MXene-based saturable absorber for femtosecond mode-locked fiber lasers

Copper phthalocyanines as a mode-locker in an Er-doped fiber laser

We demonstrate a mode-locked erbium-doped fiber laser (EDFL) utilizing copper phthalocyanines (CuPc) as a saturable absorber (SA) for the first time, to the best of our knowledge. The investigated SA was prepared using a simple, low-cost and straightforward technique, whereby the CuPc powder was embedded into polyvinyl alcohol (PVA) to form a thin film. The thin film acted as a mode-locker when it was incorporated into the EDFL cavity to produce output pulses at a repetition rate of 1.8 MHz with a pulse duration of 1.98 ps. The frequency spectrum showed a signal-to-noise ratio as high as 55 dB, which indicates the stability of the mode-locking operation. To the best of our knowledge, this is the first work to report using CuPc as a mode-locker.

Germanene saturable absorber for mode-locked operation in an all-fiber laser with multiple dispersion environments

In this paper, a high-quality germanene-polyvinyl alcohol (PVA) saturable absorber (SA) with a modulation depth of 3.05% and a saturation intensity of 17.95M W/c m 2 was prepared. Stable conventional mode-locking and harmonic mode-locking (HML) were achieved in germanene-based Er-doped fiber lasers (EDFL) using dispersion management techniques. In a cavity with a net dispersion value of -0.22p s 2, the conventional soliton had a center wavelength of 1558.2 nm, a repetition frequency of 19.09 MHz, and a maximum 3 dB spectrum bandwidth of 3.5 nm. The highest repetition frequencies achieved in cavities with net dispersion values of -2.81p s 2, -1.73p s 2, and -1.09p s 2 were 9.48 MHz, 12.75 MHz, and 12.10 MHz for HML, respectively. Furthermore, the effects of dispersion, power, and the polarization state on HML were systematically investigated. Our research results fully demonstrate the capability of germanene as an optical modulator in generating conventional mode-locked and harmonic mode-locked solitons. This provides meaningful references for promoting its application in ultrafast fiber lasers.

Ultracompact MXene V2C-Improved Temperature Sensor by a Runway-Type Microfiber Knot Resonator

We demonstrate an all-fiber, compact-structure, high-sensing-efficiency temperature sensor using a resonator structure sensor device of a runway type and MXene V2C. The high-quality functional material MXene V2C, synthesized by a simple two-step method, has excellent photothermal conversion performance. As-prepared MXene V2C is integrated into the runway section of a runway-type microfiber knot resonator based on the coupling mechanism between the surface near the field of the fiber and materials. When the temperature variation range is ~25-70 °C, the corresponding transmission light intensity variation is linear, and the maximum normalized sensing efficiency is 2.21 dB/°C/mm. Our work demonstrates that the runway-type structure ensures the compactness of the sensor device and enhances the interaction distance between the material and the microfiber, which provides additional integration strategies for functional material-based sensor devices.

MXene V2C-coated runway-type microfiber knot resonator for an all-optical temperature sensor

We present an all-optical temperature sensor device made of an MXene V₂C integrated runway-type microfiber knot resonator (MKR) for the first time. MXene V₂C is coated on the surface of the microfiber by optical deposition. The experimental results show that the normalized temperature sensing efficiency is ∼1.65 dB °C^-1 mm^-1. The high sensing efficiency of the temperature sensor we proposed benefits from the efficient coupling of the highly photothermal material MXene and the runway-type resonator structure, which provides a better idea for the preparation of all-fiber sensor devices.

MXene-Nanoflakes-Enabled All-Optical Nonlinear Activation Function for On-Chip Photonic Deep Neural Networks

2D metal carbides and nitrides (MXene) are promising material platforms for on-chip neural networks owing to their nonlinear saturable absorption effect. The localized surface plasmon resonances in metallic MXene nanoflakes may play an important role in enhancing the electromagnetic absorption; however, their contribution is not determined due to the lack of a precise understanding of its localized surface plasmon behavior. Here, a saturable absorber made of MXene thin film and a silicon waveguide with MXene flakes overlayer are developed to perform neuromorphic tasks. The proposed configurations are reconfigurable and can therefore be adjusted for various applications without the need to modify the physical structure of the proposed MXene-based activator configurations via tuning the wavelength of operation. The capability and feasibility of the obtained results of machine-learning applications are confirmed via handwritten digit classification task, with near 99% accuracy. These findings can guide the design of advanced ultrathin saturable absorption materials on a chip for a broad range of applications.

Femtosecond Pulsed Fiber Laser by an Optical Device Based on NaOH-LPE Prepared WSe2 Saturable Absorber

We report on all-optical devices prepared from WSe2 combined with drawn tapered fibers as saturable absorbers to achieve ultrashort pulse output. The saturable absorber with a high damage threshold and high saturable absorption characteristics is prepared for application in erbium-doped fiber lasers by the liquid phase exfoliation method for WSe2, and the all-optical device exhibited strong saturable absorption characteristics with a modulation depth of 15% and a saturation intensity of 100.58 W. The net dispersion of the erbium-doped fiber laser cavity is ~-0.1 ps2, and a femtosecond pulse output with a bandwidth of 11.4 nm, a pulse width of 390 fs, and a single-pulse capability of 42 pJ is obtained. Results indicate that the proposed WSe2 saturable absorbers are efficient, photonic devices to realize stable fiber lasers. The results demonstrate that the WSe2 saturable absorber is an effective photonic device for realizing stable fiber lasers, which have a certain significance for the development of potential photonic devices.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Femtosecond Pulsed Fiber Laser Based on Graphdiyne-Modified Tapered Fiber

We report the application of saturable absorbers prepared from graphdiyne-modified tapered fibers to an erbium-doped fiber laser to achieve a femtosecond pulse output. Graphdiyne quantum dots are successfully prepared by the Glaser–Hay method. The graphdiyne-based all-fiber saturable absorber device exhibited strongly saturable absorption characteristics with a modulation depth of 18.06% and a saturation intensity of 103.5 W. The net dispersion of the erbium-doped fiber laser cavity is ~0.016 ps², and a femtosecond pulse output with a bandwidth of 26.3 nm, a pulse width of 135.8 fs, and a single pulse capability of 54 pJ is obtained. This work lays the foundation for the application of the nonlinear optical material, graphdiyne, in ultrafast photonics.

Development of polarization modulator using MXene thin film

In this work, polarization modulator utilizing MXene material, namely Nb₂C is demonstrated. S band signal is injected into Nb₂C thin film and is modulated by 1400 nm laser diode. A total of 39.81° of polarization rotation is attained when the pump power is increased to 223 mW. The rotation of light is due to thermo-optic effect. The efficiency of polarization modulator is calculated at 0.1974°/mW.

Broadband and Wide-Angle Performance of a Perfect Absorber Using a MIM Structure with 2D MXene

Due to the extraordinary optoelectronic properties and surface-rich functional groups, MXene has shown great promise in many applications, such as electromagnetic shielding, catalysis, sensors, ultrafast photons, etc. In this work, we propose a wide-angle absorber based on a metal-insulator-metal (MIM) metamaterial consisting of MXene. By optimizing the design, the absorption efficiency can be further improved throughout the entire wavelength range. More importantly, the absorber exhibits high-efficiency broadband and wide-angle (20–80°) absorption in the near-infrared range (NIR: 1.1–1.7 μm) by numerical calculation. It is foreseeable that the excellent absorption characteristics and easy-to-manufacture structure of the designed absorber will bring some inspiration to the absorption device in the NIR and its practical application.

Nonlinear optical properties and passively Q-switched laser application of a layered molybdenum carbide at 639 nm

Molybdenum carbide (Mo₂C) exhibits enormous potential applications in various optoelectronic and photonic fields due to its remarkably electrical and optical characteristics. Here, we fabricate a high-quality Mo₂C film by the radio frequency magnetron sputtering deposition method. The nonlinear optical response and ultrafast dynamics are thoroughly studied based on open-aperture Z-scan and nondegenerate pump-probe experimental measurements. The open-aperture Z-scan experimental result exhibits a modulation depth of 8.5% and a saturation fluence of 0.28 mJ/cm². Simultaneously, the relaxation time constant is fitted by a biexponential decay function, showing an ultrafast intraband carrier recovery time of 0.58 ps at 530 nm. Consequently, by employing the Mo₂C film as a saturable absorber (SA), stable Q-switched Pr:YLF laser pulses with the shortest pulse width of 160 ns are generated at 639 nm. Our experimental results demonstrate excellent nonlinear optical properties of the layered Mo₂C in the visible region and will further advance their potential applications in visible nonlinear optics.

Controllable Synthesis of Narrow-Gap van der Waals Semiconductor Nb2GeTe4 with Asymmetric Architecture for Ultrafast Photonics

Ultrafast photonics has become an interdisciplinary topic of great consequence due to the spectacular progress of compact and efficient ultrafast pulse generation. Wide spectrum bandwidth is the key element for ultrafast pulse generation due to the Fourier transform limitation. Herein, monoclinic Nb₂GeTe₄, an emerging class of ternary narrow-gap semiconductors, was used as a real saturable absorber (SA), which manifests superior wide-range optical absorption. The crystallization form and growth mechanism of Nb₂GeTe₄ were revealed by a thermodynamic phase diagram. Furthermore, the Nb₂GeTe₄-SA showed reliable saturation intensity and larger modulation depth, ascribed to a built-in electric field driven by the asymmetric crystal architecture confirmed via X-ray diffraction, polarized Raman spectra, and scanning transmission electron microscopy. Based on the Nb₂GeTe₄-SA, femtosecond mode-locked operation with good overall performance was achieved by a properly designed ring cavity. These results suggest that Nb₂GeTe₄ shows great promise for ultrafast photonic applications and arouse interests in exploring the intriguing properties of the ternary van der Waals material family.

Highly Sensitive Photothermal Fiber Sensor Based on MXene Device and Vernier Effect

A photothermal fiber sensor based on a microfiber knot resonator (MKR) and the Vernier effect is proposed and demonstrated. An MXene Ti₃C₂T_x nanosheet was deposited onto the ring of an MKR using an optical deposition method to prepare photothermal devices. An MXeneMKR and a bare MKR were used as the sensing part and reference part, respectively, of a Vernier-cascade system. The optical and photothermal properties of the bare MKR and the MXeneMKR were tested. Ti₃C₂T_x was applied to a photothermal fiber sensor for the first time. The experimental results showed that the modulation efficiency of the MXeneMKR was 0.02 nm/mW, and based on the Vernier effect, the modulation efficiency of the cascade system was 0.15 nm/mW. The sensitivity was amplified 7.5 times. Our all-fiber photothermal sensor has many advantages such as low cost, small size, and good system compatibility. Our sensor has broad application prospects in many fields. The proposed stable MKR device based on two-dimensional-material modification provides a new solution for improving the sensitivity of optical fiber sensors.

Synthesis of Hexagonal Structured GaS Nanosheets for Robust Femtosecond Pulse Generation

Gallium sulfide (GaS), with a hexagonal structure, has received extensive attention due to its graphene-like structure and derived optical properties. Here, high-quality GaS was obtained via chemical vapor synthesis and then prepared as a saturable absorber by the stamp-assisted localization-transfer technique onto fiber end face. The stability of the material and the laser damage threshold are maintained due to the optimized thickness and the cavity integration form. The potential of the GaS for nonlinear optics is explored by constructing a GaS-based Erbium-doped mode-locked fiber laser. Stable femtosecond (~448 fs) mode-locking operation of the single pulse train is achieved, and the robust mode-locked operation (>30 days) was recorded. Experimental results show the potential of GaS for multi-functional ultrafast high-power lasers and promote continuous research on graphene-like materials in nonlinear optics and photonics.

2 µm sub-GHz harmonic mode-locked soliton generation based on a Bi2S3 saturable absorber

Saturable absorber (SA) based harmonic mode-locking (HML) techniques at 2 µm waveband are much less reported than those at 1.5 µm waveband, the maximum repetition rate of the harmonic pulse generated by such techniques at 2 µm waveband is also much lower than those generated at 1.5 µm waveband. In this paper, the 39th harmonic with the repetition rate of 908.6 MHz is realized in a Bi₂S₃-based thulium-doped fiber laser. The fundamental mode-locked pulse has a central wavelength of 1954.2 nm and a 3-dB bandwidth of 5.1 nm. The repetition rate is 23.27 MHz and the pulse width is 902 fs. The characteristics of the material and harmonic mode-locked pulse are investigated. To the best of our knowledge, this is the highest and the closest resonance frequency to GHz among the reported SA-based harmonic mode-locked fiber lasers operating at 2 µm waveband.

Two-Dimensional Gallium Sulfide as a Novel Saturable Absorber for Broadband Ultrafast Photonics Applications

Two-dimensional (2D) gallium sulfide (GaS) offers a plethora of exceptional electrical and optical properties, allowing it to be used in a wide range of applications, including photodetectors, hydrogen generation, and nonlinear optical devices. In this paper, ultrathin 2D GaS nanosheets are synthesized using the liquid-phase exfoliation method, and the structure, morphology, and chemical composition of the as-prepared nanosheets are extensively investigated. After depositing 2D GaS nanosheets on side polished fibers, successful saturable absorbers (SAs) are fabricated for the first time. The realized modulation depths are 10 and 5.3% at 1 and 1.5 μm, respectively, indicating the wideband saturable absorption performance of the prepared SAs. By integrating GaS-SAs into three different wavelength-based fiber laser cavities, stable mode-locked pulses are achieved, having pulse durations of 46.22 ps (1 μm), 614 fs (1.5 μm), and 1.02 ps (2 μm), respectively. Additionally, different orders of harmonic mode-locked pulses with the highest repetition rate of 0.55 GHz (45th order) and Q-switched pulses with the shortest pulse duration of 2.2 μs are obtained in the telecommunication waveband. These findings suggest that 2D GaS has a lot of potential for broadband ultrafast photonics in nonlinear photonics devices.

Femtosecond ultrafast pulse generation with high-quality 2H-TaS2 nanosheets via top-down empirical approach

Tantalum disulfide (TaS₂), an emerging group VB transition metal dichalcogenide, with unique layered structure, rich phase diagrams, metallic behavior, higher carrier concentration and mobility is emerging as a prototype for revealing basic physical phenomena and developing practical applications. However, its photonics properties and even engineering-related processes are still rare. Here, the top-down experiment demonstration, including synthesis, thickness optimization and nonlinear optical application, has been reported. In addition, the ultrafast (∼373 fs) erbium-doped fiber pulse with a small time-bandwidth product (∼0.34) and long-term stability (∼25 days) was realized using the nonlinear absorption properties of the high-quality 2H-TaS₂ nanosheet. These results suggest an experimental route for further ultrafast photonics exploration based on metallic transition metal dichalcogenides.

MXenes: synthesis, incorporation, and applications in ultrafast lasers

The rapid expansion of nanotechnology and material science prompts two-dimensional (2D) materials to be extensively used in biomedicine, optoelectronic devices, and ultrafast photonics. Owing to the broadband operation, ultrafast recovery time, and saturable absorption properties, 2D materials become the promising candidates for being saturable absorbers in ultrafast pulsed lasers. In recent years, the novel 2D MXene materials have occupied the forefront due to their superior optical and electronic, as well as mechanical and chemical properties. Herein, we introduce the fabrication methods of MXenes, incorporation methods of combining 2D materials with laser cavities, and applications of ultrafast pulsed lasers based on MXenes. Firstly, top-down and bottom-up approaches are two types of fabrication methods, where top-down way mainly contains acid etching and the chief way of bottom-up method is chemical vapor deposition. In addition to these two typical ones, other methods are also discussed. Then we summarize the advantages and drawbacks of these approaches. Besides, commonly used incorporation methods, such as sandwich structure, optical deposition, as well as coupling with D-shaped, tapered, and photonic crystal fibers are reviewed. We also discuss their merits, defects, and conditions of selecting different methods. Moreover, we introduce the state of the art of ultrafast pulsed lasers based on MXenes at different wavelengths and highlight some excellent output performance. Ultimately, the outlook for improving fabrication methods and applications of MXene-based ultrafast lasers is presented.

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.

Modulation of MXene Nb2CTx saturable absorber for passively Q-switched 2.85 µm Er:Lu2O3laser

We proposed a passively stabilized Q-switched Er:Lu₂O₃ laser at 2845 nm, applying a MXene Nb₂CT_x nanosheets saturable absorber prepared by the liquid-phase exfoliation method. The surface morphology and nonlinear properties of this nanosheet were systematically characterized. Average output power of 542 mW for the Q-switched laser was obtained under 7.26 W of absorbed pump power. Meanwhile, the Q-switched pulse duration was measured to be 223.7 ns with 142.9 kHz repetition rate corresponding to a peak power of 16.96 W.

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.

Hetero-MXenes: Theory, Synthesis, and Emerging Applications

Since their discovery in 2011, MXenes (abbreviation for transition metal carbides, nitrides, and carbonitrides) have emerged as a rising star in the family of 2D materials owing to their unique properties. Although the primary research interest is still focused on pristine MXenes and their composites, much attention has in recent years been paid also to MXenes with diverse compositions. To this end, this work offers a comprehensive overview of the progress on compositional engineering of MXenes in terms of doping and substituting from theoretical predictions to experimental investigations. Synthesis and properties are briefly introduced for pristine MXenes and then reviewed for hetero-MXenes. Theoretical calculations regarding the doping/substituting at M, X, and T sites in MXenes and the role of vacancies are summarized. After discussing the synthesis of hetero-MXenes with metal/nonmetal (N, S, P) elements by in situ and ex situ strategies, the focus turns to their emerging applications in various fields such as energy storage, electrocatalysts, and sensors. Finally, challenges and prospects of hetero-MXenes are addressed. It is anticipated that this review will be beneficial to bridge the gap between predictions and experiments as well as to guide the future design of hetero-MXenes with high performance.

Applications of Few-Layer Nb2C MXene: Narrow-Band Photodetectors and Femtosecond Mode-Locked Fiber Lasers

Although the physicochemical properties of niobium carbide (Nb₂C) have been widely investigated, their exploration in the field of photoelectronics is still at the infancy stage with many potential applications that remain to be exploited. Hence, it is demonstrated here that few-layer Nb₂C MXene can serve as an excellent building block for both photoelectrochemical-type photodetectors (PDs) and mode-lockers. We show that the photoresponse performance can be readily adjusted by external conditions and that Nb₂C NSs exhibit a great potential for narrow-band PDs. The demonstrated mechanism was further confirmed by work functions predicted by density functional theory calculations. In addition, as an optical switch for passively mode-locked fiber lasers, ultrastable pulses can be demonstrated in the telecommunication and mid-infrared regions for Nb₂C MXene, and as high as the 69th harmonic order with 411 MHz at the center wavelength of 1882 nm can be achieved. These intriguing results indicate that few-layer Nb₂C nanosheets can be used as building blocks for various photoelectronic devices, further broadening the application prospects of two-dimensional MXenes.

Recent advances in real-time spectrum measurement of soliton dynamics by dispersive Fourier transformation

Mode-locking lasers have not only produced huge economic benefits in industrial fields and scientific research, but also provided an excellent platform to study diverse soliton phenomena. However, the real-time characterization of the ultrafast soliton dynamics remains challenging for traditional electronic instruments due to their relatively low response bandwidth and slow scan rate. Consequently, it is urgent for researchers to directly observe these ultrafast evolution processes, rather than just indirectly understand them from numerical simulations or averaged measurement data. Fortunately, dispersive Fourier transformation (DFT) provides a powerful real-time measurement technique to overcome the speed limitations of traditional electronic measurement devices by mapping the frequency spectrum onto the temporal waveform. In this review, the operation principle of DFT is discussed and the recent progress in characterizing the ultrafast transient soliton dynamics of mode-locking lasers is summarized, including soliton explosions, soliton molecules, noise-like pulses, rogue waves, and mode-locking buildup processes.

ZrSe2 nanosheet as saturable absorber for soliton operations within an Er-doped passive mode-locked fiber laser

Two-dimensional materials have extensively promoted the development of ultrafast photonics in the past decade. In our work, the saturable absorption properties of ZrSe₂ were presented. The saturation intensity, modulation depth, and nonlinear absorption coefficient of the ZrSe₂ saturable absorber (SA) are about 13.14MW/cm², 6.09%, and 1.85∗10^-1cm/GW. In the experiment, based on the ZrSe₂ SA, two types of solitons were recorded. A conventional soliton with a pulse width of 985 fs and a three-pulse bound state soliton have been obtained. Our experiment reveals that ZrSe₂ can be employed for generating multiple ultrafast soliton generations and possess promising application in ultrafast photonics.

Passively Q-switched and mode-locked erbium-doped fiber lasers based on tellurene nanosheets as saturable absorber

Various two-dimensional (2D) materials show unique optical properties and excellent performance in acting as saturable absorber (SA) for demonstrating all-fiber ultra-fast lasers. Tellurene, as a new-fashioned few-layer 2D monoelemental material, was designed as an excellent saturable absorber to achieve Q-switched and mode-locked operations within erbium-doped fiber (EDF) lasers in our experiment. High-quality tellurene-based SA with a modulation depth of 0.97% was obtained by blending few-layer tellurene nanosheet solution prepared by liquid phase exfoliation method and the polyvinyl alcohol (PVA) solution. Inserting the SA into the EDF laser cavity by sandwiching the tellurene-PVA film between two fiber ferrules, either the passively Q-switched or the passively mode-locked operations can be obtained. The repetition rate varies from 15.92 to 47.61 kHz, and the pulse duration decreases from 8.915 to 5.196 µs in the passively Q-switched operation. To the best of our knowledge, this is the first demonstration focusing on the modulation application of tellurene in designing Q-switched pulsed laser operations. Additionally, mode-locked operations were also achieved by adjusting the polarization state. The obtained results fully indicate that tellurene can be developed as an efficient SA for pulsed fiber lasers.