Internal motion within ultrafast asynchronous dual wavelength mode-locked lasers.

Realtime spectroscopy access to ultrafast fiber lasers provides new opportunities for exploring complex soliton interaction dynamics. In this study, we employ a time-stretch technique that enables real-time access to both spectral and temporal dynamics, revealing rich nonlinear processes in asynchronous dual wavelength mode-locked pulses in an ultrafast fiber laser. Due to the different group velocities of the two wavelengths, the mode-locked solitons centered at different wavelengths periodically collide with each other. We recorded the entire process of soliton establishment, stabilization, and disappearance, shedding light on the mystery of stable transmission of dual-wavelength mode-locked pulses. These processes were observed for the first time in an ultrafast fiber laser, and the experimental evidence provides important insights into the understanding of nonlinear dynamics in fiber lasers, as well as the potential for improving laser performance for application in dual-comb spectroscopy.

Sensitive colorimetric assay of hydrogen peroxide and glucose in humoral samples based on the enhanced peroxidase-mimetic activity of NH2-MIL-88-derived FeS2@CN nanocomposites compared to its precursors.

By employing NH2-MIL-88 as a template, we synthesized the intermediate Fe@CN under high-temperature calcination and further fabricated the FeS2@CN nanocomposites in the presence of sulfur powder. Under varying temperatures (300-600 °C) and Fe@CN-to-S ratios (1:3-6), FeS2@CN500-5 nanocomposites had the highest peroxidase-mimetic activity. Under optimized conditions (incubation temperature 40 °C; solution pH 4.0 and nanocomposite concentration 10 µg/mL; 652-nm absorption), the Michaelis-Menten constant (Km) of FeS2@CN was much lower than that of horseradish peroxidase (HRP), therefore demonstrating that it had a higher affinity for both chromogenic substrates than conventional HRP. The limits of detection for H2O2 and glucose were 0.15 and 0.30 µmol/L, respectively, and the recoveries for glucose were 91.8-103% with RSDs <5.2%. The novelty of this study lies in (1) the FeS2@CN was confirmed to possess stronger enzyme-mimetic activity than its precursors (NH2-MIL-88 and Fe@CN); (2) the enhanced activity resulted from the unsaturated sites of N and S doping and the plentiful defects on the porous carbon surface; and (3) free radical trapping experiments evidenced that •OH played a major role in the catalytic reaction, while h+ and •O2- simultaneously participated in the catalytic process. These convincing performance metrics lead us to postulate that the FeS2@CN-based colorimetric biosensor provides a promising approach for several real-world applications, such as point-of-care diagnosis and workplace health evaluations.

Generation of noise-like pulses with a 920 fs pedestal in a nonlinear Yb-doped fiber amplifier.

We demonstrate the compression of noise-like pulses in an Yb-doped fiber master-oscillator power-amplifier (MOPA). The seed source of the MOPA is an NPR mode locked fiber laser delivering 5.94-ps dissipative soliton pulses with a repetition rate of 37.48 MHz. After amplification in the Yb-doped fiber amplifier, stable noise-like pulses with maximum power of 5 W are obtained. Subsequently a grating pair is used to tailor the spectrum and compensate the dispersion of the amplified noise-like pulses. The pedestal of de-convolution autocorrelation trace is compressed from 6.5 ps to 920 fs. To the best of our knowledge, this is the first time that the pedestal of a noise-like pulse is compressed to femtosecond region.

Noise-like pulses with a 14.5 fs spike generated in an Yb-doped fiber nonlinear amplifier.

We demonstrate noise-like pulses with a 14.5 fs spike generated in an Yb-doped fiber nonlinear amplifier after compression, while the pedestal has a width of 3.70 ps. This Yb-doped fiber nonlinear amplifier is seeded by dissipative solitons. With an average output power of 5 W, the single pulse energy is approximately 200 nJ at a repetition rate of 24.65 MHz. Mathematical research based on the coupled nonlinear Schrödinger equations about the generation and the compressibility of the noise-like pulses is also carried out.

Compact Yb-doped mode-locked fiber laser with only one polarized beam splitter.

We experimentally demonstrate a kind of simplified Yb-doped mode-locked fiber laser with a nonlinear polarization rotation technique. In the oscillator, there are no waveplates, physical bandpass filters, or polarization controlling devices except for one polarized beam splitter and a polarization-independent isolator. In the experiment, self-started stable mode locking pulse trains are obtained with four different cavity lengths: 110 m, 66 m, 50 m, and 26 m. To make clear the mechanism of pulse formation of this kind of simplified fiber laser, the characteristics of which are called an artificial saturable absorber, are discussed and the influences of these parameters on mode locking are also analyzed.

Mode-locked ytterbium-doped fiber laser based on topological insulator: Bi2Se3.

We demonstrated an all-normal-dispersion Yb-doped mode-locked fiber laser based on Bi2Se3 topological insulator (TI). Different from previous TI-mode-locked fiber lasers in which TIs were mixed with film-forming agent, we used a special way to paste a well-proportioned pure TI on a fiber end-facet. In this way, the effect of the film-forming agent could be removed, thus the heat deposition was relieved and damage threshold could be improved. The modulation depth of the Bi2Se3 film was measured to be 5.2%. When we used the Bi2Se3 film in the Yb-doped fiber laser, the mode locked pulses with pulse energy of 0.756 nJ, pulse width of 46 ps and the repetition rate of 44.6 MHz were obtained. The maximum average output power was 33.7 mW. When the pump power exceeded 270 mW, the laser can operate in multiple pulse state that six-pulse regime can be realized. This contribution indicates that Bi2Se3 has an attractive optoelectronic property at 1µm waveband.

High-repetition-rate Q-switched fiber laser with high quality topological insulator Bi2Se3 film.

We demonstrated a high-repetition-rate Q-switched fiber laser with topological insulator Bi2Se3 absorber. The absorber was made into a film structure by spin-coating method using few-layer Bi2Se3 nano-platelets which had regular shape. The uniform film had a low saturable optical intensity of 11 MW/cm(2), which is the lowest saturable optical intensity in the saturable absorbers made by topological insulator till now. By inserting the absorber film into an Erbium-doped fiber laser, a high-repetition Q-switched laser with the repetition rates from 459 kHz to 940 kHz was achieved. The maximum output power was 22.35 mW with the shortest pulse duration of 1.9 µs. To the best of our knowledge, both of the repetition rate and the output power were the highest values among the Q-switched fiber lasers with topological insulator absorber.

Sequential Regulation of Local Reactive Oxygen Species by Ir@Cu/Zn-MOF Nanoparticles for Promoting Infected Wound Healing.

Most antimicrobials treat wound infections by an oxidation effect, which is induced by the generation of reactive oxygen species (ROS). However, the potential harm of the prolonged high level of ROS should not be ignored. In this study, we presented a novel cascade-reaction nanoparticle, Ir@Cu/Zn-MOF, to effectively regulate the ROS level throughout the healing progress of the infected wound. The nanoparticles consisted of a copper/zinc-modified metal-organic framework (Cu/Zn-MOF) serving as the external structure and an inner core composed of Ir-PVP NPs, which were achieved through a process known as "bionic mineralization". The released Cu2+ and Zn2+ from the shell structure contributed to the production of ROS, which acted as antimicrobial agents during the initial stage. With the disintegration of the shell, the Ir-PVP NP core was gradually released, exhibiting the property of multiple antioxidant enzyme activities, thereby playing an important role in clearing excessive ROS and alleviating oxidative stress. In a full-layer infected rat wound model, Ir@Cu/Zn-MOF nanoparticles presented exciting performance in promoting wound healing by clearing the bacteria and accelerating neovascularization as well as collagen deposition. This study provided a promising alternative for the repair of infected wounds.

Watt-level passively Q-switched double-cladding fiber laser based on graphene oxide saturable absorber.

A watt-level passively Q-switched ytterbium-doped double-cladding fiber laser with a graphene oxide (GO) absorber was demonstrated. The structure of the GO saturable absorber mirror (GO-SAM) was of the sandwich type. A maximum output power of 1.8 W was obtained around a wavelength of 1044 nm. To the best of our knowledge, this is the highest output power in Q-switched fiber lasers based on a GO saturable absorber. The pure GO was protected from the oxygen in the air so that the damage threshold of the GO-SAM was effectively raised. The gain fiber was a D-shaped ytterbium-doped double-cladding fiber. The pulse repetition rates were tuned from 120 to 215 kHz with pump powers from 3.89 to 7.8 W. The maximum pulse energy was 8.37 µJ at a pulse width of 1.7 µs.

Multi-Scale Feature Fusion Convolutional Neural Network for Indoor Small Target Detection.

The development of object detection technology makes it possible for robots to interact with people and the environment, but the changeable application scenarios make the detection accuracy of small and medium objects in the practical application of object detection technology low. In this paper, based on multi-scale feature fusion of indoor small target detection method, using the device to collect different indoor images with angle, light, and shade conditions, and use the image enhancement technology to set up and amplify a date set, with indoor scenarios and the SSD algorithm in target detection layer and its adjacent features fusion. The Faster R-CNN, YOLOv5, SSD, and SSD target detection models based on multi-scale feature fusion were trained on an indoor scene data set based on transfer learning. The experimental results show that multi-scale feature fusion can improve the detection accuracy of all kinds of objects, especially for objects with a relatively small scale. In addition, although the detection speed of the improved SSD algorithm decreases, it is faster than the Faster R-CNN, which better achieves the balance between target detection accuracy and speed.

Real-Time Target Detection Method Based on Lightweight Convolutional Neural Network.

The continuous development of deep learning improves target detection technology day by day. The current research focuses on improving the accuracy of target detection technology, resulting in the target detection model being too large. The number of parameters and detection speed of the target detection model are very important for the practical application of target detection technology in embedded systems. This article proposed a real-time target detection method based on a lightweight convolutional neural network to reduce the number of model parameters and improve the detection speed. In this article, the depthwise separable residual module is constructed by combining depthwise separable convolution and non-bottleneck-free residual module, and the depthwise separable residual module and depthwise separable convolution structure are used to replace the VGG backbone network in the SSD network for feature extraction of the target detection model to reduce parameter quantity and improve detection speed. At the same time, the convolution kernels of 1 × 3 and 3 × 1 are used to replace the standard convolution of 3 × 3 by adding the convolution kernels of 1 × 3 and 3 × 1, respectively, to obtain multiple detection feature graphs corresponding to SSD, and the real-time target detection model based on a lightweight convolutional neural network is established by integrating the information of multiple detection feature graphs. This article used the self-built target detection dataset in complex scenes for comparative experiments; the experimental results verify the effectiveness and superiority of the proposed method. The model is tested on video to verify the real-time performance of the model, and the model is deployed on the Android platform to verify the scalability of the model.

Determination of a thiol-based ionic liquid using ultrathin graphitic carbon nitride nanosheets as a nanofluoroprobe.

Herein, a simple and green "ON-OFF-ON" sensing system was developed using ultrasonic-exfoliated g-C3N4 nanosheets (CNNS) for the determination of a thiol-based ionic liquid (THIL), which was prominently different from common organic or biothiol molecules in terms of physico-chemical properties. After addition of THIL ([HSBMIM]Br used as an example), the Ag+-quenched CNNS fluorescence ("OFF" state) was recovered to the "ON" state due to reaction between THIL and Ag+ that led to functional group activation on CNNS surfaces. This phenomenon can be explained by competition of THIL with Ag+ because of the strong and specific affinity of -SH groups in THIL for Ag+ and by the reversibility of the Ag+-CNNS coordination reaction. Relevant factors influencing fluorescent recovery were rigorously optimized, including solution pH, incubation time as well as CNNS and THIL concentrations. THIL-recovered fluorescence intensities increased with increasing THIL concentrations providing a linear range of 15-360 nM and limit of detection (LOD) of 4.28 nM (1.07 µg L-1). Testing a series of conventional imidazole-based ionic liquids indicated high specificity for the target analyte and negligible interference effects for the determination of nM-level THIL. The proposed fluorescent sensing method demonstrated excellent feasibility for trace THIL determination in real-world fresh and marine water matrices with high extraction recovery (90.3-107.9%) and high inter- and intra-day precisions (2.3-5.6% relative standard deviations). As far as our information goes, it is the first report on the development of g-C3N4-based "ON-OFF-ON" sensing platform for fast, sensitive and cost-effective determination of nM-level ionic liquids in natural waters.

Sulfur vacancy promoted peroxidase-like activity of magnetic greigite (Fe3S4) for colorimetric detection of serum glucose.

Herein, sulfur vacancies in magnetic greigite (SVs-Fe3S4) nanosheets were synthesized by a one-step solvothermal method by adjusting the ethylene glycol: water ratio. Electron paramagnetic resonance spectroscopy (EPR) and X-ray photoelectron spectroscopy (XPS) revealed that SV-rich Fe3S4 and SV-poor Fe3S4 were acquired using 100% ethylene glycol and 100% water as solvent, respectively. A peroxidase-like activity assay demonstrated that maximum reaction rates for H2O2-mediated oxidation of 3,3',5,5'-tetramethyl-benzidine (TMB) catalyzed by the SV-rich Fe3S4 was 2.3 times higher than SV-poor Fe3S4. Density functional theory (DFT) calculations and reactive oxygen species (ROS) detection confirmed that the enhanced peroxidase-like activity by SV-rich Fe3S4 was attributed to efficient adsorption of H2O2 and subsequent decomposition to hydroxyl radicals (•OH) on the SVs sites of Fe3S4. The SV-rich Fe3S4 nanozyme was employed to develop a simple, highly sensitive and selective assay for glucose detection with a linear range of 0.5-150 µM and a detection limit of 0.1 µM (S/N = 3). A smartphone application (App) was designed and applied to efficiently detect serum glucose with the integrated analytical system based on the SV-rich Fe3S4. These new findings highlight the important role of surface defects in nanozymes on generating peroxidase-like activity for glucose detection in point-of-care diagnosis.

Band-selective optical polarizer based on gold-nanowire plasmonic diffraction gratings.

We report a plasmonic diffraction grating device as a new kind of optical polarizer. This simple device consists of periodically distributed gold nanowires on top of a transparent glass substrate and is based on the strong polarization dependence of the particle plasmon resonance of the gold nanowires. A high-efficiency secondary diffraction in the same device enhances the polarization extinction ratio significantly. Linearly polarized spectrum in the red with a bandwidth of 53 nm is selectively picked up from the nonpolarized white light, where a polarization extinction ratio higher than 100 at about 650 nm has been achieved. The idea of plasmonic diffraction grating is important for exploiting new detection and sensor techniques.

Optical polarizers based on gold nanowires fabricated using colloidal gold nanoparticles.

We demonstrate optical polarization devices, consisting of gold nanowires, which are based on the strong polarization dependence of the particle plasmon resonance of the gold nanowires and the resonance of the waveguided grating structures. Using a layer of indium tin oxide underneath the gold nanowires as the waveguide, we achieved tunable polarization band-pass and band-suppression filters in the transmission and reflection configuration with a bandwidth less than 20 nm at full width at half maximum (FWHM) in the visible spectral range. Then, using side-input geometry for multiplying the absorption by the particle plasmon resonance, we achieved a strong band-suppression polarizer with an extinction ratio of up to 145. These polarization devices can be used directly in optical engineering, and potentially provide alternatives to conventional devices in some special applications. A simple solution-processible fabrication technique enables high quality and large area (>10 × 10 mm(2)) production of these polarizers.

Independently tunable 1.3 W femtosecond Ti:sapphire lasers passively synchronized with attosecond timing jitter and ultrahigh robustness.

A stable passively synchronized femtosecond laser has been realized by coupling two 1.3 W mode-locked Ti:sapphire lasers with a Kerr medium. An ultralong tolerance of 10 microm for the cavity length mismatch and a timing jitter of less than 0.4 fs were obtained. The relative carrier-envelope phase slip was directly observed by measuring the heterodyne output between the two lasers.