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Theoretical analysis of ultra-short phenomena occurring during the positive streamer propagation in atmospheric pressure air is presented. Motivated by experimental results obtained with tens-ofpicoseconds and tens-of-microns precision, it is shown that when the streamer head passes a spatial coordinate, emission maxima from N2 and N2+ radiative states follow with different delays. These different delays are caused by differences in the dynamics of populating the radiative states, due to different excitation and quenching rates. Associating the position of the streamer head with the maximum value of the self-enhanced electric field, a delay of 160 ps was experimentally found for the peak emission of the first negative system of N2+ . A delay dilatation was observed experimentally on early-stage streamers and the general mechanism of this phenomenon is clarified theoretically. In the case of the second positive system of N2, the delay can reach as much as 400 ps. In contrast to the highly nonlinear behavior of streamer events, it is shown theoretically that emission maximum delays linearly depend on the ratio of the streamer radius and its velocity. This is found to be one of the fundamental streamer features and its use in streamer head diagnostics is proposed. Moreover, radially resolved spectra are synthesized for selected subsequent picosecond moments in order to visualize spectrometric fingerprints of radial structures of N2(C3Πu) and N2+ (B2∑u+) populations created by streamer-head electrons. [ABSTRACT FROM AUTHOR]

The results of an experimental study of the evolution of surface flashover across the surface of an insulator in vacuum subject to a high-voltage pulse and the parameters of the flashover plasma are reported. For the system studied, flashover is always initiated at the cathode triple junctions. Using time-resolved framing photography of the plasma light emission the velocity of the light emission propagation along the surface of the insulator was found to be ∼2.5.108 cm/s. Spectroscopic measurements show that the flashover is characterized by a plasma density of 2-4×1014 cm-3 and neutral and electron temperatures of 2-4 eV and 1-3 eV, respectively, corresponding to a plasma conductivity of ∼0.2 Ω-1 cm-1 and a discharge current density of up to ∼10 kA/cm2. [ABSTRACT FROM AUTHOR]

Plasma-based microwave power limitation is experimentally investigated in a microstrip transmission line integrating a direct current (dc) plasma discharge for preionization. Steady state and transient high-power microwave measurements demonstrate that the preionization by the dc plasma discharge helps in reducing limiting power thresholds and time responses to about 30 dBm and tens to few microsecond, respectively. Besides, optical diagnostics also exhibit interesting behaviors of the plasma discharge such as diffusion and parasitic breakdowns. Finally, the effect of plasma confinement is discussed, and it is shown that reducing the volume devoted to plasma expansion leads to a decrease in the dynamic range of the microwave power limitation. [ABSTRACT FROM AUTHOR]

A relativistic laser pulse, confined in a cylindrical-like target, under specific conditions may perform multiple scattering along the internal target surface. This results in the confinement of the laser light, leading to a very efficient interaction. The demonstrated propagation of the laser pulse along the curved surface is just yet another example of the “whispering gallery” effect, although nonideal due to laser-plasma coupling. In the relativistic domain its important feature is a gradual intensity decrease, leading to changes in the interaction conditions. The process may pronounce itself in plenty of physical phenomena, including very efficient electron acceleration and generation of relativistic magnetized plasma structures. [ABSTRACT FROM AUTHOR]

The spectral emission properties of a droplet-based laser-produced plasma are investigated in the vacuum ultraviolet (VUV) range. Measurements are performed with a spectrograph that operates from 30 to 180 nm with a spectral resolution of 0.1 nm. The emission spectra are recorded for different metal droplet targets, namely tin, indium, and gallium. Measurements were performed at different pressure levels of the background gas. Several characteristic emission lines are observed. The spectra are also calibrated in intensity in terms of spectral radiance to allow absolute emission power estimations from the light source in the VUV region. The presented experimental results are relevant for alternative light sources that would be needed for future wafer inspection tools. In addition, the experimental results help to determine the out-of-band radiation emission of a tin-based extreme ultraviolet (EUV) source. By tuning the type of fuel, the laser energies, and the background gas, the laser-produced plasma light source shows good capabilities to be operated as a light source that covers a spectral emission range from the EUV to the sub-200 nm range. [ABSTRACT FROM AUTHOR]

The stimulated Brillouin scattering (SBS) of nonresonant beat mode in the presence of static magnetic field is investigated in a plasma. Two counter-propagating lasers of frequencies (ω1 and ω2) and wave vectors (k1 and k2) drive a nonresonant space charge beat mode at the phase matching condition of frequency ω0≈ω1~ω2 and wave number k0≈k1+k2. The driver wave parametrically excites a pair of ion acoustic wave (ω,k) and a sideband electromagnetic wave (ω3,k3). The beat wave couples with the sideband electromagnetic wave to exert a nonlinear ponderomotive force at the frequency of ion acoustic wave. Density perturbations due to ion acoustic wave and ponderomotive force couple with the oscillatory motion of plasma electron due to velocity of beat wave to give rise to a nonlinear current (by feedback mechanism) responsible for the growth of sideband wave at resonance. The growth rate of SBS was reduced (from ~1012s-1 to 1010s-1) by applying a transverse static magnetic field ~90T. The present study can be useful for the excitation of fast plasma waves (for the purpose of electron acceleration) by two counter-propagating laser beams. [ABSTRACT FROM AUTHOR]

Relativistic third-harmonics of a laser in a magnetized plasma channel is studied. Due to relativistic self-focusing in plasma, a high-intensity laser pulse pushes the plasma electrons radially outward to create a plasma channel. The relativistic mass effect and the magnetic field contribute to the nonlinear dielectric response of plasma and create a plasma channel due to the laser self-focusing. Self-sustained plasma channel is very important and may affect the efficiency of harmonic generation of the interacting laser beam. The velocity and density perturbation associated with the self-focused laser beam can generate a nonlinear current at triple fold frequency of the fundamental laser. Our results show the significant effect of the self-sustained magnetized plasma channel on the efficiency of third-harmonic generation of the laser beam. [ABSTRACT FROM AUTHOR]

Whereas noble metal films deposited directly onto an undulated photosensitive polymer exhibit plasmon-mediated resonant transmission of free-space waves accompanied by excess losses of approximately 40 %, adequate hard baking of the photosensitive polymer, together with pre- and post-deposition of nanometer-thick ZnS or MgF dielectric interfacial layers, restores excellent propagation conditions for the long-range plasmon mode, reducing resonant transmission excess losses to 10 %, and creating the conditions for the experimental evidence of the long-sought plasmonic anomalous reflection phenomenon. [ABSTRACT FROM AUTHOR]

Nanowire-loaded surface plasmon polariton waveguide is an extremely simple structure that can be naturally formed by directly dropping a dielectric cylinder onto a metallic substrate. However, despite the substantial emphasis devoted to its hybrid plasmonic counterparts, this waveguiding structure has been paid little attention to so far. Here in this paper, through comprehensive numerical analysis, we reveal that such a configuration can be leveraged to achieve deep-subwavelength field confinement with mode area more than one order of magnitude smaller than that of the conventional hybrid waveguide, while maintaining a moderate attenuation with propagation distance over tens of microns. Two-dimensional parameter mapping concerning physical dimension, shape and material of the nanowire as well as the refractive index of the cladding has disclosed the wide-range existence nature of this plasmonic mode and the feasibility to further balance its confinement and loss. [ABSTRACT FROM AUTHOR]

We demonstrate theoretically and experimentally the difference between the band compression mechanisms of plasmonic gratings and plasmonic heterogratings. The plasmonic grating can pick out a fixed polarization bandwidth with changing the incidence angle, while the plasmonic heterograting composed of two different nanogratings can pick out a tunable bandwidth with changing the incidence angle and the period ratio of two gratings. The range of the plasmonic resonances of the heterograting is about 30% wider than that of a conventional grating. Thus, the plasmonic heterograting can work efficiently in a wider wavelength range. Moreover, high polarization extinction ratio is achieved in these plasmonic devices. This gives more insights into the mechanisms in plasmonic filters and is helpful to promote the actual applications. [ABSTRACT FROM AUTHOR]

Abstract: Influence of time delay between two laser pulses on the LIBS (laser induced breakdown spectroscopy) signal inside liquids was investigated and the results are compared with data from literature. Plasma was produced by laser ablation (LA) of aluminum inside water and its emission after the second laser pulse was characterized by spectrally and time resolved detection. Light propagation through the vapor bubble formed by the first laser pulse was studied by measurements of beam scattering and transmission. Optical absorption by the evolving bubble is not significant, but its growth is accompanied by lowering of its refraction index nb with respect to surrounding liquid; this effect increases defocusing both of the incident beam and of the out-coming plasma radiation. Collection efficiency of the secondary plasma emission rapidly degrades with the cavity growth, but close to its full expansion the LIBS signal partially recovers through Snell''s reflections at the liquid–vapor interface, which produce a bright spot close to the bubble center. Such a light redistribution allows detecting of the emission from external plasma volume, otherwise deflected out of the collection system. Except for strong line transitions from the main sample constituents, self-absorbed inside the high-pressure cavity, we observed the highest LIBS signal when sending the second pulse well before the bubble is fully expanded. Transitions of the pressure wave through the focal volume, formed by the first laser pulse and reflected from the cell''s walls and sample back-plane, enhances the LIBS signal importantly. The measured lifetime of the secondary plasma rapidly decreases with the bubble expansion. Here, we also discuss the optimization of the optical collection system and some analytical aspects of double-pulse (DP) LIBS inside liquids. [Copyright &y& Elsevier]

The role of collisional absorption is shown to be important at the beginning of the interaction between a laser pulse and matter, i.e., a plasma, while later on collective absorption phenomena take place. The scope of this paper is to present first results about the long predicted AC-run-away effect, defined as a sudden transition from a collisional absorption regime to a collective one, in ideal circumstances: for an ultrashort laser pulse, i.e., subpicosecond pulses, and with a sharp plasma boundary, i.e., without a laser prepulse. For this purpose, we use a kinetic model based on the ballistic theory. The model is of relevance to laser-matter interaction including laser nuclear fusion or laser particle acceleration. [ABSTRACT FROM PUBLISHER]

A Si-based light modulator working in the near infrared at frequencies up to 300 kHz is proposed. The device proposed is a bipolar mode field effect transistor (BMFET) integrated in an Si rib waveguide. The operation principle is the light dispersion/absorption by a plasma of free carriers that can be moved in or out of the optical path. It works as a pure amplitude light modulator; hence, it can be shrunk. Its small dimensions, only 100 mum in length and less than 50 mum in width in the proposed version, make it suitable for monolithic integration for applications where the major constraints are the device dimensions. As an example, the device application to an integrated gas sensing or chemical absorption spectroscopy system is also described [ABSTRACT FROM PUBLISHER]

For the diagnostics of dense plasmas, photons in the vacuum ultraviolet (VUV) or X-ray domain are required. First experiments have demonstrated the great capacity of X-ray Thomson scattering for the determination of plasma parameters such as density, temperature, and ionization state. For a reliable interpretation of the experimental data, the interaction of energetic photons with the plasma has to be considered. The cross section for Thomson scattering is related to the dynamic structure factor S(k,ω). We improve the usual random phase approximation by including collisions and calculate a dynamic collision frequency in Born approximation. We also study the influence of dynamic screening and the relation to the concept of dynamic local-field corrections to the dielectric function. We give results for a variety of plasma parameters and determine the region where collisions are of relevance. [ABSTRACT FROM PUBLISHER]

This work describes an experiment to demonstrate, for the first time, laser wakefield acceleration (LWFA), driven by 10.6-μm light from a CO2 laser. This experiment is also noteworthy because it will operate in a pseudoresonant LWFA regime, in which the laser-pulse-length is too long for resonant LWFA, but too short for self-modulated LWFA. Nonetheless, high acceleration gradients are still possible. This experiment builds upon an earlier experiment called staged electron laser acceleration (STELLA), where efficient trapping and monoenergetic laser acceleration of electrons were demonstrated using inverse free electron lasers. The aim is to apply the STELLA approach of laser-driven microbunch formation followed by laser-driven trapping and acceleration to LWFA. These capabilities are important for a practical electron linear accelerator based upon LWFA. [ABSTRACT FROM PUBLISHER]

Electron beam based radiation sources provide electromagnetic radiation for countless applications. The properties of the radiation are primarily determined by the properties of the electron beam. Compact laser driven accelerators are being developed that can provide ultrashort electron bunches (femtosecond duration) with relativistic energies reaching toward a GeV. The electron bunches are produced when an intense laser interacts with a dense plasma and excites a large amplitude plasma density modulation (wakefield) that can trap background electrons and accelerate them to high energies. The short-pulse nature of the accelerated bunches and high particle energy offer the possibility of generating radiation from one compact source that ranges from coherent terahertz to gamma rays. The intrinsic synchronization to a laser pulse and unique character of the radiation offer a wide range of possibilities for scientific applications. Two particular radiation source regimes are discussed: coherent terahertz emission, and X-ray emission based on betatron oscillations and Thomson scattering. [ABSTRACT FROM PUBLISHER]

We have recently reported the successful development of a fully coherent X-ray laser (XRL) at 13.9 nm by an oscillator-amplifier configuration with two targets. In the experiment, a seed XRL beam from the first target is injected into a plasma amplifier at the second target. The observed XRL beam has full spatial coherence and 0.2 mrad of nearly diffraction-limited divergence. In order to improve the output fluence, the amplification properties of the XRL beam have been investigated using various plasma lengths of the second amplifier target. The output energy has been improved by a factor of ten, increasing the length of the gain region to 10 mm, resulting in about 0.2 μJ of output energy. [ABSTRACT FROM PUBLISHER]

We propose to generate as-X-ray laser pulses by beating of two or more X-ray laser lines with a frequency separation in the range of 1015 Hz. We focus on nickel-like X-ray lasers, some of which have a few almost equidistant laser gain lines with an appropriate difference frequency. It is shown that in the case of three or more lines, these can be phase-locked by means of a Langmuir wave generated in the gain medium at a suitable electron density. [ABSTRACT FROM PUBLISHER]

Recent experimental work on the development of extreme ultraviolet lasers undertaken using as the pumping source the VULCAN laser at the Rutherford Appleton Laboratory is compared to detailed simulations. It is shown that short duration (∼picosecond) pumping can produce X-ray laser pulses of a few picosecond duration and that measurement of the emission from the plasma can give an estimate of the duration of the gain coefficient. The Ehybrid fluid and atomic physics code developed at the University of York is used to simulate X-ray laser gain and plasma emission. Two postprocessors to the Ehybrid code are utilized: 1) to raytrace the X-ray laser beam amplification and refraction and 2) to calculate the radiation emission in the kiloelectronvolt photon energy range. The raytracing and spectral simulations are compared, respectively, to measured X-ray laser output and the output of two diagnostics recording transverse X-ray emission. The pumping laser energy absorbed in the plasma is examined by comparing the simulations to experimental results. It is shown that at high pumping irradiance (>1015 Wcm-2), fast electrons are produced by parametric processes in the preformed long scalelength plasmas. These fast electrons do not pump the population inversion and so pumping efficiency is reduced at high irradiance. [ABSTRACT FROM PUBLISHER]

Complex spectra of Xe and Sn, and their feasibility for use in the lithographic extreme ultraviolet (EUV) source, are investigated. By combining calculations of atomic data using the HULLAC code and the Whiam collisional radiative model with a simple radiative transfer model, the EUV spectrum is shown to originate from a large number of fine structure transitions. Satellite lines of 4d-4f and 4d-5p transitions from near ten times ionized states are found to make a significant contribution to the emission from an optically thick plasma. The wavelengths and transition probabilities of emission lines, charge state distribution, and level population in the plasma, as well as radiation intensity from a laser-produced plasma source have been calculated. The effect of opacity is taken into account assuming a local thermodynamic equilibrium population in the plasma. The calculated result reproduces the observed spectra both for Xe and Sn, showing applicability of the present method to the spectroscopy of hot dense plasmas. [ABSTRACT FROM PUBLISHER]

Quantitative phase-sensitive measurements of ultrafast optical-field ionization rates in molecular H2, N2, and O2 are obtained using a temporally gated frequency-domain interferometric pulse measurement technique: multipulse interferometric frequency-resolved optical gating (MI-FROG). By measuring the pump-induced frequency change on a weak copropagating probe pulse, the optical field ionization dynamics can be completely time-resolved with sub-pulsewidth time resolution. A one-dimensional nonrelativistic electromagnetic fluid code model is used to compute the ionization dynamics and optical field propagation through the plasma. Using the Ammosov-Delone-Krainov (ADK) tunnel ionization rate model originally developed for atoms, the relatively simple model proposed here has been shown to compare favorably with the MI-FROG measured ionization rates in noble gases in the intermediate intensity regime (10 14 W/cm2) (Siders et al, Phys. Rev. Lett.). We attempt to unify our studies in noble gases and molecules by performing experiments on N2 and O2, which have nearly identical ionization potentials to Ar and Xe, respectively. For the molecules studied here, we show that an ADK-like description of molecular ionization rates calculated from the model agree with the experimentally measured rates using the MI-FROG technique for H2 and N2. In the case of O2, however, the experimentally measured ionization rate is approximately two orders of magnitude lower than that expected from the standard ADK formula. This is in agreement with the previously observed suppressed O2 ionization rate in ion mass spectroscopy studies (Guo, 2000). We attribute the suppressed ionization rate in O2 to a multielectron screening effect and show that a modified version of the ADK formula, taking into account the electron screening as proposed by Guo, well approximates the MI-FROG O2 ionization rate data [ABSTRACT FROM PUBLISHER]

The article focuses on how researchers have observed light propagation where photons smoothly glide along a one-dimensional chain of electrons known as a Luttinger liquid. Topics covered include how researchers are trying to understand how to control and guide light at the nanoscale and how the physics behind Luttinger liquid plasmonics is located at the interface between condensed-matter theory and nanophotonics.

We study the propagation of light bundles in non-empty spacetime, as most of the Universe is filled by baryonic matter in the form of a (dilute) plasma. Here we restrict to the case of a cold (i.e. pressureless) and non-magnetised plasma. Then the influence of the medium on the light rays is encoded in the spacetime dependent plasma frequency. Our result for a general spacetime generalises the Sachs equations to the case of a cold plasma Universe. We find that the reciprocity law (Etherington theorem), the relation that connects area distance with luminosity distance, is modified. Einstein’s field equation is not used, i.e. our results apply independently of whether or not the plasma is self-gravitating. As an example, our findings are applied to a homogeneous plasma in a Robertson–Walker spacetime. We find small modifications of the cosmological redshift of frequencies and of the Hubble law. [ABSTRACT FROM AUTHOR]

Positive streamers need a source of free electrons ahead of them to propagate. Here we investigate a possible source of these electrons in high density media: field ionization. Zener's model is used to obtain the field ionization rate of free electrons from impurities. Including this effect, we simulate the inception of positive streamers in supercritical N2 , in a needle-to-plane electrode geometry. Our 3-D particle simulation shows how the discharge branches into positive streamer channels around the electrode. [ABSTRACT FROM PUBLISHER]

Streak cameras, with high-temporal resolution, are a powerful tool to image light propagation and plasma dynamics. Here, we present the image of a 2.2-TW peak power, 50-fs laser pulse, propagating in a nonlinear fashion over a distance of 1.3 m, leaving behind plasma. A direct measurement of the Rayleigh scattering of the pulse and the plasma emission are presented here. [ABSTRACT FROM PUBLISHER]

A fast imaging technique applied for RF plasma indicates that the plasma jet propagation dynamics, operating even at megahertz frequency includes the so-called plasma bullet phenomenon. Broadband emission and intensity of lines corresponding to excited Ar and OH are registered. The spatial distribution of ground state OH is also detected by means of laser-induced fluorescence diagnostics. [ABSTRACT FROM PUBLISHER]