On the generation of high-quality Nyquist pulses in mode-locked fiber lasers

Nyquist pulses have wide applications in many areas,from electronics to optics.Mode-locked lasers are ideal platforms to generate such pulses.However,how to generate high-quality Nyquist pulses in mode-locked lasers remains elusive.We address this problem by managing different physical effects in mode-locked fiber lasers through extensive numerical simulations.We find that net dispersion,linear loss,gain and filter shaping can affect the quality of Nyquist pulses significantly.We also demonstrate that Nyquist pulses experience similariton shaping due to the nonlinear attractor effect in the gain medium.Our work may contribute to the design of Nyquist pulse sources and enrich the understanding of pulse shaping dynamics in mode-locked lasers.

Electromagnetic pulses produced by a picosecond laser interacting with solid targets

A high-power laser ablating solid targets induces giant electromagnetic pulses(EMPs),which are intimately pertinent to laser parameters,such as energy and pulse width.In this study,we reveal the features of EMPs generated from a picosecond(ps)laser irradiating solid targets at the SG-Ⅱpicosecond petawatt(PSPW)laser facility.The laser energy and pulse,as well as target material and thickness,show determinative effects on the EMPs’amplitude.More intense EMPs are detected behind targets compared to those at the other three positions,and the EMP amplitude decreases from 90.09 kV/m to 17.8 kV/m with the gold target thickness increasing from 10μm to 20μm,which is suppressed when the laser pulse width is enlarged.The results are expected to provide more insight into EMPs produced by ps lasers coupling with targets and lay the foundation for an effective EMP shielding design in high-power laser infrastructures.

Generation and regulation of electromagnetic pulses generated by femtosecond lasers interacting with multitargets

Ultrashort and powerful laser interactions with a target generate intense wideband electromagnetic pulses(EMPs).In this study,we report EMPs generated by the interactions between petawatt(30 fs,1.4×10^(20) W/cm^(2))femtosecond(fs)lasers with metal flat,plastic flat,and plastic nanowire-array(NWA)targets.Detailed analyses are conducted on the EMPs in terms of their spatial distribution,time and frequency domains,radiation energy,and protection.The results indicate that EMPs from metal targets exhibit larger amplitudes at varying angles than those generated by other types of targets and are enhanced significantly for NWA targets.Using a plastic target holder and increasing the laser focal spot can significantly decrease the radiation energy of the EMPs.Moreover,the composite shielding materials indicate an effective shielding effect against EMPs.The simulation results show that the NWA targets exert a collimating effect on thermal electrons,which directly affects the distribution of EMPs.This study provides guidance for regulating EMPs by controlling the laser focal spot,target parameters,and target rod material and is beneficial for electromagnetic-shielding design.

Collision off-axis position dependence of relativistic nonlinear Thomson inverse scattering of an excited electron in a tightly focused circular polarized laser pulse

This paper presents a novel view of the impact of electron collision off-axis positions on the dynamic properties and relativistic nonlinear Thomson inverse scattering of excited electrons within tightly focused, circularly polarized laser pulses of varying intensities. We examine the effects of the transverse ponderomotive force, specifically how the deviation angle and speed of electron motion are affected by the initial off-axis position of the electron and the peak amplitude of the laser pulse. When the laser pulse intensity is low, an increase in the electron's initial off-axis distance results in reduced spatial radiation power, improved collimation, super-continuum phenomena generation, red-shifting of the spectrum's harmonic peak, and significant symmetry in the radiation radial direction. However, in contradiction to conventional understandings,when the laser pulse intensity is relatively high, the properties of the relativistic nonlinear Thomson inverse scattering of the electron deviate from the central axis, changing direction in opposition to the aforementioned effects. After reaching a peak, these properties then shift again, aligning with the previous direction. The complex interplay of these effects suggests a greater nuance and intricacy in the relationship between laser pulse intensity, electron position, and scattering properties than previously thought.

Pulsed laser interference patterning of transition-metal carbides for stable alkaline water electrolysis kinetics

We investigated the role of metal atomization and solvent decomposition into reductive species and carbon clusters in the phase formation of transition-metal carbides(TMCs;namely,Co_(3)C,Fe_(3)C,TiC,and MoC)by pulsed laser ablation of Co,Fe,Ti,and Mo metals in acetone.The interaction between carbon s-p-orbitals and metal d-orbitals causes a redistribution of valence structure through charge transfer,leading to the formation of surface defects as observed by X-ray photoelectron spectroscopy.These defects influence the evolved TMCs,making them effective for hydrogen and oxygen evolution reactions(HER and OER)in an alkaline medium.Co_(3)C with more oxygen affinity promoted CoO(OH)intermediates,and the electrochemical surface oxidation to Co_(3)O_(4)was captured via in situ/operando electrochemical Raman probes,increasing the number of active sites for OER activity.MoC with more d-vacancies exhibits strong hydrogen binding,promoting HER kinetics,whereas Fe_(3)C and TiC with more defect states to trap charge carriers may hinder both OER and HER activities.The results show that the assembled membrane-less electrolyzer with Co_(3)C∥Co_(3)C and MoC∥MoC electrodes requires~2.01 and 1.99 V,respectively,to deliver a 10 mA cm−2 with excellent electrochemical and structural stability.In addition,the ascertained pulsed laser synthesis mechanism and unit-cell packing relations will open up sustainable pathways for obtaining highly stable electrocatalysts for electrolyzers.

Electrokinetic-mechanism of water and furfural oxidation on pulsed laser-interlaced Cu_(2)O and CoO on nickel foam

The electrocatalytic oxidation of biomass-derived furfural(FF)feedstocks into 2-furoic acid(FA)holds immense industrial potential in optics,cosmetics,polymers,and food.Herein,we fabricated Co O/Ni O/nickel foam(NF)and Cu_(2)O/Ni O/NF electrodes via in situ pulsed laser irradiation in liquids(PLIL)for the bifunctional electrocatalysis of oxygen evolution reaction(OER)and furfural oxidation reaction(FOR),respectively.Simultaneous oxidation of NF surface to NiO and deposition of CoO and/or Cu_(2)O on NF during PLIL offer distinct advantages for enhancing both the OER and FOR.CoO/NiO/NF electrocatalyst provides a consistently low overpotential of~359 m V(OER)at 10 m A/cm^(2),achieving the maximum FA yield(~16.37 m M)with 61.5%selectivity,79.5%carbon balance,and a remarkable Faradaic efficiency of~90.1%during 2 h of FOR at 1.43 V(vs.reversible hydrogen electrode).Mechanistic pathway via in situ electrochemical-Raman spectroscopy on CoO/NiO/NF reveals the involvement of phase transition intermediates(NiOOH and CoOOH)as surface-active centers during electrochemical oxidation.The carbonyl carbon in FF is attacked by hydroxyl groups to form unstable hydrates that subsequently undergo further oxidation to yield FA products.This method holds promise for large-scale applications,enabling simultaneous production of renewable building materials and fuel.

Laser shaping and optical power limiting of pulsed Laguerre–Gaussian laser beams of high-order radial modes in fullerene C60

We study the strong nonlinear optical dynamics of nanosecond pulsed Laguerre–Gaussian laser beams of high-order radial modes with zero orbital angular momentum propagating in the fullerene C60molecular medium. It is found that the spatiotemporal profile of the incident pulsed Laguerre–Gaussian laser beam is strongly reshaped during its propagation in the C60molecular medium. The centrosymmetric temporal profile of the incident pulse gradually evolves into a noncentrosymmetric meniscus shape, and the on-axis pulse duration is clearly depressed. Furthermore, the field intensity is distinctly attenuated due to the field-intensity-dependent reverse saturable absorption, and clear optical power limiting behavior is observed for different orders of the input pulsed Laguerre–Gaussian laser beams before the takeover of the saturation effect;the lower the order of the Laguerre–Gaussian beam, the lower the energy transmittance.

Diagnosis of ultrafast ultraintense laser pulse characteristics by machine-learning-assisted electron spin

The rapid development of ultrafast ultraintense laser technology continues to create opportunities for studying strong-field physics under extreme conditions.However,accurate determination of the spatial and temporal characteristics of a laser pulse is still a great challenge,especially when laser powers higher than hundreds of terawatts are involved.In this paper,by utilizing the radiative spin-flip effect,we find that the spin depolarization of an electron beam can be employed to diagnose characteristics of ultrafast ultraintense lasers with peak intensities around 10^(20)–10^(22) W/cm^(2).With three shots,our machine-learning-assisted model can predict,simultaneously,the pulse duration,peak intensity,and focal radius of a focused Gaussian ultrafast ultraintense laser(in principle,the profile can be arbitrary)with relative errors of 0.1%–10%.The underlying physics and an alternative diagnosis method(without the assistance of machine learning)are revealed by the asymptotic approximation of the final spin degree of polarization.Our proposed scheme exhibits robustness and detection accuracy with respect to fluctuations in the electron beam parameters.Accurate measurements of ultrafast ultraintense laser parameters will lead to much higher precision in,for example,laser nuclear physics investigations and laboratory astrophysics studies.Robust machine learning techniques may also find applications in more general strong-field physics scenarios.

High harmonic generation in crystal SiO_(2) by sub-10-fs laser pulses

The high harmonic generation(HHG)by few-cycle laser pulses is essential for research in strong-field solid-state physics.Through comparison of high harmonic spectra of solids generated by laser pulses with varying durations,we discovered that lasers with good dispersion compensation are capable of producing a broad spectrum of high harmonics.As the pulse duration is further compressed,several interference peaks appear in the broad spectrum.Moreover,we conducted simulations using the semiconductor Bloch equation,considering the effect of Berry curvature,to better understand this process.Our work provides a valuable approach for studying HHG by few-cycle laser pulses in solid materials,expanding the application of HHG in attosecond physics.

Exploration of growth conditions of TaAs Weyl semimetal thin film using pulsed laser deposition

Ta As,the first experimentally discovered Weyl semimetal material,has attracted a lot of attention due to its high carrier mobility,high anisotropy,nonmagnetic properties and strong interaction with light.These make it an ideal candidate for the study of Weyl fermions and applications in quantum computation,thermoelectric devices,and photodetection.For further basic physics studies and potential applications,large-size and high-quality Ta As films are urgently needed.However,it is difficult to grow As-stoichiometry Ta As films due to the volatilization of As during the growth.To solve this problem,we attempted to grow Ta As films on different substrates using targets with different As stoichiometric ratios via pulsed laser deposition(PLD).In this work,we found that partial As ions of the Ga As substrate are likely to diffuse into the Ta As films during growth,which was preliminarily confirmed by structural characterization,surface topography and composition analysis.As a result,the As content in the Ta As film was improved and the Ta As phase was achieved.Our work presents an effective method for the fabrication of Ta As films using PLD,enabling possible use of the Weyl semimetal film for functional devices.

Optimization of large-area YBa_(2)Cu_(3)O_(7-δ)thin films by pulsed laser deposition for planar microwave devices

This paper presents high quality YBa_(2)Cu_(3)O_(7-δ)(YBCO)thin films on LaAlO_(3)substrate for microwave devices prepared by pulsed laser deposition(PLD).The double-sided YBCO films cover a large area and have been optimized for key parameters relevant to microwave device applications,such as surface morphology and surface resistance(R_(s)).This was achieved by improving the target quality and increasing the oxygen pressure during deposition,respectively.To evaluate the suitability of the YBCO films for microwave devices,a pair of microwave filters based on microstrip fabricated on films from this work and a commercial company were compared.The results show that the YBCO films in this work could completely meet the requirements for microwave devices.

Sensitivity study of the SiGe heterojunction bipolar transistor single event effect based on pulsed laser and technology computer-aided design simulation

The single event effect of a silicon–germanium heterojunction bipolar transistor(SiGe HBT) was thoroughly investigated. By considering the worst bias condition, the sensitive area of the proposed device was scanned with a pulsed laser.With variation of the collector bias and pulsed laser incident energy, the single event transient of the SiGe HBT was studied.Moreover, the single event transient produced by laser irradiation at a wavelength of 532 nm was more pronounced than at a wavelength of 1064 nm. Finally, the impact of the equivalent linear energy transfer of the 1064 nm pulsed laser on the single event transient was qualitatively examined by performing technology computer-aided design simulations, and a good consistency between the experimental data and the simulated outcomes was attained.

Structural Analysis of TiC and TiC-C Core-Shell Nanostructures Produced by Pulsed-Laser Ablation

This paper reports on the ablation process of a pure Ti solid target immersed in a C-enriched acetone solution, leading to the production of titanium carbide (TiC) and Ti-C core-shell nanostructures. The used route of synthesis is generally called pulsed laser ablation in liquid (PLAL). The presence of carbon structures in the solution contributed to the carbon content in the produced Ti-based nanomaterials. The atomic composition of the produced nanostructures was analyzed using SEM-EDS, while TEM micrographs revealed the formation of spherical TiC and core-shell nanostructures ranging from 40 to 100 nm. The identification of atomic planes by HRTEM confirmed a 10 nm diameter C-shell with a graphite structure surrounding the Ti-core. Raman spectroscopy allowed for the identification of D and G peaks for graphite and a Raman signal at 380 and 600 cm−1, assigned to TiC. The results contribute to the state-of-the-art production of TiC and Ti-C core-shell nanostructures using the PLAL route.

Pulsed Laser Annealed Ga Hyperdoped Poly-Si/SiO_(x)Passivating Contacts for High-Efficiency Monocrystalline Si Solar Cells

Polycrystalline Si(poly-Si)-based passivating contacts are promising candidates for high-efficiency crystalline Si solar cells.We show that nanosecond-scale pulsed laser melting(PLM)is an industrially viable technique to fabricate such contacts with precisely controlled dopant concentration profiles that exceed the solid solubility limit.We demonstrate that conventionally doped,hole-selective poly-Si/SiO_(x)contacts that provide poor surface passivation of c-Si can be replaced with Ga-or B-doped contacts based on non-equilibrium doping.We overcome the solid solubility limit for both dopants in poly-Si by rapid cooling and recrystallization over a timescale of∼25 ns.We show an active Ga dopant concentration of∼3×10^(20)cm^(−3)in poly-Si which is six times higher than its solubility limit in c-Si,and a B dopant concentration as high as∼10^(21) cm^(−3).We measure an implied open-circuit voltage of 735 mV for Ga-doped poly-Si/SiO_(x)contacts on Czochralski Si with a low contact resistivity of 35.5±2.4 mΩcm^(2).Scanning spreading resistance microscopy and Kelvin probe force microscopy show large diffusion and drift current in the p-n junction that contributes to the low contact resistivity.Our results suggest that PLM can be extended for hyperdoping of other semiconductors with low solubility atoms to enable high-efficiency devices.

Heavy-ion and pulsed-laser single event effects in 130-nm CMOS-based thin/thick gate oxide anti-fuse PROMs

Single event effects of 1-T structure programmable read-only memory(PROM) devices fabricated with a 130-nm complementary metal oxide semiconductorbased thin/thick gate oxide anti-fuse process were investigated using heavy ions and a picosecond pulsed laser. The cross sections of a single event upset(SEU) for radiationhardened PROMs were measured using a linear energy transfer(LET) ranging from 9.2 to 95.6 MeV cm^2mg^(-1).The result indicated that the LET threshold for a dynamic bit upset was ～ 9 MeV cm^2mg^(-1), which was lower than the threshold of ～ 20 MeV cm^2mg^(-1) for an address counter upset owing to the additional triple modular redundancy structure present in the latch. In addition, a slight hard error was observed in the anti-fuse structure when employing209 Bi ions with extremely high LET values(～ 91.6 MeV cm^2mg^(-1)) and large ion fluence(～ 1×10~8 ions cm^(-2)). To identify the detailed sensitive position of a SEU in PROMs, a pulsed laser with a 5-μm beam spot was used to scan the entire surface of the device.This revealed that the upset occurred in the peripheral circuits of the internal power source and I/O pairs rather than in the internal latches and buffers. This was subsequently confirmed by a ^(181)Ta experiment. Based on the experimental data and a rectangular parallelepiped model of the sensitive volume, the space error rates for the used PROMs were calculated using the CRèME-96 prediction tool. The results showed that this type of PROM was suitable for specific space applications, even in the geosynchronous orbit.

Numerical Analysis of Nd:YAG Pulsed Laser Polishing CVD Self-standing Diamond Film

Chemical vapor deposited （CVD） diamond film has broad application foreground in high-tech fields. But polycrystalline CVD self-standing diamond thick film has rough surface and non-uniform thickness that adversely affect its extensive applications. Laser polishing is a useful method to smooth self-standing diamond film. At present, attentions have been focused on experimental research on laser polishing, but the revealing of theoretical model and the forecast of polishing process are vacant. The paper presents a finite element model to simulate and analyze the mechanism of laser polishing diamond based on laser thermal conduction theory. The experimental investigation is also carried out on Nd：YAG pulsed laser smoothing diamond thick film. The simulation results have good accordance with the results of experimental results. The temperature and thermal stress fields are investigated at different incidence angles and parameters of Nd：YAG pulsed laser. The pyramidal-like roughness of diamond thick film leads to the non-homogeneous temperature fields. The temperature at the peak of diamond film is much higher than that in the valley, which leads to the smoothing of diamond thick film. The effect of laser parameters on the surface roughness and thickness of graphite transition layer is also carried out. The results show that high power density laser makes the diamond surface rapid heating, evaporation and sublimation after its graphitization. It is also found that the good polish quality of diamond thick film can be obtained by a combination of large incident angle, moderate laser pulsed energy, large repetition rate and moderate laser pulse width. The results obtained here provide the theoretical basis for laser polishing diamond film with high efficiency and high quality.

Low-pressure pulse flushing choledochoscopy combined with neodymium laser lithotripsy for the treatment of intrahepatic bile duct stones

Cholelithiasis is a common and frequently occurring disease in East Asia.Intrahepatic bile duct stone is a benign disease,but the recurrence rate is high.The incidence of postoperative residual stones after traditional manual stone removal can be30%–96%^([1]).The emergence of endoscopes is therefore a milestone in the treatment of hepatolithiasis.Among the various approaches,choledochoscopy combined with laser lithotripsy has become the main procedure for intrahepatic bile duct stone removal^([2]).Lithotripsy alone has the disadvantages of low stoneremoving efficiency and blurred choledochoscope vision while performing the procedure.Currently,our team has achieved satisfactory results in the removal of intrahepatic bile duct stones with low-pressure pulse flushing and neodymium laser lithotripsy.

Analysis of Enhanced Laser-Induced Breakdown Spectroscopy with Double Femtosecond Laser Pulses

In this paper, an experimental study of collinear geometry double-pulse femtosecond LIBS was performed on a Ni sample in ambient air in an effort to clarify the contributing processes responsible for the signal enhancement observed in comparison with the single-pulse case. Doublepulse LIBS spectra show a very clear enhancement when an optimum inter-pulse delay was used. The influences of the inter-pulse delay between two pulses on the LIBS signal intensity, electron temperature and density were investigated. It is most remarkable that the evolutions of signal enhancement and electron temperature versus the inter-pulse delay showed the same behavior and revealed two main regimes of interaction. These results provide additional insight into the possible emission enhancement mechanisms in the double pulse configuration.

Extension of high-order harmonics and generation of an isolated attosecond pulse in the chirped laser field

This paper theoretically investigates the high-order harmonic generation cutoff extension using intense few-cycle linearly chirped laser pulses. It shows that the cutoff of the harmonic can be extended remarkably by optimising the chirping parameters. The time-frequency characteristics of high-order harmonics with different chirping parameters are analysed by means of wavelet transform of the dipole acceleration. It also gives out the classical three-step model pictures of electron. By superposing a properly selected range of the harmonic spectrum, it obtains an isolated 65as pulse.

Interaction of a parabolic-shaped pulse pair in a passively mode-locked Yb-doped fiber laser

We numerically investigate the formation and interaction of a parabolic-shaped pulse pair in a passively mode-locked Yb-doped fiber laser. Based on a lumped model, the parabolic-shaped pulse pair is obtained by controlling the intercavity average dispersion and gain saturation energy, Moreover, pulse repulsive and attractive motion are also achieved with different pulse separations. Simulation results show that the phase shift plays an important role in pulse interaction, and the interaction is determined by the inter-cavity average dispersion and gain saturation energy, i.e., the strength of the interaction is proportional to the gain saturation energy, a stronger gain saturation energy will result in a higher interaction intensity. On the contrary, the increase of the inter-cavity dispersion will counterbalance some interaction force. The results also show that the interaction of a parabolic-shaped pulse pair has a larger interaction distance compared to conventional solitons.