Pulse Overlap Research Articles

Pulse Duration Dependent Formation of Laser‐Induced Periodic Surface Structures in Atomic Layer Deposited MoS2

AbstractIn this paper, the formation of laser‐induced periodic surface structures (LIPSS) on atomic‐layer deposited MoS2 layers are studied experimentally. The process parameters (laser fluence and the pulse overlap) corresponding to formation of low‐ and high‐spatial frequency LIPSS as well as ablation and modification of the layers are identified for different pulse durations in the range from 0.2 to 10 ps. The role of the temperature accumulation is evaluated by changing the repetition rate from 0.2 to 2 MHz. The negative accumulation effect, i.e., the ablation of the layers becomes more difficult at higher laser pulse overlaps, is also observed. A simple model explaining the transition between different types of the LIPSS and the decrease of the ablation efficiency with the pulse overlap is suggested.

Efficient general method for numerically modeling laser pulse propagation, overlap, and lifetime effects in amplifiers

An efficient numerical time-dependent general method is developed to address incoherent pulse overlap and lifetime effects in laser amplifiers. The alternating propagation-population laser energetics method (APPLE) has been validated against a semi-discrete coupled rate equation numerical method (SDRE) and analytic formalisms in bounding cases. APPLE is based on decoupled rates applied to a time-dependent framework where both space-time-dependent populations and pulse energetics are consistently updated in each time step. A significant advantage of APPLE lies in its conceptual simplicity, ease of implementation, and relatively small computational cost. SDRE tracks the populations through coupled rates and uses the method of lines to discretize the hyperbolic partial differential transport equations allowing for use of ordinary differential equation solvers. With reasonably sized mesh, we report both energetic and power pulse shape relative differences on the order of one percent between the models over a large range of initial conditions.

Fabrication of Multiscale and Periodically Structured Zirconia Surfaces Using Direct Laser Interference Patterning

AbstractThe functionalization of zirconia surfaces by accurate and fast printing of periodical patterns embedding sub‐micrometric features is of great interest to many engineering fields and is yet to be explored. This study aims to assess the influence of the Direct Laser Interference Patterning processing parameters on the morphology and microstructure of zirconia surfaces using a 532 nm 10 ps‐pulsed laser source. Well‐defined linear structures with a period of 3 µm are successfully produced. Depending on the laser parameters, the structures are developed at or below the surface level, with higher depths (≈1 µm) being seen for increasing values of laser fluence and pulse overlap. Line‐like hierarchical structures with smaller interference spatial structures (3 µm period) and higher secondary structures with different periods (18, 15, and 12 µm) and heights (7, 5, and 3 µm, respectively) are also obtained on zirconia surface. Ablated regions presented few traces of molten material, nano‐droplets, and sub‐micrometric (<1 µm) pores, while no (sub) micrometric cracks are detected. A slight amount of tetragonal to monoclinic phase transformation (≈5%) is detected by X‐ray diffractometry. A processability map for ps‐laser processing of zirconia is proposed based on the experimental data.

Characterization of Micro-Holes Drilled Using a UV Femtosecond Laser in Modified Polyimide Flexible Circuit Boards.

Modified polyimide (MPI) flexible printed circuits (FPCs) are used as chip carrier boards. The quality of the FPC directly affects the reliability of the integrated circuit. Furthermore, micro-holes are critical components of FPCs. In this study, an ultraviolet (UV) femtosecond laser is used to drill micro-holes in double-layer flexible circuit boards with MPI as the substrate. The morphology of the micro-hole wall in the copper foil and MPI layer is observed, and the effects of the laser processing parameters on the diameter and depth of the micro-holes are analyzed. The drilling process and mechanism of micro-holes obtained using a UV femtosecond laser in MPI FPCs are discussed. The results show that the morphology of femtosecond laser-machined copper is closely related to the laser energy, and a periodic structure is observed during the machining process. Copper, MPI, and copper oxides are the most common molten deposits in micro-holes during drilling. The depth of the micro-holes increases with an increase in the energy of a single pulse, scanning time, and scanning overlap rate of the laser beam. However, the diameter exhibits no discernible alteration. The material removal rate increased significantly when laser processing was applied to the MPI resin layer.

Experimental investigation of nanosecond pulsed laser ablation of polycrystalline diamond composite

A fundamental understanding of the laser ablation of polycrystalline diamond composite (PCD) at different laser energy intensities is indispensable for extending these findings to practical applications where, through appropriate parameter combinations, a substantial amount of material with minimal thermal damage should be removed. The objective of this study is to investigate experimentally the nanosecond ablation of polycrystalline diamond composite at different laser pulse energies and input energy densities. The research comprised two distinct categories of experiments: (1) single-point and (2) line experiments. In single-point tests, the number of the incidence pulses (N.P) was varied from 1 to 100 pulses with different pulse energies (Ep) ranging from 40 to 200 µJ. As the N.Ps increased, the ablation depth increased as well. No considerable enhancement in ablation depth was observed with the increase in pulse energies. For the line experiments, the results emphasized the differences in ablation depths by nearly a factor of 2, resulting in a corresponding change in laser input densities from 6.5 J/mm2 to 1.3 J/mm2. The ablation difference was pronounced as the number of repeats of the scanning path increased. The results emphasize that the ablation depth varies at the same pulse energy when the pulse overlap decreased from 100 % to lower percentages. Based on these results, a constant laser energy density with three different combinations of laser inputs—namely, scanning speed and average power—was selected to evaluate the ablation depth and quality. The results show that in moderate numbers of scan repeats (here 20, 30 & 40 times), it is advisable to adopt a minimum pulse energy when the same ablation depth is achieved compared to higher pulse energies. The ablation pile-up was also evaluated in these repeat ranges, indicating that higher pulse energies and repeats do not result in ablation improvement and cause excessive melt deposition.

Laser spot overlap in scanning laser ablation ICP-MS analysis: Impact on analytical signal and properties of the generated aerosol

The aim of this study was to determine whether the degree of pulse overlap affects the analytical response of LA-ICP-MS when utilized in scanning mode. The investigation focused not only on the analytical signal but also on the properties of the generated aerosol and the morphology of ablation craters. Ablation was carried out using a nanosecond ablation system on single-element samples of metals and metalloids, specifically Cr, Hf, Sb, Si, Ta, and W. Pure single-element standards were selected to eliminate elemental fractionation and ensure the chemical composition of the generated aerosol was consistent across all particle sizes. The effect was studied on the surfaces of the samples prepared in various ways, including polished and two types of roughened finishes. It was found that the degree of laser pulse overlap significantly influences the quantity of generated particles, their size, and consequently the measured analytical signal, altering it by tens of percent. Slightly different properties were observed in the groups of metal and metalloid samples. For metals, an increasing overlap of pulses resulted in a decrease in the analytical signal across all types of surfaces. For metalloids, the results were different on polished surfaces. The study shows that adjusting the pulse overlap by combining the scan speed and laser repetition rate can have a significant effect on the LA-ICP-MS analysis results.

Analytical formula for signal optimization in stimulated photon-photon scattering setup with three laser pulses

We consider a setup to detect stimulated photon-photon scattering using high-power lasers. Signal photons are emitted from an overlap of the incoming intense laser pulses focused in vacuum from three sides. We derive and justify a general approximate analytical formula for the angular distribution and total yield of such signal photons in terms of the parameters of the incoming pulses, including their intensity, carrier frequencies, durations, focusing, polarizations, mutual orientation and overlap. Using the obtained formula a parametric study of the signal is carried out and optimization is performed. Published by the American Physical Society 2024

Weak signal detection based on pulse width counting method for underwater wireless optical communication with an analog mode PMT detector

This work proposes a weak signal detection method based on pulse width counting (PWC) for the on-off keying (OOK) underwater wireless optical communication (UWOC) system with an analog mode photomultiplier tube (PMT) detector. The signal output model of the analog mode PMT in weak light communication and the influence of pulse overlap are investigated. We experimentally evaluate the proposed algorithm under different sampling rates, detection thresholds, data rates as well as received optical powers (ROPs), and compare the performance of the proposed approach with that of pulse amplitude detection and pulse peak counting. A 10-Mbps OOK UWOC link is realized with a sensitivity of -71.5 dBm in a 7-meter tank, which is 1.1-dB and 3.8-dB lower than that of pulse peak counting (PPC) and pulse amplitude detection (PAD) methods, respectively, and the total link attenuation is 94.8 dB. This system utilizes the analog mode PMT with larger dynamic range than photon-counting mode PMT to achieve weak light signal detection, which benefits design long-range UWOC systems.

Direct nanosecond laser metallization of AlN ceramics

In this paper, a study of the influence of laser processing parameters with pulsed nanosecond laser radiation on the degree of metallization and the quality of the metallized surface of aluminum nitride ceramics is presented. Experiments were carried out to create conductive structures with the lowest resistance using direct laser metallization. The dependences of resistance on duration, pulse overlap, and laser fluence were obtained and analyzed, and changes in surface roughness were considered. In addition, the composition of the surface of laser-metallized ceramics was studied using energy-dispersive x-ray spectroscopy. As a result, it was shown that the resistance is inversely proportional to the square root of the pulse duration, the thermal diffusion length was estimated as lT = 8.2 μm for 200 ns and lT = 1.2 μm for 4 ns, and the presence of optimal values of pulse overlap Oy (scanning direction) equal to 50% and pulse overlap Ox (step direction) equal to 96% and 99.7% for pulse durations of 200 and 4 ns, respectively, was determined. The choice of optimal pulse overlaps with the highest laser fluence allowed us to obtain the minimum resistance value with maximum performance.

Laser Scribing for Perovskite Solar Modules of Long‐Term Stability

Although the efficiency of hybrid lead‐halide perovskite solar cells has been significantly improved, the efficiency gap between small‐area cells and large modules continues to be a considerable challenge. Laser scribing is essential for realizing high‐quality monolithic connections; however, the laser‐induced material changes and their correlation with device performance have not been yet well understood, in particular for the perovskite material systems. In this study, the effect of P3 laser processing conditions on device performance and stability is explained. The most interesting finding is an improvement in open‐circuit voltage (VOC) after aging and long‐term stability under low‐laser‐overlap conditions that avoid direct laser exposure to perovskite material system as a source of material degradation. It is found that a high‐laser‐overlap during P3 results in a lower fill factor after aging and accelerated degradation due to larger portion of perovskite directly exposed to laser during the scribing process. The increased VOC under low‐overlap conditions is attributed to the increased PbI2 formation in the P3 region. Moreover, a minimal pulse overlap is favorable for preserving long‐term device stability. Finally, a perovskite minimodule with an efficiency of 20.24% is successfully developed as a result of these findings.

Laser peening induced mitigation of severe pitting corrosion in titanium stabilized 321 steel

Titanium stabilized 321 stainless steel, as a structural component in oil and petrochemical industries, often susceptible to severe pitting corrosion under aggressive corrosion environments (sulfuric acid and chloride ions). This study explores the applicability of laser peening without protective coating (at 6 GW cm−2 intensity and 75 % pulse overlap rate) to enhance the electrochemical stability of 321 steel against such severe pitting corrosion. Laser peening altered surface-mechanical and electrochemical properties were investigated by AFM, SEM and potentiodynamic polarization/impedance spectroscopy techniques. On the account of laser peening induced compressive residual stress (-408 MPa) hardness enhancement, a stable/dense and uniform oxide film was formed on 321 steel, subsequently mitigating the pitting corrosion. By contrast, unpeened surface exhibited more number of wider and deeper merged pits (maximum diameter of ∼ 2197 μm). Laser peening promoted oxidation has been confirmed via elemental composition analysis. A twentyfold enhancement in charge transfer resistance and improved oxide film resistance, attributed from the laser peening increased defect density as demonstrated by the improved hardness, evidenced a highly electrochemical stability on 321 steel.

Investigating surface integrity of laser-machined polycrystalline diamond using a 300 W picosecond laser

Polycrystalline diamond (PCD) is an important material in the tool industry due to its superior wear resistance and strength. Picosecond laser machining enables accurate cutting, geometric profiling, and precise dimensional features to be produced on PCD. The industrialisation of this manufacturing process necessitates the development of laser systems and process strategies that facilitate high throughput and quality. This paper investigates the feasibility of using a 300 W picosecond laser to machine PCD. It examines the influence of fluence, average power, pulse duration and number of passes on the material removal rate and surface quality. The surface integrity after machining is ascertained using a surface profilometer, optical microscopy and Raman spectroscopy. The results have shown that a high material removal rate of 33 mm3/min (0.11 mm3/min W) at a surface roughness of 2.3 μm is achievable using a fluence of 0.9 J/cm2 at 41.3 MHz and 300 W average power. Raman’s analysis of the machined sample suggests a laser-induced graphitisation which is prevalent at high pulse overlap, peak power, and the number of passes. Overall, the study strengthens the idea that with optimised process parameters, a balance is established between productivity and high surface integrity.

Microcutting of glass with high ablation efficiency by means of a high power ps-pulsed NIR laser

Microcutting of glass through ultrafast lasers represents a process more and more employed in many industrial fields, among which touchscreen displays for electronic devices. The main disadvantage is the low processing speed that does not meet the industrial requirements. In this work, a ps-pulsed NIR laser with 200 W average power was employed for cutting of thick soda-lime glass with a relatively high thickness of 1 mm submerged in water. The cut evolution with respect to the number of passes and the quality of cut kerf were analysed by considering different levels of pulse repetition rate, pulse energy and scan speed. The results show a considerable influence of the scan speed on the ablation efficiency and surface roughness, because of the thermal accumulation given by the pulse overlap. Complete cut of 1 mm-thick glass was achieved at 400 kHz, 360 μJ, with speed from 0.5 m/s to 3 m/s and number of scans from 300 to 1900 respectively. The results showed that the use of high power ps-pulsed NIR laser can provide high material removal efficiency up to 600 µm3/pulse at 0.5 mm thickness and up to 300 µm3/pulse at 1 mm thickness.

Near-infrared femtosecond laser direct writing of microchannel and controlled surface wettability

The application of femtosecond laser in fabricatingpoly (methyl methacrylate) microchannels and the alteration of wettability by inducing a change in chemical functional groups has already been established and reported in literature.However, studies on the application of femtosecond laser for modification of surface wettability without significant chemical modification,are yet to be reported. The paper reports work on an expedition of femtosecond (Ytterbium-doped fiber) laser ablation working at the near-infrared wavelength (1030 nm) tuning the wettability behavior comprising of hydrophilic and hydrophobic state on the basis of the surface morphology created below the focal beam diameter. The initial part revolves around the investigation of near-infrared femtosecond laser ablation for achieving microcavity and microchannel below the focal beam diameter. The latter part deals with laser surface modification for attaining both hydrophilic and hydrophobic surfaces. The study obtained a high degree of hydrophobicity, with a maximum water contact angle of 129.05°at a laser energy of 40 μJ and pulse laser overlap of 96%. It was also possible to attain a minimum water contact angle of 56.35°using a laser pulse energy of 40 μJ and pulse overlap of 0 %. The reported work avoids surface chemical modification for controlling the wettability.

Direct Laser Interference Patterning of Zirconia Using Infra‐Red Picosecond Pulsed Laser: Effect of Laser Processing Parameters on the Surface Topography and Microstructure

AbstractThis study investigates the influence of processing parameters when applying direct laser interference patterning (DLIP) on the morphology and microstructure of zirconia surfaces using a 10 ps‐pulsed laser source with 1064 nm wavelength. An experimental testing matrix is built with different values of laser fluence (5.7–18.2 J cm−2) and pulse overlap (66–98%). Surface morphology and microstructure are characterized by confocal microscopy and scanning electron microscopy. Homogeneous line‐like patterns with periodic spatial repetition of 5.0 µm, with varying depths, widths, and aspect ratio, are fabricated using proper processing parameters (5.7–7.6 J cm−2 and 92–96%). Structures with maximum depth of 1.5 µm and sharp edges are obtained (7.6 J cm−2 and 96% overlap). Ablated regions presented a morphology typical of photophysical ablation mechanism, with signs of molten material at the surface. Sub‐micrometric pores and nanodroplets are registered for all conditions, while sub‐micrometric cracks developed only for higher fluences. A processing window conducing to homogenous DLIP structures is set based on experimental data. Periodic structures with multiscale topographic features are successfully obtained on zirconia surfaces using DLIP technology in this study. These outcomes open new perspectives for fabrication of multifunctional zirconia surfaces for advanced biomedical and engineering applications.

Spectral Broadening Effects on Pulsed-Source Digital Holography

Using a pulsed configuration, a digital-holographic system is setup in the off-axis image plane recording geometry, and spectral broadening via pseudo-random bit sequence is used to degrade the temporal coherence of the master-oscillator laser. The associated effects on the signal-to-noise ratio are then measured in terms of the ambiguity and coherence efficiencies. It is found that the ambiguity efficiency, which is a function of signal-reference pulse overlap, is not affected by the effects of spectral broadening. The coherence efficiency, on the other hand, is affected. As a result, the coherence efficiency, which is a function of effective fringe visibility, is shown to be a valid performance metric for pulsed-source digital holography.

Methane coupling by a parallel-electrodes pulsed gliding plasma: A strong correlation between conversion ability and the plasma mobility

The coupling of methane using a highly mobile nanosecond pulsed plasma is examined. A stroboscopic imaging system is used to determine the drift velocity of the plasma column (vd). When vd varies within a narrow range of 1.2–2.2 m/s in CO2 + CH4 plasma, the energy efficiency increases from 35% to over 60% showing very sensitive correlation. The reactants conversion rates also increase linearly with vd, ranging from 40 to 60%. Yields of ethylene and ethane monotonically increases with vd when pure methane is introduced. A mathematical model predicts a high spatial overlap of successive plasma pulses, with an overlap of 78% at the highest vd obtained here. This overlap, inversely related to vd, negatively affects on the process due to energy dissipation by products and high temperatures in the overlap zone. To address the problem of low conversion rates or efficiencies of plasmas, using non-overlapping plasma reactors may be a viable solution.

Multi-Scale Structuring of CoCrMo and AZ91D Magnesium Alloys Using Direct Laser Interference Patterning

In this work, the technique of Direct Laser Interference Patterning (DLIP) was used to fabricate micrometric structures at the surface of Cobalt-Chromium-Molybdenum and AZ91D magnesium alloys. Line-like patterns with spatial periods of 5 μm were textured using an ultra-short pulsed laser (10 ps pulse duration and 1064 nm wavelength) with a two-beam interference setup. The surface topography, morphology, and chemical modifications were analysed using Confocal Microscopy, Scanning Electron Microscopy, and Energy Dispersive Spectroscopy (EDS), respectively. Laser fluence and pulse overlap were varied to evaluate their influence on the final structure. Homogeneous structures were achieved for the CoCrMo alloy for every condition tested, with deeper structures (up to 0.85 μm) being achieved for higher energy levels (higher overlap and/or fluence). For high energy, sub-micrometric secondary structures, so-called LIPSS, could also be observed on the CoCrMo. The EDS analysis showed some oxidation after the laser texturing. Regarding the AZ91D alloy, deeper structures could be achieved (up to 2.5 μm), but more melting and oxidation was observed, forming spherical oxide particles. Nonetheless, these results bring new perspectives on the fabrication of microtextures on the surface of CoCrMo and AZ91D using DLIP.

Impact of laser chirp on the polarization of terahertz from two-color plasma

Two-color plasma, induced by two lasers of different colors, can radiate ultra-broadband and intense terahertz (THz) pulses, which is desirable in many technological and scientific applications. It was found that the polarization of the emitted THz depends on the phase difference between the fundamental laser wave and its second harmonic. Recent investigation suggests that chirp-induced change of pulse overlap plays an important role in the THz yield from two-color plasma. However, the effect of laser chirp on THz polarization remains unexplored. Hereby, we investigate the impact of laser chirp on THz polarization. It is unveiled that the chirp-induced phase difference affects THz polarization. Besides, positive and negative chirps have opposite effects on the variation of the THz polarization versus the phase difference. The polarization of THz generated by a positively chirped pump laser rotates clockwise with an increasing phase difference, while it rotates anticlockwise when generated by a negatively chirped pump laser.

Pulse overlap artifacts and double quantum coherence spectroscopy.

The double quantum coherence (DQC) signal in nonlinear spectroscopy gives information about the many-body correlation effects not easily available by other methods. The signal is short-lived, consequently, a significant part of it is generated during the pulse overlap. Since the signal is at two times the laser frequency, one may intuitively expect that the pulse overlap-related artifacts are filtered out by the Fourier transform. Here, we show that this is not the case. We perform explicit calculations of phase-modulated two-pulse experiments of a two-level system where the DQC is impossible. Still, we obtain a significant signal at the modulation frequency, which corresponds to the DQC, while the Fourier transform over the pulse delay shows a double frequency. We repeat the calculations with a three-level system where the true DQC signal occurs. We conclude that with realistic dephasing times, the pulse-overlap artifact can be significantly stronger than the DQC signal. Our results call for great care when analyzing such experiments. As a rule of thumb, we recommend that only delays larger than 1.5 times the pulse length should be used.