings also provide new ingredient to understand charge and energy transfer in transition metal dichalcogenide heterobilayers, as the layer-coupled states can play important roles in the efficient transfer observed in these systems.

Interface Characterization and Control of 2D Materials and Heterostructures

2D materials and heterostructures have attracted significant attention for a variety of nanoelectronic and optoelectronic applications. At the atomically thin limit, the material characteristics and functionalities are dominated by surface chemistry and interface coupling. Therefore, methods for comprehensively characterizing and precisely controlling surfaces and interfaces are required to realize the full technological potential of 2D materials. Here, the surface and interface properties that govern the performance of 2D materials are introduced. Then the experimental approaches that resolve surface and interface phenomena down to the atomic scale, as well as strategies that allow tuning and optimization of interfacial interactions in van der Waals heterostructures, are systematically reviewed. Finally, a future outlook that delineates the remaining challenges and opportunities for 2D material interface characterization and control is presented.

Active synchronization and modulation of fiber lasers with a graphene electro-optic modulator

We report the synchronization of two actively Q-switched fiber lasers operating at 1.5 μm and 2 μm with a shared broadband graphene electro-optic modulator. Two graphene monolayer sheets separated with a high-kHfO₂ dielectric layer are configured to enable broadband light modulation. The graphene electro-optic modulator is shared by two optical fiber laser cavities (i.e., an erbium-doped fiber laser cavity and a thulium/holmium-codoped fiber laser cavity) to actively Q-switch the two lasers, resulting in stable synchronized pulses at 1.5 μm and 2 μm with a repetition rate ranging from 46 kHz to 56 kHz.

Nonlinear Optics with 2D Layered Materials

2D layered materials (2DLMs) are a subject of intense research for a wide variety of applications (e.g., electronics, photonics, and optoelectronics) due to their unique physical properties. Most recently, increasing research efforts on 2DLMs are projected toward the nonlinear optical properties of 2DLMs, which are not only fascinating from the fundamental science point of view but also intriguing for various potential applications. Here, the current state of the art in the field of nonlinear optics based on 2DLMs and their hybrid structures (e.g., mixed-dimensional heterostructures, plasmonic structures, and silicon/fiber integrated structures) is reviewed. Several potential perspectives and possible future research directions of these promising nanomaterials for nonlinear optics are also presented.

Wavelength and pulse duration tunable ultrafast fiber laser mode-locked with carbon nanotubes

Ultrafast lasers with tunable parameters in wavelength and time domains are the choice of light source for various applications such as spectroscopy and communication. Here, we report a wavelength and pulse-duration tunable mode-locked Erbium doped fiber laser with single wall carbon nanotube-based saturable absorber. An intra-cavity tunable filter is employed to continuously tune the output wavelength for 34 nm (from 1525 nm to 1559 nm) and pulse duration from 545 fs to 6.1 ps, respectively. Our results provide a novel light source for various applications requiring variable wavelength or pulse duration.

Black Phosphorus: Optical Characterization, Properties and Applications

The rise of black phosphorus (BP) as a new family member of two-dimensional (2D) materials brings new concepts and applications to the field, because of the infrared band gap and anisotropic properties of such materials. Among many excellent properties of BP, the optical property attracts special attention in recent years. Optical methods have been widely and successfully used in characterizing BP, not only to obtain the structural information (such as thickness and crystalline orientation), but also to probe the fundamental properties of BP in terms of the behavior of electrons, phonons, excitons etc. In this Review, a comprehensive understanding about the optical characterization of BP such as Raman, absorption, and photoluminescence is presented. Also, the unique optical properties and applications explored in recent years are reviewed.

All-reflective multiphoton microscope

We present the design, construction, and characterization of a multiphoton microscope that uses reflective elements for beam shaping and steering. This compact all reflective design removes the adverse effects of dispersion on laser pulse broadening as well as chromatic aberration in the focusing of broadband and multicolored laser sources. The design of this system is discussed in detail, including aberrations analysis via ray-tracing simulation and opto-mechanical design. The resolution of this mirror based all-reflective microscope is characterized using fluorescent microbeads. The performance of the system at multiple wavelengths is investigated along with some potential multiphoton imaging and writing applications.

Rapid visualization of grain boundaries in monolayer MoS2 by multiphoton microscopy

Grain boundaries have a major effect on the physical properties of two-dimensional layered materials. Therefore, it is important to develop simple, fast and sensitive characterization methods to visualize grain boundaries. Conventional Raman and photoluminescence methods have been used for detecting grain boundaries; however, these techniques are better suited for detection of grain boundaries with a large crystal axis rotation between neighbouring grains. Here we show rapid visualization of grain boundaries in chemical vapour deposited monolayer MoS₂ samples with multiphoton microscopy. In contrast to Raman and photoluminescence imaging, third-harmonic generation microscopy provides excellent sensitivity and high speed for grain boundary visualization regardless of the degree of crystal axis rotation. We find that the contrast associated with grain boundaries in the third-harmonic imaging is considerably enhanced by the solvents commonly used in the transfer process of two-dimensional materials. Our results demonstrate that multiphoton imaging can be used for fast and sensitive characterization of two-dimensional materials.

Atomically thin transition metal dichalcogenides can be grown on large scale using chemical vapour deposition which, however, determines presence of grain boundaries. Here, the authors report that third-harmonic generation imaging provides excellent sensitivity and fast speed for grain boundary visualization in MoS₂.