Direct space-time manipulation mechanism for spatio-temporal coupling of ultrafast light field.

Traditionally, manipulation of spatiotemporal coupling (STC) of the ultrafast light fields can be actualized in the space-spectrum domain with some 4-f pulse shapers, which suffers usually from some limitations, such as spectral/pixel resolution and information crosstalk associated with the 4-f pulse shapers. This work introduces a novel mechanism for direct space-time manipulation of ultrafast light fields to overcome the limitations. This mechanism combines a space-dependent time delay with some spatial geometrical transformations, which has been experimentally proved by generating a high-quality STC light field, called light spring (LS). The LS, owing a broad topological charge bandwidth of 11.5 and a tunable central topological charge from 2 to -11, can propagate with a stable spatiotemporal intensity structure from near to far fields. This achievement implies the mechanism provides an efficient way to generate complex STC light fields, such as LS with potential applications in information encryption, optical communication, and laser-plasma acceleration.

Research on safety evaluation of collapse risk in highway tunnel construction based on intelligent fusion.

To solve the problems of untimely and low accuracy of tunnel project collapse risk prediction, this study proposes a method of multi-source information fusion. The method uses the PSO-SVM model to predict the surrounding rock displacement. With the prediction index as the benchmark, the Cloud Model (CM) is used to calculate the basic probability assignment value. At the same time, the improved D-S theory is used to fuse the monitoring data, the advanced geological forecast, and the tripartite information indicators of site inspection patrol. This method is applied to the risk assessment of Jinzhupa Tunnel, and the decision-makers adjust the risk factors in time according to the prediction level. In the end, the tunnel did not collapse on a large scale.

396 MHz CPA femtosecond laser system based on a single crystal fiber rod-type amplifier.

The paper presents the results of multiplying the repetition frequency of the seed source to 396 MHz using an extra-cavity fiber multiplication method. The single crystal fiber rod-type amplifier was able to boost the average output power to approximately 15.9 W. Using a transmission gratings-based pulse compressor, an average output power of 9.56 W was achieved, corresponding to a burst energy of up to 382 µJ and a compression efficiency of 81.7%. The shortest pulse duration was optimized to be 594 fs; however, an obvious pedestal was also observed. This system reduces the impact of nonlinear effects on the device by increasing the repetition frequency, thereby increasing the injection energy of a single process in industrial processing.

Research on blasting cumulative dynamic damage of surrounding rock in step construction tunnel.

In the process of cyclic blasting during tunnel excavation, the reserved surrounding rock sustains irreparable damage accumulation. For safe tunnel construction, it is imperative to understand the characteristics of blasting dynamic cumulative rock damage. Sonic wave test and numerical simulation methods were applied to the research. The JH-2 model was adopted as the damage model of surrounding rock. Based on the data transfer method between solvers in ABAQUS software, the cumulative damage was calculated. The damage characteristics were obtained by combining the sonic wave test results. According to the research findings, the entire reserved surrounding rock has periodic damage characteristics. Each periodic damage area has a funnel shape along the tunnel's longitudinal direction, with a length of 160 cm, and 1.07 times the excavation footage. The latter excavation footage's blasting effect on the damaged area of the previous footage rock is 40 cm long, with three cumulative damage patterns. The three cumulative damage patterns more clearly reveal the surrounding rock's additional damage law, the degree of additional damage is greatest with the distance of 5-20 cm from the latter excavation footage. The research can provide appropriate theoretical guidance for the design of the step-blasting construction tunnel's blasting scheme and lining.

Asymmetry spin-polarization Fermi arc and topological chiral beam splitter with the triply degenerate point in metamaterial.

Dirac points (DPs) and Weyl points (WPs) have received much attention in photonic crystals (PhCs) and three-dimensional (3D) metamaterials research due to the robust surface states and Fermi arcs. In this work, two pairs of triply degenerate points (TDPs) have been proposed in a 3D metamaterial by breaking the time reversal symmetry (T) with an external magnetic field. Based on these TDPs, two pairs of asymmetric surface states with spin-polarization are revealed, and a topological chiral beam splitter is demonstrated showing the different propagating directions of the right-handed polarization (RCP) and left-handed polarization (LCP) lights. Remarkably, we can achieve unidirectional propagation with RCP or LCP even excited by a linear source owing to the asymmetry surface state. Our work provides a new, to the best of our knowledge, platform to study spin-polarization surface states and the enhanced spin photonic Hall effect in the metamaterials.

Research on structural parameter optimization of elliptical bipolar linear shaped charge based on machine learning.

Numerical simulation based on SPH method, compared with laboratory experiments, using the grey correlation theory to analyze the correlation between the parameters of the elliptical bipolar linear shaped charge and the performance of the shaped charge jet. The structure of shaped charge is optimized by machine learning to obtain the optimal structural parameters, and it is compared with the rock crack development of shaped charge blasting in practical application. The results show that the structural parameters of the shaped charge have the same influence on the jet head velocity, and there are certain differences in the impact on the jet length. The fitted curve of the support vector machine (SVM) regression model based on the genetic algorithm (GA) is high prediction accurate. By comparing the optimization results with the actual engineering application of the shaped charge structure, the rock breaking effect has been significantly improved, which has important guiding significance for the actual engineering application.

A numerical study on CO migration after blasting in high-altitude tunnel by inclined shaft.

On the western plateau of China, ventilation problems brought on by low atmospheric pressure must be overcome. And CO migration after blasting in high-altitude tunnel by inclined shaft has become a significant scientific issue. In this study, the Computational Fluid Dynamics (CFD) method was used to analyze the flow field characteristics at the junction of the inclined shaft and tunnel. In addition, the effects of different fan opening modes and different initial CO concentration distributions on the ventilation were discussed. The simulation results showed that the main difference in the ventilation wind field was reflected in the position of the vortex region due to the different fan opening modes. Meanwhile, various initial CO concentration distributions showed different migration when there was no air volume difference between the left and right tunnels. Eliminating vortex zones and fully using high velocity airflow could improve relative ventilation efficiency by at least 18%. CO would accumulate in the opposite direction of the tunnel if only one of the fans was turned on. Therefore, a two-stage ventilation scheme was proposed, and the energy consumption was reduced by at least 33%. This research can provide guidance on high-altitude tunnel construction with multiple working faces to improve ventilation efficiency and reduce energy consumption.

Orthogonality of diffractive deep neural network.

Some rules of the diffractive deep neural network (D2NN) are discovered. They reveal that the inner product of any two optical fields in D2NN is invariant and the D2NN acts as a unitary transformation for optical fields. If the output intensities of the two inputs are separated spatially, the input fields must be orthogonal. These rules imply that the D2NN is not only suitable for the classification of general objects but also more suitable for applications aimed at optical orthogonal modes. Our simulation shows the D2NN performs well in applications like mode conversion, mode multiplexing/demultiplexing, and optical mode recognition.

A multi-source information fusion approach in tunnel collapse risk analysis based on improved Dempster-Shafer evidence theory.

The tunneling collapse is the main engineering hazard in the construction of the drilling-and-blasting method. The accurate assessment of the tunneling collapse risk has become a key issue in tunnel construction. As for assessing the tunneling collapse risk and providing basic risk controlling strategies, this research proposes a novel multi-source information fusion approach that combines Bayesian network (BN), cloud model (CM), support vector machine (SVM), Dempster-Shafer (D-S) evidence theory, and Monte Carlo (MC) simulation technique. Those methods (CM, BN, SVM) are used to analyze multi-source information (i.e. statistical data, physical sensors, and expert judgment provided by humans) respectively and construct basic probability assignments (BPAs) of input factors under different risk states. Then, these BPAs will be merged at the decision level to achieve an overall risk evaluation, using an improved D-S evidence theory. The MC technology is proposed to simulate the uncertainty and randomness of data. The novel approach has been successfully applied in the case of the Jinzhupa tunnel of the Pu-Yan Highway (Fujian, China). The results indicate that the developed new multi-source information fusion method is feasible for (a) Fusing multi-source information effectively from different models with a high-risk assessment accuracy of 98.1%; (b) Performing strong robustness to bias, which can achieve acceptable risk assessment accuracy even under a 20% bias; and (c) Exhibiting a more outstanding risk assessment performance (97.9% accuracy) than the single-information model (78.8% accuracy) under a high bias (20%). Since the proposed reliable risk analysis method can efficiently integrate multi-source information with conflicts, uncertainties, and bias, it provides an in-depth analysis of the tunnel collapse and the most critical risk factors, and then appropriate remedial measures can be taken at an early stage.

Local OAM manipulation of a terahertz wave from the air filament by chirping the few-cycle vortex pump laser.

We propose a scheme to manipulate the local orbital angular momentum (OAM) of the ultra-broadband (0.1-30 THz) terahertz (THz) waves from the laser-induced short air filament via chirping the few-cycle vortex laser pump. The simulation results show that either the THz vortex pulses with linear azimuth-dependent phases or the THz angular accelerating vortex beams (AAVBs) with nonlinear azimuth-dependent phases can be produced by tuning the chirp parameter of the pump. Thus, the dominant physical mechanism for THz generation can be determined. The THz temporal and transverse spatial distributions can be also controlled by the chirp parameter. Furthermore, their local OAM density distributions present very complex structures because most of the modulated azimuthal intensity and the corresponding local angular helicity distributions are not able to cancel out completely. Via analyzing the simulated THz results at the different pump intensities, we classify the initial pump intensity into three cases. For the low intensity case, the Kerr effect comes into prominence, so the generated THz radiation shall be vortex pulses. While for the high intensity case, the leading plasma effect dominates. In contrast, when the pump intensity is at the medium level, the Kerr nonlinearity and the plasma effect may be comparable and competitive. Basically, THz AAVBs are generated for both high and medium intensity cases. Our study will provide the possibility for studying the optically induced rotation technology more intuitively from the perspective of angular momentum transfer.

Dynamic risk evaluation method for collapse disasters of drill-and-blast tunnels: a case study.

The tunnel collapse is one of the most frequent and harmful geological hazards during the construction of highway rock tunnels. As for reducing the occurrence probability of tunnel collapse, a new dynamic risk assessment methodology for the tunnel collapse was established, which combines the Cloud model (CM), the Membership function, and the Bayesian network (BN). During the preparation phase, tunnel collapse risk factors are identified and an index system is constructed. Then, the proposed novel assessment method is used to evaluate the probability of tunnel collapse risk for on-site construction. The probability of tunnel collapse risk in the dynamic process of construction can provide real-time guidance for tunnel construction. Moreover, a typical case study of the Yutangxi tunnel is performed, which belongs to the Pu-Yan Highway Project (Fujian, China). The results show that the dynamic evaluation model is well validated and applied. The risk value of tunnel collapse in a construction cycle is predicted successfully, and on-site construction is guided to reduce the occurrence of tunnel collapse. Besides, it also proves the feasibility of the dynamic evaluation method and its application potential.

All-optical high spatial-temporal resolution photography with raster principle at 2 trillion frames per second.

A novel single-shot ultrafast all-optical photography with raster principle (OPR) that can capture real-time imaging of ultrafast phenomena is proposed and demonstrated. It consists of a sequentially timed module (STM), spectral-shaping module (SSM), and raster framing camera (RFC). STM and SSM are used for linearly encoding frequency-time mapping and system calibration, respectively. The function of the RFC is sampling the target by microlens arrays and framing on the basis of frequency-time-spatial positions conversion. We demonstrated the recording of transient scenes with the spatial resolution of ∼90lp/mm, the frame number of 12 and the frame rate of 2 trillion frames per second (Tfps) in single-shot. Thanks to its high spatial-temporal resolution, high frame rate (maximum up to 10 Tfps or more) and sufficient frame number, our OPR can observe the dynamic processes with complex spatial structure at the atomic time scale (10 fs∼1ps), which is promising for application in plasma physics, shock waves in laser-induced damage, and dynamics of condensed matter materials.

Tailoring the ultrafast and nonlinear photonics of MXenes through elemental replacement.

Due to the outstanding electronic properties, unique chemical surface termination units and rich elemental compositions, MXenes have become promising candidates for the development of new generation optoelectronic devices. However, there is still a gap between advanced photonics applications and fundamental understanding of ultrafast carrier photo-physics dynamics and a nonlinear optical response in layered MXenes. Here, we present insight into the excited state relaxation processes and nonlinear optical response of few-layer Ti3CN and Ti3C2 nanosheets (NSs) via transient absorption spectroscopy and Z-scan measurements. Owing to similar structural compositions, the transient absorption and nonlinear absorption characteristics behave totally opposite. In addition, photo-induced bandgap renormalization and Pauli blocking phenomena exist in Ti3C2 and Ti3CN NSs, respectively. The element replacement may be a new strategy for tunable carrier kinetics and nonlinear optical response of MXenes. These research studies may provide insight into ultrafast carrier photo-physics dynamics as well as promote MXene-based advanced photonics and their applications in optoelectronic devices.

Enhance terahertz radiation and its polarization- control with two paralleled filaments pumped by two-color femtosecond laser fields.

We present experimentally an obvious enhancement of the terahertz (THz) radiation with two paralleled filaments pumped by two-color laser fields for a full use of a high laser power, compared with single filament. By mapping the 3-dimensional electric trajectories of generated THz fields with a (111) ZnTe crystal, we observe that the total THz polarization from two filaments can be manipulated by varying the time delay between the two orthogonally polarized pumps, which agrees well with the simulations under the photocurrent model. Notably, the power and spectrum of the THz field almost keep unchanged while manipulating the ellipticity of the THz polarization, which is important for a polarization-controllable THz source.

Generation and evolution of different terahertz singular beams from long gas-plasma filaments.

We theoretically and numerically investigate the generation and evolution of different pulsed terahertz (THz) singular beams with an ultrabroad bandwidth (0.1-40 THz) in long gas-plasma filaments induced by a shaped two-color laser field, i.e., a vortex fundamental pulse (ω0) and a Gaussian second harmonic pulse (2ω0). Based on the unidirectional propagation model under group-velocity moving reference frame, the simulating results demonstrate that three different THz singular beams, including the THz necklace beams with a π-stepwise phase profile, the THz angular accelerating vortex beams (AAVBs) with nonlinear phase profile, and the THz vortex beams with linear phase profile, are generated. The THz necklace beams are generated first at millimeter-scale length. Then, with the increase of the filament length, THz AAVBs and THz vortex beams appear in turn almost periodically. Our calculations confirm that all these different THz singular beams result from the coherent superposition of the two collinear THz vortex beams with variable relative amplitudes and conjugated topological charges (TCs), i.e., +2 and -2. These two THz vortex beams could come from the two four-wave mixing (FWM) processes, respectively, i.e., ω0+ω0-2ω0→ωTHz and -(ω0+ω0) + 2ω0→ωTHz. The evolution of the different THz singular beams depends on the combined effect of the pump ω0-2ω0 time delay and the separate, periodical, and helical plasma channels. And the TC sign of the generated THz singular beams can be easily controlled by changing the sign of the ω0-2ω0 time delay. We believe that these results will deepen the understanding of the THz singular beam generation mechanism and orbital angular momentum (OAM) conversion in laser induced gas-filamentation.

Ultra-Small 2D PbS Nanoplatelets: Liquid-Phase Exfoliation and Emerging Applications for Photo-Electrochemical Photodetectors.

2D PbS nanoplatelets (NPLs) form an emerging class of photoactive materials and have been proposed as robust materials for high-performance optoelectronic devices. However, the main drawback of PbS NPLs is the large lateral size, which inhibits their further investigations and practical applications. In this work, ultra-small 2D PbS NPLs with uniform lateral size (11.2 ± 1.7 nm) and thickness (3.7 ± 0.9 nm, ≈6 layers) have been successfully fabricated by a facile liquid-phase exfoliation approach. Their transient optical response and photo-response behavior are evaluated by femtosecond-resolved transient absorption and photo-electrochemical (PEC) measurements. It is shown that the NPLs-based photodetectors (PDs) exhibit excellent photo-response performance from UV to the visible range, showing extremely high photo-responsivity (27.81 mA W-1 ) and remarkable detectivity (3.96 × 1010 Jones), which are figures of merit outperforming currently reported PEC-type PDs. The outstanding properties are further analyzed based on the results of first-principle calculations, including electronic band structure and free energies for the oxygen evolution reaction process. This work highlights promising applications of ultra-small 2D PbS NPLs with the potential for breakthrough developments also in other fields of optoelectronic devices.

NiPS3 nanoflakes: a nonlinear optical material for ultrafast photonics.

Ultrafast photonics based on two-dimensional (2D) materials has been used to investigate light-matter interactions and laser generation, as well as light propagation, modulation, and detection. Here, 2D metal-phosphorus trichalcogenides, which are known for applications in catalysis and electrochemical storage, also exhibit advantageous photonic properties as nanoflakes that are only a few layers thick. By using an open-aperture Z-scan system, few-layer NiPS3 nanoflakes exhibited a large modulation depth of 56% and a low saturable intensity of 16 GW cm-2 at 800 nm. When NiPS3 nanoflakes were used as a saturable absorber at 1066 nm, highly stable mode-locked pulses were generated. Thus, these results revealed the nonlinear optical properties of NiPS3 nanoflakes which have potential photonics applications, such as modulators, switches, and thresholding devices.

Liquid crystal-based order electrically controlled q-plate system.

This paper proposes a liquid crystal-based order adjustable q-plate system. The system, which is solid-state and electrically controlled without any mechanical components, consists of several bit cells and one symbol cell. The bit cells can be electrically selected whether to modulate the beam. The magnitude of the order of the q-plate system can be controlled by activating specific bit cells. And the sign of the order can be changed by controlling the voltages in the symbol cell. The whole system can realize the function of the order adjustable q-plate with the order ranging from -2n + 1 to 2n-1 with n bit cells. In our experiment, the system with 4 bits is verified. Based on the q-plate system, the vector beams and optical vortexes with the orders ranging from -15 to 15 can be generated.

Deep learning based atmospheric turbulence compensation for orbital angular momentum beam distortion and communication.

Atmospheric transmission distortion is one of the main challenges hampering the practical application of a vortex beam (VB) which carries orbital angular momentum (OAM). In this work, we propose and investigate a deep learning based atmospheric turbulence compensation method for correcting the distorted VB and improving the performance of OAM multiplexing communication. A deep convolutional neural network (CNN) model, which can automatically learn the mapping relationship of the intensity distributions of input and the turbulent phase, is well designed. After trained with loads of studying samples, the CNN model possesses a good generalization ability in quickly and accurately predicting equivalent turbulent phase screen, including the untrained turbulent phase screens. The results show that through correction, the mode purity of the distorted VB improves from 39.52% to 98.34% under the turbulence intensity of Cn2 = 1 × 10-13. Constructing an OAM multiplexing communication link, the bit-error-rate (BER) of the transmitted signals in each OAM channel is reduced by almost two orders of magnitude under moderate-strong turbulence, and the demodulated constellation diagram also converges well after compensated by the CNN model.

Beta-lead oxide quantum dot (ß-PbO QD)/polystyrene (PS) composite films and their applications in ultrafast photonics.

Polymer composite films, particularly those based on polymers and layered nanomaterials, are attractive materials for exploiting the properties of multiple materials for applications in electronics and photonics. In this work, a beta-lead oxide quantum dot (ß-PbO QD)/polystyrene (PS) composite film is successfully fabricated by a solution blending method. The ß-PbO QDs are well-distributed within a ß-PbO QD/PS composite film and the composite film is transparent and flexible. Owing to the almost complete insolubility of both ß-PbO QDs and PS, the as-fabricated ß-PbO QD/PS composite film holds the nonlinear photonic response from 540 nm to 1060 nm under complete water immersion, confirming its excellent stability to high humidity. Additionally, the ß-PbO QD/PS composite film exhibits a considerable capacity for optical modulation owing to a strong nonlinear absorption coefficient compared with those of other two-dimensional (2D) materials. On the basis of a home-made ß-PbO QD/PS composite film saturable absorber, stable mode-locked pulses at 1060 nm are generated under humid conditions. It is anticipated that the ß-PbO QD/PS composite films enable the exploitation of new waterproof, flexible photonic devices based on functional 2D materials and polymers.