Ps Laser Pulses Research Articles

Dynamics of the Electromagnetic Fields Induced by Fast Electron Propagation in Near-Solid-Density Media.

The propagation of fast electron currents in near solid-density media was investigated via proton probing. Fast currents were generated inside dielectric foams via irradiation with a short (∼0.6 ps) laser pulse focused at relativistic intensities (Iλ^{2}∼4×10^{19} W cm^{-2} μm^{2}). Proton probing provided a spatially and temporally resolved characterization of the evolution of the electromagnetic fields and of the associated net currents directly inside the target. The progressive growth of beam filamentation was temporally resolved and information on the divergence of the fast electron beam was obtained. Hybrid simulations of electron propagation in dense media indicate that resistive effects provide a major contribution to field generation and explain well the topology, magnitude, and temporal growth of the fields observed in the experiment. Estimations of the growth rates for different types of instabilities pinpoints the resistive instability as the most likely dominant mechanism of beam filamentation.

Conical microstructuring of titanium by reactive gas assisted laser texturing.

Femtosecond laser micromachining is an important and flexible method to generate precisely targeted surfaces on various materials. On titanium, the laser structuring process strongly depends on the laser parameters. For example, an increasement of the pulse length and repetition rate favors melting processes instead of ablation and microstructuring. We report on an investigation of reactive halogens (iodine, bromine, chlorine) and halocarbons as additives to the laser structuring process of pure titanium and the common alloy Ti-6Al-4V with 0.75 ps laser pulses. The choice of the halogen allows control of whether solely the chemical composition or the surface microstructure should be altered. Bromine was found to be an efficient additive to generate homogeneous microstructures based on micropillars at convenient conditions (air, atmospheric pressure). The resulting surfaces have been characterised by scanning electron microscopy, energy dispersive X-ray spectroscopy, thermal emission infrared photography, reflective UV/Vis spectroscopy and contact angle measurements. The bromine/air processed titanium surfaces revealed superhydrophilicity, strongly increased thermal emissivity and a high absorptivity (“black metal”).

Ionization behavior and dynamics of picosecond laser filamentation in sapphire

Currently, laser-induced structural modifications in optical materials have been an active field of research. In this paper, we reported structural modifications in the bulk of sapphire due to picosecond (ps) laser filamentation and analyzed the ionization dynamics of the filamentation. Numerical simulations uncovered that the high-intensity ps laser pulses generate plasma through multi-photon and avalanche ionizations that leads to the creation of two distinct types of structural changes in the material. The experimental bulk modifications consist of a void like structures surrounded by cracks which are followed by a submicrometer filamentary track. By increasing laser energy, the length of the damage and filamentary track appeared to increase. In addition, the transverse diameter of the damage zone increased due to the electron plasma produced by avalanche ionizations, but no increase in the filamentary zone diameter was observed with increasing laser energy.

UV irradiation induce NLO modulation in photochromic styrylquinoline-based polymers: Computational and experimental studies

Abstract Functionalized photochromic materials have attracted a growing interest during the last few decades since they are promising materials to be used in optoelectronics and photonics. In this paper, the results of the nonlinear optical (NLO) investigation (second and third harmonic generation) of spin deposited high-quality thin films of styrylquinoline containing methacrylic polymers studied by using Maker fringes technique employing 30 ps laser pulses at fundamental wavelength of 1064 nm are presented. Photochromic styrylquinoline units were oriented in the thin films by corona poling technique promoting second order NLO activity. Strong dependence of the NLO response upon the structure of the polymers has been found, which is related to the different charge transfer within the styrylquinoline fragments. These investigations were completed by theoretical studies using HOMO-LUMO energy levels theory and the first and second order hyperpolarizability values. Good compatibility has been achieved between the theoretical and experimental results. Studied styrylquinoline-based polymers showed the contrast in NLO response after their trans-cis photoisomerization making them interesting for using in photonic devices.

Microdrilling of Through-Holes in Flexible Printed Circuits Using Picosecond Ultrashort Pulse Laser.

High density and high quality interconnects are necessary for the preparation of miniaturized and lightweight electronic products. Therefore, small-diameter and high-density through-holes in FPCs (Flexible Printed Circuits) are required. However, the current processing methods cannot further decrease the diameters and improve the quality of through-holes. Comparatively, ultrashort pulse laser is a good choice. In this paper, the processing technology for the microdrilling of through-holes in FPCs using a 10 ps pulse laser was systematically studied. The effects of laser parameters, including the wavelength, energy, pulses and polarization, on the drilling of through-holes were investigated. The various processing parameters were optimized and the plausible reasons were discussed. Finally, the desired small-diameter and high-density through-holes in FPCs were obtained. The experimental results showed that, through-holes with diameters of less than 10 µm and inlet interconnection pitches of 0~2 µm could be successfully drilled in FPCs using ultrashort pulse laser.

Study of thermal physical parameters in high energy domain femtosecond laser ablation on Au film

On the basis of the two-temperature model (TTM), the influence of electronic heat capacity(Ce), electroacoustic coupling coefficient(G) and electron thermal conductivity(Ke) is simulated, contrasted and analyzed by finite element method and method of control variables on three ablation properties in femtosecond laser ablation on 800nm Au film with a 60 ps laser pulse at 1.5 Jcm−2. Maximum electron temperature, electroacoustic coupling time and electroacoustic coupling temperature are selected to describe the three ablation properties. The results show that all the three thermal physical parameters (Ce, G and Ke) have different degrees of effects on three ablation properties simultaneously. While Ce has the biggest influence on maximum electronic temperature, the effects of G on electroacoustic coupling time is the most prominent, and Ke is the key factor for electroacoustic coupling temperature change. Finally the physical mechanism of three parameters influencing ablation results is analyzed and explained, the main reason for these conclusions is that the electron thermal capacity describes the thermal energy capacity of electronic subsystem, the electroacoustic coupling coefficient represents the energy conversion capacity between the electronic subsystem and the lattice subsystem, and the physical nature of the electronic thermal conductivity is the speed of energy transmission within the electronic subsystem.

Optical limiting properties of hybrid nickel naphthalocyanine-titania nanoparticals thin films

In this work, an ultrathin nanocomposite films containing tetracarboxylic Nickel (Ⅱ) naphthalocyanine (NiNc) and titania nanoparticles are fabricated through electrostatic self-assembled layer-by-layer (LBL) technique. UV–Vis spectroscopy, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) are utilized to characterize the film. The optical limiting properties of the composite films are investigated through intensity-dependent transmittance measurements at 600 nm with 18 ps laser pulses. The results show that the composite films have notable optical limiting response (β ∼ 5.4 × 10−7 m/W), which is strongly enhanced compared to both TiO2 and NiNc-only samples. Our results suggest that the nanocomposite films possess the potential for optical limiting applications in the phototherapeutic window.

X-ray backlighter requirements for refraction-based electron density diagnostics through Talbot-Lau deflectometry.

Talbot-Lau x-ray interferometers can map electron density gradients in High Energy Density (HED) samples. In the deflectometer configuration, it can provide refraction, attenuation, elemental composition, and scatter information from a single image. X-ray backlighters in Talbot-Lau deflectometry must meet specific requirements regarding source size and x-ray spectra, amongst others, to accurately diagnose a wide range of HED experiments. 8 keV sources produced in the high-power laser and pulsed power environment were evaluated as x-ray backlighters for Talbot-Lau x-ray deflectometry. In high-power laser experiments, K-shell emission was produced by irradiating copper targets (500 × 500 × 12.5 μm3 foils, 20 μm diameter wire, and >10 μm diameter spheres) with 30 J, 8-30 ps laser pulses and a 25 μm copper wire with a 60 J, 10 ps laser pulse. In the pulsed power environment, single (2 × 40 μm) and double (4 × 25 μm) copper x-pinches were driven at ∼1 kA/ns. Moiré fringe formation was demonstrated for all x-ray sources explored, and detector performance was evaluated for x-ray films, x-ray CCDs, and imaging plates in context of spatial resolution, x-ray emission, and fringe contrast.

Photoluminescence and Nonlinear Optical Properties of Transition Metal (Ag, Ni, Mn) Doped ZnO Nanoparticles.

ZnO nanoparticles doped with transition metal (Ag, Ni, Mn) were prepared by using co-precipitation method. Absorption spectra showed exciton peak for ZnO nanoparticles at 367 nm and for Ag, Ni, Mn doped ZnO nanoparticles at 328, 354 nm and 342 nm respectively. Photoluminescence dynamics showed shallow level-trap and deep-level emission where the intensity of deep-level trap increases with transition metal doping due to the oxygen deficiency. Nonlinear absorption measurements showed the two-photon absorption increasing with transition metal following the trend Ag > Ni > Mn when excited with 532 nm, 30 ps laser pulses. The transition metal doped and un-doped ZnO nanoparticles can be used as optical limiters.

Nonlinear Properties Of Water-soluble Ag2S And PbS Quantum Dots Under Picosecond Laser Pulses

In this paper, the water-soluble Ag2S and PbS quantum dots(QDs) are synthesized by aqueous phase synthesis. The nonlinear optical properties of Ag2S and PbS QDs were investigated by Top-hat Z-scan technique under ps laser pulses (λ = 532 nm, t = 21 ps). The open Z-scan results show that Ag2S and PbS QDs all have a valley value which indicate that they have strong reverse saturable absorption(RSA). The nonlinear absorption coefficient of Ag2S QDs is 6.72×10-13 esu, however, the valley value of PbS QDs at the focal point is similar to 1, so its nonlinear absorption coefficient is small, is 7.13×10-14 esu. The close Z-scan results show that Ag2S QDs has self-defocusing effect and PbS QDs has self-focusing effect, their nonlinear refractive index are on the order of -6.13×10-13 esu and 2.41×10-13 esu. After fitting calculation, the third order nonlinear polarizability of Ag2S and PbS QDs are 2.40×10-14 esu and 6.02×10-15 esu respectively. The physical mechanisms responsible for the nonlinear optical response of the QDs are discussed in detail. In summary, the nonlinear absorption and refraction properties of Ag2S QDs are better than that of PbS QDs, but both of these materials are of great significance for the research of photoelectric, biosensor, optical limiting, and self-adaptive technologies.

On a New Method of Acoustic Monitoring of the Nanosecond Laser Ablation of Metals

The possibility of acoustic detection of submicron displacement of a metal surface during ablation induced by a nanosecond (∼550 ns) train of ∼70 short (∼60 ps) laser pulses has been demonstrated experimentally for the first time. The detection is based on the comparison of time structures of modulation of the laser intensity and recoil pressure in an irradiated target observed by a piezoelectric sensor. The displacement at low intensities of irradiation is due to thermal expansion. With an increase in the intensity, the displacement changes sign because of the development of ablation processes in the irradiated region. The proposed method makes it possible to obtain new information on the behavior of a material, in particular, under the conditions of extreme phase nonequilibrium under intense laser irradiation.

A novel longitudinal laserwire to non-invasively measure 6-dimensional bunch parameters at high current hydrogen ion accelerators

Optical methods for non-invasive beam diagnostics of high current H− ion accelerators have been developed in recent years. Such laserwires typically measure a 1D beam profile and/or 2D transverse emittance from the products of photo-detached ions as a laser beam is scanned across the H− beam. For laser pulse durations (∼80 ns) longer than the RF period (∼3ns), the detector integrates many complete bunches, enabling only transverse beam monitoring. This paper presents a new technique to capture a series of time resolved transverse emittance measurements along the bunch train. A fast (∼10 ps) pulsed laser photo-detaches ions within each bunch and is synchronised to sample consecutive bunches at certain longitudinal positions along each bunch. A fast detector records the spatial distribution and time-of-flight of the neutralised H0, thus both the transverse and longitudinal emittance are reconstructed. We present simulations of a time varying pulsed laser field interacting within an H− bunch, and estimate the yield, spatial and time distributions of H0 arriving at the detector. We summarise the design of a recently funded longitudinal laserwire being installed in FETS at RAL, UK.

Inscription of silicon waveguides using picosecond pulses.

Direct writing of single-mode waveguides into crystalline silicon using ps laser pulses is presented. The embedded structures were fabricated by moving the focal position along the beam axis with the help of a long distance microscope objective. In situ monitoring during inscription was performed to analyze the processing dynamics. The waveguide generation is based on pronounced multi-pulse interaction at moderate pulse energies around 100 nJ. All samples were characterized in terms of mode field distribution and damping losses. Calculations indicate an induced refractive index change in the range of 10-3 to 10-2. Moreover, a Y-splitter was realized to demonstrate the potential of this process.

Picosecond all-optical switching and dark pulse generation in a fibre-optic network using a plasmonic metamaterial absorber

Coherent interaction of two light waves on a film of subwavelength thickness provides remarkable opportunities for controlling intensity and polarization of light beams as well as all-optical image processing. Here, we show that such interactions can be used for optical dark pulse generation and basic all-optical signal processing in fully fiberized coherent information networks with 1 THz bandwidth. With an encapsulated plasmonic metamaterial absorber operating in the telecommunications C-band, we demonstrate switching and dark pulse generation with 1 ps laser pulses.

Invited Article: CARS molecular fingerprinting using sub-100-ps microchip laser source with fiber amplifier

We have developed an ultrabroadband multiplex coherent anti-Stokes Raman scattering (CARS) microspectroscopic system using a supercontinuum (SC) seeded by sub-100-ps (85 ps) laser pulses with a sub-MHz (0.82 MHz) repetition rate. Because of the high peak power and ultrabroadband spectral profile of the SC, we can efficiently generate multiplex CARS signals in the spectral range of 600–3600 cm−1, which covers the entire molecular fingerprint region, as well as the C—H and O—H stretching regions. Due to the high peak power of the new laser source, the exposure time (pixel dwell time) for CARS imaging of polymer beads was reduced to less than 1 ms (0.8 ms), which was limited by the readout time of a CCD camera. Owing to the improvement in CARS spectral quality, clear molecular fingerprinting was achieved for living HeLa cells at different phases in the cell cycle.

Short-pulse-induced solute migration in the C49H43ClO6 + 1,2 dichloroethane solution.

Using the Z-scan technique with 532 nm 19 ps laser pulses separated by two time intervals τp-p's (0.1 s and 1.0 s) sandwiching the mass diffusion time constant of the C49H43ClO6 + 1,2 dichloroethane solution, we investigate short-pulse-induced solute migration in the sample by measuring its transmittance change with τp-p variation. Preparing the sample at two concentrations, we find that τp-p reduction, from 1.0 s to 0.1 s, increases its transmittance when input pulse energy ε1 exceeds a threshold εT, which is lower for the dilute solution than the concentrated one. At two ε1's above εT for the dilute solution, τp-p-reduction-induced transmittance increase in the dilute solution, as compared to that in the concentrated solution, is more at the lower ε1 and less at the higher ε1. This differs from continuous-wave-driven thermal diffusion which always causes a larger transmittance increase in the concentrated solution by inducing a larger temperature gradient. From this study, we predict that solute migration induced by short pulses at 1064 nm is one of the undesired heating effects occurring when this solution is used to simultaneously Q-switch and mode-lock Nd:YAG lasers.

Cavity-enhanced absorption spectroscopy for shocktubes: Design and optimization

Cavity-enhanced absorption spectroscopy (CEAS) has generated much interest in shocktube kinetics studies because of its recent success in achieving improved sensitivity and high time resolution with robust optical alignment. While recent progress demonstrated experimental schemes including off-axis scanned-wavelength approach and on-axis ps-pulsed laser approach, that both successfully suppressed the laser-cavity coupling noise, this paper develops a theoretical model to predict the CEAS sensor performance that can be used as a design tool applicable to more generalized cases. The method models the optical field in the cavity based on the decentered Gaussian beam model, from which the cavity transmission spectrum and the laser-cavity coupling noise can be numerically calculated. The simulation results predict sensor performance for different cavity configurations and laser characteristics, including various degrees of laser-cavity mode-matching, laser linewidths, scanning rates, and cavity filling conditions. Simulation with example wavelengths in the ultraviolet, near-infrared, and mid-infrared showed increasing mode-matched beam waist size for increasing wavelengths. An off-axis alignment scheme was found to be capable of suppressing the coupling noise by two orders-of-magnitude at a moderate laser linewidth of 1 GHz. Coupling noise level on the order of 1e-5 for scanned-wavelength off-axis alignment case with a narrowband mid-infrared laser was obtained by model calculation and agreed with experimental results within acceptable uncertainty range. The developed method can serve to guide future design and optimization of CEAS system in shocktube studies.

Combination of short and ultrashort pulse laser processing for productive large scale structuring of 3D plastic mould steel

Surface functionality is an increasing and crucial factor for the success and acceptance of a product. Through structured surfaces, products can gain additional functions: for example, friction reduction in combustion engines and bearings or optimizing lighting efficiency in light-emitting diodes. Furthermore, not only technical properties but also optical and haptic functions can be generated by surface structuring. Especially in the area of consumer products, these functions determine essentially the product quality. The most common way to create surface functionality in mass production is replication processes via structured mold tools. Currently used manufacturing processes for tool structuring like photochemical etching are limited in precision (manual preparation and execution) and in flexibility (limited design). This is the reason why laser ablation with (ultra-) short laser radiation is becoming an increasingly important technology that is able to generate structure sizes in the range of several hundred nanometers to several hundred micrometers. In this paper, the possibility of combining nanosecond (ns) pulsed laser radiation with picosecond (ps) pulsed laser radiation for microstructuring is investigated. This approach promises an increase of the productivity of the manufacturing process, similar to the strategy of roughing and finishing in milling processes. Within a first laser process, the workpiece is pretreated by short nanosecond laser pulses and a large amount of the volume is removed. A second, following laser process using picosecond pulses is utilized to ablate the remaining volume and to finish the surface by fabricating precise microstructures. This combination brings the advantages of both processes together: (comparable) short processing time (short pulses = roughing) while achieving high precision (ultrashort pulses = finishing). Main research activities presented are different approaches for combination strategies, discussions about achievable and from industrial site acceptable processing times, and accessible surface qualities for the combined process.

Microstructure evolution of the crystallization of amorphous Ge2Sb2Te5 thin films induced by single picosecond pulsed laser

Phase-change materials (PCMs) have been widely used in optical and electronic applications for their extraordinary properties. However, the procedure of fast transition about PCMs is still not clear for the time limit. Pulse laser, especially ultra-short pulse laser, is an excellent tool to study the crystallization characteristics of PCMs for its high heating rate. In this paper, picosecond pulsed laser was employed to study the crystallization of amorphous Ge2Sb2Te5 (a-GST) thin films via the evolution of morphology and microstructure. The threshold for inducing crystallization was 7.9 mJ/cm2 and amorphization occured at 17.2 mJ/cm2. With the aid of high-resolution transmission electron microscopy (HRTEM), the growth of grains were observed to own a preferable growth direction of [100]. The grain size and laser fluence showed an exponential relationship under the ps laser pulse, and the maximal grain reached to ~5 μm when approaching the melting point. The morphology and grain distribution of GST film under 18.8 mJ/cm2 indicated a solid-state phase transition. Besides, the matrix of a-GST film changed into crystalline state even though the surface melted into amorphous state when irradiated by high laser pulse. Finally, the reason for fast transition of a-GST film under ultra-short laser pulse was discussed in terms of heating rate and temperature. The present study is fundamental for the understanding of the fast phase transition of PCMs.

Influence of pulse repetition rate and pulse energy on the heat accumulation between subsequent laser pulses during laser processing of CFRP with ps pulses

Heat accumulation between subsequent laser pulses (HAP) is one of the major reasons for the formation of a heat-affected zone during laser processing of carbon fiber-reinforced plastics (CFRP) with picosecond (ps) laser pulses. In the case of CFRP, the formation of a so-called matrix evaporation zone (MEZ) can be observed. Based on a theoretically derived equation that describes the heat accumulation effect and presuming one-dimensional heat flow, an expression for the critical feed rate at which the HAP effect sets in was derived as a function of the pulse repetition rate and the pulse energy. This expression provides a simple dependency of the critical feed rate on the pulse energy and the repetition rate that is a useful tool for process development. To verify this relation, we used dedicated experimental conditions that ensured one-dimensional heat flow and irradiated the material with fluences below the ablation threshold to ensure that the absorbed energy completely remains in the material and is not removed by an ablation process. By doing so, we were able to confirm the validity of the theoretical model. During the actual cutting of CFRP, however, hence, when material is removed, the theoretically predicted dependency of the critical feed rate on the pulse energy and the repetition rate deviates from experimental results when one assumes the fraction of pulse energy that is left in the workpiece as heat (hence not removed with the ablated material) to be constant. The difference, therefore, is most likely attributed to the fact that the fraction of heat remaining in the material after each ablation processes, the so-called residual heat, depends on both the pulse repetition rate and the pulse energy. These dependencies are reported for the first time for pulsed laser processing of CFRP within the presented study.