Wide spectral and wavelength-tunable dissipative soliton fiber laser with topological insulator nano-sheets self-assembly films sandwiched by PMMA polymer

Green Synthesis and Morphological Evolution for Bi2Te3 Nanosystems via a PVP-Assisted Hydrothermal Method

Bi2Te3 has been extensively used because of its excellent thermoelectric properties at room temperature. Here, 230-420 nm of Bi2Te3 hexagonal nanosheets has been successfully synthesized via a "green" method by using ethylene glycol solution and applying polyvinyl pyrrolidone (PVP) as a surfactant. In addition, factors influencing morphological evolution are discussed in detail in this study. Among these parameters, the reaction temperature, molar mass of NaOH, different surfactants, and reaction duration are considered as the most essential. The results show that the existence of PVP is vital to the formation of a plate-like morphology. The reaction temperature and alkaline surroundings played essential roles in the formation of Bi2Te3 single crystals. By spark plasma sintering, the Bi2Te3 hexagonal nanosheets were hot pressed into solid-state samples. We also studied the transport properties of solid-state samples. The electrical conductivity σ was 18.5 × 103 Sm-1 to 28.69 × 103 Sm-1, and the Seebeck coefficient S was -90.4 to -113.3 µVK-1 over a temperature range of 300-550 K. In conclusion, the observation above could serve as a catalyst for future exploration into photocatalysis, solar cells, nonlinear optics, thermoelectric generators, and ultraviolet selective photodetectors of Bi2Te3 nanosheet-based photodetectors.

Wavelength-tunable broadband lasers based on nanomaterials

Nanomaterials are widely used in the fields of sensors, optoelectronics, biophotonics and ultrafast photonics due to their excellent mechanical, thermal, optical, electrical and magnetic properties. Particularly, owing to their nonlinear optical properties, fast response time and broadband operation, nanomaterials are ideal saturable absorption materials in ultrafast photonics, which contribute to the improvement of laser performance. Therefore, nanomaterials are of great importance to applications in wavelength-tunable broadband pulsed lasers. Herein, we review the integration and applications of nanomaterials in wavelength-tunable broadband ultrafast photonics. Firstly, the two integration methods, which are direct coupling and evanescent field coupling, and their characteristics are introduced. Secondly, the applications of nanomaterials in wavelength-tunable broadband lasers are summarized. Finally, the development of nanomaterials and broadband tunable lasers is reviewed and discussed.

Single and Bunch Soliton Generation in Optical Fiber Lasers Using Bismuth Selenide Topological Insulator Saturable Absorber

In this work, we present the generation of two distinct types of soliton pulses using a Bismuth Selenide (Bi₂Se₃) saturable absorber (SA) synthesized in our laboratory. The soliton pulses were generated in two different laser cavity configurations, resulting in two types of solitons: a soliton pulse with Kelly sidebands and a bunched soliton pulse with peak-dip sidebands. Both solitons operated at the fundamental repetition rate-23.3 MHz (for the soliton with Kelly sidebands) and 13 MHz (for the bunched soliton with peak-dip sidebands). We observed that the accumulation of nonlinear phase shift from the added single mode fiber (SMF) split the single soliton pulse into 44 pulses in a bunched oscillation envelope. At the same time, peak-dip sidebands were imposed on the bunched soliton spectrum due to constructive and destructive interferences between soliton pulse and dispersive waves. The measured pulse width for both solitons were 0.63 ps (for the soliton with Kelly sidebands) and 1.52 ps (for the bunched soliton with peak-dip sidebands), respectively. Our results demonstrate the potential of Bi₂Se₃ SAs in generating different types of soliton pulses, which could have potential applications in various areas of optical communication and spectroscopy.

GaSb Film is a Saturable Absorber for Dissipative Soliton Generation in a Fiber Laser

Nanotechnology is at the forefront of scientific research and offers great prospects for the development of technology. As a type of III-V semiconductor, GaSb materials exhibit numerous outstanding optical and electrical characteristics that are very promising for nonlinear optical device applications. In this study, the electronic band structures of GaSb are theoretically calculated, and its application in dissipative soliton fiber lasers is validated. A GaSb thin film is deposited on a microfiber using magnetron sputtering deposition, and the morphology, chemical composition, structure, and nonlinear optical characteristics of the proposed microfiber-GaSb device are investigated. After incorporating it into an Er-doped fiber laser, dissipative soliton laser pulses are readily obtained with a fundamental frequency of 43.5 MHz. With increasing pump power, the fiber laser could work in the fundamental frequency mode-locking state. At a pump power of 570 mW, the pulse width and the output power are measured to be 917 fs and 49.75 mW, separately. These results reveal that GaSb can be used as an efficient saturable absorber, which will have potential applications in ultrafast optics.

Integration of the Rhombohedral BiSb(0001) Topological Insulator on a Cubic GaAs(001) Substrate

Bismuth-antimony alloy (Bi_1 - xSb_x) is the first reported 3D topological insulator (TI). Among many TIs reported to date, it remains the most promising for spintronic applications thanks to its large conductivity, its colossal spin Hall angle, and the possibility to build low-current spin-orbit-torque magnetoresistive random access memories. Nevertheless, the 2D integration of TIs on industrial standards is lacking. In this work, we report the integration of high-quality rhombohedral BiSb(0001) topological insulators on a cubic GaAs(001) substrate. We demonstrate a clear epitaxial relationship at the interface, a fully relaxed TI layer, and the growth of a rhombohedral matrix on top of the cubic substrate. The antimony composition of the Bi_1 - xSb_x layer is perfectly controlled and covers almost the whole TI window. For optimized growth conditions, the sample generates a semiconductor band structure at room temperature in the bulk and exhibits metallic surface states at 77 K.

Nonlinear Photonics Using Low-Dimensional Metal-Halide Perovskites: Recent Advances and Future Challenges

Low-dimensional metal-halide perovskites have exhibited significantly superior nonlinear optical properties compared to traditional semiconductor counterparts, thanks to their peculiar physical and electronic structures. Their exceptional nonlinear optical characteristics make them excellent candidates for revolutionizing widespread applications. However, the research of nonlinear photonics based on low-dimensional metal-halide perovskites is in its infancy. There is a lack of comprehensive and in-depth summary of this research realm. Here, the state-of-the-art research progress related to third-and higher-order nonlinear optical properties of low-dimensional metal-halide perovskites with diverse crystal structures from 3D down to 0D, together with their practical applications, is summarized comprehensively. Critical discussions are offered on the fundamental mechanisms beneath their exceptional nonlinear optical performance from the physics viewpoint, attempting to disclose the role of intrinsic attributes (e.g., composition, bandgap, size, shape, and structure) and external modulation strategies (e.g., developing core-shell structures, transition metal ion doping, and hybridization with dielectric microspheres) in tuning the response. Additionally, their potential applications in nonlinear photonics, nonlinear optoelectronics, and biophotonics are systematically and thoroughly summed up and categorized. Lastly, insights into the current technical challenges and future research opportunities of nonlinear photonics based on low-dimensional metal-halide perovskites are provided.

Advances in Solar-Driven Hygroscopic Water Harvesting

Water scarcity is one of the greatest global challenges at this time. Significant efforts have been made to harvest water from the air, due to widely available water sources present in the atmosphere. Particularly, solar-driven hygroscopic water harvesting based on the adsorption-desorption process has gained tremendous attention because of the abundance of solar energy in combination with substantial improvements in conversion efficiency enabled by advanced sorbents, improved photothermal materials, interfacial heating system designs, and thermal management in recent years. Here, recent developments in atmospheric water harvesting are discussed, with a focus on solar-driven hygroscopic water harvesting. The diverse structural designs and engineering strategies that are being used to improve the rate of the water production, including the design principles for sorbents with high adsorption capacity, high-efficiency light-to-heat conversion, optimization of thermal management, vapor condensation, and water collection, are also explored. The current challenges and future research opportunities are also discussed, providing a roadmap for the future development of solar-driven hygroscopic water harvesting technology.

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.

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.

CVD-Bi2Te3 as a saturable absorber for various solitons in a mode-locked Er-doped fiber laser

In this work, we report about high energy and various solitons' operation by using high-efficiency topological insulator bismuth telluride (Bi₂Te₃) nanofilms as broadband saturable absorbers in the passively mode-locked Er-doped fiber laser. The Bi₂Te₃ film was successfully synthesized by chemical vapor deposition (CVD). Excellent characteristics of the dark-bright pulse pairs, bright pulses, and multiharmonics have been investigated experimentally by adjusting the polarization state. At the same time, the maximum average output power was 40.18 mW, and the single-pulse energy was 20.91 nJ. As we all know, it is the various solitons of the first generation with large pulse energy in an Er-doped fiber laser with Bi₂Te₃ as saturable absorber. The experimental results show that CVD Bi₂Te₃ can be used as an excellent candidate in mode-locked fiber lasers.

Recent Progress, Challenges, and Prospects in Two-Dimensional Photo-Catalyst Materials and Environmental Remediation

The successful photo-catalyst library gives significant information on feature that affects photo-catalytic performance and proposes new materials. Competency is considerably significant to form multi-functional photo-catalysts with flexible characteristics. Since recently, two-dimensional materials (2DMs) gained much attention from researchers, due to their unique thickness-dependent uses, mainly for photo-catalytic, outstanding chemical and physical properties. Photo-catalytic water splitting and hydrogen (H2) evolution by plentiful compounds as electron (e-) donors is estimated to participate in constructing clean method for solar H2-formation. Heterogeneous photo-catalysis received much research attention caused by their applications to tackle numerous energy and environmental issues. This broad review explains progress regarding 2DMs, significance in structure, and catalytic results. We will discuss in detail current progresses of approaches for adjusting 2DMs-based photo-catalysts to assess their photo-activity including doping, hetero-structure scheme, and functional formation assembly. Suggested plans, e.g., doping and sensitization of semiconducting 2DMs, increasing electrical conductance, improving catalytic active sites, strengthening interface coupling in semiconductors (SCs) 2DMs, forming nano-structures, building multi-junction nano-composites, increasing photo-stability of SCs, and using combined results of adapted approaches, are summed up. Hence, to further improve 2DMs photo-catalyst properties, hetero-structure design-based 2DMs' photo-catalyst basic mechanism is also reviewed.

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.

Solution-gated transistors of two-dimensional materials for chemical and biological sensors: status and challenges

Two-dimensional (2D) materials have been the focus of materials research for many years due to their unique fascinating properties and large specific surface area (SSA). They are very sensitive to the analytes (ions, glucose, DNA, protein, etc.), resulting in their wide-spread development in the field of sensing. New 2D materials, as the basis of applications, are constantly being fabricated and comprehensively studied. In a variety of sensing applications, the solution-gated transistor (SGT) is a promising biochemical sensing platform because it can work at low voltage in different electrolytes, which is ideal for monitoring body fluids in wearable electronics, e-skin, or implantable devices. However, there are still some key challenges, such as device stability and reproducibility, that must be faced in order to pave the way for the development of cost-effective, flexible, and transparent SGTs with 2D materials. In this review, the device preparation, device physics, and the latest application prospects of 2D materials-based SGTs are systematically presented. Besides, a bold perspective is also provided for the future development of these devices.

Single-axis soliton molecule and multiple solitons generation from a vector fiber laser

We investigate various patterns of vector solitons arising in a passively mode-locked fiber laser based on semiconductor saturable absorber mirror (SESAM). By properly adjusting the cavity parameters including the pump power and intra-cavity birefringence, the fundamental vector solitons, vector soliton molecules, and macroscopic vector solitons can be separately observed. In particular, both vector soliton molecule and macroscopic vector solitons exhibit multi-pulse structure along one polarization axis while there occurs single pulse profile at its orthogonal polarization component. Thus, they can be treated as "1 + 2" and "1+n" vector solitons. Moreover, the size of the macroscopic solitons can be manipulated from half of the cavity to even the whole cavity. The generation mechanisms of these vector soliton patterns are also investigated.

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.

Controllable Polarization of Lasing Emission From a Polymer Microfiber Laser

Microlasers with controllable polarization of output emission are vital for on-chip optical communications, optical sensors and optical switches. In this work, we report a high quality (Q) factor, low-threshold polymer microfiber laser and the possibility of achieving laser emission with a desired polarization. The microfiber is fabricated by direct drawing from a dye-doped polymer solution and it can generate whispering gallery mode (WGM) lasing under optical pulse excitation. When the microfiber is pumped from the side with pumping direction perpendicular to the microfiber’s axis, the polarization direction of the output laser is found to be the same as that of the pump laser. Lasing emission with either transverse electric (TE) or transverse magnetic (TM) modes can be obtained and these two polarization states can be switched over by tuning the pumping laser. Furthermore, emission with both TE and TM modes can also be observed by changing the orientation of the microfiber relatively to pumping direction. Our finding provides an effective approach for achieving microlasers that have high Q lasing modes with anticipated polarization.

Mechanism of dissipative soliton stabilization by nonlinear gradient terms

We present a feedback mechanism for dissipative solitons in the cubic complex Ginzburg-Landau (CGL) equation with a nonlinear gradient term. We are making use of a mechanical analog containing contributions from a potential and from a nonlinear viscous term. The feedback mechanism relies on the continuous supply of energy as well as on dissipation of the stable pulse. Our picture is corroborated by the numerical solution of the full equation. A quintic contribution is not necessary for stabilization in the presence of a suitable nonlinear gradient term as we show by using a linear stability analysis for the stationary pulses. We find that the limit of vanishing magnitude of the nonlinear gradient term is singular: For exactly vanishing nonlinear gradient terms stable pulses do not exist. This situation is qualitatively different from that found for the cubic-quintic complex Ginzburg-Landau equation with a nonlinear gradient term: In this case the limit of a vanishing nonlinear gradient contribution is completely smooth. We demonstrate that, for small magnitude of the nonlinear gradient term simple types of scaling behavior are found for the amplitude, the full width at half maximum (FWHM), the velocity, and the effective frequency of the stable pulse as a function of the magnitude of the nonlinear gradient term.

Generation of high-energy rectangular pulses in a nonlinear polarization rotation mode-locked ring fiber laser

In this paper, a novel high-energy mode-locked fiber laser based on the nonlinear polarization rotation (NPR) technique is presented to generate 331 nJ rectangular pulses. When pump power was 2659 mW, the maximum output power would be 102.3 mW; the maximum peak power was 41.74 W under the pump power of 1766 mW. In this study, the use of two homemade laser diodes and other common fiber devices was a vital step to achieve the low-cost and high-efficiency NPR mode-locked fiber laser. Based on these results, a novel approach could be developed to realize a high-energy rectangular pulse and promote the practical applications of the NPR mode-locked fiber laser in the field of ultrafast photonics.

Improved Laser Damage Threshold of In2Se3 Saturable Absorber by PVD for High-Power Mode-Locked Er-Doped Fiber Laser

In this study, a double-end pumped high-power passively mode-locked erbium-doped fiber laser (EDFL) was realized by employing a few-layered In₂Se₃ flakes as a saturable absorber (SA). Herein, the uniform large-scale In₂Se₃ flakes were synthesized by the physical vapor deposition (PVD) method. The PVD-In₂Se₃ SA exhibited a remarkable damage threshold of higher than 24 mJ/cm². Meanwhile, the PVD-In₂Se₃ SA had a modulation depth and saturable intensity of 18.75% and 6.8 MW/cm², respectively. Based on the In₂Se₃ SA, the stable bright pulses emitting at 1559.4 nm with an average output power/pulse energy/pulse duration of 122.4 mW/5.8 nJ/14.4 ns were obtained successfully. To our knowledge, 122.4 mW was the new major breakthrough of mode-locked Er-doped fiber lasers. In addition, this is the first demonstration of the dark-bright pulse pair generation based on In₂Se₃ SA. The maximum average output power of the dark-bright pulse reached 121.2 mW, which also showed significant enhancement in comparison with previous works. Our excellent experiment results fully prove the superiority of our experimental design scheme and indicate that the PVD-In₂Se₃ could operate as a promising highly-nonlinear photonic material for a high-power fiber laser.

Output energy enhancement in a mode-locked Er-doped fiber laser using CVD-Bi2Se3 as a saturable absorber

In this study, the output energy in topological insulators (TIs)-based Erbium-doped fiber laser (EDFL) was improved using two strategies: bidirectional pumped laser cavity and saturable absorber (SA) with high damage threshold and large modulation depth. Using the chemical vapor deposition (CVD) method, Bismuth Selenide (Bi₂Se₃) film was synthesized and improved to a SA. Employing this CVD-Bi₂Se₃ SA in an EDFL, bright and bright-dark soliton operations were achieved. The average output power/pulse energy was 82.6 mW/48.3 nJ and 81.2 mW/47.5 nJ, respectively. The results demonstrate that CVD-Bi₂Se₃ can act as an excellent performance material to improve output power performance in TISA-based EDFL.

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.

23 MHz widely wavelength-tunable L-band dissipative soliton from an all-fiber Er-doped laser

Via careful choice of Er-doped fiber length in the cavity, a widely wavelength-tunable L-band dissipative soliton all-fiber Er-doped laser incorporating a L-band optimized polarizing fiber grating device is experimentally demonstrated. The laser delivers 15.38 ps dissipative soliton pulses centered at 1597.34 nm with 3 dB bandwidth of 34.6 nm under 622 mW pump power. The pulse repetition rate is 23 MHz. After using single mode fiber at external cavity, the pulse duration is compressed to 772 fs. With nonlinear polarization rotation-based intracavity comb filter, the central wavelength of the generated dissipative soliton can be tuned from 1567 nm to 1606 nm with a spectral tuning range of 39 nm, which, to the best of our knowledge, is the widest tuning range yet reported for a dissipative soliton fiber laser working in communication band.

Emerging 2D materials beyond graphene for ultrashort pulse generation in fiber lasers

Ultrafast fiber lasers have significant applications in ultra-precision manufacturing, medical diagnostics, medical treatment, precision measurement and astronomical detection, owing to their ultra-short pulse width and ultra-high peak-power. Since graphene was first explored as an optical saturable absorber for passively mode-locked lasers in 2009, many other 2D materials beyond graphene, including phosphorene, antimonene, bismuthene, transition metal dichalcogenides (TMDs), topological insulators (TIs), metal-organic frameworks (MOFs) and MXenes, have been successively explored, resulting in rapid development of novel 2D materials-based saturable absorbers. Herein, we review the latest progress of the emerging 2D materials beyond graphene for passively mode-locked fiber laser application. These 2D materials are classified into mono-elemental, dual-elemental and multi-elemental 2D materials. The atomic structure, band structure, nonlinear optical properties, and preparation methods of 2D materials are summarized. Diverse integration strategies for applying 2D materials into fiber laser systems are introduced, and the mode-locking performance of the 2D materials-based fiber lasers working at 1-3 μm are discussed. Finally, the perspectives and challenges facing 2D materials-based mode-locked fiber lasers are highlighted.

3 GHz passively harmonic mode-locked Er-doped fiber laser by evanescent field-based nano-sheets topological insulator

In this paper, we report on the experimental observation of harmonic mode locking (HML) fiber laser based on evanescent field of tapered fiber using topological insulator (TI) Bi₂Te₃ saturable absorber (SA). The optical deposition method was adopted to fabricate the tapered fiber-based TISA. By significant nonlinear optical property of the tapered fiber TISA and high nonlinear fiber (HNLF), the fiber laser could operate at the pulse repetition of 3.125 GHz under 976 nm pump power of 148 mW, the pulse width of 1.754 ps, and the output power of 6.4 mW. Our results demonstrate that the tapered fiber TI device and HNLF can act as both high nonlinear optical component and SA in fiber lasers, and could also have better performance to achieve ultrashort pulses with ultrahigh repetition.

Fiber optics frequency comb enabled linear optical sampling with operation wavelength range extension

Although the linear optical sampling (LOS) technique is powerful enough to characterize various advanced modulation formats with high symbol rates, the central wavelength of a pulsed local oscillator (LO) needs to be carefully set according to that of the signal under test, due to the coherent mixing operation. Here, we experimentally demonstrate wideband LOS enabled by a fiber optics frequency comb (FOFC). Meanwhile, when the broadband FOFC acts as the pulsed LO, we propose a scheme to mitigate the enhanced sampling error arising in the non-ideal response of a balanced photodetector. Finally, precise characterizations of arbitrary 128 Gbps PDM-QPSK wavelength channels from 1550 to 1570 nm are successfully achieved, when a 101.3 MHz frequency spaced comb with a 3 dB spectral power ripple of 20 nm is used.

Optical nonlinearity engineering of a bismuth telluride saturable absorber and application of a pulsed solid state laser therein

Saturable absorbers (SAs) have interesting applications for the realization of pulsed lasers in various wavelengths of fiber and solid-state lasers. Topological insulators (TIs) have been recently discovered to feature saturable absorption due to their unique band structure. In this study, high-purity layers of Bi₂Te₃ thin film SA have been successfully prepared using the spin coating-coreduction approach (SCCA). Compared with the typical method of preparing SAs, the SCCA can be used to prepare topological insulator saturable absorbers (TISAs) with high optical quality, large area consistency, and controllable thickness, which is critical for pulsed lasers. To the best of our knowledge, this study is the first observation and discussion of clear thickness-dependent optical nonlinearity. In this study, a Q-switched bulk Nd:YAG laser is demonstrated and investigated using the prepared TISA as the absorber. The timing jitter and amplitude fluctuation of the stable pulse laser indicated that the SCCA is suitable for fabricating a Bi₂Te₃ SA. Furthermore, the SCCA enables the establishment of a pulsing laser through saturation intensity engineering.

The Property, Preparation and Application of Topological Insulators: A Review

Topological insulator (TI), a promising quantum and semiconductor material, has gapless surface state and narrow bulk band gap. Firstly, the properties, classifications and compounds of TI are introduced. Secondly, the preparation and doping of TI are assessed. Some results are listed. (1) Although various preparation methods are used to improve the crystal quality of the TI, it cannot reach the industrialization. Fermi level regulation still faces challenges; (2) The carrier type and lattice of TI are affected by non-magnetic impurities. The most promising property is the superconductivity at low temperature; (3) Magnetic impurities can destroy the time-reversal symmetry of the TI surface, which opens the band gap on the TI surface resulting in some novel physical effects such as quantum anomalous Hall effect (QAHE). Thirdly, this paper summarizes various applications of TI including photodetector, magnetic device, field-effect transistor (FET), laser, and so on. Furthermore, many of their parameters are compared based on TI and some common materials. It is found that TI-based devices exhibit excellent performance, but some parameters such as signal to noise ratio (S/N) are still lower than other materials. Finally, its advantages, challenges and future prospects are discussed. Overall, this paper provides an opportunity to improve crystal quality, doping regulation and application of TI.

Switchable thulium-doped fiber laser from polarization rotation vector to scalar soliton

We experimentally demonstrate switchable temporal soliton generation from a thulium-doped fiber laser (TDFL), using carbon nanotubes as the mode-locker. With the help of residual polarization dependent loss of a wavelength division multiplexer, a weak nonlinear polarization rotation (NPR) effect can be achieved within the laser cavity, which may provide joint contribution for passive mode-locking operation. By finely adjusting the polarization to alter the strength of NPR-based saturable absorption, the TDFL either approaches the operation regime of scalar soliton with strong NPR effect, or generates polarization rotation locked vector soliton (PRLVS) with weak NPR effect. The scalar solitons and PRLVSs possess 3-dB optical spectrum bandwidth of 2.2 nm and 2 nm, pulse-width of 1.8 ps and 2 ps, respectively. Moreover, the PRLVSs demonstrate a typical energy exchange between two polarized components on optical spectra and a period-doubling feature in time domain. Such operation principle can also be used in 1550 nm band fiber lasers and other nonlinear systems.

Scalar-vector soliton fiber laser mode-locked by nonlinear polarization rotation

We report a passively mode-locked fiber laser by nonlinear polarization rotation (NPR), where both vector and scalar soliton can co-exist within the laser cavity. The mode-locked pulse evolves as a vector soliton in the strong birefringent segment and is transformed into a regular scalar soliton after the polarizer within the laser cavity. The existence of solutions in a polarization-dependent cavity comprising a periodic combination of two distinct nonlinear waves is first demonstrated and likely to be applicable to various other nonlinear systems. For very large local birefringence, our laser approaches the operation regime of vector soliton lasers, while it approaches scalar soliton fiber lasers under the condition of very small birefringence.

Polarization domain wall pulses in a microfiber-based topological insulator fiber laser

Topological insulators (TIs), are novel two-dimension materials, which can act as effective saturable absorbers (SAs) in a fiber laser. Moreover, based on the evanescent wave interaction, deposition of the TI on microfiber would create an effective SA, which has combined advantages from the strong nonlinear optical response in TI material together with the sufficiently-long-range interaction length in fiber taper. By using this type of TI SA, various scalar solitons have been obtained in fiber lasers. However, a single mode fiber always exhibits birefringence, and hence can support two orthogonal degenerate modes. Here we investigate experimentally the vector characters of a TI SA fiber laser. Using the saturated absorption and the high nonlinearity of the TI SA, a rich variety of dynamic states, including polarization-locked dark pulses and their harmonic mode locked counterparts, polarization-locked noise-like pulses and their harmonic mode locked counterparts, incoherently coupled polarization domain wall pulses, including bright square pulses, bright-dark pulse pairs, dark pulses and bright square pulse-dark pulse pairs are all observed with different pump powers and polarization states.

3-μm Mid-infrared pulse generation using topological insulator as the saturable absorber

We report an 1150-nm diode-pump passively Q-switched Ho3+-doped ZBLAN fiber laser using topological insulator (TI): Bi2Te3 as the saturable absorber (SA). The TI: Bi2Te3 prepared using the cost-effective hydrothermal intercalation/exfoliation method was dropped onto a CaF2 substrate to fabricate the free-space SA component. It has a low saturable peak intensity of 2.12  MW/cm2 and a high modulation depth of 51.3% measured at 2 μm. Inserting this component into a linear-cavity Ho3+-doped ZBLAN fiber laser, stable Q-switched pulses at 2979.9 nm were obtained with the repetition rate of 81.96 kHz and pulse duration of 1.37 μs. The achieved maximum output power and pulse energy were 327.4 mW at a slope efficiency of 11.6% and 3.99 μJ, respectively, only limited by the available pump power. Our work reveals that the TIs are absolutely a class of promising and reliable SAs for pulse generation at 3-μm mid-infrared waveband.

Dissipative soliton generation in Er-doped fiber laser mode-locked by Sb2Te3 topological insulator

We report on the generation of dissipative solitons in an Er-doped fiber laser mode-locked by antimony telluride (Sb 2 Te 3) topological insulator in the near-zero dispersion regime. Layers of Sb 2 Te 3 were deposited on a side-polished (D-shaped) fiber using a pulsed magnetron sputtering technique. Sub-170-fs pulses with 34-nm full width at half-maximum (FWHM) and 0.21 nJ of pulse energy were obtained from an all-fiber, ring-shaped laser cavity after the compression in a single-mode fiber.

Dual-state dissipative solitons from an all-normal-dispersion erbium-doped fiber laser: continuous wavelength tuning and multi-wavelength emission

We propose and experimentally demonstrate switchable operation of dissipative solitons (DSs) in an erbium-doped fiber laser. By managing normal dispersion of laser cavity, the 3-dB spectral bandwidth up to 8.1 nm can be obtained with the help of a semiconductor saturable absorber mirror. Using an inline polarizer, the fiber laser can be separately operated at either wavelength-tunable or multi-wavelength emission. The central wavelength of DS can be continuously tuned from 1554 to 1561 nm with its spectra maintaining standard rectangular shape. Alternatively, triple-wavelength DSs at 1535, 1544, and 1553 nm can be simultaneously obtained.