Volume 9, Issue 8 - Special Issue: 2D materials for nanophotonics: from fundamentals to applications, II. Applications; Guest Editors: Xiantao Jiang, Meng Zhang, Xi Ling, Pablo Alonso González, Han Zhang

Optical modulation technique plays a crucial role in photonics technologies, and there is an ever-increasing demand for broadband and ultrafast optical modulation in the era of artificial intelligence. All-optical modulation is known to be able to operate in an ultrafast way and has a broadband response, showing great potential in applications for ultrafast information processing and photonic computing. Two-dimensional (2D) materials with exotic optoelectronic properties bring tremendous new opportunities for all-optical modulators with excellent performance, which have attracted lots of attention recently. In this review, we cover the state-of-art all-optical modulation based on 2D materials, including graphene, transitional metal dichalcogenides, phosphorus, and other novel 2D materials. We present the operations mechanism of different types of all-optical modulators with various configurations, such as fiber-integrated and free-space ones. We also discuss the challenges and opportunities faced by all-optical modulation, as well as offer some future perspectives for the development of all-optical modulation based on 2D materials.

Transition metal carbides and nitrides (MXenes), which comprise a rapidly growing family of two-dimensional materials, have attracted extensive attention of the scientific community, owing to its unique characteristics of high specific surface area, remarkable biocompatibility, and versatile applications. Exploring different methods to tune the size and morphology of MXenes plays a critical role in their practical applications. In recent years, MXenes have been demonstrated as promising nanomaterials for cancer therapy with substantial performances, which not only are helpful to clarify the mechanism between properties and morphologies but also bridge the gap between MXene nanotechnology and forward-looking applications. In this review, recent progress on the preparation and properties of MXenes are summarized. Further applications in cancer therapy are also discussed. Finally, the current opportunities and future perspective of MXenes are described.

Two-dimensional (2D) materials have undergone a rapid development toward real applications since the discovery of graphene. At first, graphene is a star material because of the ultrahigh mobility and novel physics, but it always suffered from zero bandgap and limited device application. Then, 2D binary compounds such as transition-metal chalcogenides emerged as complementary materials for graphene due to their sizable bandgap and moderate electrical properties. Recently, research interests have turned to monoelemental and ternary 2D materials. Among them, monoelemental 2D materials such as arsenic (As), antimony (Sb), bismuth (Bi), tellurium (Te), etc., have been the focus. For example, bismuthene can act as a 2D topological insulator with nontrivial topological edge states and high bulk gap, providing the novel platforms to realize the quantum spin-Hall systems. Meanwhile, ternary 2D materials such as Bi 2 O 2 Se, BiOX and CrOX (X=Cl, Br, I) have also emerged as promising candidates in optoelectronics and spintronics due to their extraordinary mobility, favorable band structures and intrinsic ferromagnetism with high Curie temperature. In this review, we will discuss the recent works and future prospects on the emerging monoelemental and ternary materials in terms of their structure, growth, physics and device applications.

Since graphene was first reported as a saturable absorber to achieve ultrafast pulses in fiber lasers, many other two-dimensional (2D) materials, such as topological insulators, transition metal dichalcogenides, black phosphorus, and MXenes, have been widely investigated in fiber lasers due to their broadband operation, ultrafast recovery time, and controllable modulation depth. Recently, solution-processing methods for the fabrication of 2D materials have attracted considerable interest due to their advantages of low cost, easy fabrication, and scalability. Here, we review the various solution-processed methods for the preparation of different 2D materials. Then, the applications and performance of solution-processing-based 2D materials in fiber lasers are discussed. Finally, a perspective of the solution-processed methods and 2D material-based saturable absorbers are presented.

The burgeoning research into two-dimensional (2D) materials opens a door to novel photonic and optoelectronic devices utilizing their fascinating electronic and photonic properties in thin-layered architectures. The hybrid integration of 2D materials onto integrated optics platforms thus becomes a potential solution to tackle the bottlenecks of traditional optoelectronic devices. In this paper, we present the recent advances of hybrid integration of a wide range of 2D materials on integrated optics platforms for developing high-performance photodetectors, modulators, lasers, and nonlinear optics. Such hybrid integration enables fully functional on-chip devices to be readily accessible researchers and technology developers, becoming a potential candidate for next-generation photonics and optoelectronics industries.

Transition-metal dichalcogenides (TMDCs) and black phosphorus (BP) are typical 2D materials with layer-dependent bandgaps, which are emerging as promising saturable absorption materials for pulsed fiber lasers. In this review, we discuss the nonlinear saturable absorption properties of TMDCs and BP, and summarize the recent progress of saturable absorbers from fabrication methods to incorporation strategies. The performances of saturable absorbers and the properties of Q-switched/mode-locked fiber lasers at different wavelengths are summarized and compared to give a comprehensive insight to optical modulators based on TMDCs/BP, and to promote their practical applications in nonlinear optics.

MXenes, a new family of two-dimensional materials, are also known as transition metal carbides and nitride, with a general formula of M n +1 X n T x ( n  = 1–3). Their inherent metallic conductivity and hydrophilic nature endow MXenes with fascinating physicochemical properties (optical, electronic, magnetic, light-to-heat conversion. etc.). The ultrathin layer structure and photothermal property attract many interests in biomedical applications, especially as phototherapeutic agents for cancer treatment. In this review, we summarize the recent progress of MXenes in the field of photothermal therapy and highlight the crucial biotic index for their preparation and evaluation. First, we introduce the main strategies for the preparation and surface modifications of biologically applied MXenes. Then, representative cases in the field of MXene-based photothermal application, such as photothermal therapy, synergistic therapy, and targeting treatments, are reviewed. Finally, the cytotoxicity and in vivo long-term biosafety are introduced. We also propose the underlying challenges and perspectives for MXene applications in terms of photothermal therapy.

The rapid advancement of perovskite-based optoelectronics devices has caught the world’s attention due to their outstanding properties, such as long carrier lifetime, low defect trap density, large absorption coefficient, narrow linewidth and high optical gain. Herein, the photonic lasing properties of perovskites are reviewed since the first stimulated emission of perovskites observed in 2014. The review is mainly focused on 3D structures based on their inherently active microcavities and externally passive microcavities of the perovskites. First, the fundamental properties in terms of crystal structure and optical characteristics of perovskites are reviewed. Then the perovskite lasers are classified into two sections based on the morphology features: the ability/inability to support lasing behaviors by themselves. Every section is further divided into two kinds of cavities according to the light reflection paths (Standing wave for the Fabry–Pérot cavity and travelling wave for the Whispering-Gallery-Mode cavity). The lasing performance involves fabrication methods, cavity sizes, thresholds, quality factors, pumping sources, etc. Finally, some challenges and prospects for perovskite lasers are given.

The recent renaissance in pulsed lasers operating in the visible spectral region has been driven by their significant applications in a wide range of fields such as display technology, medicine, microscopy, material processing, and scientific research. Low-dimensional nanomaterials as saturable absorbers are exploited to create strong nonlinear saturable absorption for pulse generation at visible wavelengths due to their absorption peaks located in visible spectral region. Here we provide a detailed overview of visible-wavelength pulsed lasers based on low-dimensional nanomaterials, covering the optical properties and various integration strategies of these nanomaterials saturable absorbers, and their performance from solid-state as well as fiber pulsed lasers in the visible spectral range. This emerging application domain will undoubtedly lead to the rapid development of visible pulsed lasers.

Silicon photonics is becoming more and more attractive in the applications of optical interconnections, optical computing, and optical sensing. Although various silicon photonic devices have been developed rapidly, it is still not easy to realize active photonic devices and circuits with silicon alone due to the intrinsic limitations of silicon. In recent years, two-dimensional (2D) materials have attracted extensive attentions due to their unique properties in electronics and photonics. 2D materials can be easily transferred onto silicon and thus provide a promising approach for realizing active photonic devices on silicon. In this paper, we give a review on recent progresses towards hybrid silicon photonics devices with 2D materials, including two parts. One is silicon-based photodetectors with 2D materials for the wavelength-bands from ultraviolet (UV) to mid-infrared (MIR). The other is silicon photonic switches/modulators with 2D materials, including high-speed electro-optical modulators, high-efficiency thermal-optical switches and low-threshold all-optical modulators, etc. These hybrid silicon photonic devices with 2D materials devices provide an alternative way for the realization of multifunctional silicon photonic integrated circuits in the future.

Due to the unique properties of two-dimensional (2D) materials, much attention has been paid to the exploration and application of 2D materials. In this review, we focus on the application of 2D materials in mode-locked fiber lasers. We summarize the synthesis methods for 2D materials, fiber integration with 2D materials and 2D materials based saturable absorbers. We discuss the performance of the diverse mode-locked fiber lasers in the typical operating wavelength such as 1, 1.5, 2 and 3 μm. Finally, a summary and outlook of the further applications of the new materials in mode-locked fiber lasers are presented.

In this article, a high-quality saturable absorber (SA) based on a two-dimensional ternary chalcogenide Ta 2 NiS 5 nanosheet has been successfully fabricated and used in 1- and 1.5-μm spectral regions to generate ultrafast laser pulses. The Ta 2 NiS 5 -based SA is fabricated by mechanical exfoliation and sandwiched between two fiber ferrules to form a fiber-compatible SA. On the basis of the twin-detector technique, nonlinear optical absorption of the Ta 2 NiS 5 -SA is characterized by 64.7% and 11.95% modulation depths with 1.3 and 0.72 MW/cm 2 saturation intensities at 1028 and 1570 nm, respectively. When Ta 2 NiS 5 -SA is integrated into Yb- and Er-doped fiber laser cavities, stable self-starting Q-switched pulses are observed. Furthermore, by adjusting the cavity structure and optimizing dispersion in the cavity, we obtain hybrid mode-locking and mode-locking fiber laser operation at 1029 and 1569 nm, respectively. These results validate the performance of Ta 2 NiS 5 as a broadband SA for the generation of ultrafast laser pulses, offering new opportunities of ternary transition-metal dichalcogenide alloys in future photonic devices.

Strong quantum confinement and coulomb interactions induce tightly bound quasiparticles such as excitons and trions in an atomically thin layer of transitional metal dichalcogenides (TMDs), which play a dominant role in determining their intriguing optoelectronic properties. Thus, controlling the excitonic properties is essential for the applications of TMD-based devices. Here, we demonstrate the all-optical tuning of the local excitonic emission from a monolayer MoS 2 hybridized with phase-change material Ge 2 Sb 2 Te 5 (GST) thin film. By applying pulsed laser with different power on the MoS 2 /GST heterostructure, the peak energies of the excitonic emission of MoS 2 can be tuned up to 40 meV, and the exciton/trion intensity ratio can be tuned by at least one order of magnitude. Raman spectra and transient pump-probe measurements show that the tunability originated from the laser-induced phase change of the GST thin film with charge transferring from GST to the monolayer MoS 2 . The dynamic tuning of the excitonic emission was all done with localized laser pulses and could be scaled readily, which pave a new way of controlling the excitonic emission in TMDs. Our findings could be potentially used as all-optical modulators or switches in future optical networks.

We report on the experimental observation of dual-wavelength dissipative soliton operation of a fiber laser with net anomalous cavity dispersion. Different from the dual- or multi-wavelength soliton operation of fiber lasers where mode locking is used to initiate soliton formation, no mode locking occurs in our fiber laser. Instead, soliton formation is through the dissipative mechanism caused by the effective gain bandwidth limitation. Either dual-wavelength scalar, or vector, or induced dissipative solitons are experimentally obtained. Their robustness is experimentally confirmed.

Lead selenide (PbSe) is a new emerging semiconductor with layer-dependent bandgap that has attracted much interest due to its high infrared response and good environmental stability. We have prepared large-scale PbSe films with the area of 7 cm 2 and thickness of 25 nm based on physical vapor deposition approach at 160°C. The PbSe films exhibit saturable absorption property at 1.55 μm and a polarization-sensitive saturable absorber is obtained by growing PbSe on D-shaped fiber. Single-pulse with the duration of 490 fs is generated at the pump of 12 mW and the mode-locking operation is maintained at the pump of 1500 mW, indicating the high damage threshold of the D-shaped fiber based saturable absorber. Two polarization-insensitive saturable absorbers are achieved by depositing PbSe on fiber facet and polyvinyl alcohol film, respectively. For fiber facet (polyvinyl alcohol film) based saturable absorber, the repetition rate of Q-switched pulses increases from 8.6 (16.3) kHz to 45.4 (59.2) kHz while the duration decreases from 7.92 (12) μs to 2.06 (3.12) μs by tuning the pump from 15 mW to 90 (60) mW. Such large-scale PbSe films possess features of low cost and high modulation ability, and can find important applications in infrared optical modulators and detectors.

Graphene has been widely used in silicon-based optical modulators for its ultra-broadband light absorption and ultrafast optoelectronic response. By incorporating graphene and slow-light silicon photonic crystal waveguide (PhCW), here we propose and experimentally demonstrate a unique double-layer graphene electro-absorption modulator in telecommunication applications. The modulator exhibits a modulation depth of 0.5 dB/μm with a bandwidth of 13.6 GHz, while graphene coverage length is only 1.2 μm in simulations. We also fabricated the graphene modulator on silicon platform, and the device achieved a modulation bandwidth at 12 GHz. The proposed graphene-PhCW modulator may have potentials in the applications of on-chip interconnections.

Two-dimensional material tungsten diselenide (WSe 2 ) nanosheets are coated on a microfibre knot resonator (MKR) to achieve an all-optical power modulation functionality. On account of the strong absorption property of WSe 2 and the resonance enhancement properties of MKR, the transmitted optical power of signal light within the WSe 2 -based MKR can be effectively modulated. The sensitivities of light–control–light experiments with 405- and 660-nm lasers are as high as 0.32 and 0.12 dB/mW, respectively. The sensitivities and power tuning can be enhanced by a higher resonance Q and a larger extinction ratio of MKR. In terms of the response time, the average rise and fall times are 3.5/3.7 and 3.5/4 ms with 405- and 660-nm lasers, respectively. This proposed structure is expected to achieve potential applications in all-fibre-optic–based tunable device such as optical modulator, detector, and so on.

Rapid and high-sensitive detection of mycotoxins is believed to be of vital importance in assuring food safety. In this study, we developed a novel fluorescence immunochromatographic sensor (ICS) for the mycotoxin of zearalenone (ZEN) in cereals. This was done by using a black phosphorus-Au nanocomposite (BP-Au) as the 2D quenching platform. Herein, gold nanoparticles (AuNPs) were directly reduced on the surface of BP nanosheets (BPNSs) to form BP-Au nanocomposites, showing higher fluorescence (quantum dots, λ Em  = 525 nm) quenching efficiency compared to the BPNSs and AuNPs. The fluorescence quenching efficiency of the prepared BP-Au nanocomposite reached 73.8%, which was 1.73-fold and 1.44-fold higher than AuNPs and BPNSs, respectively. The density functional theory was also successfully used to explore the formation mechanism of the BP-Au nanocomposite. By introducing the quantum dots/BP-Au signal/quencher pair, a high-sensitive fluorescence quenching ICS (B-FICS) was developed for the detection and discrimination of ZEN with the limit of detection of 0.1 μg/l in pure working buffer. This was 2.5-fold more sensitive than AuNPs-based FICS (A-FICS). The B-FICS was successfully applied in real cereals detection with the sample limit of detection of 2 μg/kg. The successful construction of B-FICS offers a novel method for a rapid and high-sensitive detection of ZEN in cereals. It also provides a new practical application of 2D BPNSs in food safety sensing.

Ultrathin two-dimensional (2D) materials have drawn great attention in recent years due to their promising applications in biomedicine and atomically optoelectronic devices. In this work, we have fabricated a 2D In 2 Se 3 nanosheet doped with Nd 3+ ions via the two-step method of solid phase synthesis and liquid exfoliation. Owing to the special inner 4f-4f energy level transitions, lanthanide ions can emit photons with almost the same energy in different environments. Here, a stable near-infrared luminescence from Nd 3+ -doped 2D In 2 Se 3 nanosheets has been realized, which includes emission bands around 910, 1057, and 1324 nm. The doping of Nd 3+ ions extends the emission region of In 2 Se 3 nanosheets. Moreover, the photoluminescence mechanism of Nd 3+ ions was investigated through a series of optical measurements. This work not only provides a reliable method to fabricate lanthanide ion-doped 2D materials but also possesses a great significance for luminescence study of lanthanide ions in the 2D matrix.

Two-dimensional layered materials (2DLM) have become the subject of intensive research in various applications such as electronics, photonics and optoelectronics due to their unique physical properties. As a new class of 2DLM, MXenes have attracted great interest due to their superior performance in a wide variety of applications such as batteries, supercapacitors, catalysts, electronics and optics. Here, we have investigated the broadband spatial self-phase modulation (SSPM) and ultrafast response of the MXene Ti 3 C 2 T x (T=O, OH or F) experimentally. The MXene Ti 3 C 2 T x exhibited the broadband nonlinear optical response via SSPM from 400 nm to ~1 μm under the ultrafast laser excitation, and ultrafast carrier characteristics with an ultrafast recovery time with femtosecond transient absorption spectroscopy. The experimental results have shown that the MXenes have the broadband nonlinear optical response, which can lay a foundation for the application prospect for the MXene-based ultrafast optoelectronic devices.

Biological systems have high transparence to 700–1100-nm near-infrared (NIR) light. Black phosphorus quantum dots (BPQDs) have high optical absorbance in this spectrum. This optical property of BPQDs integrates both diagnostic and therapeutic functions together in an all-in-one processing system in cancer theranostic approaches. In the present study, BPQDs were synthesized and functionalized by targeting moieties (PEG-NH 2 -FA) and were further loaded with anticancer drugs (doxorubicin) for photodynamic–photothermal–chemotherapy. The precise killing of cancer cells was achieved by linking BPQDs with folate moiety (folic acid), internalizing BPQDs inside cancer cells with folate receptors and NIR triggering, without affecting the receptor-free cells. These in vitro experiments confirm that the agent exhibited an efficient photokilling effect and a light-triggered and heat-induced drug delivery at the precise tumor sites. Furthermore, the nanoplatform has good biocompatibility and effectively obliterates tumors in nude mice, showing no noticeable damages to noncancer tissues. Importantly, this nanoplatform can inhibit tumor growth through visualized synergistic treatment and photoacoustic and photothermal imaging. The present design of versatile nanoplatforms can allow for the adjustment of nanoplatforms for good treatment efficacy and multiplexed imaging, providing an innovation for targeted tumor treatment.

We report experimental observation of polarization attractors in the form of vector dissipative solitons from a carbon nanotube mode locked fiber laser. At a time scale of 14–14,000 roundtrips, the typical polarization locked vector solitons, and other appealing attractors are shown. In addition, we observe the vector dissipative solitons operated in dual-wavelength regime with central wavelength of 1533 and 1557 nm which can be related to a fixed point polarization attractor. The results unveiled complex oscillatory behavior of dissipative solitons in the polarization domain which may help understand laser physics, nonlinear optics. The demonstrated work may underpin a new type of high energy laser source leading to possible applications in nanoparticle manipulation, micromachining, spectroscopy etc.

Black phosphorus (BP) attracts huge interest in photonic and optoelectronic applications ranging from passive switch for ultrafast lasers to photodetectors. However, the instability of chemically unfunctionalized BP in ambient environment due to oxygen and moisture remains a critical barrier to its potential applications. Here, the parylene-C layer was used to protect inkjet-printed BP-saturable absorbers (BP-SA), and the efficacy of this passivation layer was demonstrated on the stable and continuous operation of inkjet-printed BP-SA in harsh environmental conditions. BP-SA was integrated in an erbium-doped ring laser cavity and immersed in water at ~60°C during operation for investigation. Mode-locked pulses at ~1567.3 nm with ~538 fs pulse width remained stable for >200 h. The standard deviation of spectral width, central wavelength, and pulse width were 0.0248 nm, 0.0387 nm, and 2.3 fs, respectively, in this period, underscoring the extreme stability of BP-SA against high temperature and humidity. This approach could enable the exploitation of BP-based devices for photonic applications when operating under adverse environmental conditions.

MXene is a promising two-dimensional (2D) material that is widely used in electro-photonic devices due to its unique properties. In this contribution, V 2 CT x , a novel MXene, was employed as a saturable absorber (SA) for hybrid passively mode-locked fiber lasers. An ultra-stable and self-starting mode-locked laser system with low threshold can be achieved using V 2 CT x nanosheets and nonlinear polarization evolution (NPE). Signal to noise ratio increased 13 dB compared with using only NPE SA. A 72 fs pulse duration is easily achieved from this hybrid mode-locked fiber laser system. To the best of our knowledge, this is the shortest pulse duration generated from the Yb-doped mode-locked fiber lasers using a hybrid or 2D SAs. This study proves that MXene V 2 CT x nanosheets can be developed as suitable SAs and served as potential advanced ultrafast photonic devices in the future.

Solar-blind deep ultraviolet (DUV) photodetectors with high responsivity (R) and fast response speed are crucial for practical applications in astrophysical analysis, environmental pollution monitoring, and communication. Recently, 2D tellurium has emerged as a potential optoelectronic material because of its excellent photoelectric properties. In this study, solar-blind DUV photodetectors are demonstrated based on solution-synthesized and air-stable quasi-2D Te nanosheets (Te NSs). An R of 6.5 × 10 4 A/W at 261 nm and an external quantum efficiency (EQE) of higher than 2.26 × 10 6 % were obtained, which are highest among most other 2D material-based solar-blind DUV photodetectors. Moreover, the photoelectric performance of the quasi-2D Te-based photodetector exhibited good stability even after ambient exposure for 90 days without any encapsulation. These results indicate that quasi-2D Te NSs provide a viable approach for developing solar-blind DUV photodetectors with ultrahigh R and EQE.

Recently, layered two-dimensional (2D) palladium diselenide (PdSe 2 ), with a unique low- symmetry puckered pentagon atomic morphology, has emerged as a promising candidate for next-generation nanoelectronics and optoelectronics because of its chemical stability and extraordinary electrical properties. Moreover, PdSe 2 possesses a strong thickness-dependent bandgap that varies from 0 eV for bulk to 1.3 eV for monolayer, which can further render its potential applications in optoelectronics. However, the layer-dependent optoelectronic properties of PdSe 2 are still lacking up to date. Herein, we studied the optoelectronics transport characteristics of high-quality PdSe 2 -based photodetectors with different thicknesses. We demonstrated an enhancement of PdSe 2 photodetector performance owing to the band engineering via a thickness reduction. The highest responsivity of 5.35 A/W can be achieved with an external quantum efficiency of 1250% at the wavelength of 532 nm. We attribute such high performance in photoresponsivity to the high valley convergence in the conduction band of layered PdSe 2 , in agreement with first-principles calculation. Our results offer new insight into the layer-dependent optoelectronic properties of PdSe 2 and open new avenues in engineering next-generation 2D-based electronics and optoelectronics.

The synapse is one of the fundamental elements in human brain performing functions such as learning, memorizing, and visual processing. The implementation of synaptic devices to realize neuromorphic computing and sensing tasks is a key step to artificial intelligence, which, however, has been bottlenecked by the complex circuitry and device integration. We report a high-performance charge-trapping memory synaptic device based on two-dimensional (2D) MoS 2 and high-k Ta 2 O 5 –TiO 2 (TTO) composite to build efficient and reliable neuromorphic system, which can be modulated by both electrical and optical stimuli. Significant and essential synaptic behaviors including short-term plasticity, long-term potentiation, and long-term depression have been emulated. Such excellent synaptic behaviors originated from the good nonvolatile memory performance due to the high density of defect states in the engineered TTO composite. The 2D synaptic device also exhibits effective switching by incident light tuning, which further enables pattern recognition with accuracy rate reaching 100%. Such experimental demonstration paves a robust way toward a multitask neuromorphic system and opens up potential applications in future artificial intelligence and sensing technology.

The fast-developing information technology has imposed an urgent need for effective solutions to overcome the limitations of integration density in chips with smaller size but higher performance. van der Waals heterojunctions built with two-dimensional (2D) semiconductors have been widely studied due to their 2D nature, and their unique electrical and photoelectronic properties are quite attractive in realizing multifunctional devices toward multitask applications. In this work, black phosphorus (BP)/MoS 2 heterojunctions have been used to build electronic devices with various functionalities. A p-n diode is achieved based on the vertically stacked BP/MoS 2 heterojunction exhibiting an ideal factor of 1.59, whereas a laterally stacked BP/MoS 2 heterojunction is implemented to fabricate a photodetector that shows a photodetection responsivity of 2000 mA/W at a wavelength of 1300 nm. Furthermore, a ternary inverter has been realized using a BP field-effect transistor in-series with a lateral BP/MoS 2 heterojunction. Such results have unambiguously demonstrated the superiority of BP/MoS 2 heterojunction in realizing multiple functionalities and have offered a new pathway for the design and engineering of future circuitry and device integration based on novel 2D semiconductors.

We report on an Er:Y 2 O 3 ceramic channel waveguide laser operating at continuous-wave and passively Q-switched regimes, for the first time to the best of our knowledge. The Er:Y 2 O 3 ceramic channel waveguide has been fabricated by a femtosecond laser direct-writing technique. The maximum output power reached 123.5 mW in continuous-wave mode with a slope efficiency of about 21%, and the lasing behavior has been found simultaneously at 2710.28, 2716.76, and 2723.18 nm. We have further fabricated an MXene Ti 3 C 2 T x saturable absorber mirror to operate the Er:Y 2 O 3 waveguide in passively Q-switched regime. At the same time, property of the MXene Ti 3 C 2 T x saturable absorber has been characterized, and nonlinear saturable absorption of the saturable absorber has also been measured. The average output power is about 67.6 mW, and the shortest pulse width is about 58.1 ns. This work indicates that femtosecond laser direct writing is a reliable technique for processing mid-infrared ceramic waveguides, and MXene Ti 3 C 2 T x is promising for operating pulsed mid-infrared lasers.

MXene are a class of metal carbide and metal nitride materials with a two-dimensional layered structure. MXene Ti 3 C 2 T x has the characteristics of good metal conductivity and adjustable chemical composition, which has attracted the attention of scientists. Recently, Mxene have shown strong nonlinear photonics and optoelectronic effect, which can be used to generate ultrashort pulsed laser. However, soliton molecules pulse in laser cavity based on Mxene have not been reported at present. In this article, MXene have been characterized systematically, and the nonlinear optical characters were measured. In addition, we combined MXene with taper fiber to make a saturable absorber device for an erbium-doped fiber laser. The modulation depth and saturation absorption intensity of MXene are 10.3% and 197.5 MW/cm 2 , respectively. Thanks to the outstanding character of MXene, a three-order soliton molecules pulse were generated in laser cavity. The center wavelength, pulse interval and spectral modulation period of soliton molecules are 1529.4 nm, 15.5 ps and 0.5 nm, respectively. The above experimental results show that MXene have broad application prospects in the fields of optical fiber communication, laser material processing and high-resolution optics.

Two-dimensional (2D) transition metal dichalcogenide materials have attracted much attention in recent years due to their excellent electro-optical properties. FeS 2 , the ideal composition of iron pyrite, is a 2D transition metal dichalcogenide which has been potentially used in the electronic, optical, and chemical fields. On the other hand, the narrow band gap of FeS 2 (≈0.96 eV) makes it very suitable and promising for the ultrafast application in near-infrared regimes. However, the potential application of FeS 2 in laser technology has not been explored till now. Ultrashort pulse lasers have great applications in industry and science because of its stability, ease of operation, and portability. Passively mode-locked fiber lasers using 2D materials (such as MoS 2 , CuS 2 , and WS 2 ) as saturable absorber are intensively investigated. Here, layered FeS 2 has been characterized systematically. It is successfully applied in ultrafast photonics and plays a key component in the passively mode-locked laser for the first time. The single pulse can be obtained with 1.7-ps pulse duration, 1.89-nm spectral width, and fundamental repetition of 6.4 MHz at 1563 nm central wavelength. Through controlling the pump power, the evolution of the pulse train can be observed, which can be transformed from single pulse to bound states. Also, the harmonic mode-locked fiber laser is observed with the pump power high enough.

As a new member of saturable absorber (SA), molybdenum carbide (Mo 2 C) has some excellent optical properties. Herein, we report a new type of Mo 2 C/fluorine mica (FM) SA device. Uniform and compact Mo 2 C films were deposited on the FM by magnetron sputtering method. In order to increase the laser damage threshold, an additional protective layer of silicon oxide was deposited on the Mo 2 C. The FM is a single-layer structure of 20 μm, and its high elasticity makes it not easy to fracture. The transmission rate of FM is as high as 90% at near infrared wavelength. FM has better heat dissipation and softening temperature than organic composite materials, so it can withstand higher laser power without being damaged. In this work, Mo 2 C/FM SA was cut into small pieces and inserted into erbium-doped fiber laser to achieve mode-locked operation. The pulse duration and average output power of the laser pulses were 313 fs and 64.74 mW, respectively. In addition, a 12th-order sub-picosecond harmonic mode-locking was generated. The maximum repetition rate was 321.6 MHz and the shortest pulse duration was 338 fs. The experimental results show that Mo 2 C/FM SA is a broadband nonlinear optical mode-locker with excellent performance.

Position-sensitive detectors (PSDs) are essential components to the realization of displacement and vibration detection, optical remote control, robot vision, etc. The light sensitivity of PSDs is a crucial parameter, which determines the operating range or detection accuracy of the measurement systems. Here, we devise an ultrasensitive PSD based on graphene/Si hybrid structure by using the synergistic effect of charge injection and interfacial gating. Photogenerated carriers in Si are separated by the built-in electric field at the surface. Holes diffuse laterally in inversion layer and then inject into graphene to form photoresponse. Meanwhile, the electrons in bulk Si that move to the area under graphene cause a gating effect, thus introducing a high gain. With the benefit of synergistic effect, the detection limit power of our device can be pushed to pW level, which is reduced by two orders of magnitude compared to previously reported graphene based PSD. Furthermore, even for infrared light of 1064 nm, the PSD still retains position sensitivity to 1 nW weak light, as well as fast response speed at the μs level. This work provides the potential of graphene as a promising material for ultraweak light position sensitive detection.

Localized surface plasmon resonance (LSPR) of metallic nanoparticles (NPs) can generate and enhance the nonlinear optical (NLO) response and has been widely used in biosensing, optical bistability, optical switch, and modulator, surface-enhanced spectroscopies, etc. Here, the two-dimensional (2D) Au & Ag hybrid plasmonic NP network (Au & Ag HPNN) were synthesized by assembling Au and Ag NPs in ethanol solvent and transferring onto a CaF2 substrate. The frequency-domain finite element method (FEM) simulations were performed to explore their LSPR properties, demonstrating the broadband optical responses throughout visible, near-infrared and mid-infrared regions. The ultrafast carrier relaxation times were determined to be 3.9, 5.6, and 8.6 ps, while the nonlinear absorption coefficients were −1.12 × 104, −1.71 × 104, and −2.54 × 104 cm/GW, respectively, for the three wavelengths matching the LSPRs peaks at 1.0, 2.0, and 3.0 μm bands. Furthermore, passively Q-switched (PQS) solid-state lasers operating at 1062.8, 1990.8, and 2947 nm were demonstrated with 2D Au & Ag HPNN based saturable absorbers. This work not only reveals desirable ultrafast broadband NLO responses of 2D HPNN, but also provides a platform for investigating their applications in nanophotonic devices.

IV–VI semiconductors have attracted widespread attention in basic research and practical applications, because of their electrical and optoelectronic properties comparable to graphene. Herein, an optical modulator based on SnSSe with strong nonlinearity is prepared by chemical vapor transfer method. The modulation depth of proposed SnSSe saturable absorber (SA) is up to 57.5%. By incorporating SnSSe SA into the laser, the Q-switched pulses as short as 547.8 ns are achieved at 1530.07 nm. As far as we know, this is the first successful application of SnSSe in Q-switched lasers. Our investigation not only prove the optical nonlinearity of SnSSe, but also reveal the potential of SnSSe SA in ultrafast photonics.

Air-stable broadband saturable absorbers (SAs) exhibit a promising application potential, and their preparations are also full of challenges. Palladium selenide (PdSe 2 ), as a novel two-dimensional (2D) layered material, exhibits competitive optical properties including wide tunable bandgap, unique pentagonal atomic structure, excellent air stability, and so on, which are significant in designing air-stable broadband SAs. In our work, theoretical calculation of the electronic band structures and bandgap characteristics of PdSe 2 are studied first. Additionally, PdSe 2 nanosheets are synthesized and used for designing broadband SAs. Based on the PdSe 2 SA, ultrafast mode-locked operations in 1- and 1.5-μm spectral regions are generated successfully. For the mode-locked Er-doped operations, the central wavelength, pulse width, and pulse repetition rate are 1561.77 nm, 323.7 fs, and 20.37 MHz, respectively. Meanwhile, in all normal dispersion regions, mode-locked Yb-doped fiber laser with 767.7-ps pulse width and 15.6-mW maximum average output power is also generated successfully. Our results fully reveal the capacity of PdSe 2 as a broadband SA and provide new opportunities for designing air-stable broadband ultra-fast photonic devices.

Transition metal dichalcogenides have been widely utilized as nonlinear optical materials for laser pulse generation applications. Herein, we study the nonlinear optical properties of a VS 2 -based optical device and its application as a new saturable absorber (SA) for high-power pulse generation. Few-layer VS 2 nanosheets are deposited on the tapered region of a microfiber to form an SA device, which shows a modulation depth of 40.52%. After incorporating the microfiber-VS 2 SA into an Er-doped fiber laser cavity, passively Q-switched pulse trains could be obtained with repetition rates varying from 95 to 233 kHz. Under the pump power of 890 mW, the largest output power and shortest pulse duration are measured to be 43 mW and 854 ns, respectively. The high signal-to-noise ratio of 60 dB confirms the excellent stability of the Q-switching state. To the best of our knolowdge, this is the first illustration of using VS 2 as an SA. Our experimental results demonstrate that VS 2 nanomaterials have a large potential for nonlinear optics applications.

MXene as a novel two-dimensional (2D) material exhibits a lot of advantages in nonlinear optics. However, the common MXene, Ti 3 C 2 T x and Ti 2 CT x nanosheets, easily suffer from degradation under ambient conditions, greatly limiting their practical applications. Here, we demonstrated one of MXene compounds, V 2 CT x , which has a strong modulation depth (nearly 50%), can serve as an excellent saturable absorber (SA) in passively mode-locked (PML) fiber lasers. More importantly, 206th harmonic order has been successfully generated in Er-doped mode-locked fiber laser, exhibiting maximum repetition rate of 1.01 GHz and pulse duration of 940 fs, which to the best of our knowledge, is the highest harmonic mode-locked fiber laser from the MXene SA so far. In addition, the high harmonic order mode-locked operation can maintain at least 24 h without any noticeable change, suggesting MXene V 2 CT x nanosheets have excellent stability in this mode-locked fiber laser. It is anticipated that the present work can pave the way to new design for MXene-based heterostructures for high-performance harmonic mode-locked lasers.

Monolayer transition metal dichalcogenides (TMDs) have emerged as a promising platform for chip-integrated optoelectronics and non-linear optics. Here, we demonstrate a two-dimensional (2D) monolayer tungsten disulfide (WS 2 ) efficiently coupled to a dielectric circular Bragg resonator (CBR). The coupling of the WS 2 and CBR leads to pronounced enhancements in both photoluminescence (PL) and second harmonic generation (SHG) by a factor of 34 and 5, respectively. Our work provides a powerful tool to enhance the interactions between light and the 2D materials, paving the way for efficient on-chip optoelectronic devices.

Two-dimensional (2D) mono-elemental materials (Xenes) show remarkable potential in the fields of fundamental science and technology, have been regarded as a wide range of building blocks for electronic technologies due to their unique chemical, physical, electrical, and optical properties. Here, 2D tellurene/black phosphorus (Te/BP) heterojunctions are successfully fabricated through liquid-phase exfoliation (LPE) method. Their nonlinear optical absorption properties at 1.0, 2.0, and 2.8 μm have been studied by an open-aperture Z -scan method. The results revealed the excellent broadband saturable absorption responses of the prepared BP/Te heterojunctions, which are further confirmed by using them as saturable absorbers (SAs) for passively Q-switched all-solid-state lasers operating at 1.0, 2.0, and 2.8 μm, respectively. In particular, 1.04 μm continuous-wave (CW) mode-locked lasers with a pulse width of 404 fs is realized for the first time, to our best knowledge. Our work indicates that 2D Xenes especially 2D Xenes based heterojunctions have great potential in the fields of pulsed laser generation/modulation and other optoelectronic and photonic devices.