High Mobility 3D Dirac Semimetal (Cd3As2) for Ultrafast Photoactive Terahertz Photonics

The Dirac semimetal cadmium arsenide (Cd₃As₂), a 3D electronic analog of graphene, has sparked renewed research interests for its novel topological phases and excellent optoelectronic properties. The gapless nature of its 3D electronic band facilitates strong optical nonlinearity and supports Dirac plasmons that are of particular interest to realize high-performance electronic and photonic devices at terahertz (1 THz = 4.1 meV) frequencies, where the performance of most dynamic materials are limited by the tradeoff between power-efficiency and switching speed. Here, all-optical, low-power, ultrafast broadband modulation of terahertz waves using an ultrathin film (100 nm, λ/3000) of Cd₃As₂ are experimentally demonstrated through active tailoring of the photoconductivity. The measurements reveal the photosensitive metallic behavior of Cd₃As₂ with high terahertz electron mobility of 7200 cm² (Vs)⁻¹. In addition, optical fluence dependent ultrafast charge carrier relaxation (15.5 ps), terahertz mobility, and long momentum scattering time (157 fs) comparable to superconductors that invoke kinetic inductance at terahertz frequencies are demonstrated. These remarkable properties of 3D Dirac topological semimetal envision a new class of power-efficient, high speed, compact, tunable electronic, and photonic devices.     

Dynamical control of directional nonlinear scattering from metallic nanoantennas by three-dimensional focal polarization orientation

We demonstrate that the directionality of far-field second harmonic(SH) emission generated from individual gold nanosphere can be flexibly engineered by manipulating three-dimensional(3D) focal polarization orientation of the excitation field, which is explained by the coherent interference between SH dipolar and quadrupolar emission modes. The SH dipolar emission mode is independent of the polarization direction of the fundamental field whereas the evolution of the focal polarization orientation can dramatically modify the quadrupolar emission pattern. Therefore, the resultant SH emission pattern has a polarization-dependent behavior and side scattering almost perpendicular to the propagation direction of the incident light can be observed under a specific condition. Our findings provide a novel degree of freedom for all-optical control of directional nonlinear scattering from single nanoantenna, thereby opening new possibilities for future potential applications.     

Single mode fiber dislocation Mach-Zehnder interfero- meter cascaded with fiber Bragg grating for monitoring metal electrochemical corrosion

An optical fiber sensor composed of a dislocation Mach-Zehnder interferometer (MZI) cascaded with a fiber Bragg grating (FBG) is proposed, and it is used to monitor the electrochemical corrosion of metals in experiments. The dislocation interferometer is composed of two segments of single-mode fiber (SMF) and one segment of dislocation SMF. The contact surface is increased between the fiber and the environment, which helps to improve the interference sensitivity. The relationship between the dislocation amount and the refractive index sensitivity of the interferometer is discussed through simulation. In the experiment, the sensitivity of the interferometer reaches more than 10 000 nm/RIU, and the monitoring of metal electrochemical corrosion is also realized in 3.5% NaCl solution. The proposed measurement scheme has the advantages of small structure, low cost and high sensitivity. It has good prospects in chemical reaction monitoring.     

Seawater temperature and salinity sensing based on in-fiber Michelson-Fabry-Perrot hybrid interferometer employing frequency domain decomposition method

We propose an in-fiber Michelson-Fabry-Perrot (M-FP) hybrid interferometer for the simultaneous measurement of seawater temperature and salinity. The sensor head consists of two parallel hetero Fabry-Perot (FP) cavities fabricated on the end face of the twin core fiber (TCF). A fiber fusion taper is used to split and recouple the light in the two cores. In this case, the Vernier effect can be obtained which can greatly enhance the sensitivity and solve the problem of temperature cross-sensitivity. Different from the traditional demodulation method based on envelop detection, we employed frequency domain decomposition method (FDDM) to demodulate the sensing signal. The simulation results indicate that the proposed sensor has high sensitivity to salinity and temperature. Thanks to the merits of high sensitivity, ease of fabrication and small footprint, the proposed seawater temperature and salinity sensor would have potential applications in marine science, food industry and ocean ranching.     

飞秒激光超声时域分析用于无损检测研究

为了进一步探究飞秒激光诱导超声波在无损检测领域中的应用,本文提出了一种基于飞秒激光的超声时域分析方法,实现了对物质的无损检测。该方法首先将飞秒激光聚焦到被测物体表面激发出超声脉冲,然后通过采集超声脉冲的时域信号来对被测物体的内部结构进行可视化无损检测。为了验证方法的有效性,我们进行了以下两个实验:首先,我们对水泥样品进行了扫描并分析得到了其中的孔洞的位置;此外,我们还对未知内部结构的金属镜筒进行了扫描,检测出了其中透镜组的位置。理论分析和实验结果均证明了实验方法的有效性,与传统的超声无损检测技术相比,基于飞秒激光的超声时域分析方法能够简单快速地实现无损检测,具有良好的应用前景。     

An electrically controlled tunable photonic crystal filter based on thin-film lithium niobate

In this paper, we present an electrically controlled tunable narrowband filter based on a thin-film lithium niobate two-dimensional(2D) photonic crystal. The filter incorporates a photonic crystal microcavity structure within the straight waveguide, enabling electronic tuning of the transmitted wavelength through added electrode structures. The optimized microcavity filter design achieves a balance between high transmission rate and quality factor, with a transmission center wavelength of 1 551....     

Integration of Alloy Segregation and Surface Co?O Hybridization in Carbon-Encapsulated CoNiPt Alloy Catalyst for Superior Alkaline Hydrogen Evolution

Constructing an efficient alkaline hydrogen evolution reaction (HER) catalyst with low platinum (Pt) consumption is crucial for the cost reduction of energy devices, such as electrolyzers. Herein, nanoflower-like carbon-encapsulated CoNiPt alloy catalysts with composition segregation are designed by pyrolyzing morphology-controlled and Pt-proportion-tuned metal–organic frameworks (MOFs). The optimized catalyst containing 15% CoNiPt NFs (15%: Pt mass percentage, NFs: nanoflowers) exhibits outstanding alkaline HER performance with a low overpotential of 25 mV at a current density of 10 mA cm⁻², far outperforming those of commercial Pt/C (47 mV) and the most advanced catalysts. Such superior activity originates from an integration of segregation alloy and Co-O hybridization. The nanoflower-like hierarchical structure guarantees the full exposure of segregation alloy sites. Density functional theory calculations suggest that the segregation alloy components not only promote water dissociation but also facilitate the hydrogen adsorption process, synergistically accelerating the kinetics of alkaline HER. In addition, the activity of alkaline HER is volcanically distributed with the surface oxygen content, mainly in the form of Co_3d O_2p hybridization, which is another reason for enhanced activity. This work provides feasible insights into the design of cost-effective alkaline HER catalysts by coordinating kinetic reaction sites at segregation alloy and adjusting the appropriate oxygen content.     

Reconstruction performance for image transmission through multimode fibers

Due to the applications in the fields of optical communication, neuronal imaging, and medical endoscopic imaging, the study of multimode fiber (MMF) wavefront transmission is crucial for image reconstruction and wavefront generation in user terminals. State of art studies in this area focus on high-quality image reconstruction and wavefront shaping. Besides the way of imaging reconstruction, the performances of image reconstruction and wavefront shaping are also dependent on system and environment parameters. This paper numerically analyzes the influence of key factors, such as the numerical aperture (NA) of the near-end objective lens of the MMF imaging system, the charge coupled device (CCD) noise of the acquired image, and the ambient temperature at close distances. This work would help to the optimization of the MMF-based imaging system and provide a theoretical basis for potential applications in optical communication systems, neuron imaging and endoscopic diagnosis.     

Holographic grating fabrication for wide angular bandwidth using polymer thin films

To increase the angular bandwidth of volume holographic grating, we fabricate holographic gratings based on grating multiplexing technique by using thin films of photopolymers and polymer dispersed liquid crystals. Experimental results confirm that the liquid crystal materials increase the refractive index modulation of the grating, enabling high diffraction efficiency with wide angular response compared to pure polymer materials. We observe that the fabricated holographic grating has near 80% of diffraction efficiency and about 18° of angular bandwidth, which can be further improved by modifying the liquid crystal/polymer mixtures and the grating multiplexing technique. The grating can be used to fabricate holographic waveguide structures for emerging applications in the near-eye display systems.