Increasing Pulse Duration Research Articles

Tunable Doppler shift using a time-varying epsilon-near-zero thin film near 1550 nm.

We experimentally investigate the tunable Doppler shift in an 80 nm thick indium-tin-oxide (ITO) film at its epsilon-near-zero (ENZ) region. Under strong and pulsed excitation, ITO exhibits a time-varying change in the refractive index. A maximum frequency redshift of 1.8 THz is observed in the reflected light when the pump light has a peak intensity of ∼140GW/cm2 and a pulse duration of ∼580fs, at an incident angle of 40°. The frequency shift increases with the increase in pump intensity and saturates at the intensity of ∼140GW/cm2. When the pump pulse duration increases from ∼580fs to ∼1380fs, the maximum attainable frequency shift decreases from 1.8 THz to 0.7 THz. In addition, the pump energy required to saturate the frequency shift decreases with the increase in pump pulse duration for ∼x<1ps and remains unchanged for ∼x>1ps durations. Tunability exists among the pump pulse energy, duration, and incident angle for the Doppler shift of the ITO-ENZ material, which can be employed to design efficient frequency shifters for telecom applications.

Increase in the CuBr Laser Pulse Duration in the Pulse Train Mode

Increase in the CuBr Laser Pulse Duration in the Pulse Train Mode

Controlling quantum numbers and light emission of Rydberg states via the laser pulse duration

High Harmonic Generation (HHG) creates coherent high frequency radiation via the process of strong field ionization followed by recombination. Recently, a complementary approach based on Frustrated Tunnel Ionization (FTI) was demonstrated (Nature Photonics 12, 620 (2018)). It uses spectrally separated peaks created by lower quantum number Rydberg states to produce coherent extreme ultraviolet (EUV) light. While much is understood about enhancing emission from HHG by controlling recombining electron trajectories, relatively little is known about controlling the quantum number distribution of Rydberg states. This distribution is generally believed to be determined primarily by field strength and laser frequency. We show that, in fact, it also changes significantly with the duration of the laser pulse: increasing pulse duration depletes lower-lying Rydberg states, thereby substantially decreasing EUV yield. Using electron trajectory analysis, we identify elastic recollision as the underlying cause. Our results open a door to greater control over production of coherent high frequency radiation, by combining FTI and HHG mechanisms, and also improved the interpretation of molecular imaging experiments that rely on elastic electron recollision.

Integrated control of molten pool morphology and solidification texture by adjusting pulse duration in laser additive manufacturing of Inconel 718

Both single-track and bulk Inconel 718 samples were fabricated using pulsed laser additive manufacturing to investigate the effect of pulse duration on the molten pool morphology and the solidification texture. For single-track depositions, clad width, remelting depth, dilution ratio and peak molten pool temperature decrease, while clad height, clad area, minimum temperature and molten pool lifetime increase with the increase of the pulse duration. The fusion line of the molten pool in transverse direction is pan-like at a short pulse duration (20 ms), and gradually turns circular (60 ms) as the pulse duration increases. For bulk samples, uniform grains with a strong fiber texture are established for a short pulse duration. In contrast, alternatively distributed coarse grains in the overlapping region and fine grains at the center of the molten pool with a weak cube texture are obtained for a long pulse duration. In addition, samples fabricated with short pulse durations contain less volume fraction of Laves phase than that with long pulse durations. Pulsed laser additive manufacturing has the potential for texture control and in-situ production of components with functional graded textures.

Design guidelines for ultrashort pulse generation by a Mamyshev regenerator.

We study numerically the possibility of using various gain-switched seed laser pulse parameters and fibers for a low-cost, all-fiber Mamyshev regenerator scheme. We find that for increasing pulse durations, sufficient spectral broadening will be difficult to achieve in practice and careful design of the system parameters is required for the regenerator to function. Furthermore, an optimal input peak power level can be defined for a given fiber and pulse duration that results from a balance of competing Kerr effect and stimulated Raman scattering. We also demonstrate experimental results of 3 ps pulse generation seeded by an 80 ps gain-switched diode. Our results pave the way for designing pulse-on-demand picosecond scale fiber sources for applications.

Spectral shape of quasi-monochromatic radiation from electron colliding with tightly focused laser pulses

We theoretically investigate spectra of emitted light in nonlinear Thomson backscattering, which is generated during the head-on collision of the electron and the tightly focused circularly polarized laser pulse. With numerical analysis, a novel approach of generating quasi-monochromatic x-rays is obtained. By properly setting the electron–laser interaction point, limiting the laser intensity, and increasing pulse duration, the emission spectral bandwidth would be effectively narrowed without reducing the radiant energy of the electrons. In the end, the optimal situations for obtaining the quasi-monochromatic x-ray source are presented.

Ultrashort single-pulse laser ablation of stainless steel, aluminium, copper and its dependence on the pulse duration

In this work, we investigate single-pulse laser ablation of bulk stainless steel (AISI304), aluminium (Al) and copper (Cu) and its dependence on the pulse duration. We measured the reflectivity, ablation thresholds and volumes under the variation of pulse duration and fluence. The known drop of efficiency with increasing pulse duration is confirmed for single-pulse ablation in all three metals. We attribute the efficiency drop to a weakened photomechanically driven ablation process and a stronger contribution of photothermal phase explosion. The highest energetic efficiency and precision is achieved for pulse durations below the mechanical expansion time of 3-5 ps, where the stress confinement condition is fulfilled.

Effect of Laser Pulse Duration on Tensile and Electrochemical Behavior of Laser-Welded Dual-Phase Steel

Dual-phase steels having excellent combination of strength and ductility are extensively used in automotive industry for the manufacture of car body structure involving welding and joining significantly. The present study assesses the tensile and electrochemical behavior of butt welded dual-phase 780 steel grade produced using pulsed Nd:YAG laser with varying pulse duration in the range of 1.5 to 3.6 ms. Numerical modeling has been done to know the temperature profile during welding. Systematic analysis of welded joint bead profile, microstructure, tensile test results, and fractographic features revealed the effect of pulse duration on the fusion zone, heat-affected zone softening, and also allowed the estimation of critical size of the soft zone near to the failure location under tensile loading. With an increase in pulse duration, welded joint concavity reduced from 35 to 6% which improved the mechanical performance of the welded joints. Welded joints with higher pulse duration enhanced the resistance against galvanic and pitting corrosion due to the formation of the oxide layer, finer martensitic microstructure in the fusion zone and a smaller amount of weld defects.

Dissociation dynamics of multiply charged CH3I in moderately intense laser fields

We report on the fragmentation of multiply charged ${\mathrm{CH}}_{3}\mathrm{I}$ ions through dissociative ionization and Coulomb explosion induced by moderately intense (${10}^{12}--{10}^{13} \mathrm{W}/{\mathrm{cm}}^{2}$) ultrashort laser fields. Velocity map imaging of the fragment ions as a function of pulse duration, ranging from 25 fs to 1.5 ps, leads to kinetic energies, angular distributions, and the relative ion yield in different channels of these fragments. We propose possible pathways for the fragmentation channels based on the kinetic energies and theoretical potential energy curves. For the energetic fragments, we observe an enhanced yield with increasing pulse duration. A simple one-dimensional classical model of wave-packet propagation over the proposed intermediate state potential energy curve is used to estimate the ionization probability as a function of pulse duration. Our results suggest that a delayed enhanced ionization is a consequence of rearrangement of the energies of the molecular orbitals following bond stretching. The resultant energy upshift of the inner orbitals at larger internuclear separation gives rise to resonant multiorbital coupling at a critical distance, which enables enhanced ionization for longer pulses.

Does Double-Pass Pulsed-Dye Laser With Long and Short Pulse Duration Increase Treatment Efficacy of Port-Wine Stain? A Randomized Clinical Trial.

Although pulsed-dye laser (PDL) 595 nm is known as the gold standard for treatment of port-wine stains (PWS), complete clearance of lesions occurs in a minority of cases. To compare the efficacy and safety of double-pass pulsed-dye laser (DPL), long pulse duration (20 m/s) followed by short pulse duration (1.5 m/s) within 20 minutes interval, with single-pass pulsed-dye laser (SPL) for (1.5 m/s) in the treatment of PWS. Twenty-four patients with PWS underwent 3 sessions of PDL. Each lesion was randomly divided into 2 portions to receive DPL or SPL. Colorimetric and dermoscopic evaluations were used to determine the response objectively. In addition, improvement was scored subjectively using the visual analog scale (VAS). According to colorimetric analysis, the mean blanching rates for DPL and SPL treated sites were 48% (SD = 0.215) and 37% (SD = 0.213), respectively (p = .001). With VAS, 3.79 (SD = 0.93) and 3.33 (SD = 0.91) improvement scores were reported in the DPL and SPL treated areas, respectively (p = .008). Dermoscopic images showed that larger deep vessels were the most common remnant vessels in both treatment areas. Compared with SPL, DPL with 20 minutes interval seems to be a more effective and safe method for the treatment of PWS.

Study on weld formation and segregation mechanism for dissimilar pulse laser welding of NiTi and Cu wires

In this study, laser microwelding was adopted to join NiTi and Cu wires using different laser pulse duration. The cross section of welded samples showed no visible defect, but some intermetallic compounds were formed inside the fusion zone. An excellent metallurgic bonding was achieved during laser welding for NiTi and Cu wires with increasing pulse duration. Local element segregation like peninsulas and beaches was observed in the fusion zone for the pulse duration of 5 and 8 ms, but the segregation was absent with the pulse duration increased to 15 ms. The liquidus temperatures of weld metals in molten pool had a significant effect on the segregation formation. If TLW < TLNiTi/TLCu, a NiTi/Cu peninsula can form near the fusion boundary of NiTi/Cu side, while a NiTi/Cu beach can form if TLW > TLNiTi/TLCu. With increasing pulse duration, the fraction of Ni-Ti brittle intermetallic compounds in the fusion zone decreased, but Cu-Ti intermetallic compounds with lower hardness increased. Little hardness variation was achieved and peak hardness values decreased (~630 → ~560 → ~490 HV). The joint with the pulse duration of 5 ms displayed poor mechanical properties due to uncompleted penetration. Fracture occurred in Cu base metal for the pulse duration of 8 ms and 15 ms. This study showed that it was potential to diminish the brittle intermetallic compounds and improve the mechanical properties of fusion zone by adjusting the laser pulse duration for the dissimilar laser welding of NiTi and Cu wires.

Influence of laser pulse on solidification of molten pool during laser welding of dissimilar Ti-based amorphous alloys

Dissimilar Ti-based amorphous alloy sheets were butt welded by a pulsed laser and metallurgical joints were obtained. The β-Ti crystal grains in nanometre scale unevenly distribute on the amorphous background in the fusion zone. With the increase of laser pulse energy and pulse duration, the crystalline β-Ti grains grow up from spherical to dendritical profile, but the crystalline size reaches its maximum of about 1 µm, which is much smaller than the grains in base metal. The refinement of the grains in fusion zone results from the fast cooling speed during welding. The microhardness in the welded joints ranges from 470 to 525 HV. Average tensile strength of the welded joint is about 1510 MPa (86% of pure Ti-based amorphous alloy BM and 91% of β-Ti dendrite reinforced Ti-based amorphous alloy BM).

Increase in the CuBr-laser pulse duration in the pulse train mode

Increase in the CuBr-laser pulse duration in the pulse train mode

Experimental Analysis of Breakdown With Nanosecond Pulses for Spark-Ignition Engines

The influence of pulse rise rate and pulse duration for ignition purposes in engines is investigated. A constant volume cell is used to characterize the breakdown voltage under nanosecond pulsed voltages with automotive sparkplugs having electrode gaps ranging from 0.2 mm to 1 mm. Two pulse generators are used to compare pulses with durations of 10 ns and 50 ns. Different pulse amplitudes are used, and air gaps with breakdown voltages ranging from 4 kV to 15 kV are investigated. The cell is filled with synthetic air with densities gap distances products that are relevant for internal combustion engines. This study shows that the pulse shape and rise rate influence the breakdown voltage. Under pulsed discharge, the breakdown voltage is always above the static breakdown voltage. The probability of pulsed discharge breakdown increases as both the pulse amplitude and duration increases. Furthermore, the breakdown voltage value increases with increasing pulse rise rate. The delay time between reaching the static breakdown voltage and the actual breakdown voltage decreases with increasing overvoltage. The delay time is constituted by statistical and formative times. Both the statistical and formative times decrease with increasing overvoltage. For ignition purposes, the pulse rise rate should be as high as possible to deliver a larger energy input in the breakdown phase. Furthermore, for reduced electrode erosion, the pulse duration should be short (10-20 ns) to reduce the probability for a transition to an arc.

Nonlinear Thomson scattering from a tightly focused circularly polarized laser with varied incident-pulse durations

In this paper, nonlinear Thomson scattering in an intense () tightly focused () Gaussian laser pulse is theoretically and numerically investigated to demonstrate varying spatial characteristics in the few-cycle and multi-cycle domains. We find that within ultrashort incident laser pulses, electrons have the potential to radiate a single attosecond pulse with a large signal-to-noise ratio, which turns into a train of attosecond pulses with a multi-cycle laser. Furthermore, a novel asymmetry phenomenon is discovered for the first time in the few-cycle domain whereby spatial radiation lasts for only one circle with apparent peaks and valleys, in contrast to the helical radiation pattern apparent in the multi-cycle domain, which gradually turns into a common fourfold-symmetry pattern. This means that the difference between tightly and non-tightly focused laser pulses can be bridged utilizing an appropriate incident pulse duration under specific conditions. The above difference is demonstrated again in the polar plane, where electrons change from a light-spot to a light-band radiation feature when the incident pulse duration increases.

Numerical Study on the Evolution Mechanism of the Crater under a Millisecond Laser

A two-dimensional numerical model considering recoil pressure and Hertz-Knudsen ablation rate was established on the foundation of the laser remelting model to investigate the influence of laser processing parameters on crater feature and melted zone, and it was verified through experiments. The temperature and flow velocity distribution of the molten pool during the formation of the crater were analyzed. The results showed that the ablation velocity could be considered under a higher laser peak power density or higher pulse width due to the metal evaporation caused by heat accumulation. The depth and diameter of the crater were significantly affected by laser peak power density and laser pulse duration. Simultaneously, the height of the edge bulge decreased with the increase in pulse duration after 1.5 ms, and the growth rate of central depth was more rapid than that of edge bulge height with the increase of laser peak power density. In the texture with the same depth, a larger melted zone could be obtained with a longer laser duration than the higher peak power density.

The structure of perturbations in boundary layer created by a pulsed electric arc in the transversal magnetic field

The paper presents results of experimental study of surface MHD arc actuator as vortex generator in boundary layer. The study was held at flow velocities 20 to 50 m/s. The pulsed arc discharge was organized in external magnetic field. The amplitude of current was varied in the range from 8A to 160A, while pulse duration was from 40 μs up to 380 μs. The flow velocity was measured by PIV method. It was found that the increase of pulse duration leads to increase circulation vortex. The destruction of induced vortex was observed at 40 mm downstream from spark gap.

Increasing Energy Density of Electrochemical Capacitors By Advanced Nanomanufacturing and Ionic Liquid Electrolyte

The growing number of energy consuming devices is driving an ever increasing demand for high efficiency, high capacity, and portable energy storage devices. In addition to higher levels of energy storage, there is a growing need for the capability to produce sustained high power pulses. For many applications, the energy storage devices must also provide a high cycle life to provide the sustained operation life, along with increased capability of providing high power over ever increasing pulse durations. Moreover, these energy storage devices are in demand over a wide range of sizes from chip-scale power storage to large-scale, kilovolt-level power delivery. Rechargeable batteries, in particular lithium ion, are well known and have been extensively developed for their high energy density, but their major drawback remains in their limited power density and cycle-life capability. Electrostatic capacitors, on the other hand, provide a high power density, and research has shown they have great potential for meeting the power and cycle life demands for the new energy storage device demands. However, for conventional electrostatic capacitors, the energy density and thus pulse power duration capability is very limited. Alternatively, electrochemically-based double-layer capacitors (EDLCs) or ultracapacitors, provide a pathway to an energy storage device that shows significant promise for delivering the high power density and high cycle life that is achievable with electrostatic capacitors, while providing a path to much greater energy storage and thus sustained long duration pulsed power delivery. Through the surface charge adsorption-based mechanism of energy storage utilized in EDLCs, it is possible to get high frequency response times while providing the ability to supply power for much longer periods of time than can be achieved with conventional capacitors. Despite this major benefit, currently available commercial EDLC devices still provide energy densities that are much lower than that of batteries. To increase the number of potential applications for EDLCs and displace batteries, the energy density must be increased. Thus the main focus of our ultracapacitor research, presented here, is increasing the energy density of ultracapacitors through the use of highly ordered nanostructured electrode materials, which are combined with a high voltage window ionic liquid-based electrolytes to increase the power and energy density.In EDLCs, the capacitance and corresponding energy storage capability are directly proportional to the electrode’s solution accessible surface area. To maximize this area, we have developed electrode structures with a tunable, highly controlled porosity that provides very high, electrolyte-accessible, surface area. We’ve developed a robust and scalable templated approach to allow the fabrication of high surface area carbon nanotube (CNT)-based electrodes with highly aligned and vertically-oriented CNTs. This approach provides an areal CNT density that is much greater than typical carpet-grown CNTs. Furthermore, through our templated fabrication approach, we can tune the electrode pore size, thus allowing us to optimize it for the specific electrolyte. Our approach, combining tuning of both the electrode and electrolyte, ensures maximum charge adsorption for energy storage, while maintaining excellent solution access for fast charge transport during charging and discharging, thereby ensuring power capability.Ionic liquids are becoming popular electrolytes for a range of applications because of their large voltage window, and chemical properties that can be tailored for the specific application. The use of ionic liquid electrolytes allows operation at higher cell voltages than the conventional aqueous or organic solution-based capacitors. Larger operation voltage further enhances the power and energy density in our capacitors. Additionally, since ionic liquids are nonflammable and have a negligible vapor pressure, they make the ideal electrolyte choice in higher temperature applications or for micro- and on-chip devices, where the removal of heat is difficult because of the confined space operating conditions. Through the use of designer electrolytes based on pure and mixed ionic liquids, and combining and optimizing the interaction of the electrolyte with our templated CNT electrodes, we are able to produce an energy storage technology which combines the optimal balance and benefits of high energy density and high-power density. This talk will detail our electrode development and device optimization efforts for our large format capacitors.[DISTRIBUTION STATEMENT A. Approved for public release; Distribution is unlimited 412TW-PA-20122]

High-throughput laser fabrication of Ti-6Al-4V alloy: Part I. Numerical investigation of dynamic behavior in molten Pool

In the present work, an innovative high-throughput method was developed to fabricate a Ti-6Al-4V alloy via pulsed laser melting with water-cooled copper mold. Moreover, a three-dimensional transient model was established via numerical simulation to reveal the dynamic mechanism of heat transfer and fluid flow of the molten pool during high-throughput laser fabrication processes. Different laser powers, laser pulse durations and Marangoni coefficients were adopted to perform the numerical simulations, revealing the underlying physics of the heat transfer and fluid flow during the melting and solidification process. Special attention was paid to the cooling rate of the molten pool affected by cooling water velocity. Numerical results indicated that a “button” sample can be prepared in a short time of 200 ms by adjusting the processing parameters in this high-throughput preparation process. The maximum temperature of the molten pool was attained at the center underneath the laser beam and decreased radially outward. There was vigorous fluid flow mainly driven by thermal-capillary force in the molten pool. With increases in laser power and laser pulse duration, both the peak temperature and velocity of the molten pool increased. Accordingly, a high cooling rate was obtained under high cooling water velocity. Furthermore, the Marangoni effect fully changed the flow direction of the molten pool fluid, and the Marangoni convection flow influenced the temperature distribution of the molten pool. A new principle of the metallurgical dynamic behavior for the high-throughput laser fabrication process was established in this study and can provide technical guidance for high-throughput preparation experiments, such as laser parameter selection, structural control, and performance optimization.

In Vitro Dusting Performance of a New Solid State Thulium Laser Compared to Holmium Laser Lithotripsy.

Purpose: To examine the dusting performance of a novel solid state Thulium laser device compared to a standard holmium:yttrium-aluminum-garnet (Ho:YAG) device. Methods: This study compares a Dornier Medilas H Solvo 35 with an evaluation model of a pulsed solid state thulium:yttrium-aluminum-garnet (Tm:YAG) laser (Dornier MedTech Laser GmbH, Wessling, Germany). The in vitro model consisted of a mold with irrigated water at 37°C. For 2-9 minutes, laser fibers were guided by an xy-plotter in spirals over BegoStones. Stone mass was measured before and after laser application. Comparisons to Ho:YAG and further Tm:YAG investigations were performed. Results: Identical settings with similar pulse durations yielded a significant 14% advantage for Ho:YAG in slow fiber speeds and a tendency toward 15% higher efficiency of Tm:YAG in fast fiber speeds. Increased pulse duration in Tm:YAG led significantly to 32%-54% higher ablation rates in comparison to Tm:YAG in both setups. Ablated mass loss range is 102-1107 mg for slow fiber speeds and 22-528 mg for fast speeds. Mass loss is proportional to pulse energy, frequency, and pulse duration, whereas pulse energy defines the penetration depth into the model stones. Frequency characterizes the ablation homogeneity and possible working speeds. Conclusion: Tm:YAG is significantly more efficient when longer pulse durations are used. Identical settings revealed a strong connection to fiber movement speeds. In addition, the Tm:YAG device enables a broader range of settings with the possibility of minimal pulse energy of 100 mJ for low retropulsion and fine dusting with possible frequencies ≤200 Hz.