Search

Zn vapour is easily generated on the surface by fusion welding galvanized steel sheet, resulting in the formation of defects. The present study develops a novel method for automatically detect typical undercuts, pores and burn-through defects from CCD using concepts of Convolutional Neural Network(CNN) and active vision. Four deep learning methods of training a defects classifier using 1) an AlexNet built from scratch, 2) a Visual Geometry Group 16 (VGG16) built from scratch, 3) the output features of the VGG16 network architecture previously trained on the general ImageNet dataset, 4) the convolutional block attention module adaptively refining the intermediate feature map of VGG16 are investigated. To detect the weld defects, laser streak images of different weld defects are collected and two types of data augmentation are used to reduce overfitting caused by the limited training dataset. In the method 1 and 2, the improvement of accuracy by data augmentation is gradually slow. VGG16 model in combination with data augmentation 2 achieve the state-of-the-art performance for defects detection in the galvanized steel sheets lap joint. Transfer learning methods using the output features of VGG16 and attention mechanism methods introducing the convolutional block attention module achieve improved accuracy in the case of a small number of training dataset. And all classifiers have a 100 % recognition rate for good welds. This shows that the method of combining CNN and laser vision is feasible, and this method can be used to record the number and location of typical defects in continuous welds.

The atomically thin layered transition metal dichalcogenide (TMDCs) PtSe2 is a new emerging two-dimension (2D) material, which has a transition from indirect-gap semiconductor to semimetal with increasing thickness. Defects in 2D TMDCs are very ubiquitous and play a crucial role in understanding many electronics and optoelectronics in TMDCs. In this article, PtSe2 films with different thickness are obtained by selenizing the variously thick Pt films. Extrapolation of the onset absorption from the visible-infrared spectrum demonstrates that the selenization of 1 and 3 nm Pt films shows semiconductor-like character, while those of thick Pt films (with thickness of 10 and 15 nm) show metallic behavior. The transient absorption (TA) spectroscopy reveals that all films show immediately photobleaching signals after photoexcitation at 800 nm, and the subsequent relaxation process shows strongly thickness dependence. The thinnest film shows biexponential relaxation with typical time constants of 1.28 and 101.2 ps. In contrast, the photobleaching signals are developed into photoinduced absorption signals in thicker films, and the absorption magnitude increases with the thickness of the films. The optical pump and terahertz (THz) probe spectroscopy reveals that all samples show positive photoconductivity after photoexcitation of 800 nm, the subsequent recovery is completed within 2.0 ps, and the recovery time constant decreases with the increase of the films' thickness. Our TA and THz spectroscopy results reveal that the defect states of the films play dominated role in the relaxation of photocarrier after photoexcitation, and edgesite states are inferred to make dominated contributions to the defect states in the selenization of platinum films.

The intelligentized robotic multipass welding process (IRMWP) involves adjustments of welding parameters, posture adjustments of the welding torch, real-time decision making of the tracking point, etc. It constructs a typical mixed logical dynamical (MLD) process. In order to smooth the welding process with thick steel plates, the typical process of implementing two continuous welds with T-joints is first modeled based on finite state machines (FSMs) at software and hardware levels. This process consists of four stages: preparation, determination and adjustment, tracking, and return stage. In these stages, some algorithms that are integrated into the peripheral software system (PSS) terminate the welding process when empirical parameters set in them are inappropriate. Since the weld profile extraction algorithm is the prerequisite for subsequent operations, this paper then presents a strategy to adaptively alter the empirical parameters arranged in this algorithm. The strategy implements fault detection and diagnosis (FDD) for the extraction process. Welding experiments are conducted under the framework of the proposed model, and results show that the proposed method leads to better stability of the PSS and higher ratios of successful weld profile extraction, over 95%. This research is of practical significance for strengthening the stability of the IRMWP with thick steel plates and improving welding quality.

The ultrafast photocurrents in a typical 3D topological insulator (TI) Sb2Te3 were probed with THz emission spectroscopy by tuning the polarization of excitation pulses and the sample’s azimuthal angle. The different ultrafast photocurrents driven by the linear photogalvanic effect, circular photogalvanic effect, and thermoelectric effect were distinguished from THz emission signal. Results potentially provide insights into the ultrafast all optical modulation of THz emission with non-external bias field, which could play a vital role in future TI-based high-speed THz optoelectronic and optospintronic devices.

We demonstrate that the tunneling magneto-resistance (TMR) can be used to modulate terahertz (THz) wave propagation through a magnetic tunnel junctions (MTJ). Operating at the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with the thickness of 77.45 nm, by using a small magnetic field of 30 mT. The THz conductivity spectrum of the MTJ is governed by the spin-dependent electron tunneling. This findings open that the MTJ device will have potential applications in THz magnetic modulators, filtering and sensing.

By employing ultrafast time-resolved optical pump terahertz (THz) probe spectroscopy, we report a comparative THz response on CVD grown topological insulator (TI) Sb 2 Te 3 under different temperatures. Both photo induced carrier dynamics attributed to the surface and bulk electrons are observed. The electron system undergoes an ultrafast excitation and relaxation and releases through intra-band and inter-band scattering.

Magnetic tunnel junctions (MTJs) are a key technology in modern spintronics because they are the basis of read-heads of modern hard disk drives, nonvolatile magnetic random access memories, and sensor applications. In this paper, we demonstrate that tunneling magnetoresistance can influence terahertz (THz) wave propagation through a MTJ. In particular, various magnetic configurations between parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the MTJ have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. Operating in the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with a thickness of 77.45 nm, using a magnetic field as low as 30 mT. The THz conductivity spectrum of the MTJ is governed by spin-dependent electron tunneling. It is anticipated that the MTJ device and its tunability scheme will have many potential applications in THz magnetic modulators, filtering, and sensing.

A Polarizing beam splitter (PBS) based on pendulum effect in volume holographic grating (VBG) is proposed. Using the modified coupled-wave equations of Kogelnik, expressions of transmitted and diffracted intensities are derived when a polarized femtosecond pulse incidents on a VHG. Simulation results show that the intensities of the transmitted and diffracted pulses are periodic oscillation and exchange along the propagation depth, and this property is pendulum effect. Period of the pendulum effect is sensitive to polarization state of the incident pulse. By suitably using this property, it is possible to realize a PBS where the incident TE polarization outputs in transmitted direction of the VHG and TM component outputs in diffracted direction.

Magnon-polaritons are shown to play a dominant role in the propagation of terahertz (THz) waves through TmFeO3 orthoferrite, if the frequencies of the waves are in the vicinity of the quasi-antiferromagnetic spin resonance mode. Both time-domain THz transmission and emission spectroscopies reveal clear beatings between two modes with frequencies slightly above and slightly below this resonance, respectively. Rigorous modeling of the interaction between the spins of TmFeO3 and the THz light shows that the frequencies correspond to the upper and lower magnon-polariton branches. Our findings reveal the previously ignored importance of propagation effects and polaritons in such heavily debated areas as THz magnonics and THz spectroscopy of electromagnons. It also shows that future progress in these areas calls for an interdisciplinary approach at the interface between magnetism and photonics.

In this paper, a femtosecond laser line-scanning irradiation was used to induce the periodic surface microstructure on HgCdTe crystal. Low spatial frequency laser induced periodic surface structures of 650–770 nm and high spatial frequency laser induced periodic surface structures of 152–246 nm were respectively found with different scanning speeds. The evolution process from low spatial frequency laser induced periodic surface structures to high spatial frequency laser induced periodic surface structures is characterized by scanning electron microscope. Their spatial periods deduced by using a two-dimensional Fourier transformation partly agree with the predictions of the Sipe-Drude theory. Confocal micro-Raman spectral show that the atomic arrangement of induced low spatial frequency laser-induced structures are basically consistent with the crystal in the central area of laser-scanning line, however a new peak at 164 cm −1 for the CdTe-like mode becomes evident due to the Hg vaporization when strong laser ablation happens. The obtained surface periodic ripples may have applications in fabricating advanced infrared detector.

In the robotic welding process with thick steel plates, laser vision sensors are widely used to profile the weld seam to implement automatic seam tracking. The weld seam profile extraction (WSPE) result is a crucial step for identifying the feature points of the extracted profile to guide the welding torch in real time. The visual information processing system may collapse when interference data points in the image survive during the phase of feature point identification, which results in low tracking accuracy and poor welding quality. This paper presents a visual attention feature-based method to extract the weld seam profile (WSP) from the strong arc background using clustering results. First, a binary image is obtained through the preprocessing stage. Second, all data points with a gray value 255 are clustered with the nearest neighborhood clustering algorithm. Third, a strategy is developed to discern one cluster belonging to the WSP from the appointed candidate clusters in each loop, and a scheme is proposed to extract the entire WSP using visual continuity. Compared with the previous methods the proposed method in this paper can extract more useful details of the WSP and has better stability in terms of removing the interference data. Considerable WSPE tests with butt joints and T-joints show the anti-interference ability of the proposed method, which contributes to smoothing the welding process and shows its practical value in robotic automated welding with thick steel plates.

The speed of optical modulation on a terahertz (THz) pulse is mainly dominated by the optical response of the photocarrier. In order to achieve ultrafast THz modulation, the effective method is to reduce the lifetime of the photocarrier by introducing defects that can trap the photocarriers efficiently. In this paper, we reported the ultrafast optical modulation of THz switching in a 10 nm CdTe nanostructure film. After photoexcitation at 800/400 nm, the THz response of the film is extremely fast with a lifetime of ${\sim}{1.3}\;{\rm ps}$∼1.3ps. Further, the ultrafast transient THz transmission shows almost temperature independence down to 100 K. On the other hand, the transient absorption spectroscopy reveals that the lifetime of photocarriers in CdTe nanostructure film lasts as long as several ns. The 1.3 ps THz photoconductivity response is due to the substantial decrease of photocarrier mobility in a CdTe nanostructure, which comes from the increase of the photocarrier scattering between the photocarrier and the surface states of CdTe nanostructural film. Our experimental results provide a new method to design optically driven ultrafast THz response devices, such as THz switch and THz modulator.

THz time-domain spectroscopy (THz-TDS) is used to study the THz-optical properties of a single crystal bismuth ferrite (BiFeO 3 ). The anisotropy of BiFeO 3 is strongly dependent on the temperature. A giant birefringence is observed up to around 3.6.

The cooling mechanism is crucial for nanoelectronics, which determines the response time limited for the devices. The cooling process after optical pump in graphene, which is a well-known material with ultrahigh carrier mobility, is therefore an urgent research topic. Ultrafast thermalization and relaxation process in a monolayer graphene film grown on a quartz substrate is investigated by femtosecond laser pump, terahertz probe technique. A typical photo induced negative terahertz conductivity is found, and the electron-phonon-impurity supercollision process is determined to about 2.8 picosecond at different pump photon energy and pump fluence. The competition between the impurity supercollision and the interband-intraband scattering in graphene could give new insight into the design of the application of graphene photonic devices.

In this paper, based on Kogelnik’s coupled-wave theory and transfer matrix method, we discussed the temporal diffraction properties of multilayer volume holographic gratings under femtosecond pulses readout. Results show that grating layers generate diffracted pulses, with each grating layer diffracting out one sub-pulse, whose waveform is determined only by parameters of the corresponding grating layer. The buffer layers placed on the right side of the grating layer translates the former diffracted sub-pulse along the negative time axis, and the translation is proportional to the total thickness of buffer layers placed on the right side of it, whereas the buffer layers placed on the left side of the grating layer has no effect on the diffracted sub-pulse. We also give explanations of these phenomena based on the diffraction efficiency of the grating layers and the phase shift of the buffer layers.

In this paper, a novel terahertz metamaterial structure composed of a pair of sub-wavelength reverse U-shaped split ring resonators (RUSRs) and cut wire (CW) resonator is designed to realize electromagnetically induced transparency (EIT) effect in weak coupling region. Theoretical and simulated results show that by modulating the relative coupling distance between CW and SRR or mutual distance between SRR pair, the EIT-like phenomenon can be tailored. Furthermore, by introducing photosensitive silicon (Si) cell into the units of the dark mode resonator, actively optical control of the EIT-like effect is realized through increasing the dark mode damping rate. The present work provides an alternative method to design ultrasensitive sensors, filters and slow-light devices in the THz regime.

In this study, we have reported a newly ultrafast optically modulated terahertz (THz) switch based on the transition metal dichalcogenide (TMD) material platinum diselenide ( P t S e 2 ) with different thicknesses. The high-quality P t S e 2 thin films with centimeter scale are fabricated on sapphire substrate by the chemical vapor deposition method. The optical pump and THz probe (OPTP) spectroscopy reveals that the THz response of the thin films is as fast as ∼ 2.0 p s after photoexcitation of a 780 nm pulse. Interestingly, we found that the THz response time of the P t S e 2 semimetal phase is faster than that of the semiconducting phase. In addition, the THz response time becomes faster when increasing the film thickness for the semimetal phase P t S e 2 , while for the semiconducting phase, the response time becomes slower with film thickness. Moreover, degenerate optical pump and optical probe spectroscopy (OPOP) demonstrated that the ultrafast photoinduced negative absorption (photoinduced bleaching) occurs after photoexcitation of 780 nm, and the subsequent recovery consists of two relaxation processes: the fast component with more than 85% of weight has a lifetime of ∼ 1.5 p s for semiconducting-phase films and less than 1 ps for the semimetal phase, similar to the response time obtained from OPTP measurement. The slow component with less than 15% of weight has a lifetime of a few hundred picoseconds. The subpicosecond response time observed in both OPTP and OPOP is ascribed to the carrier trapping by defect states, and the slow relaxation process appearing in OPOP arises from the defect state relevant relaxation that is insensitive to the THz photoconductivity due to the frozen carrier mobility in defect states. Our experimental results demonstrate a new application of TMD materials such as P t S e 2 in THz technology, for instance, the design and fabrication of THz modulators and THz switches.

The layered transition metal dichalcogenide has attracted tremendous attention for its unique structure and electrical and optoelectronic properties. As an emerging two-dimensional material, PdSe2 plays a key role in optoelectronic applications due to its distinct optical and tunable electrical properties. The carrier dynamic and low-frequency phonon modes and how they evolve with the number of layers are important for future device fabrication in photonics, optoelectronics, and nanomechanics. Here, by employing ultrafast optical pump–probe spectroscopy, we have investigated systematically the photocarrier dynamics as well as the thickness dependent interlayer coherent phonon modes in PdSe2 films. Two low-frequency phonon modes in PdSe2 films are identified after photoexcitation at 780 nm. The higher-frequency mode is ascribed to the interlayer breathing mode, and the lower one is assigned to the standing wave mode, and both of the mode frequencies decrease with increasing the number of layers of films. Analysis based on simple one-dimensional chain model produces interlayer force constant K = 5.74 × 1019 N/m3 for the interlayer breathing mode, and sound velocity of v = 8.27 × 104 cm/s for the standing wave mode in PdSe2 film. Our experimental finding paves the way for designing and developing PdSe2-based optoelectronic and nanomechanic devices.

Graphene oxide (GO) is an attractive option for large scale production of graphene. On the other hand, the graphene obtained by the reduction of GO has inevitable structural defects, and the vacant lattice sites will significantly restrict its conductivity. It has been demonstrated that thermal annealing in hydrogen is an efficient method to reduce defects and heal the lattice in GO samples. However, it is still not clear how the defects and/or disordering influence the photoelectric conversion efficiency and the carrier relaxation pathway in GO. Herein, time-domain terahertz (THz) spectroscopy is employed to characterize the properties of the multilayer GO films which were annealed in hydrogen at various temperatures. Upon photoexcitation, a transient increase of the conductivity was observed for the reduced graphene oxide (RGO) samples. The ultrafast carrier relaxation process can be well assigned to the carrier–carrier scattering and carrier–phonon coupling. Our results demonstrated that the RGO films ...

In this study, we report the ultrafast excitation and dynamics of the carrier and photocurrent in a 3D typical topological insulator Sb2Te3. We utilize time-resolved optical pump-THz probe spectroscopy to explore the nonequilibrium carrier dynamics of Sb2Te3. The electron system undergoes an ultrafast relaxation and releases through intra-band and inter-band scattering. Additionally, THz emission spectroscopy is employed to investigate the different ultrafast photocurrents in Sb2Te3 through tuning the polarization of excitation pulses and rotating the sample's azimuthal angle. We distinguish the different ultrafast photocurrents driven by the linear photogalvanic effect, circular photogalvanic effect, and thermoelectric effect. Our results potentially enable an all-optical modulation of THz emission without any external bias field, which could play an important role in the development of topological insulator-based high-speed THz optoelectronic and opto-spintronic devices.

The influence of doping on the ultrafast carrier dynamics in InSb has been studied by time-resolved terahertz spectroscopy with photogenerated carrier densities from 1.5 × 1 0 18 to 9.5 × 1 0 19 c m − 3 at 800 nm. The photoinduced absorption and carrier recovery process show doping type dependence. The carrier recovery time of intrinsic InSb is greater than that of p-doped InSb but less than that of n-doped InSb at low carrier densities. At high carrier densities, compared with intrinsic InSb, the doped InSb is more prone to transient Auger recombination, which indicates that the appearance of the fast decay process depends on the carrier densities. The photoinduced absorption of terahertz probe pulse of n-doped InSb is significantly less than that of p-doped and intrinsic InSb; however, that of p-doped InSb is close to that of intrinsic InSb, which demonstrates that the high concentration of electrons can accelerate the efficiency of transient Auger recombination. Our analysis provides assistance to the design, manufacture, and improvement of photovoltaic detectors.

We present experimental observations of the high-performance magneto-optic (MO) CeF3 crystal using terahertz time-domain spectroscopy (THz-TDS) in the temperature range 40–300 K for the first time. The complex refractive index strongly depends on temperature, which increases with increasing temperature. Intriguingly, the changes of complex refractive index values reach more than 0.1 between 40 K and 300 K, which is larger than terbium scandium aluminum garnet (TSAG) crystal. The weak loss tangent indicates that the crystal has a high potential as a high-dielectric-constant photovoltaic device in the terahertz domain.

We report the efficient broadband terahertz (THz) radiation in metallic ferromagnetic CoFeB/Pt heterostructures, which can be significantly modulated by metal-block-pattern through transient charge current confinement.

By using optical pump THz probe spectroscopy, Cd 3 As 2 film on mica was investigated systematically at room temperature. After photoexcitation, the film shows a photoinduced absorption initially, and bleaching signal appears around 40 ps. The THz photoconductivity shows a peak and dip around 1.2 TH for the absorption and bleaching signals, respectively, both of signals do not shift under different pump intensity.

In this paper, we report the birefringence effect of surface self-assembly periodic nanostructures induced on 6H-SiC by femtosecond laser irradiation. Birefringence characteristic (e.g. cross-polarized image), measured by cross polarized microscopy, was found to be controlled by both single pulse energy and scanning velocity. Comparing birefringence measurement results of nanostructures and morphology characterization by Scanning electron microscopy, it is shown that ∼200 nm-period deep-subwavelength periodic ripples (DSWR) plays a dominating role in the birefringence effect. Raman spectra show that the change of retardance with pulse energy and scanning velocity is most possibly caused by the thickness variation of DSWR. Finally, a light attenuator based on a single layer of DSWR structure on 6H-SiC surface was constructed and tested by light source of 800 nm to have a tunable attenuating ratio of 69–100%.

Multiphoton fluorescence imaging of tissues offers an essential tool for studying biological systems. However, autofluorescence becomes a serious issue in complicated tissue imaging where biological molecules compete with the fluorophore. Such a critical issue can be resolved through the design of phosphorescent molecules with long emission lifetime. In this work, the nonlinear optical properties of small phosphorescent polymer dots (Pdots) with strong red-emitting phosphorescence (620 nm) have been disclosed. An analysis of decay data from fs pump-probe experiment yields a value of 164 ps for the singlet excited state lifetime of the Pdots. Notably, thanks to efficient two-photon excited energy transfer and moderate phosphorescence quantum yield (η = 1.36%), the Pdots exhibit large two-photon absorption action cross-section (ηδ max = 15 GM) in aqueous solution. Results of living cell imaging experiments show the potential of these Pdots in two-photon time-resolved phosphorescent microscopy.

We have used transient terahertz (THz) photoconductivity measurements to demonstrate that upon optical excitation of CH3NH3PbI3 perovskite, the hole transfers from CH3NH3PbI3 into organic hole-transporting material (HTM) Spiro-OMeTAD.

By using optical pump-terahertz (THz) probe spectroscopy, we investigated the photo-excited THz conductivity dynamics of chemical vapor deposition grown SnS2 laminate, which is a semiconducting 2D material with earth abundant, nontoxic constituent elements.

Destabilizing the polar order unexpectedly boosts the ferroelectric photovoltaic performance in bismuth ferrite., Ferroelectric materials for photovoltaics have sparked great interest because of their switchable photoelectric responses and above-bandgap photovoltages that violate conventional photovoltaic theory. However, their relatively low photocurrent and power conversion efficiency limit their potential application in solar cells. To improve performance, conventional strategies focus mainly on narrowing the bandgap to better match the solar spectrum, leaving the fundamental connection between polar order and photovoltaic effect largely overlooked. We report large photovoltaic enhancement by A-site substitutions in a model ferroelectric photovoltaic material, BiFeO3. As revealed by optical measurements and supported by theoretical calculations, the enhancement is accompanied by the chemically driven rotational instability of the polarization, which, in turn, affects the charge transfer at the band edges and drives a direct-to-indirect bandgap transition, highlighting the strong coupling between polarization, lattice, and orbital order parameters in ferroelectrics. Polar order engineering thus provides an additional degree of freedom to further boost photovoltaic efficiency in ferroelectrics and related materials.

We have used time-domain THz spectroscopy to study the electronic transition in the ground multiplet of terbium–scandium–aluminum garnet (TSAG) crystal. In addition, by the time-resolved pump-probe measurement to operate an all-optical magneto-optical switching in TSAG crystal.

Based on coupled-wave equations, the diffracted induced femtosecond splitting and its physical origin in the volume grating are discussed. To get two pulses with different duration, we include the chirp in the incident femtosecond pulse.

Terahertz (THz) time-domain spectroscopy can be used to investigate the spintronic effects, such as low-energy magnons and magnetotransports, in the ultrafast operation regime, sub-picosecond time scale and/or terahertz frequency range.

In this paper, the effect of the irradiation pulse number on the morphology of the near-wavelength ripples perpendicular to the laser polarization was investigated. The results show that the ripple spacing first fast decreases with the increased irradiation pulse number and finally reaches a stable value. The change of ripple morphology with different incidence angles was also studied. The ripples are found keeping always perpendicular to the laser polarization and have respective ripple spacings of 650, 480 and 390 nm upon the laser irradiation with pulse number N = 16 and fluence of 1.33 J/cm2 at the incidence angles of 0°, 30° and 60°. The change of the ripple spacing with different excitation levels was also studied. The theory for interference of the incident wave with the surface plasmon polariton wave (SPP) and the Sipe–Drude theory are both employed to analyze the experimental results. Detailed calculations show that the Sipe–Drude theory works better for explaining the main features of the ripples on silicon wafer surface. We suggest that the interaction of the incident wave with some form of surface electromagnetic wave except SPP is most possibly responsible for the ripple formation on silicon surface.

Based on the coupled wave theory of Kogelnik and Fourier optics, the time-domain diffraction characteristics of tramsmitted multilayer volume holographic grating (MVHG) under an ultrashort pulse readout are investigated. It is shown that the temporal diffraction characteristics depend not only on the numbers of the grating layers, but also on the thicknesses of the grating layers and buffer layers, grating period and the refractive index modulation. Furthermore, using group velocity dispersion we give explanation on the time-delay of diffraction pulse with respect to the center of the readout pulse. Results of our discussion may find applications in optical communications, pulse shaping and processing.

Ultrafast carrier dynamics and terahertz conductivity of ZnSe nanocrystal are investigated by means of the optical-pump terahertz-probe spectroscopy at room temperature. With the laser pulse excitation at 400 nm, the negative transmission of terahertz pulse shows an ultrafast rising followed by a biexponential recovery. The fast decay component with time constant of 5 ps is dominated by the photocarriers backscattering at interfaces of ZnSe nanoparticles, and the slow one with time constant longer than 1 ns can be assigned to the carrier recombination from band-to-band transition. The evolution of conductivity with time demonstrates that this kind of nanostructure is a good candidate for fabricating ultrafast terahertz switching.

Recently, ferromagnetic (FM) heterostructures have been demonstrated to emit THz radiation induced by femtosecond laser pulses. In such spintronic THz emitters, an in-plane external magnetic field is technological required to assist the orientations of the magnetization, which could be an obstacle for practical micro-electrooptic applications. Here, as a proof-of-concept, THz generation is based on the inverse spin Hall effect of the FM/Ta within a magnetic tunneling junction (MTJ) without external magnetic field. Furthermore, THz emission can be engineered by modulating of the tunneling conductivity within a photoexcited MTJ as the relative magnetizations of two FM layers change their alignments.

The as-grown atomically thin transition metal dichalcogenides (TMDs) usually have extremely large surface-to-volume ratios. Hence, the electrical and optical properties are always affected by the adsorption of atmospheric gases. By using optical pump terahertz (THz) probe spectroscopy, THz photoconductivity dynamics of chemical vapor deposition grown monolayer MoS2 has been investigated in nitrogen, air and oxygen atmospheric environments. Our study reveals that the photoconductivity of MoS2 at THz frequencies is dramatically altered by the adsorption of oxygen. The relaxation dynamics of photoconductivity strongly depends on atmospheres, which is attributed to photo-excited different quasi-particles, such as free charge carriers, excitons and trions. This study highlights the role of oxygen passivation of the photoconductivity in monolayer MoS2, which may have important applications for the design of future MoS2-based electronic devices.

Terbium scandium aluminum garnet (TSAG) crystals have been widely used in magneto-optical systems. We investigate the complex refractive index of the TSAG crystal in the terahertz frequency range using terahertz (THz) time-domain spectroscopy in the temperature range 100–300 K. It is observed that the refractive index and the absorption coefficient increase with the THz frequency. The refractive index increases with the temperature. We measure the temperature coefficient of the refractive index of the TSAG crystal in the frequency range 0.4–1.4 THz. Furthermore, the loss tangent, i.e., the ratio of experimental values of the imaginary and real part of the dielectric permittivity, is found to be almost independent of frequency. TSAG is very promising for applications in THz optoelectronics because it has a high dielectric constant, low loss, and low thermal coefficient of the dielectric constant.

We investigate the morphology change of Au film on sapphire substrate by irradiating with a 1 kHz femtosecond pulse laser. Under observation of a scanning electron microscope, a textured nanostructure was formed in the exposed area on Au film due to laser ablation and subsequent stress relaxation. This process was strongly determined by the laser intensity profile and the dynamics of molten liquid. With the increasing of laser pulses number, the Au film was broken down and then a few polarization-dependent nanoripples arranged in the same direction appeared on the sapphire surface, which may result from a spatial modulation of energy due to the interference between the incident light and the excited surface plasmon polaritons. In addition, we used an energy dispersive spectrometer to analyze the chemical composition of nanoripples on the surface and in the ablated crater, respectively. The changes of O and Al elements implied that a complicated chemical transformation participated in the nanoripples formation process. We believe that present results are very useful for the analysis of the interaction between femtosecond laser and solids, especially the film material.

This paper proposes a new sequential similarity detection algorithm (SSDA), which can overcome matching error caused by grayscale distortion; meanwhile, time consumption is much less than that of regular algorithms based on image feature. The algorithm adopts Sobel operator to deal with subgraph and template image, and regards the region which has maximum relevance as final result. In order to solve time-consuming problem existing in original algorithm, a coarse-to-fine matching method is put forward. Besides, the location correlation keeps updating and remains the minimum value in the whole scanning process, which can significantly decrease time consumption. Experiments show that the algorithm proposed in this article can not only overcome gray distortion, but also ensure accuracy. Time consumption is at least one time orders of magnitude shorter than that of primal algorithm.

In this paper, we report a non-reciprocal writing process for inducing asymmetric microstructure using a femtosecond laser with tilted pulse fronts in fused silica. The shape of the induced microstructure at the focus closely depends on the laser scan direction. An elongated end is observed as a kind of structural difference between the written lines with two reverse scans along +x and –x, which further leads to a birefringence intensity difference. We also find a bifurcation in the head region of the induced microstructure between the written lines along x and y. That process results from the focal intensity distortion caused by the pulse front tilt by comparing the simulated intensity distribution with the experimental results. The current results demonstrate that the pulse front tilt not only affects the free electron excitation at the focus but also further distorts the shape of the induced microstructure during a high-energy femtosecond laser irradiation. These results offer a route to fabricate optical elements by changing the spatiotemporal characteristics of ultrashort pulses.

Based on the modified Kogelnik’s coupled-wave theory, expressions of diffraction field distribution of volume holographic grating (VHG) are derived when it is illuminated by an ultrashort pulse. It is found that the diffracted pulse is involved into two pulses, and the two-pulse interval is found to be proportional to the refractive index modulation of the VHG. Moreover, the emergence of double pulses is periodic. An overmodulation effect of refractive index modulation of transmitted VHG is used to explain this particular diffraction behavior of the VHG. Results of our study provide a feasible method to generate interval adjustable double pulse with a simple system.

Optical pump-terahertz probe spectroscopy is employed to investigate the optical characteristics of silicon wafer. The wafer surface undergoes a phase transition from insulator to metal for terahertz wave with increasing pump fluence. The real part of the pump-induced conductivity shows strong frequency dependence, which can be well described with Drude–Smith model. Our results also demonstrate that the photoexcited Si layer acts as a broadband terahertz pulse antireflection coating with proper pump fluence. In addition, it is observed that the terahertz pulse apparently arrives at the detector earlier when silicon is optically excited.

A method of transmission and monitoring in local area network (LAN) for the welding seam video information of binocular vision welding robot is proposed. This paper uses MPEG-4 video CODEC functions of DivX, video for Windows (VFW) video capture technology, multi-threaded network communication idea and WinSock Socket in Visual C++ 6.0. In the end, client/server (C/S) communication model based on TCP/IP protocol has been developed. Experimental results show that the method can achieve the multiple welding seam video’s real-time transmission in LAN. Moreover, the Canny operator is used to realize the edge feature extraction of the seam image which is captured by the client, so that the edge of the seam extracted is continuous and clear.

In this paper, a modified spectral holography structure is demonstrated. Combining the direct space-to-time pulse shaping theory with the modified structure, we can convert a spatial domain x–y image into a y–t image, where one spatial dimension is now transformed into the time domain. Thus we realize the space-to-time or parallel-to-serial conversion. As an example, we generate the temporal equivalent of letter “A”, where each pixel of the image is now represented by a short optical pulse. As a possible application of our scheme, we demonstrate the generation of trains of a femtosecond pulse sequence by our structure. The results of the paper can be applied in ultrashort pulse shaping, ultrafast communication and other relevant areas.