Powerful Ultrashort Laser Pulses Research Articles

High power ultra-short pulse laser ablation of IN718 using high repetition rates

Laser ablation using ultra-short pulsed laser radiation with pulse durations below 10ps is known to be capable of high-precision machining with very small thermal load for the processed workpiece. Although the processing quality is excellent, the productivity is too small for many applications. Therefore, an upscaling of the achievable ablation rate is one major research topic in the field of ultra-short pulse laser ablation. In this study, we identify and investigate a novel high-speed ablation process for the ablation of Inconel 718 using ultra-short pulsed laser radiation. The formation of a thin layer of molten material avoids the formation of rough microstructures on the workpiece surface that usually appear for the processing with high average powers and high spatial pulse overlaps. Thus, processing with high average power of more than 50W using conventional, high efficient and high flexible beam deflection technology becomes applicable. The presented process is approximately 20 times faster than conventional ultra-short pulse laser ablation with average powers of less than 5W.The influence of the repetition rate, average power, pulse duration and hatch distance on the high-speed ablation process is investigated. The potential of an optional finishing process to remove the molten layer by melt-free ablation with the same laser system but different processing parameters is demonstrated.

Lowering of stimulated Raman scattering threshold as a result of light capture

Stimulated Raman Scattering in globular photonic crystals and globular photonic glasses at different diameters of globules (250 – 400 nm) with embedded molecular liquids is studied under excitation by nanosecond or picoseconds laser pulses. Substantial decrease of Stimulated Raman Scattering threshold was observed. Such phenomenon was explained as the result of laser radiation field increase in globular photonic structures due to photonic density of states enhancement near the edges of photonic stop bands of photonic crystals and due to Mie resonance or whispering gallery modes effect revealing in photonic glasses. Stimulated Raman Scattering threshold lowering as a result of light capture in globular photonic crystals and photonic glasses opens the way to new efficient laser sources created on the base of composite globular photonic structures. Experimental data on spectra of Stimulated Raman Scattering in light and heavy waters are presented. As sources of exciting light the powerful ultra short solid state laser pulses with 532.0 nm wavelength and giant pulses of Ruby laser (694.3 nm) have been used. Several Stokes and anti-Stokes satellites were observed. Libration modes have been excited and resulted in some additional Raman bands at low frequency region and also as combining tones.

Transient optical properties of dielectrics and semiconductors excited by an ultrashort laser pulse

The transient permittivity of dielectrics and semiconductors excited by a powerful ultrashort laser pulse is introduced here in explicit form, which shows a decreasing contribution of valence electrons and an increasing contribution of free carriers with rising laser fluence. We describe the evolution of permittivity from the initial state up to transformation into plasma before ablation. A two orders of magnitude change in the electrons’ collision rate during this transition is taken into account explicitly. The interplay between ionization nonlinearity and electron collisions dominates the transient optical properties of a swiftly excited material and results in unexpected minima and maxima in the permittivity and reflectivity. Our analysis of the transient permittivity of silica and silicon at 800 and 1300 nm reveals the differences in the femtosecond excitation of narrow- and wide-band-gap material, and also distinguishes a metal-like state of the ionized dielectric from the excited state in metals. The dependence of transient permittivity on electron density obtained can be directly mapped onto the fluence distribution in the time and space domains. We briefly discuss the possibility to measure these transient properties in pump–probe experiments.

The extreme nonlinear optics of gases and femtosecond optical filamentation

Under certain conditions, powerful ultrashort laser pulses can form greatly extended, propagating filaments of concentrated high intensity in gases, leaving behind a very long trail of plasma. Such filaments can be much longer than the longitudinal scale over which a laser beam typically diverges by diffraction, with possible applications ranging from laser-guided electrical discharges to high power laser propagation in the atmosphere. Understanding in detail the microscopic processes leading to filamentation requires ultrafast measurements of the strong field nonlinear response of gas phase atoms and molecules, including absolute measurements of nonlinear laser-induced polarization and high field ionization. Such measurements enable the assessment of filamentation models and make possible the design of experiments pursuing applications. In this paper, we review filamentation in gases and some applications, and discuss results from diagnostics developed at Maryland for ultrafast measurements of laser-gas interactions.

Radiation by electrons channeled in a plasma-ion cavity

This work is devoted to the electron motion in an infinite plasma-ion channel formed by powerful ultra short laser pulse. Analytical expressions for both transverse energy levels and widths of plasma-ion channeled electrons in simplified approximation of a scalar particle have been derived. The spectra of electromagnetic radiation by channeled electrons in a plasma-ion cavity in the direction of relativistic motion with different initial conditions have been calculated.

Ultraviolet conical emission produced by high-power femtosecond laser pulse in transparent media

In this work, supercontinuum conical emission (SC CE) accompanying the filamentation of powerful ultrashort laser pulse in BK7 glass and fused silica is studied. The SC CE is controlled by the laser power density and the sample thickness, and the minimum SC CE cut-off wavelength is about 309 nm in the BK7 glass and 237 nm in the fused silica. The angular distributions of the SC CE in the wavelength range less than 510 nm are measured by using a new method, and it cannot be explained by the Cerenkov emission theory but the unabridged X-Waves solution theory. Meanwhile numerical simulations of the propagation of femtosecond laser pulse in sample are performed to provide theoretical support to our results.

The features of nonlinear absorption during resonance one- and two-photon excitation of the basic exciton transition in colloidal CdSe/ZnS quantum dots

The features of the nonlinear absorption of CdSe/ZnS quantum dots (colloidal solution) in the case of resonant one- and two-photon excitation of the basic exciton transition by powerful ultra-short laser pulses were determined. In one-photon excitation, with an increasing intensity of impulses, a decrease in absorption (bleaching) is relayed by an increase in absorption, which is associated with the process of the filling of the states (saturation) of a two-level system with the lifetime of the excited state depending on the light intensity. The arising Fresnel or Fraunhofer diffraction of the laser ray that pass through a colloidal solution with a high concentration of quantum dots is associated with the formation of the transparency channel and self-diffraction of laser ray on an induced diaphragm. In two-photon excitation, the features of the nonlinear absorption and luminescence tracks (the dependence of luminescence intensity on distance) were explained by the influence, in addition to the two-photon absorption, of the processes that are responsible for the slower growth of nonlinear absorption and luminescence quenching at high intensities of laser pulses.

Stimulated Raman scattering of light in artificial opal filled by water

We investigate stimulated scattering of light in globular photonic crystals infiltrated by water. Excitation of stimulated scattering of light is realized using powerful ultrashort (70 ps) laser pulses with an energy of 35 mJ and a frequency repetition of 15 Hz. We use the second-harmonic generation (532 nm) of the master oscillator and amplifier with a wavelength of 1064 nm. The photonic crystals under study are artificial opals filled by water or ethanol. We characterize the sample structures employing an electronic microscope along with the fiber-optics reflectance-spectroscopy technique. Photonic crystals have a stop band near the spectral positions of the exciting line (532 nm) and the first satellite of stimulated Raman scattering of light in water (649 nm). We observe a substantial reduction of the threshold of stimulated Raman scattering of light in water-infiltrated artificial-opal matrices in comparison with that of pure water. Such a reduction is explained as the result of a sharp increase in the photonic density of states near the stop-band edges of investigated photonic crystals. The reduction in the threshold of stimulated Raman scattering of light in water-infiltrated artificial-opal matrices opens up the opportunity to observe stimulated Raman scattering in numerous water media, including water solutions, biological and medical samples, heavy waters, and others.

Ultrafast Manipulation of Self‐Assembled Form Birefringence in Glass

Ultrashort pulse lasers have allowed probing of molecular dynamics in real time on the femtosecond time scale, with exotic behavior ranging from alignment of molecules and clusters, structural deformation, phase transitions on solid, and electron localization in magnetic materials. A recent progress in high power ultrashort pulse lasers has opened new frontiers in physics and technology of light-matter interactions from X-ray generation, nuclear fusion, laser surgery, integrated and fiber optics, optical data storage, to 3D micro- and nano-structuring. An intriguing phenomenon that currently attracts a lot of interest is the self-assembly of periodic nanostructures in the direction perpendicular to the light polarization. Uniaxial birefringence observed after femtosecond laser irradiation of silica glass has been explained by induced nanogratings and referred as self-assembled form birefringence. Self organization process has been interpreted in terms of the interference of electron plasma waves resulting in electron concentration modulation, followed by freezing of the interference pattern by structural change in glass. However, the mechanism including dynamics of self-organized nanostructures formation is still not fully understood. Recently, a double-pulse pump-probe configuration was used to enhance ablation in fused silica and silicon. In similar experiments molecular ensembles with an oriented angular momentum were produced. Here, we describe the ultrafast writing dynamics of form birefringence produced by self-organized nanogratings in double pulse experiments. Rewritable five-dimensional (5D) optical data storage using self-assembled form birefringence was demonstrated.

Femtosecond filament and supercontinuum generated in cobalt phthalocyanine chloroform solution

Filamentation and supercontinuum generation in cobalt phthalocyanine chloroform solution by 800 nm 50-fs Ti: sapphire laser pulse is investigated for the first time to the best of our knowledge. It is found that filamentation is triggered by self-focusing and defocusing. The main mechanism of supercontinuum and conical emission generation is self-phase modulation (SPM) enhanced by self-steepening of the pulse and group velocity dispersion (GVD). This investigation of organic dye solutions by fs laser is a good complementarity to study the propagation of powerful ultrashort laser pulses in liquid media.

Breakdown of a nanoparticle partially ionized by a powerful ultrashort laser pulse

This paper presents the results of an analytical calculation and numerical modeling of the disintegration of a nanoparticle of a condensed medium partially ionized by a powerful ultrashort pulse (USP) of laser radiation. The mechanism of the process by which the nanoparticle is disintegrated by the Coulomb forces that result from the disruption of charge equilibrium in it under the action of the USP is established by comparing the results obtained by analytical and numerical methods. The kinetics of the energy spectrum of the dispersing ions is analyzed, taking into account the deceleration of the ions in the nanoparticle.

Self-guiding of 100TW femtosecond laser pulses in centimeter-scale underdense plasma

An experiment for studying laser self-guiding has been carried out for the high power ultrashort pulse laser interaction with an underdense plasma slab. Formation of an extremely long plasma channel and its bending are observed when the laser pulse power is much higher than the critical power for relativistic self-focusing. The long self-guiding channel formation is accompanied by electron acceleration with a low transverse emittance and high electric current. Particle-in-cell simulations show that laser bending occurs when the accelerated electrons overtake the laser pulse and modify the refractive index in the region in front of the laser pulse.

Dynamics of the disruption of a nanoparticle totally ionized by a powerful ultrashort laser pulse

This paper presents the results of a model investigation of the consequences of total ionization of a nanoparticle of a condensed medium under the action of a powerful ultrashort pulse of laser radiation. Analytical and numerical methods are used to investigate the evolution of the spatial and energy distributions of ions of the medium in the course of their dispersal for various particle sizes and geometries.

Production of hollow cylindrical plasmas for laser guiding in acceleration experiments

One of the major tasks for the progress of laser acceleration of electrons in plasmas is the guiding of focused pulses along path lengths much larger than the depth of focus. We have tested experimentally the production of hollow plasmas in gas to be used as guiding medium. We induced optical breakdown in Helium with a nanosecond laser pulse similar to the Amplified Spontaneous Emission (ASE) pedestal of a powerful ultrashort laser pulse. The plasma produced in this way was carefully characterized by high resolution interferometry. Plasma channels several millimeters in length whose density, depth and width match the requirements for an efficient guiding have been obtained. Channels with a variety of parameters can be created according to the well-known dynamics of the plasmas produced by optical breakdown.

Anomalous transmission of an ultrashort ionizing laser pulse through a thin foil.

The formation of a highly anisotropic photoelectron velocity distribution as a result of the interaction of a powerful ultrashort laser pulse with a thin foil is found to yield a large skin-layer depth and an anomalous increase of the transmission coefficient. The physical reason for the effect is the influence of the incident wave magnetic field, through the Lorenz force, on the electron kinetics in the skin layer.

Experiments Detail How Powerful Ultrashort Laser Pulses Propagate through Air

Thanks to self-focusing, laser pulses can be launched upward into clouds where they can be used to measure pollutants.

Nuclear reactions triggered by laser-accelerated high-energy ions

A technique is suggested for triggering nuclear reactions by accelerating ions with a powerful ultrashort laser pulse in a plasma. The underlying idea of the suggested compact “reactor” is utilization of high-energy ions accelerated by the charge-separation electrostatic field in the direction perpendicular to the laser beam axis in a gas-filled capillary. Accelerated ions with energies of several MeV penetrating the target from the inside surface of a channel give rise to nuclear reactions which can be used to create a compact source of fast neutrons and neutrons of intermediate energies for generating various (short-and long-lived, light and heavy) isotopes, for generating gamma radiation over a broad energy range, for making sources of light ion and induced radioactivity. The yield of the corresponding nuclear reactions as a function of the laser beam parameters has been investigated. The suggested technique for triggering nuclear reactions provides a practical tool for studies of nuclear transformation on the pico-and nanosecond scales, which cannot be achieved using other methods.

Four-wave interaction in gas and vacuum: definition of a third-order nonlinear effective susceptibility in vacuum: χvacuum(3)

Semiclassical methods are used to study the nonlinear interaction of light in vacuum in the context of four wave mixing. This study is motivated by a desire to investigate the possibility of using recently developed powerful ultrashort (femtosecond) laser pulses to demonstrate the existence of nonlinear effects in vacuum, predicted by quantum electrodynamics (QED). An approach, similar to classical nonlinear optics in a medium, is developed in this article. A third-order nonlinear effective susceptibility of vacuum is then introduced.

Acceleration of electrons in thin metal films by strong ultra-short laser pulses

The possibility of effective electron acceleration in a thin solid film exposed to irradiation by two power ultra-short laser pulses is discussed. The acceleration effect is caused by resonant action of the radiation on electrons oscillating across the film with frequencies close to the laser frequency. The relativistic equations of electron motion are numerically analyzed.

FILAMENTATION AND SUPERCONTINUUM GENERATION DURING THE PROPAGATION OF POWERFUL ULTRASHORT LASER PULSES IN OPTICAL MEDIA (WHITE LIGHT LASER)

The fundamental physical mechanism responsible for the self-focussing, filamentation, supercontinuum generation and conical emission of a powerful ultrashort laser pulse in a transparent optical medium is reviewed. The propagation can be described by the model of moving focus modified by the defocussing effect of the self-induced plasma through multiphoton interaction with the medium. Spatial and temporal self-phase modulation in both the neutral Kerr medium and the plasma transform the pulse into a chirped (elongated) and strongly deformed pulse both temporally and spatially. The manifestation of the deformation is supercontinuum generation and conical emission. A new phenomenon of refocussing was observed. It is due to the diffraction of the trailing part of the pulse by the plasma that results in a ring structure of positive index changes surrounding the plasma column. This ring structure refocuses the pulse partially. The measured coherence lengths of the various frequencies components of the supercontinuum are independent of the optical media used and are essentially equal to that of the pump laser pulse when compared to an incoherent white light source. We thus justify that such a deformed pulse with a very broad spectrum could be called a chirped white light laser pulse.