Pulses In Air Research Articles

Periodic surface structure bifurcation induced by ultrafast laser generated point defect diffusion in GaAs

The formation of high spatial frequency laser induced periodic surface structures (HSFL) with period <0.3 λ in GaAs after irradiation with femtosecond laser pulses in air is studied. We have identified a point defect generation mechanism that operates in a specific range of fluences in semiconductors between the band-gap closure and ultrafast-melt thresholds that produces vacancy/interstitial pairs. Stress relaxation, via diffusing defects, forms the 350–400 nm tall and ∼90 nm wide structures through a bifurcation process of lower spatial frequency surface structures. The resulting HSFL are predominately epitaxial single crystals and retain the original GaAs stoichiometry.

Generation of scalable terahertz radiation from cylindrically focused two-color laser pulses in air

We demonstrate scalable terahertz (THz) generation by focusing terawatt, two-color laser pulses in air with a cylindrical lens. This focusing geometry creates a two-dimensional air plasma sheet, which yields two diverging THz lobe profiles in the far field. This setup can avoid plasma-induced laser defocusing and subsequent THz saturation, previously observed with spherical lens focusing of high-power laser pulses. By expanding the plasma source into a two-dimensional sheet, cylindrical focusing can lead to scalable THz generation. This scheme provides an energy conversion efficiency of 7 × 10−4, ∼7 times better than spherical lens focusing. The diverging THz lobes are refocused with a combination of cylindrical and parabolic mirrors to produce strong THz fields (>21 MV/cm) at the focal point.

Energy deposition from focused terawatt laser pulses in air undergoing multifilamentation.

Laser filamentation is responsible for the deposition of a significant part of the laser pulse energy in the propagation medium. We found that using terawatt laser pulses and moderately strong focusing conditions in air, more than 60 % of the pulses energy is transferred to the medium, eventually degrading into heat. This results in a strong hydrodynamic reaction of air with the generation of shock waves and associated underdense channels for each of the generated multiple filaments. In the focal zone, where filaments are close to each other, these discrete channels eventually merge to form a single cylindrical low-density tube over a ~ 1 µs timescale. We measured the maximum lineic deposited energy to be more than 1 J·m-1.

Aging Characteristics on Epoxy Resin Surface Under Repetitive Microsecond Pulses in Air at Atmospheric Pressure**supported by the Natural Science Foundation of Hebei Province (No. E2015502081), National Natural Science Foundation of China (Nos. 51222701, 51307060), and the National Basic Research Program of China (No. 2014CB239505-3)

Research on aging characteristics of epoxy resin (EP) under repetitive microsecond pulses is important for the design of insulating materials in high power apparatus. It is because that very fast transient overvoltage always occurs in a power system, which causes flashover and is one of the main factors causing aging effects of EP materials. Therefore, it is essential to obtain a better understanding of the aging effect on an EP surface resulting from flashover. In this work, aging effects on an EP surface were investigated by surface flashover discharge under repetitive microsecond pulses in atmospheric pressure. The investigations of parameters such as the surface micro-morphology and chemical composition of the insulation material under different degrees of aging were conducted with the aid of measurement methods such as atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Results showed that with the accumulation of aging energy on the material surface, the particles formed on the material surface increased both in number and size, leading to the growth of surface roughness and a reduction in the water contact angle; the surface also became more absorbent. Furthermore, in the aging process, the molecular chains of EP on the surface were broken, resulting in oxidation and carbonisation.

Ultrafast imaging the light-speed propagation of a focused femtosecond laser pulse in air and its ionized electron dynamics and plasma-induced pulse reshaping

The light-speed propagation of a focused femtosecond (fs) laser pulse in air was recorded by a pump–probe shadowgraph imaging technique with femtosecond time resolution. The ultrafast dynamics of the laser-ionized electrons were studied, which revealed a strong reshaping of the laser field due to laser–air nonlinear interaction. The influence of laser fluence and focusing conditions on the pulse reshaping was studied, and it was found that: (1) double foci are formed due to the refocusing effect when the laser fluence is higher than 500 J/cm2 and the focusing numeric aperture (NA) is higher than 0.30; and (2) a higher NA focusing lens can better inhibit the prefocusing effect and nonlinear distortion in the Gaussian beam waist.

Spectroscopic and electrical characters of SBD plasma excited by bipolar nanosecond pulse in atmospheric air

In this paper, an atmospheric surface barrier discharge (SBD) generated by annular electrodes in quartz tube is presented through employing bipolar nanosecond pulse voltage in air. The discharge images, waveforms of pulse voltage and discharge current, and optical emission spectra emitted from the discharges are recorded and calculated. A spectra simulation method is developed to separate the overlap of the secondary diffraction spectra which are produced by grating in monochromator, and N2 (B3Πg→A3Σu+) and O (3p5P→3s5S2o) are extracted. The effects of pulse voltage and discharge power on the emission intensities of OH (A2Σ+→X2Пi), N2+ (B2Σu+→X2Σg+), N2 (C3Πu→B3Πg), N2 (B3Πg→A3Σu+), and O (3p5P→3s5S2o) are investigated. It is found that increasing the pulse peak voltage can lead to an easier formation of N2+ (B2Σu+) than that of N2 (C3Πu). Additionally, vibrational and rotational temperatures of the plasma are determined by comparing the experimental and simulated spectra of N2+ (B2Σu+→X2Σg+), and the results show that the vibrational and rotational temperatures are 3250±20K and 350±5K under the pulse peak voltage of 28kV, respectively.

Modelling the impact of non-equilibrium discharges on reactive mixtures for simulations of plasma-assisted ignition in turbulent flows

This article presents a model to describe the effects of non-equilibrium plasma discharges on gas temperature and species concentration, in the set of equations governing the combustion phenomena. Based on the results reported in the literature, the model is constructed by analysing the channels through which the electric energy is deposited. The two main channels by which the electrons produced during the discharge impact the flow are considered: (1) the excitation and the subsequent relaxation of electronic states of nitrogen molecules which leads to an ultrafast increase of gas temperature and species dissociation within the discharge characteristic time; and (2) the excitation and relaxation of vibrational states of nitrogen molecules which causes a much slower gas heating. The model is fully coupled with multi-dimensional flow balance equations with detailed transport coefficients and detailed combustion chemical kinetic mechanisms. This high level of NRP discharge modelling allows computing high Reynolds flows by means of Direct Numerical Simulations and, therefore, a better understanding of plasma-assisted ignition phenomena in practical configurations. A sequence of discharge pulses in air and methane–air mixture in quiescent and turbulent flow configurations are studied with this model. The results show the minor impact of the vibrational energy on mixture ignition and how the increase of the turbulence spreads this vibrational energy and intermediate combustion species around the discharge zone, minimizing the cumulative effect of multiple pulses. In contrast, the production of O atoms during the discharge has a strong impact on the ignition delays and ignition energies (number of discharge pulses). The results also underscore the impact of the initial turbulent flow Reynolds number and the spatial distribution of turbulent eddies, relative to the discharge channel, on the number of pulses needed to ignite the mixture.

Acoustic emission of ultrashort laser pulses in air and aerosols

The effectiveness of photoacoustic diagnosis in studying ultrashort laser pulse propagation in different media is considered on the basis of results from laboratory experiments on recording acoustic emissions induced by terawatt femtosecond laser pulses affecting aerosols, individual droplets, and air.

Nitrogen fluorescence induced by the femtosecond intense laser pulses in air

Our experiments show that external focusing and initial laser energy strongly influences filament generated by the femtosecond Ti–sapphire laser in air. The experimental measurements show the filament length can be extended both by increasing the laser energy and focal length of focusing lens. On the other hand, the plasma fluorescence emission can be enhanced by increasing the laser energy with fixed focal length or decreasing the focal length. In addition, the collapse distance measured experimentally are larger than the calculated ones owing to the group-velocity-dispersion effect. In addition, we find that the line widths of the spectral lines from $\text{N}_{2}$ is independent of filament positions, laser energies and external focusing.

Laser-induced breakdown spectroscopy system for remote measurement of salt in a narrow gap

We performed remotely measured, with a 5-m optical path, the chlorine concentration of a sea salt attached to stainless steel (SS) located at the side wall of a narrow gap (width ~50mm) by using laser-induced breakdown spectroscopy (LIBS) in two configurations. One uses mirrors for transmitting laser pulses in air, while the other uses multimode fiber. A compact optical device was developed to access the surface of SS for focusing laser pulses and collecting laser-induced plasma. With the configuration in which laser pulses pass through the fiber, the chlorine spectrum could be detected by fiber-coupled LIBS. In addition, with the configuration in which laser pulses pass through air, chlorine concentrations from 0 to 100mg/m2 could be evaluated quantitatively by using the calibration data of chlorine emission intensity. These results show that the proposed system enables the measurement of chlorine at the surface of SS remotely, instantly, and quantitatively.

Intrafilm separation of solgel film under nanosecond irradiation.

We have observed large-scale intrafilm separation after the irradiation of solgel film with a single Nd:YAG pulse (1064nm, 12ns) in air. The irradiated but undamaged surface or the surface after intrafilm separation is densified. These damage features are distinctly different from the scalding surface of the electron beam evaporation coating or the ripple structures on the rear surface of fused silica, which indicates the extreme pressure gradients at the free surface-film interface. The submicrometer size melted cavity in the center of damage site is related with the nanoscale absorber. A phenomenological description that combines the defect-induced incubation phase and laser-supported surface breakdown wave is used to explain the damage process.

Role of coherent resonant nonlinear processes in the ultrashort KrF laser pulse propagation and filamentation in air

Recent experiments on multiple filamentation of sub-picosecond terawatt-level KrF laser pulse in air and multi-photon ionization of air revealed an extremely low electron density in filaments, which is out of the conventional filamentation model considering Kerr self-focusing and plasma de-focusing. We propose here the coherent resonant scattering and ionization processes at the pulse durations significantly less than the polarization relaxation time to be possible explanation of the observed filamentation peculiarities. Namely, we argue that the plasma production results from the resonance enhanced (2+1)-photon ionization of the oxygen molecules through the two-photon excitation of the 3s metastable Rydberg state. Coherent Raman self-scattering at rotational transitions of nitrogen molecules provides self-induced focusing of the ultrashort UV laser pulse and filament formation.

Femtosecond filament array generated in air

Patterning multiple filamentation of femtosecond pulses in air is studied using a microlens array for modulation of the spatial profile and a single lens for power concentration. We generate a stable array of filaments containing a maximum of five hotspots per mm2 from a modest 68-GW input power. The evolution of the pattern along the axis of propagation as well as the means to control the inter-filament spacing is discussed. It is also shown in numerical simulations that besides the filamentation in the proximities of the focus, there is a region of early ionization around the central hotspots in the beam profile and a revival afterwards, caused by the spatial distribution of the laser energy.

Generation of broadband supercontinuum light by double-focusing of a femtosecond laser pulse in air

Generation of broadband supercontinuum light has been of great interest in recent years. In this work, we investigated an interesting way for generation of broadband supercontinuum light by double-focusing of a femtosecond (fs) laser pulses in air. In this method, the fs laser pulse is focused in air and the produced supercontinuum light from the interaction of the laser pulse and air is focused again in air. In this way, we found that the spectral broadening of the supercontinuum light can be a lot more enhanced even with a relatively low laser energy of a few mJ/pulse, compared with the single-focusing method. In this paper, we present the experimental results for the broadband supercontinuum light by the double-focusing method, in addition to numerical simulation results.

On the parameters of runaway electron beams and on electrons with an “anomalous” energy at a subnanosecond breakdown of gases at atmospheric pressure

The generation of runaway electron beams in gases at atmospheric pressure has been studied with a real picosecond accuracy. Their main parameters have been determined. It has been found that three groups of electrons can be separated at a subnanosecond voltage pulse in a runaway electron beam generated in air at atmospheric pressure. It has been proven that the duration of a beam pulse in air at atmospheric pressure behind an anode foil is ~100 ps.

Onset of nonlinear self-focusing of femtosecond laser pulses in air: Conventional vs spatiotemporal focusing

Onset of nonlinear self-focusing of femtosecond laser pulses in air: Conventional vs spatiotemporal focusing

The life cycle of infrared ultra-short high intensity laser pulses in air

The life cycle of ultra-short high intensity laser pulses propagation in air is studied. As the controversial of the high-order Kerr indices measured by Loriot et al. [Opt. Express 18, 3011 (2010)], we focus on two models which are high-order Kerr effect included and not included. Two factors are mainly analyzed, group-velocity-dispersion and the energy evolution of the pulse. It is found that the group-velocity-dispersion can not be simply ignored even though the pulse’s duration is as long as several hundreds femtoseconds. The energy loss due to the multi-photon-absorption is very small, and it may hardly change the propagation length of the pulse. Another contribution of this work is to introduce a probability quantity, which may be useful in validating the positive and negative alternating of the Kerr and high-order Kerr indices.

Carrier field shock formation of long-wavelength femtosecond pulses in single-crystal diamond and air

We numerically demonstrate the formation of carrier field shocks in single-crystal diamond and air for a wide variety of input conditions using two different electric field propagation models. We investigate the impact of numerous physical effects on the carrier wave shock. It is shown that, in many cases, a field shock is essentially unavoidable and therefore extremely important in the propagation of intense long-wavelength pulses in weakly dispersive nonlinear media. We found that single-crystal diamond is the ideal nonlinear medium for carrier wave shock formation using mid-IR laser pulses carrying moderate energies.

Super high power mid-infrared femtosecond light bullet

Mid-infrared ultrashort high energy laser sources are opening up new opportunities in science, including keV-class high harmonic generation and monoenergetic MeV-class proton acceleration. As new higher energy sources become available, potential applications for atmospheric propagation can dramatically grow to include stand-off detection, laser communications, shock-driven remote terahertz enhancement and extended long-lived thermal waveguides to transport high power microwave and radiofrequency waves. We reveal a new paradigm for long-range, low-loss, ultrahigh power ultrashort pulse propagation at mid-infrared wavelengths in the atmosphere. Before the onset of critical self-focusing, energy in the fundamental wave continually leaks into shock-driven spectrally broadened higher harmonics. A persistent near-invariant solitonic leading edge on the multi-terawatt pulse waveform transports most of the power over hundred-metre-long distances. Such light bullets are resistant to uncontrolled multiple filamentation and are expected to spark extensive research in optics, where the use of mid-infrared lasers is currently much under-utilized. A mechanism for the propagation of mid-infrared femtosecond laser pulses in air is theoretically investigated. A numerical simulation predicts that the propagation of multiple-terawatt pulses is possible over hundreds of metres.

Experimental study of the behavior of two laser produced plasmas in air

The interactions among two laser ablated Al plasmas and their shock wave fronts (SWFs) induced by double laser pulses in air were studied experimentally. The evolution processes, including the expansion and interaction of the two plasmas and their shocks, were investigated by laser shadowgraphs, schlieren images, and interferograms. Remarkably, the distribution of the compressed air and the laser plasmas during the colliding process was clearly obtained using the Mach-Zehnder interferometer. From the refractive index profiles, typical plasmas density and gas density behind the shock front were estimated as ∼5.2 × 1018 cm−3 and ∼2.4 × 1020 cm−3. A stagnation layer formed by the collision of gas behind the shock front is observed. The SWFs propagated, collided, and reflected with a higher velocity than plasmas. The results indicated that the slower plasma collided at middle, leading to the formation of the soft stagnation.