Moderate Laser Intensities Research Articles

Relativistic soliton formation in laser magnetized plasma interactions

The laser plasma interactions in the presence of strong magnetic field are studied by employing particle-in-cell simulations. Simulations show that the energy absorption of strong laser pulse is mainly characterized by the electron cyclotron resonance heating (ECRH) when the magnetic field is large enough. However, it is found that for a weaker magnetic field, a standing or moving soliton can be generated in some moderate laser intensity regions, greatly enhancing the laser absorption. The laser intensity for the soliton heating decreases as the magnetic field increases. Furthermore, the soliton position moves towards the front boundary when the laser intensity or magnetic field strength increases.

Experimental investigation of stimulated Raman and Brillouin scattering instabilities driven by two successive collinear picosecond laser pulses.

Backward stimulated Raman and Brillouin scattering (SRS and SBS) are experimentally investigated by using two successive 1-μm, 1.5-ps FWHM laser pulses. The collinear pulses, separated by 3 or 6 ps and of moderate laser intensities (∼2×10^{16}Wcm^{-2}), are fired into a preionized He plasma of density ∼2.5-6×10^{19}cm^{-3}. The electron plasma waves and ion acoustic waves, respectively driven by SRS and SBS, are analyzed through space- and time-resolved Thomson scattering. Depending on the laser and plasma parameters, we observe the effect of the first pulse on the time-resolved SRS and SBS signals of the second pulse. The measurements are found to qualitatively agree with the results of a large-scale particle-in-cell simulation.

Anisotropic electron-distribution function in inverse-bremsstrahlung-heated plasmas.

The electron-distribution function in homogeneous plasmas heated by a high-frequency laser field is calculated in velocity space from the Vlasov-Landau equation. The kinetic model is valid for moderate laser intensity defined by the relevant parameter α=v_{0}^{2}/v_{t}^{2}<0.5 where v_{0} and v_{t} are the peak velocity of oscillation in the high-frequency electric field and the thermal velocity, respectively. The results obtained constitute an improvement of the results reported in the literature devoted to weak electric field intensities. The electron-distribution function is calculated solving the kinetic equation with the use of the Legendre polynomial expansion within the laser field dipole approximation. It results in an infinite set of equations for the isotropic component f_{0}(v) and the anisotropic components f_{n≥1}(v) that we have solved numerically with appropriate truncation. For the second anisotropy f_{2}(v), we found that its maximum increases from the weak electric field intensity (α<0.01) to a moderate one (α=0.5) by a factor f_{2max}(α=0.5)/f_{2max}(α=0.01)≈48. Applications to the radiation pressure, electromagnetic instabilities, and photoabsorption are also considered.

Redistribution of fluorescent molecules at the solid/liquid interface with total internal reflection illumination

Many intriguing physical and chemical processes commonly take place at the solid/liquid interface. Total internal reflection illumination, together with single molecule spectroscopy, provides a robust platform for the selective exploration of kinetic processes close the interface. With these techniques, it was observed that the distribution of Rhodamine B molecules close to a solid/liquid interface could be regulated in a photo-induced route. The laser-induced repulsion force at this interface is enough to compromise the Brownian diffusion of single molecules in a range of several hundred nanometers normal to the solid/liquid interface. This observation is fundamentally and practically interesting because moderate laser intensity is enough to initiate this repulsion effect. Therefore, it might display extensive applications in the development of photo-modulation technique with high throughput capability.

Analytical description of attosecond pulse generation on a plasma surface irradiated by high-intense laser pulses

We study theoretically the process of turning a laser pulse into a train of attosecond or even zeptosecond pulses due to high harmonic generation (HHG) upon backreflection of intense laser radiation from a plasma surface. It is shown that under appropriate conditions these attosecond pulses may have an amplitude that is several orders of magnitude larger than that of the laser pulse. We study this process in detail, especially the nanobunching of the plasma electron density. We derive the analytical expression that describes the electron density profile and obtain a good agreement with particle-in-cell simulations. We investigate the most efficient case of HHG at a moderate laser intensity (normalised vector potential ) on the overdense plasma slab with an exponential pre-plasma profile. Subsequently we calculate the spectra of single attosecond pulses from back radiation using our expression for density shape in combination with the equation for spectrum of nanobunch radiation.

Semi-analytical fluid study of the laser wake field excitation in the strong intensity regime

We present an analytical and numerical study of the interaction of a multi-petawatt, pancake-shaped laser pulse with an unmagnetized plasma. The study has been performed in the ultrarelativistic regime of electron jitter velocities, in which the plasma electrons are almost completely expelled from the pulse region. The calculations are applied to a laser wake field acceleration scheme with specifications that may be available in the next generation of Ti:Sa lasers and with the use of recently developed pulse compression techniques. A set of novel nonlinear equations is derived using a three-timescale description, with an intermediate timescale associated with the nonlinear phase of the electromagnetic wave and with the spatial bending of its wave front. They describe, on an equal footing, both the strong and the moderate laser intensity regimes, pertinent to the core and to the edges of the pulse.

Collapse and revival of a Dicke-type coherent narrowing in potassium vapor confined in a nanometric thin cell

A nanometer-thin cell (in the direction of laser beam propagation) has been elaborated with the thickness of the atomic vapor column varying smoothly in the range of . The cell allows one to study the behavior of the resonance absorption over the D1 line of potassium atoms by varying the laser intensity and the cell thickness (from to with the step where is the resonant wavelength of the laser). It is shown that despite the huge Doppler broadening ( at the cell temperature ), at low laser intensities a narrowing of the resonance absorption spectrum is observed for ( at FWHM) and , whereas for and the spectrum broadens. At moderate laser intensities narrowband velocity selective optical pumping (VSOP) resonances appear at and with the linewidth close to the natural one. A comparison with saturated absorption spectra obtained in a 1.4 cm-sized K cell is presented. The developed theoretical model well describes the experiment.

Phase-dependent fluctuations of intermittent resonance fluorescence

Electron shelving gives rise to bright and dark periods in the resonance fluorescence of a three-level atom. The corresponding incoherent spectrum contains a very narrow inelastic peak on top of a two-level-like spectrum. Using the theories of balanced and conditional homodyne detection we study ensemble averaged phase-dependent fluctuations of intermittent resonance fluorescence. The sharp peak is found only in the spectra of the squeezed quadrature. In balanced homodyne detection that peak is positive, which greatly reduces the squeezing, also seen in its variance. In conditional homodyne detectionCHD, for weak to moderate laser intensity, the peak is negative, enhancing the squeezing, and for strong fields the sidebands become dispersive which, together with the positive sharp peak dominate the spectrum. The latter effect is due to non-negligible third order fluctuations produced by the atom-laser nonlinearity and the increased steady state population of the shelving state. A simple mathematical approach allows us to obtain accurate analytical results.

Some Words Hurt More Than Others: Semantic Activation of Pain Concepts in Memory and Subsequent Experiences of Pain.

Theory suggests that as activation of pain concepts in memory increases, so too does subsequent pain perception. Previously, researchers have found that activating pain concepts in memory increases pain perception of subsequent painful stimuli, relative to neutral information. However, they have not attempted to quantify the nature of the association between information studied and ensuing pain perception. We subliminally presented words that had either a low or high degree of association to the word 'pain,' although this was only partially successful and some words were consciously perceived. Participants then received randomized laser heat stimuli, delivered at 1 of 3 intensity levels (low, moderate, high), and we measured the effect of this on behavioral and electrophysiological measures of pain. Participants (N=27) rated moderate- and high-intensity laser stimuli as more painful after viewing high relative to low associates of pain; these effects remained present when we controlled for measures of mood, anxiety, and physical symptom reporting. Similar effects were observed physiologically, with higher stimulus negativity preceding after high relative to low associates and greater amplitudes for the N2 component of the laser-evoked potential after presentation of high associates in the moderate and high laser intensity conditions. These data support activation-based models of the effects of memory on pain perception. Consistent with current theories of memory and pain, we found that high, relative to low activation of pain concepts in memory increased psychological and physiological responses to laser-induced pain. The effect remained regardless of whether participants showed conscious awareness of activation. Theoretical and clinical implications are discussed.

Self-phase modulation in various regimes of intense laser–plasma interactions

Creation of plasmas by intense laser pulses and consequent laser–plasma interactions involve highly non-linear processes. In particular, a variation of the refractive index induced by the laser action turns into a self-phase modulation (SPM) of the laser field. This effect, already observed with nanosecond laser pulses, achieves striking evidence with ultra-short pulses whose intensity can produce index changes in a time as short as a single optical cycle. In this condition, spectral modifications of the laser pulse produced by SPM may strongly modify the laser–plasma interaction. At the same time, the spectral analysis of the transmitted and scattered laser radiation gives valuable information on a variety of processes occurring in the plasma. In this paper we simply consider a few results, which can be attributed to SPM, with the aim of comparing nanosecond and sub-picosecond laser interaction regimes at moderate laser intensities.

Role of Multi-Electron Effects in the Asymmetry of Strong-Field Ionization and Fragmentation of Polar Molecules: The Methyl Halide Series.

We report angle- and momentum-resolved measurements of the dissociative ionization and Coulomb explosion of methyl halides (CH3F, CH3Cl, CH3Br, and CH3I) in intense phase-controlled two-color laser fields. At moderate laser intensities, we find that the emission asymmetry of low-energy CH3(+) fragments from the CH3(+) + X(+) (X = F, Cl, Br, or I) channel reflects the asymmetry of the highest occupied molecular orbital of the neutral molecule with important contributions from the Stark effect. This asymmetry is correctly predicted by the weak-field asymptotic theory, provided that the Stark effect on the ionization potentials is calculated using a nonperturbative multielectron approach. In the case of high laser intensities, we observe a reversal of the emission asymmetries for high-energy CH3(+) fragments, originating from the dissociation of CH3X(q+) with q ≥ 2. We propose ionization to electronically excited states to be at the origin of the reversed asymmetries. We also report the measurements of the emission asymmetry of H3(+), which is found to be identical to that of the low-energy CH3(+) fragments measured at moderate laser intensities. All observed fragmentation channels are assigned with the help of CCSD(T) calculations. Our results provide a benchmark for theories of strong-field processes and demonstrate the importance of multielectron effects in new aspects of the molecular response to intense laser fields.

Light-Induced Heating of Gold Nanoparticles in Colloidal Solution: Dependence on Detuning from Surface Plasmon Resonance

The absorption of light by gold nanoparticles in colloidal water solution has been studied at the simultaneous illumination of nanoparticles by the continuous wave laser beam in dependence on the detuning of laser frequency from the surface plasmon resonance. The appreciable blue shift, broadening, and increase of intensity of the plasmonic absorption band have been observed at approaching of the laser frequency to surface plasmon resonance in Au nanoparticles. The plasmon band broadening reveals the heating of gold nanoparticles that has an evident resonant character. The strong increase of nanoparticle temperature of 316 K has been obtained at moderate laser intensity of 5 × 103 W/cm2 and detuning of 28.5 nm. The sharp blue shift of plasmon band was observed when the temperature exceeded the water boiling point. Such effect is probably due to formation of the vapor bubbles around the Au nanoparticles occurring at water boiling.

Surface modification of a-CN/TiAlN double layer coating on ASP 30 steel induced by femtosecond laser with 1013–1014 W/cm2 intensity in vacuum

AbstractA double layer a-CN/TiAlN coating deposited on ASP30 steel substrate was irradiated by femtosecond laser and surface modification effects were observed. Moderate laser intensities used were in the range of 1014–1013 W/cm2, while the total thickness of double layer coating was 4.8 µm (a-CN = 0.6 and TiAlN = 4.2 µm). Laser-induced changes of the surface showed dependence on laser intensity and number of accumulated pulses. Irradiations at the highest intensity resulted in preservation of one or both layers up to 10 pulses, while at lower intensity (1013 W/cm2) a-CN layer is removed after several pulses and TiAlN is preserved up to 50 pulses. Evaluated damage threshold of the target was 0.49 J/cm2. Lower laser intensity irradiation produced periodic surface structures (LIPSS) over the entire irradiated spot with periodicity of ~700 nm, almost in agreement with the laser wavelength used. Irradiations carried out at the highest laser intensity (1014 W/cm2) and laser pulse count of ≥50 resulted in the creation of crater like damages with depth up to 20 µm. Craters were conically shaped, implying intensive processes which took place at the surface. Generation of LIPSS as well as craters can be of great interest for contemporary technologies.

Light-induced heating of dense 2D ensemble of gold nanoparticles: dependence on detuning from surface plasmon resonance

The extinction of white light by dense 2D ensemble of gold nanoparticles with size of 22 nm and interparticle distance of 40 nm has been studied at the simultaneous illumination of nanoparticles by the continuous-wave laser beam in dependence of the detuning of laser frequency from the surface plasmon resonance (SPR). The appreciable red shift, broadening, and increase of intensity of the plasmonic extinction band have been observed at approaching of the laser frequency to SPR. The plasmon band shift and broadening reveal the heating of gold nanoparticles that has an evident resonant character. The strong increase of nanoparticle temperature of 590 K has been observed at moderate laser intensity of 5 × 103 W/cm2 and detuning of 24.6 nm. Such strong heating is probably due to the accumulative effect and the light extinction enhancement by intense local plasmonic field of coupled Au nanoparticles in dense 2D ensemble.

Semianalytical study of the propagation of an ultrastrong femtosecond laser pulse in a plasma with ultrarelativistic electron jitter

The interaction of a multi-petawatt, pancake-shaped laser pulse with an unmagnetized plasma is studied analytically and numerically in a regime with ultrarelativistic electron jitter velocities, in which the plasma electrons are almost completely expelled from the pulse region. The study is applied to a laser wakefield acceleration scheme with specifications that may be available in the next generation of Ti:Sa lasers and with the use of recently developed pulse compression techniques. A set of novel nonlinear equations is derived using a three-timescale description, with an intermediate timescale associated with the nonlinear phase of the electromagnetic wave and with the spatial bending of its wave front. They describe, on an equal footing, both the strong and the moderate laser intensity regimes, pertinent to the core and to the edges of the pulse. These have fundamentally different dispersive properties since in the core the electrons are almost completely expelled by a very strong ponderomotive force, and the electromagnetic wave packet is imbedded in a vacuum channel, thus having (almost) linear properties. Conversely, at the pulse edges, the laser amplitude is smaller, and the wave is weakly nonlinear and dispersive. New nonlinear terms in the wave equation, introduced by the nonlinear phase, describe without the violation of imposed scaling laws a smooth transition to a nondispersive electromagnetic wave at very large intensities and a simultaneous saturation of the (initially cubic) nonlocal nonlinearity. The temporal evolution of the laser pulse is studied both analytically and by numerically solving the model equations in a two-dimensional geometry, with the spot diameter presently used in some laser acceleration experiments. The most stable initial pulse length is estimated to exceed ≳1.5–2 μm. Moderate stretching of the pulse in the direction of propagation is observed, followed by the development of a vacuum channel and of a very large electrostatic wake potential, as well as by the bending of the laser wave front.

Detecting radiation reaction at moderate laser intensities.

We propose a new method of detecting radiation reaction effects in the motion of particles subjected to laser pulses of moderate intensity and long duration. The effect becomes sizable for particles that gain almost no energy through the interaction with the laser pulse. Hence, there are regions of parameter space in which radiation reaction is actually the dominant influence on charged particle motion.

High-Quality Laser-Driven Electron Beams by Ionization Injection in Low-Density Nitrogen Gas Jet

We report a laser wakefield acceleration of electron beams up to 130 MeV from laser-driven 4 mm long nitrogen gas jet. Using moderate laser intensity ( $\sim 35\times 10^{18}~{\rm W}\cdot {\rm cm}^{-2}$ ) and relatively low plasma densities ( $0.8\times 10^{18}~\mathrm{cm}^{-3} -2.7\times 10^{18}~\mathrm{cm}^{-3}$ ), we have achieved stable laser propagation and consequently stable acceleration of electron beams. We experimentally studied the dependence of the electron beam parameters on the laser beam energy. Evidence of the ionization-induced electron injection has been recognized from the characteristic long-tail beam energy spectrum, however, the majority of the electrons were contained in quasi-monoenergtic peaks.

Fast ion generation in femtosecond laser ablation of a metallic target at moderate laser intensity

The generation of ions during laser ablation of a metallic target (copper) with ≈50 fs Ti:Sa laser pulses of moderate intensity (≈1014 W cm−2) is studied by simultaneous fast-imaging and ion-probe techniques. The spatiotemporal distribution of excited ions and neutrals in the laser-produced plasma plume is analyzed by exploiting appropriate band-pass filters in the imaging set-up, while the ion flux angular distribution is characterized by angle-resolved ion probe measurements. An interesting feature of our results is the generation of a fast ion population separated from the neutral component of the atomic plasma plume and characterized by sub-keV kinetic energies, which is interpreted in the frame of a simple model of ambipolar diffusion.

Optimizing laser produced plasmas for efficient extreme ultraviolet and soft X-ray light sources

Photon sources produced by laser beams with moderate laser intensities, up to 1014 W/cm2, are being developed for many industrial applications. The performance requirements for high volume manufacture devices necessitate extensive experimental research supported by theoretical plasma analysis and modeling predictions. We simulated laser produced plasma sources currently being developed for several applications such as extreme ultraviolet lithography using 13.5% ± 1% nm bandwidth, possibly beyond extreme ultraviolet lithography using 6.× nm wavelengths, and water-window microscopy utilizing 2.48 nm (La-α) and 2.88 nm (He-α) emission. We comprehensively modeled plasma evolution from solid/liquid tin, gadolinium, and nitrogen targets as three promising materials for the above described sources, respectively. Results of our analysis for plasma characteristics during the entire course of plasma evolution showed the dependence of source conversion efficiency (CE), i.e., laser energy to photons at the desired wavelength, on plasma electron density gradient. Our results showed that utilizing laser intensities which produce hotter plasma than the optimum emission temperatures allows increasing CE for all considered sources that, however, restricted by the reabsorption processes around the main emission region and this restriction is especially actual for the 6.× nm sources.

Coherent acceleration by laser pulse echelons in periodic plasma structures

We consider a possibilty to use an echelon of mutually coherent laser pulses generated by the emerging CAN (Coherent Amplification Network) technology for direct particle acceleration in periodic plasma structures. The plasma structure survives a single shot only. However, due to it's simplicity and projected very low production costs, the structure can be replaced for every laser shot at a kiloherz repetition rate. We discuss resonant and free streaming configurations. The resonant plasma structures can trap energy of longer laser pulses but are limited to moderate laser intensities of about 10^{14}\,{\rm W/cm^{2}} and are very sensitive to the structure quality. The free streaming configurations can survive laser intensities above 10^{18}\,{\rm W/cm^{2}} for several tens of femtoseconds so that sustained accelerating rates well above {\rm TeV/m} are feasible. In our full electromagnetic relativistic particle-in-cell (PIC) simulations we show a test electron bunch gaining up to 120\,{\rm GeV} over a distance of 5.3\,{\rm cm} only.