Ultrafast Laser Beam Research Articles

Filament-induced breakdown spectroscopy with structured beams.

Filament-induced ablation represents an attractive scheme for long-range material identification via optical spectroscopy. However, the delivery of laser energy to the target can be severely hindered by the stochastic nature of multiple-filamentation, ionization of ambient gas, and atmospheric turbulence. In order to mitigate some of these adverse effects, we examine the utility of beam shaping for femtosecond filament-induced breakdown spectroscopy with Gaussian and structured (Laguerre-Gaussian, Airy, and Bessel-Gaussian) beams in the nonlinear regime. Interaction of filaments with copper, zinc, and brass targets was studied by recording axially-resolved broadband emission from the filament-induced plasma. The laser-solid coupling efficacy was assessed by inferring thermodynamic parameters such as excitation temperature and electron density. While under our experimental conditions the ablation rate with Gaussian- and Laguerre-Gaussian beams is found to be similar, the Airy and Bessel-Gaussian beams offer the advantage of longitudinally extended working zones. These results provide insights into potential benefits of structuring ultrafast laser beams for standoff sensing applications.

Autofluorescence guided welding of heart tissue by laser pulse bursts at 1550 nm.

Wound healing and other surgical technologies traditionally solved by suturing and stapling have recently been enhanced by the application of laser tissue welding. The usage of high energy laser radiation to anastomose tissues eliminates a foreign body reaction, reduces scar formation, and allows for the creation of watertight closure. In the current work, we show that an ultrafast pulsed fibre laser beam with 183 µJ·cm-2 energy fluence at 1550 nm provides successful welding of dissected chicken heart walls with the tensile strength of 1.03±0.12 kg·cm-2 equal to that of native tissue. The welding process was monitored employing fluorescence spectroscopy that detects the biochemical composition of tissues. We believe that fluorescence spectroscopy guided laser tissue welding is a promising approach for decreasing wound healing times and the avoiding risks of postoperative complications.

Femtosecond laser damage resistance of beam dump materials for high-peak power laser systems

Ultrahigh peak power laser systems require highly absorptive and high-damage threshold beam dumps, which allow safe and reliable termination of the beam path in vacuum. We present promising optical materials for high-power laser beam dump designs. In order to evaluate the optimal beam dump performance of the various materials, we characterized the optical properties of these materials and measured their damage thresholds. The tested beam dump materials include highly absorptive (>99.8 % ) Vantablack coatings and absorptive glasses with different optical densities. The laser-induced damage threshold (LIDT) tests were performed using 800-nm ultrafast laser beams with a pulse duration of 40 fs at a 1-kHz repetition rate. The LIDT test methods, such as S-on-1 and R-on-1, were used for determining the damage threshold. The Vantablack coatings showed a moderate ultrafast damage threshold while being highly absorptive and applicable on metallic substrates. The absorptive glasses with high optical density have higher damage resistance and can be used for low-repetition rate applications with the highest intensities. By comparing the optical properties and LIDT results, we discuss the suitability of these beam dump materials for ultrafast high-power laser systems.

Near-Infrared Laser-Based Spatially Targeted Nano-enhanced Optical Delivery of Therapeutic Genes to Degenerated Retina

Non-viral delivery of therapeutic genes into targeted areas of retina is essential for re-functionalizing the retinal circuitry. While a focused ultrafast laser beam has been recently used for intra-ocular delivery of molecules, it poses the significant technical challenge of overcoming aberrations of the eye and maintaining a tightly focused spot on the retinal cell membrane. Furthermore, to minimize collateral damage and increase the throughput of gene delivery, we introduced a weakly focused near-infrared (NIR) continuous wave (CW) or pulsed laser beam on to the cells wherein the intensity is locally enhanced by gold nanorods bound to the cell membranes to permit gene insertion. Parametric optimization of nano-enhanced optical delivery (NOD) was carried out by varying the exposure time, as well as the power of the CW NIR beam or the energy of the pulsed NIR beam. Using this NOD method, therapeutic genes encoding for multi-characteristic opsins (MCOs) were delivered to spatially targeted regions of degenerated retina ex vivo as well as in vivo. NOD-mediated cell membrane-specific expression of MCOs in targeted retinal regions with photoreceptor degeneration will allow functional recovery in an ambient light environment.

Free-Electron-Bound-Electron Resonant Interaction.

Here we present a new paradigm of free-electron-bound-electron resonant interaction. This concept is based on a recent demonstration of the optical frequency modulation of the free-electron quantum electron wave function (QEW) by an ultrafast laser beam. We assert that pulses of such QEWs correlated in their modulation phase, interact resonantly with two-level systems, inducing resonant quantum transitions when the transition energy ΔE=ℏω_{21} matches a harmonic of the modulation frequency ω_{21}=nω_{b}. Employing this scheme for resonant cathodoluminescence and resonant EELS combines the atomic level spatial resolution of electron microscopy with the high spectral resolution of lasers.

Automated free-space beam delivery system for ultrafast laser beams in the kW regime

In contrast to cw laser beams, the beam delivery of ultrafast laser beams with high average power or high pulse energies using optical fibers is still a challenge. In this paper, we present a free-space delivery and distribution system, which is able to deliver ultrafast laser beams with an average power of up to 5 kW and pulse energies of up to 5 mJ. This system is designed for automated switching between up to three laser sources and four processing stations with state of the art safety measures. Currently it is in use at the IFSW, with one kW class ultrafast laser source and three processing stations with a distance of up to 15 m from the laser source. Due to its active beam stabilization, high precision processes are possible, like Direct Laser Interference Patterning with structure periods below 1 µm, OCT based ablation control or micro drilling.

Investigation of interactions between ultrafast laser beams and screen-printed silver nanopaste films

This study aims to investigate the interaction between ultrafast laser beams and screen-printed silver (Ag) nanopaste films for strain sensors. After single pulse ablation of Ag nanopaste films with a thickness of 4.58 μm, the ablation threshold was approximately 0.2 J/cm2. By increasing single pulse laser fluences from 0.78 J/cm2 to 1.87 J/cm2, the laser-ablated line widths and depths of Ag nanopaste films were ranging from 22 ± 0.1 μm and 4 ± 0.09 μm to 44.8 ± 1.4 μm and 4.2 ± 0.05 μm, respectively. The sheet resistance of laser-processed Ag nanopaste films was measured by a four-point probe instrument. When the single pulse laser fluences set from 4.7 mJ/cm2 to 1.97 J/cm2, the measured sheet resistances near the laser-ablated line edge of 0.5 mm were 59.52 ± 0.76 mΩ/sq and 115.83 ± 6.11 mΩ/sq, respectively. The optimal ultrafast laser processing parameters consisting of the areal laser fluence of 47.4 J/cm2, the laser pulse repetition rate of 300 kHz, the scanning speed of galvanometers of 500 mm/s, and the overlapping rate of laser spots of 96.1% were used to pattern strain sensors with transversal and longitudinal electrode structures on Ag film/glass substrates.

Development of an ultrafast electron source based on a cold-field emission gun for ultrafast coherent TEM

We report on the design of a femtosecond laser-driven electron source for ultrafast coherent transmission electron microscopy. The proposed architecture allows introducing an ultrafast laser beam inside the cold field emission source of a commercial TEM, aligning and focusing the laser spot on the apex of the nanoemitter. The modifications of the gun assembly do not deteriorate the performances of the electron source in conventional DC mode and allow easy switching between the conventional and ultrafast laser-driven emission modes. We describe here this ultrafast electron source and discuss its properties.

A simple technique to overcome self-focusing, filamentation, supercontinuum generation, aberrations, depth dependence and waveguide interface roughness using fs laser processing

Several detrimental effects limit the use of ultrafast lasers in multi-photon processing and the direct manufacture of integrated photonics devices, not least, dispersion, aberrations, depth dependence, undesirable ablation at a surface, limited depth of writing, nonlinear optical effects such as supercontinuum generation and filamentation due to Kerr self-focusing. We show that all these effects can be significantly reduced if not eliminated using two coherent, ultrafast laser-beams through a single lens - which we call the Dual-Beam technique. Simulations and experimental measurements at the focus are used to understand how the Dual-Beam technique can mitigate these problems. The high peak laser intensity is only formed at the aberration-free tightly localised focal spot, simultaneously, suppressing unwanted nonlinear side effects for any intensity or processing depth. Therefore, we believe this simple and innovative technique makes the fs laser capable of much more at even higher intensities than previously possible, allowing applications in multi-photon processing, bio-medical imaging, laser surgery of cells, tissue and in ophthalmology, along with laser writing of waveguides.

Scopine Isolated in the Gas Phase.

The rotational spectrum of the tropane alkaloid scopine is detected by Fourier transform microwave spectroscopy in a pulsed supersonic jet. A nonconventional method for bringing the molecules intact into the gas phase is used in which scopine syrup is mixed with glycine powder and the solid mixture is vaporized with an ultrafast UV laser beam. Laser vaporization prevents the easy isomerization to scopoline previously observed with conventional heating methods. A single conformer is unambiguously observed in the supersonic jet and corresponds to the energetically most stable species according to quantum chemical calculations. Rotational and centrifugal distortion constants are accurately determined. The spectrum shows fine and hyperfine structure due to the hindered rotation of the methyl group and the presence of a quadrupolar nucleus (14 N), respectively. This additional information allows the angle of N-methyl inversion between the N-CH3 bond and the bicyclic C-N-C plane to be determined (131.8-137.8°), as well as the internal rotation barrier of the methyl group (6.235(1) kJ mol-1 ).

Laser-Induced Spatiotemporal Dynamics of Magnetic Films.

We present a theory for the coherent magnetization dynamics induced by a focused ultrafast laser beam in magnetic films, taking into account nonthermal (inverse Faraday effect) and thermal (heating) actuation. The dynamic conversion between spin waves and phonons is induced by the magnetoelastic coupling that allows efficient propagation of angular momentum. The anisotropy of the magnetoelastic coupling renders characteristic angle dependences of the magnetization propagation that are strikingly different for thermal and nonthermal actuation.

A promising energetic X-ray source: the non-linear Thomson scattering of ultrafast laser beams on relativistic electron bunches

In a previous paper we proved a periodicity property of the Lienard-Wiechert equation in the case of the relativistic interaction between electromagnetic field and electron. This property predicts the existence of harmonics of the nonlinear Thomson scattered radiations in head on collisions between very intense laser beams and relativistic electron bunches. In this paper we present the properties of these radiations and show that our method for the calculation of the angular and spectral distributions of the backscattered radiations is in good agreement with the existing experimental data from literature. In the case of incident radiations having relativistic parameters of the order of few units and electron energies up to 100 MeV, our calculations predict the possibility of producing hard X-rays, having relatively high intensities, which are comparable to the intensities of the first harmonics, and energies higher than 1 MeV, using the current technology. Our theoretical model uses only one approximation, that of the neglecting of the radiative corrections.

Ultrafast laser beam shaping for material processing at imaging plane by geometric masks using a spatial light modulator

We have demonstrated an original ultrafast laser beam shaping technique for material processing using a spatial light modulator (SLM). Complicated and time-consuming diffraction far-field phase hologram calculations based on Fourier transformations are avoided, while simple and direct geometric masks are used to shape the incident beam at diffraction near-field. Various beam intensity shapes, such as square, triangle, ring and star, are obtained and then reconstructed at the imaging plane of an f-theta lens. The size of the shaped beam is approximately 20µm, which is comparable to the beam waist at the focal plane. A polished stainless steel sample is machined by the shaped beam at the imaging plane. The shape of the ablation footprint well matches the beam shape.

Far-Field Nanostructuring in Dielectric Materials Beyond the Diffraction Limit Using Femtosecond Laser

We review the latest progress in the development of far-field nanostructuring technologies based on femtosecond laser direct writing. The principles for breaking through the diffraction limit in the interaction of a focused ultrafast laser beam with dielectric materials are presented, and applications of the femtosecond laser nanostructuring are discussed.

Ultrafast laser micro-cutting of stainless steel and PZT using a modulated line of multiple foci formed by spatial beam shaping

Femtosecond laser surface processing of materials allows for precise micrometer cutting with restricted detrimental side effects. An efficient way to increase the ablation or cutting speed and energy efficiency of the process is the generation of N parallel laser spots in the focal plane of a lens on the sample surface, each spot simultaneously following a defined cutting trajectory. However, the cutting scheme is then limited in area to avoid overlap of the spot trajectories. We present in this paper a technique to overcome this limitation. The ultrafast laser beam wavefront is spatially modulated, generating a line of multiple processing spots in the lens focal plane. Micro-cutting is then conducted by translation of the sample along the spot line. That way, no restriction on the cutting trajectory length is imposed while preserving the increase in the cutting speed and energy efficiency of the process compared to single spot machining. Successful cutting of PZT ceramic and stainless steel is achieved with the spot array and compared to single spot machining in terms of processing time. In the case of PZT, the cutting efficiency and speed is compared to single spot laser machining with various focal lenses. We also investigate the effect of intensity gradient on the spots line for micro-cutting on stainless steel. The method efficiency is discussed relying on surface characterization using scanning electron microscopy.

Helical filaments

The shaping of laser-induced filamenting plasma channels into helical structures by guiding the process with a non-diffracting beam is demonstrated. This was achieved using a Bessel beam superposition to control the phase of an ultrafast laser beam possessing intensities sufficient to induce Kerr effect driven non-linear self-focusing. Several experimental methods were used to characterize the resulting beams and confirm the observed structures are laser air filaments.

Single-step fabrication of stressed waveguides with tubular depressed-cladding in phosphate glasses using ultrafast vortex laser beams

We report on the fabrication of the stressed optical waveguide with tubular depressed- refractive-index cladding in phosphate glasses by use of femtosecond vortex beam. Strained regions were emerged in domains surrounding the tubular track. Waveguiding occurs mainly within the tube induced by femtosecond laser.

Fiber-optic rotation of micro-scale structures enabled microfluidic actuation and self-scanning two-photon excitation

Here, we report non-restricted, controlled fiber optic rotation of micro-motor, in counter-propagating fiber-optic beams having transverse-offset, for actuation of microfluidic flow. Ray-optics based simulations of the torque (and angular velocity) were conducted for different fiber transverse-offsets in order to determine optimal geometry for effective actuation. Further, self-scanning two-photon excitation of the fiber-optically rotated microscopic object is achieved by use of an ultrafast laser beam in one of the fiber arm.

High Speed Laser Micro Processing Using High Brilliance Continuous Wave Laser Radiation

Laser micro processing using a high power single-mode continuous wave fibre laser in combina-tion with a fast galvanometer scanner as well as an ultra fast polygon scan systems was investigated. As a key technology in high rate laser ablation a maximum laser power up to 3 kW and scan speeds up to 18,000 m/minwere applied. With the ultra fast laser beam deflection and laser a smallfocal spot diameter of 21 µm laser dwell times less than 100 nanoseconds were achieved. As a result laser intensities comparable to the q-switched lasers in the range of

Purely Coherent Nonlinear Optical Response in Solution Dispersions of Graphene Sheets

We have developed an efficient chemical exfoliation approach for the high-throughput synthesis of solution-processable, high-quality graphene sheets that are noncovalently functionalized by alkylamine. Purely coherent nonlinear optical response of these graphene sheets has been investigated, using near-infrared, visible, and ultraviolet continous wave and ultrafast laser beams. Spatial self-phase modulation has been unambiguously observed in the solution dispersions. Our results suggest that this coherent light scattering is due to a broadband, ultrafast, and remarkably huge third-order optical nonlinearity χ(3), which is a manifestation of the graphene's cone-shaped large-energy-scale band structure. Our experimental findings endow graphene new potentials in nonlinear optical applications.