Overlapping Laser Pulses Research Articles

0.35% THz pulse conversion efficiency achieved by Ti:sapphire femtosecond laser filamentation in argon at 1 kHz repetition rate

In this study, an optical setup for generating terahertz (THz) pulses through a two-color femtosecond laser filament was carefully designed to achieve a precise overlap of two-color laser pulses in space and time. β-barium borate (BBO), α-BBO, and a dual-wavelength plate were used to compensate the phase delay of the two-color lasers. Tilting of α-BBO could further realize the precise spatial overlap of the two beams by counteracting the walk-off effect. The maximum output THz pulse energy reached 21 μJ in argon gas when using a commercial Ti:sapphire laser with a pulse energy of 6 mJ at a 1 kHz repetition rate. The corresponding conversion efficiency exceeded 0.35%.

Control of photodissociation of the molecular ion HD+ with two completely overlapping laser pulses

The dissociation dynamics of molecular ion HD+ in two completely overlapping THz and infrared pulses are investigated by using the time-dependent quantum wave packet method. The angular distribution of fragments depends on the carrier-envelope phase (CEP) of the THz pulse. The infrared pulse is employed to control the dissociation probability and the direction of the products D+H+. When the intensity of infrared pulse is 2.0×1014 W/cm2, the molecule HD+ is pre-oriented by the THz pulse and most of products D+H+ and H+D+ are distributed in the same direction. When the intensity is increased to 4.6×1014 W/cm2, the angular distribution is mainly generated by the THz pulse after molecular ions are dissociated, and the direction of products D+H+ is opposite to that of products H+D+.

Reducing engine oil and fuel consumptions by multidimensional laser surface texturing on cylinder surface

• Proposed the multidimensional laser surface texturing (MLST) process, based on the engine motion characteristic. • Established hardware and software systems and optimized the parameters of MLST process. • The ablating position of each laser pulse is manipulated, and various depths of micro dimples are fabricated by controlling the repeat times of overlapped laser pulses. • Developed the post-process for MLST process, based on the finishing honing process to remove the recast bulges (RCB) induced by laser ablation. • Tested four schemes of cylinder liners (one prototype, three MLST) on a single cylinder diesel engine at rated power and speed. • One of MLST scheme reduced engine oil and fuel consumptions simultaneously. The optimization of cylinder surface topography draws immense amounts of researches, since it has far-reaching effects on engines’ overall performances. In order to promote the lubricity of ring/bore interface, the present study developed the multidimensional laser surface texturing (MLST) process, which fabricates controllable arrays of micro dimples on cylinder surface. The MLST process was achieved by a series of innovations, including its hardware and software systems, and optimization of process parameters and efficiency. A post-process was developed based on the finishing honing process to remove the recast bulges (RCB) induced by laser ablation. Additionally, the cylinder liners with different surface texturing schemes were bench tested on a single cylinder diesel engine. The experimental results indicated that near TDC (top dead center) region and piston skirt contacting region (SKIRT) were two high-impact regions for the technical efficiency of the MLST process. Benefiting from the reasonable optimization of oil distribution and enhanced hydrodynamic lubrication, the MLST process was able to reduce engine oil and fuel consumptions simultaneously. This paper provides a comprehensive introduction of equipment, pre- and post-process, and capacities of the newly developed surface modification technology.

Femtosecond inscription of semi-aperiodic multi-notch fiber Bragg gratings using a phase mask.

We present an innovative concept of a semi-aperiodic phase mask design that enables the realization of multi-notch fiber Bragg gratings (FBG). This design utilizes the overlap and interference of near-infrared ultrashort laser pulses diffracted by short sequenced phase mask sections, which not only allows for a highly stable and reproducible inscription of a large number of wavelength filters but also paves the way towards full aperiodic phase masks. The semi-aperiodic FBG inscribed by this phase mask enables versatile notch filters showing multiple non-equidistant resonances. Those filters target applications, for instance in ground-based telescopes, where a large number of hydroxyl emission lines emitted in the upper atmosphere at near-infrared wavelengths restrict the observation of faint extraterrestrial objects.

URSA-PQ: A Mobile and Flexible Pump-Probe Instrument for Gas Phase Samples at the FLASH Free Electron Laser

We present a highly flexible and portable instrument to perform pump-probe spectroscopy with an optical and an X-ray pulse in the gas phase. The so-called URSA-PQ (German for ‘Ultraschnelle Röntgenspektroskopie zur Abfrage der Photoenergiekonversion an Quantensystemen’, Engl. ‘ultrafast X-ray spectroscopy for probing photoenergy conversion in quantum systems’) instrument is equipped with a magnetic bottle electron spectrometer (MBES) and tools to characterize the spatial and temporal overlap of optical and X-ray laser pulses. Its adherence to the CAMP instrument dimensions allows for a wide range of sample sources as well as other spectrometers to be included in the setup. We present the main design and technical features of the instrument. The MBES performance was evaluated using Kr M4,5NN Auger lines using backfilled Kr gas, with an energy resolution ΔE/E ≅ 1/40 in the integrating operative mode. The time resolution of the setup at FLASH 2 FL 24 has been characterized with the help of an experiment on 2-thiouracil that is inserted via the instruments’ capillary oven. We find a time resolution of 190 fs using the molecular 2p photoline shift and attribute this to different origins in the UV-pump—the X-ray probe setup.

Plasmon-induced enhancement of nonlinear optical processes in a double-resonant metallic nanostructure grating

Nanostructured gratings in a metal surface can highly enhance nonlinear optical processes. The geometrical parameters that characterize a grating can be optimized to achieve intense near-fields, which in turn enhance the nonlinear optical signals. For a nonlinear process that involves multiple frequencies, like four-wave mixing (FWM), the optimization of grating parameters necessary to enhance the radiation in-coupling for both frequencies is not trivial. Here, we propose, compute, and experimentally demonstrate a grating design that is resonant to two excitation frequencies and thus enhances the frequency mixing processes more efficiently. Second- and third-order nonlinear mechanisms are studied using two spatially and temporally overlapped laser pulses with different frequencies. Using our grating design, we achieve an unprecedented nonlinear FWM enhancement factor of 7×103.

Photoelectron Circular Dichroism with Two Overlapping Laser Pulses of Carrier Frequencies ω and 2ω Linearly Polarized in Two Mutually Orthogonal Directions.

Using a model methanelike chiral system, we theoretically demonstrate a possibility to access photoelectron circular dichroism (PECD) by a single experiment with two overlapping laser pulses of carrier frequencies ω and 2ω, which are linearly polarized in two mutually orthogonal directions. Depending on the relative phase, the resulting electric field can be tailored to have two different rotational directions in the upper and lower hemispheres along the polarization of the ω pulse. We predict a strong forward-backward asymmetry in the emission of photoelectrons from randomly oriented samples, which has an opposite sign in the upper and lower hemispheres. The predicted PECD effect is phase and enantiomer sensitive, providing new insight in this fascinating fundamental phenomenon. The effect can be optimized by varying relative intensities of the pulses.

Holographic measurement of distortion during laser melting: Additive distortion from overlapping pulses

Laser - material interactions such as welding, heat treatment and thermal bending generate thermal gradients which give rise to thermal stresses and strains which often result in a permanent distortion of the heated object. This paper investigates the thermal distortion response which results from pulsed laser surface melting of a stainless steel sheet. Pulsed holography has been used to accurately monitor, in real time, the out-of-plane distortion of stainless steel samples melted on one face by with both single and multiple laser pulses. It has been shown that surface melting by additional laser pulses increases the out of plane distortion of the sample without significantly increasing the melt depth. The distortion differences between the primary pulse and subsequent pulses has also been analysed for fully and partially overlapping laser pulses.

Amplitude modulation of femtosecond laser pulses in the megahertz range for frequency-multiplexed two-photon imaging.

We present a frequency-multiplexed multi-site two-photon imaging method utilizing amplitude modulation of femtosecond laser pulses in the MHz range to tag each excitation beam and the corresponding fluorescence signals with specific frequencies. The frequency tags are generated with an interferometric scheme employing acousto-optic deflectors (AODs) to achieve precise spatial overlap of femtosecond laser pulses with periodically varying phase shift. Creating matching excitation beam patterns in each interferometer arm using multiple AOD driving frequencies, and subsequently overlapping these matching patterns, results in multiple encoded excitation beams with unique beat frequencies available for scanning. As a proof-of-concept, we demonstrate multiplexed two-photon image acquisition using test samples, and compare the performance of this approach to conventional two-photon laser scanning microscopy.

Damage morphology and mechanism in ablation cutting of thin glass sheets with picosecond pulsed lasers

We experimentally investigated the morphology and mechanism of laser-induced damage in the ablation cutting of thin glass sheets with picosecond pulsed lasers and we compared the experimental results to our models. After several passes of laser ablation, we observed two different kinds of damage morphologies on the cross-section of the cut channel. They are distinguished to be the damage region caused by high-density free-electrons and the heat-affected zone due to the heat accumulation, respectively. Furthermore, micro-cracks can be observed on the top surface of the workpiece near the cut edge. The nano-cracks could be generated by high energy free-electrons but opened and developed to be visible micro-cracks by thermal stress generated in the heat-affected zone. The crack length was proportional to the volume of heat-affected zone. Heat-affected-zone and visible-cracks free conditions of glass cutting were achieved by controlling the repetition rate and spatial overlap of laser pulses.

Phosphorus out-diffusion in laser molten silicon

Laser doping via liquid phase diffusion enables the formation of defect free pn junctions and a tailoring of diffusion profiles by varying the laser pulse energy density and the overlap of laser pulses. We irradiate phosphorus diffused 100 oriented p-type float zone silicon wafers with a 5 μm wide line focused 6.5 ns pulsed frequency doubled Nd:YVO4 laser beam, using a pulse to pulse overlap of 40%. By varying the number of laser scans Ns = 1, 2, 5, 10, 20, 40 at constant pulse energy density H = 1.3 J/cm2 and H = 0.79 J/cm2 we examine the out-diffusion of phosphorus atoms performing secondary ion mass spectroscopy concentration measurements. Phosphorus doping profiles are calculated by using a numerical simulation tool. The tool models laser induced melting and re-solidification of silicon as well as the out-diffusion of phosphorus atoms in liquid silicon during laser irradiation. We investigate the observed out-diffusion process by comparing simulations with experimental concentration measurements. The result is a pulse energy density independent phosphorus out-diffusion velocity vout = 9 ± 1 cm/s in liquid silicon, a partition coefficient of phosphorus 1 < kp < 1.1 and a diffusion coefficient D = 1.4(±0.2)cm2/s × 10−3 × exp[−183 meV/(kBT)].

Steering Vibrational Population Transfer via Double-∑-Type Laser Scheme

The vibrational state-selected population transfer from a highly vibrationally excited level to the ground level is of great importance in the preparation of ultra-cold molecules. By using the time-dependent quantum-wave-packet method, the population transfer dynamics is investigated theoretically for the HF molecule. A double-∑-type laser scheme is proposed to transfer the population from the ∣v=16〉 level to the ground vibrational level ∣v=0〉 on the ground electronic state. The scheme consists of two steps: The first step is to transfer the population from ∣v=16〉 to ∣v=7〉 via an intermediate level ∣v=11〉, and the second one is to transfer the population from ∣v=7〉 to ∣v=0〉 via ∣v=3〉. In each step, three vibrational levels form a ∑-type population transfer path under the action of two temporally overlapped laser pulses. The maximal population-transfer efficiency is obtained by optimizing the laser intensities, frequencies, and relative delays. Cases for the pulses in intuitive and counterintuitive sequences are both calculated and compared. It is found that for both cases the population can be efficiently (over 90%) transferred from the ∣v=16〉 level to the ∣v=0〉 level.

Investigation on the Effect of Overlapping Laser Pulses in Laser Shock Peening with Finite Element Method

Laser shock peening (LSP) is one of the modern surface treatment methods in which surface of the work-piece is peened using short and intense laser pulses. In this process, favorable compressive residual stresses are induced on and near the surface of the material. This process is usually used to improve fatigue life of the component and has been applied in different industries such as aerospace, automotive, nuclear, medical, etc. Many researchers have used the finite element method (FEM) to simulate the LSP. In the majority of the conducted researches, results are in good agreement with the experimental data which shows FEM is capable of accurately simulating the LSP process. Simulations reduce the costs related to experimental measurement. Moreover with simulations, it is easier to understand, and is possible to analyze and optimize the process. To conduct a parametric study, it is first necessary to be able to simulate a single shot. After verification of the results, it is necessary to investigate the effect of multiple close impacts on the final results. Therefore, three dimensional analysis should be used. Since deformation is caused by the generated shockwave, there is a high strain rate (in the order of 106 s-1) in this process which necessitates the use of a very fine mesh. This in conjunction with a 3D simulation significantly increases the computational cost. This analysis is crucial for the design and optimization process of laser shock peening for a specific application. The present article, investigates the effect of overlapping laser pulses. Residual stress distribution on a range of overlap ratios (0 to 80% overlap) with square laser spot shape has been presented. With a laser intensity of 8GW/cm2, and a desirable compressive stress of 300MPa, it was found that the optimum choice of overlap is between 0 to 20 percent.

Phase-controlled two-qubit quantum gates

We consider an implementation of a two-qubit entangling gate between trapped ions controlled by fully overlapping laser pulses. We particularly address the role of a relative phase between the pulses in the gate design. Though, in general, the relative phase determines the exact form of the two-qubit operation, it is shown that it affects only its single qubit part and has no effect on its entangling content. Based on a general canonical decomposition of the evolution operator we demonstrate that the controlled-not (cnot) gate can be decomposed in terms of a two-qubit entangled gate and single qubit phase gates. Our study shows that any relative phase errors in two-qubit operations related to phase fluctuations of the external fields can be successfully corrected locally by single-qubit operations, which are usually easy to implement.

Instability-triggered transformations in thin chromium film on glass under laser irradiation

Metal films on transparent substrates are widely applied for mask production in lithography, and lasers are frequently used for their patterning. Quality of the patterning is limited by fundamental phenomena taking place close to edges of the laser ablated area. We experimentally and numerically investigated transformations in metal films during their irradiation with the nanosecond laser beam with fluences above the ablation threshold. Ridges of the resolidified metal with not uniform thickness were always formed on edges of the cleaned area. Instabilities during the ablation process forced the molten metal in the ridges to break up into droplets with the periodicity predicted by the Plateau–Rayleigh instability. The droplets on ridges were starting points for formation of ripples of metal film by irradiation with partially overlapping laser pulses. The initial droplets and later the self-organized parallel lines of chromium metal were heat sinks that cooled down the metal in their close proximity. Temperature modulation along the laser irradiation spot was high enough to initiate the Marangoni effect which resulted in movement of the molten metal from hot to colder areas.

Nanosecond pulsed laser ablation of brass in a dry and liquid-confined environment

The effect of ambient environment (dry or wet) and overlapping laser pulses on the laser ablation performance of brass has been investigated. For this purpose, a Q-switched, frequency doubled Nd:YAG laser with a wavelength of 532 nm, pulse energy of 150 mJ, pulse width of 6 ns and repetition rate of 10 Hz is employed. In order to explore the effect of ambient environments, brass targets have been exposed in deionized water, methanol and air. The targets are exposed for 1000, 2000, 3000 and 4000 succeeding pulses in each atmosphere. The surface morphology and chemical composition of ablated targets have been characterized by using Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM) and Attenuated Total Reflection (ATR) techniques. In case of liquid environment, various features like nano- and micro-scale laser-induced periodic surface structures with periodicity 500 nm–1 μm, cavities of size few micrometers with multiple ablative layers and phenomenon of thermal stress cracking are observed. These features are originated by various chemical and thermal phenomena induced by laser heating at the liquid–solid interfaces. The convective bubble motion, explosive boiling, pressure gradients, cluster and colloid formation due to confinement effects of liquids are possible cause for such kind of features. The metal oxides and alcohol formed on irradiated surface are also playing the significant role for the formation of these kinds of structure. In case of air one huge crater is formed along with the redeposition of sputtered material and is ascribed to laser-induced evaporation and oxide formation.

Prediction and characterization of residual stresses from laser shock peening

The use of laser shock peening (LSP) to enhance the fatigue resistance of metals offers several potential advantages over more conventional surface enhancement techniques such as shot peening, including deeper penetration of the residual stresses, more reliable surface coverage, and the potential for reduced microstructural damage. In the last decade, computational hardware and software resources have advanced to a state that permits numerical simulation of practical LSP processing at a reasonable level of detail, including complex geometric features, multiple and overlapping laser pulses, and intensity variations within the individual laser spots. This article offers some further developments in simulating LSP processes on a realistic scale, as well as some simple methods for distilling and interpreting results from such simulations. A key point of interest is the local variations in residual stress that occur within the processed region, which are quite sensitive to processing variables, and not easily measured experimentally. The simulations suggest that X-ray diffraction measurements of the residual stress field offer only a coarse description of the final residual stress field, and should be interpreted with some caution. We propose some methods for interpreting the simulation results statistically, to provide a clear but accurate characterization of the surface treatment and its effect on fatigue behavior.

Nano- and Microstructuring of SiO2 and Sapphire with Fs-laser Induced Selective Etching

Sub wavelength ripples perpendicular to the polarisation of the laser radiation are obtained by scanning a tightly focused beam (~1µm) of femtosecond laser radiation over the surface and through the volume of various materials. The ripple patterns extend coherently over many overlapping laser pulses and scanning tracks. The cross-sections and the surface of the ripples and in volume nanoplanes are investigated using electron microscopy. By using microscope objectives with higher Numerical Apertures micro-channels can be fabricated with a technique called In Volume Selective Laser Etching (ISLE).

Two-color laser desorption of nanostructured MgO thin films

Neutral magnesium atom emission from nanostructured MgO thin films is induced using two-color nanosecond laser excitation. We find that combined vis/UV excitation, for single-color pulse energies below the desorption threshold, induces neutral Mg-atom emission with hyperthermal kinetic energies in the range of 0.1–0.2 eV. The observed metal atom emission is consistent with a mechanism involving rapid electron transfer to three-coordinated Mg surface sites. The two-color Mg-atom signal is significant only for parallel laser polarizations and temporally overlapped laser pulses indicating that intermediate excited states are short-lived compared to the 5 ns laser pulse duration.

Decoherence-free preparation of Dicke states of trapped ions by collective stimulated Raman adiabatic passage

We propose a simple technique for the generation of arbitrary-sized Dicke states in a chain of trapped ions. The method uses global addressing of the entire chain by two pairs of delayed but partially overlapping laser pulses to engineer a collective adiabatic passage along a multi-ion dark state. Our technique, which is a many-particle generalization of stimulated Raman adiabatic passage (STIRAP), is decoherence-free with respect to spontaneous emission and robust against moderate fluctuations in the experimental parameters. Furthermore, because the process is very rapid, the effects of heating are almost negligible under realistic experimental conditions. We predict that the overall fidelity of synthesis of a Dicke state involving ten ions sharing two excitations should approach 98% with currently achievable experimental parameters.