State Key Laboratory of Pulsed Power Laser Technology

Magnetite nanoparticles of Fe3O4 were found to assemble into monodisperse hollow Fe3O4 microspheres with tunable diameters ranging from 200 to 400 nm and open pores on the shells in ethylene glycol in the presence of dodecylamine (DDA). The oriented assembly of nanoparticles conferred the individual hollow Fe3O4 microspheres a remarkable feature of single crystals. The morphologies of the products could be easily manipulated by varying the synthesis parameters. Increasing the concentration of DDA led to an obvious shape evolution of the products from rhombic nanoparticles to hollow microspheres, solid microspheres, and finally irregular nanoparticles, which were mainly attributed to the special self-assembly phenomenon of Fe3O4 nanoparticles in the solvothermal process.

Due to their low losses, dielectric metamaterials provide an ideal resolution to construct ultra-narrowband absorbers. To improve the sensing performance, we present numerically a near-infrared ultra-narrowband absorber by putting ultra-sparse dielectric nanowire grids on metal substrate in this paper. The simulation results show that the absorber has an absorption rate larger than 0.99 with full width at half-maximum (FWHM) of 0.38 nm. The simulation field distribution also indicates that the ultra-narrowband absorption is originated from the low loss in the guided-mode resonance. Thanks to the ultra-narrow absorption bandwidths and the electric field mainly distributed out of the ultra-sparse dielectric nanowire grids, our absorber has a high sensitivity S of 1052 nm/RIU and a large figure of merit (FOM) of 2768 which mean that this ultra-narrowband absorber can be applied as a high-performance refractive index sensor.

Current oriented visual tracking depends on segmentation-driven framework brings about expensive computation cost, which becomes the bottleneck in the practical application. This paper proposes a simple and effective Siamese oriented Region Proposal Network (Siamese-ORPN) for visual tracking. Specifically, we propose to use oriented RPN on the similarity feature maps to directly generate high-quality oriented proposals in a nearly cost-free manner. Moreover, a top-down feature fusion network is proposed as the backbone for feature extraction and feature fusion, which can achieve substantial gains from the diversity of visual-semantic hierarchies. The Siamese-ORPN runs at 85 fps while achieving leading performance on the benchmark datasets including VOT2018 (44.6% EAO) and VOT2019 (39.6% EAO).

Fourier transform spectrometers (FTS), mostly working in infrared (IR) or near infrared (NIR) range, provide a variety of chemical or material analysis with high sensitivity and accuracy and are widely used in public safety, environmental monitoring and national border security, such as explosive detection. However, because of being bulky and expensive, they are usually used in test centers and research laboratories. Miniaturized FTS have been developed rapidly in recent years, due to the increasing demands. Using micro-electromechanical system (MEMS) micromirrors to replace the movable mirror in a conventional FTS system becomes a new realm. This paper first introduces the principles and common applications of conventional FTS, and then reviews various MEMS based FTS devices.

Real-time tracking of fast-moving object have many important applications in various fields. However, it is a great challenge to track of fast-moving object with high frame rate in real-time by employing single-pixel imaging technique. In this paper, we present the first single-pixel imaging technique that measures zero-order and first-order geometric moments, which are leveraged to reconstruct and track the centroid of a fast-moving object in real time. This method requires only 3 geometric moment patterns to illuminate a moving object in one frame. And the corresponding intensities collected by a single-pixel detector are equivalent to the values of the zero-order and first-order geometric moments. We apply this new approach of measuring geometric moments to object tracking by detecting the centroid of the object in two experiments. The root mean squared errors in the transverse and axial directions are 5.46 pixels and 5.53 pixels respectively, according to the comparison of data captured by a camera system. In the second experiment, we successfully track a moving magnet with a frame rate up to 7400 Hz. The proposed scheme provides a new method for ultrafast target tracking applications.

Constrained layer dampers (CLD) are in widespread use for passive vibration damping, in applications including aerospace structures. However, the introducing of the damping layer can reduce the stiffness of the sandwich structures. A viscoelastic material filling (VMF) is chosen to balance structural and vibrational performance of lattice truss in this work. The recently brought forward 3D Kagome truss with face sheet was manufactured by selective laser sintering technology and the thermosetting polyurethane was chosen as the viscoelastic filling material. A novel complex modal analysis finite element method for Hybrid composite lattice truss sandwich is introduced in this paper. Dynamic analysis experiment results show that the VMF method is found to be effective in reducing the vibration amplitude and it has the potential for band-gap design. The VMF method can provide high stiffness at low mass and considerable vibrational performance at low cost and it can be considered as a general vibration design method in lattice truss manufacture.

The transverse mode instability (TMI) has been one of the main limitations for the power scaling of single mode fiber lasers. In this work, we report a 6 kW single mode monolithic fiber laser enabled by effective mitigation of the TMI. The fiber laser employs a custom-made wavelength-stabilized 981 nm pump source, which remarkably enhanced the TMI threshold compared with the wavelength of 976 nm. With appropriately distributing bidirectional pump power, the monolithic fiber laser is scaled to 6 kW with single mode beam quality (M 2 <1.3). The stability is verified in a continuous operation for over 2 hours with power fluctuation below 1%.

Fibre lasers operating at the mid-IR have attracted enormous interest due to the plethora of applications in defence, security, medicine, and so on. However, no continuous-wave (CW) fibre lasers beyond 4 μm based on rare-earth-doped fibres have been demonstrated thus far. Here, we report efficient mid-IR laser emission from HBr-filled silica hollow-core fibres (HCFs) for the first time. By pumping with a self-developed thulium-doped fibre amplifier seeded by several diode lasers over the range of 1940-1983 nm, narrow linewidth mid-IR emission from 3810 to 4496 nm has been achieved with a maximum laser power of about 500 mW and a slope efficiency of approximately 18%. To the best of our knowledge, the wavelength of 4496 nm with strong absorption in silica-based fibres is the longest emission wavelength from a CW fibre laser, and the span of 686 nm is also the largest tuning range achieved to date for any CW fibre laser. By further reducing the HCF transmission loss, increasing the pump power, improving the coupling efficiency, and optimizing the fibre length together with the pressure, the laser efficiency and output power are expected to increase significantly. This work opens new opportunities for broadly tunable high-power mid-IR fibre lasers, especially beyond 4 μm.

In this letter, we proposed a deep learning wavefront sensing approach for the Shack-Hartmann sensors (SHWFS) to predict the wavefront from sub-aperture images without centroid calculation directly. This method can accurately reconstruct high spatial frequency wavefronts with fewer sub-apertures, breaking the limitation of d / r 0 ≈ 1 ( d is the diameter of sub-apertures and r 0 is the atmospheric coherent length) when using SHWFS to detect atmospheric turbulence. Also, we used transfer learning to accelerate the training process, reducing training time by 98.4% compared to deep learning-based methods. Numerical simulations were employed to validate our approach, and the mean residual wavefront root-mean-square (RMS) is 0.08 λ . The proposed method provides a new direction to detect atmospheric turbulence using SHWFS.

We have demonstrated a 5 kW high-power monolithic fiber amplifier employing a homemade spindle-shaped ytterbium-doped fiber (YDF) based on the main oscillator power amplifier configuration. The YDF consists of a spindle-shaped core and cladding along the fiber length, with a core/cladding diameter of 27/410 µm at both ends and 39.5/600 µm in the middle. An output power of over 5 kW and beam quality of about 1.9 and an optical-to-optical conversion efficiency of 66.6% were achieved in the amplifier under a bidirectional pump scheme. While operating at the maximum power, the laser performance was evaluated, and the transverse mode instability and stimulated Raman scattering effects were well mitigated. To the best of our knowledge, this is the highest power demonstration in a continuous-wave fiber laser employing a tapered fiber. Further power scaling is promising by optimizing the structure of the YDF.

Abstract Metal phosphorous tri‐chalcogenides are a category of new ternary 2D layered materials with a wide range of tuneable bandgaps (1.2–3.5 eV). These wide‐bandgap semiconductors exhibit great potential applications in solar‐blind ultraviolet (SBUV) photodetection. However, these 2D solar‐blind photodetectors suffer from low photoresponsivity, slow photoresponse speed, and narrow operation spectral region, thereby limiting their practical applications. Here, an ultra‐broadband photodetection based on a FePSe 3 /MoS 2 heterostructure with coverage ranging from solar‐blind ultraviolet 265 nm to longwave infrared (LWIR) 10.6 µm is reported. Notably, the device exhibits excellent weak light detection capability. A high photoresponsivity of 33 600 A W −1 and an external quantum efficiency of 1.57 × 10 7 % are demonstrated. A noise‐equivalent power as low as 5.7 × 10 –16 W Hz −1/2 and a specific detectivity up to 1.51 × 10 13 cm Hz 1/2 W −1 are realized in the SBUV region. The room temperature LWIR photoresponsivity of 0.12 A W −1 is realized. This work opens a route to design high‐performance SBUV photodetectors and wide spectral photoresponse applications.

N-alkyl chains have been successfully grafted into the interlayer space of a Ruddlesden–Popper-type double-layered peroviskite, H2CaTa2O7, via a hydrolysis–esterification process. During the chemical graft process, the parent layered structure is well preserved with magnifying the tailored c lattice parameter. IR and solid-state 13C CP/MAS NMR spectra indicated that oxyalkyl chains were successfully introduced. Thermogravimetric curves of the products exhibit the amount of n-alkoxyl groups per perovskite unit [CaTa2O7] is approximated to 1. A linear relationship with a slope of 0.478 nm per carbon atom is observed between the c lattice parameter and the number of carbon atoms in the n-alkyl chains, which illustrates that the n-alkyl chains form bilayers with a tilt angle of 70°. The photocatalytic activities of these products are also discussed. The 1-octadecanol derivative of H2CaTa2O7 is found to serve as an excellent catalyst for the catalytic reduction of rhodamine B (RhB) and methyl orange (MO), which is set as a novel example of an application of this tailored n-propoxy derivative of H2CaTa2O7.

Abstract Compared with conventional uniform fibers, long tapered fibers provide an effective way to maintain beam quality and suppress the generation of nonlinear effects in fiber lasers. In this work, the transverse mode instability (TMI) of two amplifiers based on ytterbium-doped fiber (YDF) with uniform core diameter and tapered core diameter has been investigated experimentally. For a reasonable comparison of TMI in these two fiber amplifiers, the same effective core diameters and design parameters are applied to both. In the uniform YDF amplifier, the TMI threshold is around 1046 W, and the beam quality M 2 factor increases with the power and reaches 3.2 when the power is around 1500 W. In a tapered YDF (T-YDF) amplifier, the maximum output power is up to 2170 W with a beam quality of M 2 ~ 2.2 and no sign of TMI is observed. To the best of our knowledge, this is the first reported all-fiber tapered fiber laser with a 2 kW-level average output power. The experimental results and theoretical analysis indicate that the TMI threshold of the T-YDF amplifier is higher than the uniform YDF amplifier in spite of sharing the same effective core diameter.

Hyperfine wind structure detection is important for aerodynamic and aviation safety. Pulse coherent Doppler wind LIDAR (PCDWL) is a widespread wind remote sensing method with tunable spatial and temporal resolutions. However, meter scale and sub-second resolution are still challenging for PCDWL. This is because of the constraints among short laser pulse duration, spectral broadening, detection accuracy, and real-time processing. In this Letter, to further improve the spatial and temporal resolution of PCDWL, we optimize the optical design of a nanosecond fiber laser and telescope and adopt a new, to the best of our knowledge, algorithm called the even-order derivative peak sharpening technique. During the experiment, all-fiber PCDWL with spatial and temporal resolutions of 3 m and 0.1 s, respectively, is demonstrated. Two-day continuous observation of the wakes of the Chinese high-speed train shows detailed hyperfine wind structures. This is similar to a computational fluid dynamics simulation.

For the first time, colloidal CdTe quantum dots are incorporated into an inorganic matrix, BaSO₄, through a co-precipitation method.

The present paper describes a hyper-spectral polarization imaging system based on a non-collinear, acousto-optic tunable filter (AOTF) and a linear polarizer. The paper begins with an analysis of the equivalent relationship of the AOTF to describe the principle of polarization detection of this system. Appropriate parameters of hardware components are assigned later. This system, being electronically controllable and tunable, can not only ensure the precision of imaging but also obtain hyper-spectral polarization signatures. The prototype has two optional working modes, namely, hyper-spectral polarization imaging and hyper-spectral intensity imaging. Moreover, parameters of this system – such as diffraction efficiency, spectral resolution and modulation precision – are tested using a halogen tungsten lamp and an optical fibre spectrometer. The results indicate that this instrument is compact, vibration-insensitive, robust and precisely controllable. The system designed in this paper has further application in polarization detection techniques in military.

To achieve competitive fluorescence carbon dots (CDs), studies on regulating fluorescence of CDs under controlled, comparable conditions are in great demand. Herein, by changing the functional groups and nitrogenous existence forms in the precursors, three efficient yellow-green emissive N-doped CDs which have the same fluorescence peak wavelength but different photoluminescence quantum yields were realized through a facile hydrothermal method. The as-prepared CDs exhibit not only excited-independent emissions but also similar surface states. The best-performing CDs among the three products exhibits photoluminescence quantum yields of up to 24.4% in water and 53.3% in ethanol, abundant surface functional groups and its high N-doping degree would be the reason for its excellent performances. By washing and reduction processes, the emission evolution of the CDs was studied linking the changes of surface states. The fluorescence can certainly be attributed to the surface of the carbon dots, and the surface states control the photoluminescence features. Serving as a yellow-green colour conversion layer, the best CDs in the three products was used to fabricate a white light-emitting diode. The white light-emitting diode shows an excellent colour rendering index up to 93.3, suggesting broad application prospects of the CDs in lighting and display fields.

We propose an ultra-narrowband dielectric metamaterial absorber with dielectric-dielectric-metal (DDM) structure where a dielectric cavity layer is inserted between the top compound periodic dielectric microstructures and bottom metal substrate. The results show that this absorber has an ultra-narrowband absorption with a full width at half-maximum (FWHM) of 0.028 nm and quality factor larger than 50,000 in the near-infrared regime. The electric field distribution shows that the phase resonance excited in the microstructures slits is coupled with the cavity resonance occurring in the middle dielectric layer which can greatly narrow the absorption bandwidth. More importantly, the absorption bandwidth can be flexibly manipulated by adjusting the thickness of the dielectric cavity layer. Due to the ultra-narrow absorption bandwidths and electric field concentrated in the slits, we can get a huge figure of merit (FOM) of 6731 which is much larger than those of the reported absorbers in the near-infrared regime. In addition, the larger FOM of 20,366 can be attained with higher-order mode resonance in the dielectric cavity layer. The proposed metamaterials have the great potentials in sensing applications.

Abstract Incorporating auxiliary all‐optical modulation speeds as optional response modes into a single metamaterial is a promising research route towards advanced terahertz (THz) applications ranging from spectroscopy and sensing to communications. Particularly, a plethora of dynamically tunable optical functionalities are determined by the resonant light‐matter interactions. Here, an electromagnetically induced transparency (EIT) resonator stacked with two traditional semiconductor films, namely silicon (Si) and germanium (Ge), is experimentally demonstrated. A giant switching feature of the EIT window with a peak at 0.65 THz occurs when the Si or Ge film is excited by ultrafast optical pulses, allowing for an optically tunable group delay of the THz wave packet. The recovery time for the slow and fast on‐off‐on switching cycles is 1.7 ns and 11 ps, respectively, which are mapped as the pump delay time of Si and Ge. Two optional response modes are integrated on the same device, where the modulation speed varies by three orders of magnitude, endowing the modulator more compact. This work provides new prospects for the design and construction of novel chip‐scale THz devices based on EIT and their applications in areas of sophisticated optical buffering and active filtering.

In this paper, a fs-laser phase mask inscription system based on a galvanometer scanning strategy is designed and set up for the fabrication of large-core fiber Bragg gratings (FBGs). Based on this setup, a homogeneous cross-sectional refractive index modulation can be achieved in the core of a large-mode-area fiber, and a pair of FBGs are fabricated in fibers with a core diameter of 30 µm. To investigate the performance of the fabricated FBGs, a high power all-fiber oscillator is built using a pure backward pumping structure. The FBGs work well, and the maximum output power of 7920 W is achieved with an optical–optical conversion efficiency of 77.3%. To the best of our knowledge, this is the highest power of all-fiber oscillators based on fs-written FBGs. This work provides a flexible, stable, and economic scanning strategy for large-core FBG inscription and exhibits excellent performance for high power fiber lasers.