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.

Aluminum nanoparticle films with an enhanced hot-spot intensity for high-efficiency SERS

The weak plasmonic coupling intensity in an aluminum (Al) nanostructure has limited potential applications in excellent low-cost surface-enhanced Raman scattering (SERS) substrates and light harvesting. In this report, we aim to elevate the plasmonic coupling intensity by fabricating an Al nanoparticle (NP)-film system. In the system, the Al NP are fabricated directly on different Al film layers, and the nanoscale-thick alumina interlayer obtained between neighboring Al films acts as natural dielectric gaps. Interestingly, as the number of Al film layers increase, the plasmonic couplings generated between the Al NP and Al film increase as well. It is demonstrated that the confined gap plasmon modes stimulated in the nanoscale-thick alumina region between the adjacent Al films contribute significantly to elevating the plasmonic coupling intensity. The finite-difference time-domain (FDTD) method is used to carry out the simulations and verifies this result.

Toward the highly sensitive SERS detection of bio-molecules: the formation of a 3D self-assembled structure with a uniform GO mesh between Ag nanoparticles and Au nanoparticles

We report a structure to form a hybrid system in which a mesh is sandwiched between Au nanoparticles (AuNPs) and Ag nanoparticles (AgNPs). This self-assembly method uses smaller and denser AgNPs "hot spots" that are spin-coated on a AuNPs@GO mesh nanostructure formed by the reaction of GO@MoS₂ and HAuCl₄ to form AuNPs@GO mesh@AgNPs SERS substrates. Sub-40-nm mesh and 10-nm gaps ensure the landing sites and spacing of the AgNPs. Consequently, the design integrates the strong plasmonic effects of AgNPs and AuNPs with the biological compatibility of the GO mesh. Crystal violet (CV) as low as 10^-15 M can be detected, which confirms the ultrahigh sensitivity of AuNPs@GO mesh@AgNPs. Furthermore, the reproducibility, stability, and finite-difference time-domain (FDTD) simulations confirm the value of this SERS substrate. This material can be used for label-free DNA detection, and the AuNPs@GO mesh@AgNPs substrate facilitated single-molecule DNA detection limits.

Quasi Optical Cavity of Hierarchical ZnO Nanosheets@Ag Nanoravines with Synergy of Near- and Far-Field Effects for in Situ Raman Detection

The vertically interlaced hierarchical structure (HS) of ZnO nanosheets (NSs)@Ag nanoravines (NRs) as a quasi optical cavity (QOC) for Raman enhancement has been studied experimentally and theoretically in this work. A novel synergism of near- and far-field effects of Ag NRs is facilitated by the multiple oscillation of light inside the ZnO QOC, providing wide distributions of "hot spots" in a large space. The "spatial hot spots" in the HS bring reliable signal collection in in situ Raman detection. Without any specific materials and methods adopted, this HS provides researchers a new way to adjust the light in the fields of Raman enhancement.

Versatile Mode-Locked Operations in an Er-Doped Fiber Laser with a Film-Type Indium Tin Oxide Saturable Absorber

We demonstrate the generation of versatile mode-locked operations in an Er-doped fiber laser with an indium tin oxide (ITO) saturable absorber (SA). As an epsilon-near-zero material, ITO has been only used to fashion a mode-locked fiber laser as an ITO nanoparticle-polyvinyl alcohol SA. However, this type of SA cannot work at high power or ensure that the SA materials can be transmitted by the light. Thus, we covered the end face of a fiber with a uniform ITO film using the radio frequency magnetron sputtering technology to fabricate a novel ITO SA. Using this new type of SA, single-wavelength pulses, dual-wavelength pulses, and triple-wavelength multi-pulses were achieved easily. The pulse durations of these mode-locked operations were 1.67, 6.91, and 1 ns, respectively. At the dual-wavelength mode-locked state, the fiber laser could achieve an output power of 2.91 mW and a pulse energy of 1.48 nJ. This study reveals that such a proposed film-type ITO SA has excellent nonlinear absorption properties, which can promote the application of ITO film for ultrafast photonics.

In situ detection of trace pollutants: a cost-effective SERS substrate of blackberry-like silver/graphene oxide nanoparticle cluster based on quick self-assembly technology

To realize fast detection of trace hazardous chemicals, a SERS substrate with the structure of a blackberry-like silver/graphene oxide nanoparticle cluster (Ag/GO NPC) has been designed and prepared through a quick capillarity-assistant self-assembly technology in this paper. Benefitting from the abundant "hot spots" and active oxygen sites brought by this Ag/GO NPC, the substrate shows good Raman performance for malachite green (MG), a common abusive germicide in aquaculture, with lowest limit of detection below 0.1 µg/L (3.48 × 10^-10 mol/L). Detailed analyses are taken on both the formation process and enhancement mechanism of this SERS substrate, and the finite-difference time-domain simulations are utilized as well to prove our hypotheses. Further constructing this structure on polyethylene terephthalate (PET) film, a translucent flexible SERS substrate can be obtained, realizing a fast in situ detection of trace MG in the fishpond subsequently. In consideration of the facile preparation process, good SERS enhancement and affordable materials (PET, Cu, Ag and GO, etc.), this substrate presents high cost performance and a promising application prospect.

Large-energy mode-locked ytterbium-doped linear-cavity fiber laser based on chemical vapor deposition-Bi2Se3 as a saturable absorber

We reported on the generation of pulse bunch and large-energy dark pulses in a mode-locked ytterbium-doped linear-cavity fiber laser based on Bi₂Se₃ as a saturable absorber (SA). Bi₂Se₃ nanosheets were successfully synthesized by the chemical vapor deposition (CVD) method and transferred to the end facet of a fiber connector for the proposed SA. Its saturation intensity and modulation depth were measured to be 52  MW/cm² and 14.5%, respectively. By inserting the Bi₂Se₃-based SA into the Yb-doped all-fiber linear cavity, stable pulse bunches were observed. In addition, dark soliton operation with a maximum average output power of 32.6 mW and a pulse energy of 61.8 nJ were also achieved. To the best of our knowledge, this is the first demonstration of a dark soliton within a linear cavity with much larger pulse energy than previous works. Our study fully indicated that CVD-Bi₂Se₃ could be an excellent SA for achieving large-energy pulse operations.

Experimental and theoretical investigation for surface plasmon resonance biosensor based on graphene/Au film/D-POF

A D-shape plastic optical fiber (D-POF) surface plasmon resonance (SPR) biosensor based on the graphene/Au film (G/Au) was proposed and experimentally demonstrated for detection of DNA hybridization process. To improve the detection performance of SPR sensors, the Physical Vapor Deposition (PVD) method was used to evaporate the Au film directly onto the graphene grown on copper foil, and the Au film acted as a role of traditional Polymethyl Methacrylate (PMMA). The process made graphene and Au film form seamless contact. Next, the G/Au was transferred onto the D-shape fiber together. We explored the G/Au SPR sensor by using the finite element method (FEM) and obtained the optimum materials thickness to form configuration. Compared to other plastic optical fiber experiments, the proposed sensor's sensitivity was improved effectively and calculated as 1227 nm/RIU in a range of glucose solution. Meanwhile, our proposed sensor successfully distinguishes hybridization and single nucleotide polymorphisms (SNP) by observing the resonance wavelength change. It also exhibits a satisfactory linear response (R² = 0.996) to the target DNA liquids with respective concentrations of 0.1nM to1µM, which shows this method's wide potential in medical diagnostics.

Graphene-Ag nanoparticles-cicada wings hybrid system for obvious SERS performance and DNA molecular detection

In recent years, biomaterials have increasingly attracted attention on surface-enhanced Raman spectroscopy (SERS) due to their well Raman performance while metal particles are combined with biological substrates. Therefore, we propose an environmentally friendly substrate based on silver-plated cicada wings with seamless graphene layer (Gr-AgNPs-C.w.), which can be prepared with a simple and inexpensive method. Compared with AgNPs-C.w., Gr-AgNPs-C.w. hybrids show better SERS performance with high sensitivity, good uniformity and good stability with R6G detection. The minimum detected concentration can reach 10^-15 M, and the value of R² can reach 0.996, respectively. Theoretical simulation demonstrates the situation of electromagnetic field through COMSOL software. In addition, due to the affinity of graphene for biomolecules, we can successfully detect the DNA biomolecules through a simple process. Therefore, this cheap and efficient natural SERS substrate has great potential for a considerable number of biochemical SERS applications and can broaden the way in which multiple SERS platforms derived from other natural materials are prepared.

Tuning the electronic and magnetic properties of MoS2 nanotubes with vacancy defects

The effects of vacancy defects on the electronic and magnetic properties of MoS₂ nanotubes were studied by DFT calculations. The rich semiconducting, metallic, and half-metallic properties make them suitable for electronic and spintronic devices.

Surface-Enhanced Raman Spectroscopy of Two-Dimensional Tin Diselenide Nanoplates

Surface-enhanced Raman spectroscopy (SERS) is a powerful spectroscopy technique to detect and characterize molecules at a very low concentration level. The two-dimensional (2D) semi-conductor layered material, tin diselenide (SnSe₂), is used as a new substrate for enhancing the Raman signals of adsorbed molecules. Three kinds of molecules-Rhodamine 6G (R6G), crystal violet (CV), and methylene blue (MB)-are used as probe molecules to evaluate the SERS performance of SnSe₂. The Raman signals of different molecules can be enhanced by SnSe₂ nanoplates (NPs). The distinguishable Raman signal of R6G molecules can be obtained for adsorbent concentrations as low as 10^-17mol/L. Based on a detailed analysis of the bandgap structure and opto-electrical properties of SnSe₂ NPs, we discuss the process of charge transfer and the Raman enhancement mechanism of SnSe₂ NP. The high Raman sensitivity of SnSe₂ NPs is related to the charge transfer between molecules and SnSe₂, 2D layered structure, and indirect bandgap of few-layered SnSe₂. The research results will help to expand the application of SnSe₂ in microanalysis, improve the measurement accuracy of SERS, and possibly find use in optoelectronic device integration.

Various large-energy soliton operations within an Er-doped fiber laser with bismuth selenide as a saturable absorber

Different large-energy mode-locked operations were successfully obtained within a Bi₂Se₃-based Er-doped fiber laser. First, mode-locked operation with maximum pulse energy of 17.2 nJ and pulse width of 187 ns under a pulse repetition rate of 537.6 kHz was obtained under the pump power of 680 mW. In addition, the characteristics of dark solitons and soliton rains, which also exhibit large pulse energies, have been investigated experimentally. Our results fully proved that Bi₂Se₃ was an excellent candidate for investigating various mode-locked operations with large pulse energy due to its high nonlinear effect and high damage threshold.

Heterogeneous and cross-distributed metal structure hybridized with MoS2 as high-performance flexible SERS substrate

The heterogeneous metal nanostructures have attracted great interest in various applications due to the synergistic effects between two noble metals, especially in surface enhanced Raman scattering (SERS) region. Herein, we prepared a 3D SERS active substrate based on heterogeneous and cross-distributed metal structure hybridized with MoS₂by in situ synthesizing gold nanoparticles (AuNPs) on MoS₂ membrane. The AuNPs-AgNPs/MoS₂/P-Si hybrid SERS substrate were characterized by a scanning electron microscope (SEM), a transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) to investigate the character and the content of elements. In virtue of the heterogeneous and cross-distributed structure and ultra-narrow interparticle gap generating strong electric fields enhancement, the ultra-low concentration of probe molecule were detected (the LOD of 10^-12 M for R6G and CV, 10^-11 M for MG), serving the optimal SERS performance. The excellent uniformity and reproducibility were achieved by the proposed substrate. Moreover, the flexible MoS₂/AuNPs-AgNPs/PMMA pyramidal SERS substrate was applied to detect melamine molecule in liquid milk (the LOD reached 10^-9 M), which revealed great potential to be an outstanding SERS substrate for biological and chemical detection.

Label-free and stable serum analysis based on Ag-NPs/PSi surface-enhanced Raman scattering for noninvasive lung cancer detection

Surface-enhanced Raman scattering (SERS) has a broad application prospect in the field of tumor detection owing to its ultrahigh detective sensitivity. However, SERS analysis of serum remain a challenge in terms of repeatability and stability due to the maldistribution of the silver nanoparticles (Ag-NPs)-serum. With the aim to make up for this shortcoming, we report a new method for obtaining stable serum Raman signals utilizing the ordered arrays of pyramidal silicon (PSi) and Ag-NPs. We prove the practicability of this method by detecting the samples of serum from 50 lung cancer patients and 50 normal healthy people. Principal component analysis (PCA) of the serum SERS spectra shows that the spectral data of the two sample groups can form obvious and completely separated clusters. The receiver operating characteristic curve provides the sensitivity (100%) and specificity (90%) from the PCA-LDA method. This research indicates that a stable and label-free analysis technique of serum SERS based on Ag-NPs/PSi and PCA-LDA is promising for noninvasive lung cancer diagnoses.

SERS substrate based on the flexible hybrid of polydimethylsiloxane and silver colloid decorated with silver nanoparticles

Various flexible SERS sensors have attracted widespread concern in performing the direct identification of the analytes adsorbed on arbitrary surfaces. Here, a sample method was proposed to integrate plasmonic nanoparticles into polydimethylsiloxane (PDMS) to fabricate flexible substrate for the decoration of silver nanoparticles (AgNPs). The flexible SERS sensor based on AgNPs/AgNPs-PDMS offers highly sensitive Raman detection with enhancement factor up to 8.3 × 10⁹, which can be attributed to the integrative effects from both the increase of the light absorption of the embedded AgNPs in PDMS substrate and the EM enhancement from the adjacent top-top, bottom-bottom and top-bottom AgNPs. After undergoing the cyclic mechanical deformation, the SERS substrate still maintains high mechanical stability and stable SERS signals. However, upon stretching the flexible substrate, there was an amusing phenomenon that SERS signals can be highly increased, which results from that the reduction of lateral nanogaps between top and bottom of the PDMS boundary strengthens the trigger of the plasmon coupling as demonstrated by the simulated result. This result reveals that the tuning and the coupling of the electromagnetic fields can be effectively controlled by the macroscopic mechanical solicitation. That will have an important significance for practical applications in strain-dependent sensors and detectors.

Sensitive, reproducible, and stable 3D plasmonic hybrids with bilayer WS2 as nanospacer for SERS analysis

The highly enhanced local electromagnetic field occurring through nanometer gap between the plamonic nanostructures provides the dominant contribution in surface enhancement Raman scattering (SERS) enhancement. Thence, we designed the remarkable SERS platform (AuNPs/WS₂@AuNPs hybrids) by introducing bilayer WS₂ film as the precise nanospacer. Bilayer WS₂ film can realize the facile and tight combination with AuNPs via the thermal decomposition approach. Dense three-dimension (3D) hot spots provided by this hybrid plasmonic nanostructures are responsible for the extremely satisfying SERS performances. Using rhodamine 6G (R6G) as the probe molecules, the AuNPs/WS₂@AuNPs hybrids perform the excellent sensitivity with the minimum detectable concentration as low as 10^-11 M. Uniform and reproducible SERS signals illustrate that the synthesized SERS hybrids perform the splendid spot-to-spot reproducibility (RSD~5.4%) and substrate-to-substrate reproducibility (RSD~5.7%). The stability of AuNPs and the protection of WS₂ film endow this hybrid plasmonic nanostructures with the brilliant anti-oxidation stability. Moreover, the enhanced electric field distribution simulated with the COMSOL software proves the remarkable SERS performance in theory. Therefore, AuNPs/WS₂@AuNPs substrate not only widens the SERS research filed of WS₂, but also shows vast potential as excellent SERS sensor for practical applicability.

High stability luminophores: fluorescent CsPbX3 (X = Cl, Br and I) nanofiber prepared by one-step electrospinning method

The novel fluorescent nanofiber membranes of CsPbX₃ (FNMs/CPX, X = Cl, Br, I) with a wide photoluminescence range from 405 nm to 675 nm are fabricated by a one-step electrospinning method in this paper. Owing to the polymer cladding, these FNMs/CPX show much better thermal and humid stability compared to the common CsPbX₃ particles, and the corresponding white light-emitting diode prepared by them also exhibits excellent optical properties. Without adopting any complicated processes, this method opens up a brand new way for the perovskite materials using in lighting and display fields.

Synthesis and Surface-Enhanced Raman Scattering of Ultrathin SnSe₂ Nanoflakes by Chemical Vapor Deposition

As a new atomically layered, two-dimensional material, tin (IV) diselenide (SnSe₂) has attracted extensive attention due to its compelling application in electronics and optoelectronics. However, the great challenge of impurities and the preparation of high-quality ultrathin SnSe₂ nanoflakes has hindered far-reaching research and SnSe₂ practical applications so far. Therefore, a facile chemical vapor deposition (CVD) method is employed to synthesize large-scale ultrathin SnSe₂ flakes on mica substrates using SnSe and Se powder as precursors. The structural characteristics and crystalline quality of the product were investigated. Moreover, Raman characterizations indicate that the intensity of A_1g peak and E_g peak, and the Raman shift of E_g are associated with the thickness of the SnSe₂ nanoflakes. The ultrathin SnSe₂ nanoflakes show a strong surface-enhanced Raman spectroscopy (SERS) activity for Rhodamine 6G (R6G) molecules. Theoretical explanations for the enhancement principle based on the chemical enhancement mechanism and charge transfer diagram between R6G and SnSe₂ are provided. The results demonstrate that the ultrathin SnSe₂ flakes are high-quality single crystal and can be exploited for microanalysis detection and optoelectronic application.

Multifunctional paper strip based on GO-veiled Ag nanoparticles with highly SERS sensitive and deliverable properties for high-performance molecular detection

The development of paper-based SERS substrates that can allow multi-component detection in real-word scenarios is of great value for applications in molecule detection under complex conditions. Here, a multifunctional SERS-based paper sensing substrate has been developed through the uniform patterning of high-density arrays of GO-isolated Ag nanoparticles on the hydrophilic porous cellulose paper strip (GO@AgNP@paper). Wet-chemical synthesis was used to provide the cover of SERS hot spots on any part of the paper, not just limited surface deposition. In virtue of the inherent ability of paper to deliver analytes by the capillary force, the detection ability of the GO@AgNP@paper substrate was greatly promoted, allowing as low as 10^-19M R6G detection from microliter-volume (50 μL) samples. For the components with different polarity, the paper substrate can be used as an all-in-one machine to achieve the integration of separation and high-sensitive detection for ultralow mixture components, which improves the practical application value of SERS-based paper devices.

Mode-locked Er-doped fiber laser based on PbS/CdS core/shell quantum dots as saturable absorber

Previously, PbS/CdS core/shell quantum dots with excellent optical properties have been widely used as light-harvesting materials in solar cell and biomarkers in bio-medicine. However, the nonlinear absorption characteristics of PbS/CdS core/shell quantum dots have been rarely investigated. In this work, PbS/CdS core/shell quantum dots were successfully employed as nonlinear saturable absorber (SA) for demonstrating a mode-locked Er-doped fiber laser. Based on a film-type SA, which was prepared by incorporating the quantum dots with the polyvinyl alcohol (PVA), mode-locked Er-doped operation with a pulse width of 54 ps and a maximum average output power of 2.71 mW at the repetition rate of 3.302 MHz was obtained. Our long-time stable results indicate that the CdS shell can effectively protect the PbS core from the effect of photo-oxidation and PbS/CdS core/shell quantum dots were efficient SA candidates for demonstrating pulse fiber lasers due to its tunable absorption peak and excellent saturable absorption properties.

3D silver nanoparticles with multilayer graphene oxide as a spacer for surface enhanced Raman spectroscopy analysis

We report a three-dimensional (3D) SERS substrate with different numbers of silver nanoparticle (Ag NP) layers using multilayer graphene oxide (GO) as a spacer. The SERS performance of the 3D nanostructure was investigated and it was found that the SERS effect increased as the number of Ag NP layers increased, and showed almost no change for more than four layers. We found that the SERS performance of the 3D nanostructures can be mainly attributed to the topmost hot spots which are closely related to the Ag NP layers in the 3D nanostructure. Furthermore, we explored 3D nanostructures with different Ag NP layers using the finite difference time domain method (FDTD). The 3D SERS substrates also exhibit excellent detection capability. The limit of detection (LOD) was calculated down to 10-15 M for R6G and 10-12 M for CV. In addition, the reproducibility of the 3D SERS substrate was attributed obviously to the increasing number of Ag NP layers. Based on these promising results, the highly sensitive detection of molecules such as malachite green was demonstrated for food safety inspection.

Ag gyrus-nanostructure supported on graphene/Au film with nanometer gap for ideal surface enhanced Raman scattering

The physical phenomenon, surface-enhanced Raman scattering (SERS), is mainly based on the local electromagnetic fields enhancement located at the nano-gaps between metal nanostructures attributed to localized surface plasmon resonance. Therefore, nano-gaps are very important for obtaining high-density hot spots and optimal and uniform SERS signals. However, it remains a challenge to form the three-dimensional ultra-narrow nano-gaps. Here, a gyrus-inspired Gyrus-SERS substrate was fabricated with the nanostructure of Ag gyrus/graphene/Au film using an extremely simple method. The lateral and vertical hot spots respectively were obtained from the dense nano-gaps (~3 nm) between gyrus and the coupling of Ag gyrus and Au film in bilayer graphene nano-gaps (0.68 nm), which were demonstrated in experiment and theory. The proposed Gyrus-SERS platform performs an excellent SERS activity (EF~5 × 10⁹), high sensitivity (the minimum detected concentration of R6G and CV respectively is 10^-13 and 10^-12 M), and outstanding reproducibility (RSD~7.11%). For practical application, the in situ detection of Malachite green (MG) residue on prawn skin was executed using the prepared flexible Gyrus-SERS substrate, which shows the wide potential in food safety field.

Adsorbable and self-supported 3D AgNPs/G@Ni foam as cut-and-paste highly-sensitive SERS substrates for rapid in situ detection of residuum

We have proposed a synthetic approach to produce self-supported and bendable surface-enhanced Raman scattering (SERS)-based 3D chemical sensors with high adsorptivity. Such 3D substrates consist of foam-like graphene macrostructures obtained by template-directed chemical vapour deposition on nickel foams (interconnected 3D scaffold of nickel) and uniform and high-density Ag nanoparticles wrapping around the foam graphene, via seed-mediated in situ growth process. Such 3D AgNPs/G@Ni foam substrates show high-quality SERS performance in terms of Raman signal reproducibility and sensitivity for the analyte, resulting from the high density and homogeneity of "hot spots" on AgNPs/G@Ni foam, multiple cascaded amplication (localized surface plasmon mode and optical standing waves or optical refraction) of incident laser to the 3D foam structures and powerful support from nickel scaffold. Moreover, in virtue of the high adsorptivity and sensitivity of AgNPs/G@Ni foam, the low-concentration crystal violet molecules can be easily traced in the curvilinear fish surface, by simply swabbing the surface to achieve molecules concentration effect in the practical applicability. This work shows promising potential in developing the applications of SERS in the foodstuffs processing and security field.