Critical Power For Self-focusing Research Articles

Electron acceleration in cavitated laser produced ion channels

This paper is concerned with the channeling of a relativistic laser pulse in an underdense plasma and with the subsequent generation of fast electrons in the cavitated ion channel. The laser pulse has a duration of several hundreds of femtoseconds and its power PL exceeds the critical power for laser channeling Pch, with Pch≈1.1Pc, Pc denoting the critical power for relativistic self-focusing. The laser pulse is focused in a plasma of electron density n0 such that the ratio n0/nc lies in the interval [10−3,10−1], nc denoting the critical density. The laser-plasma interaction under such conditions is investigated by means of three dimensional Particle-In-Cell (PIC) simulations. It is observed that the steep laser front gives rise to the excitation of a surface wave which propagates along the sharp radial boundaries of the electron free channel created by the laser pulse. The mechanism responsible for the generation of relativistic electrons observed in the PIC simulations is also analyzed by means of a test particles code. The fast electrons are found to be generated by the combination of a surface wave and of the betatron resonance. The maximum electron energy observed in the simulations is scaled as a function of PL/Pc; it reaches 350–600 MeV for PL/Pc = 70–140.

Raman effect in self-focusing of few-cycle laser pulses in air

Self-focusing of ultrashort pulses in air is investigated by means of numerical simulations. The role of the vibrational Raman effect and its dependence on pulse chirp is studied, with results shedding new light on the interpretation of the measurements of the critical self-focusing power. We also discuss computational modeling issues important specifically for few-cycle pulses.

Effect of power losses on self-focusing of high-intensity laser beam in gases

A theoretical study of power loss from periphery of an ultrashort pulse laser beam and temporally resolved defocusing produced by laser-induced plasma are performed using paraxial approximation. Our analysis incorporates consideration of spatial distribution of the laser beam irradiance and the results show that substantial power losses (10–80%) occur from the beam periphery limiting the length of a filament. It was also shown that the generally accepted concept of self-focusing critical power is inconsistent with consideration of self-induced refraction of spatially distributed laser beam. A new criterion for self-focusing and hypothesis for multiple filamentation are proposed.

Multiple refocusing in BK9 glass generated by a loosely focused high-intensity femtosecond pulse

Multiple refocusing in BK9 glass induced by a loosely focused high-intensity femtosecond pulse is studied experimentally and numerically. Multiple refocusing and small filament are clearly observed in the sample irradiated by loosely focused 120 fs femtosecond laser pulses with high power, up to 500 times the self-focusing critical power. Up to five refocusing peaks are observed along the direction of the pulse propagation. The dependence of the intensity distribution of the plasma luminescence on the pulse energy is investigated.

Self-focusing length in highly multimode ultra-large-mode-area fibers

Recent theoretical researches reveal that the self-focusing critical power in the fiber waveguide is identical to that in the bulk medium. However, the delivery of peak power much higher than the self-focusing critical power has been demonstrated experimentally in ultra-large-mode-area fiber (ULMAF). And no experimental observation of self-focusing effect has been reported in recent pulsed fiber laser system whose peak power has reached or even exceeded the critical power. In this paper, we try to address this issue by studying the self-focusing length theoretically in the ULMAF which is highly multimode. Nonlinear beam propagation method employing PÁDE(2,2) approximation is applied in the numerical simulation. The results show that the self-focusing length of the fundamental mode is typically a few millimeters which is almost identical to that in the bulk medium. However, the self-focusing length of the summation of numerous modes can be as long as a few meters.

Numerical investigation of relativistic self-focusing of intense laser beam with LG01 mode in plasmas

In this paper, we numerically investigate the relativistic self-focusing of laser beam with LG01 mode in plasmas. An effective potential is introduced to identify the critical power. Numerical methods are employed to solve the governing equations. To verify the numerical procedures, we compare the known analytical solution of the relativistic critical power for TEM00 mode with our numerical method. It is shown that there is a good agreement between our numerical results and the analytical solution. The critical power for relativistic self-focusing of a LG01 laser beam is about 6 times of that for a TEM00 laser beam.

Self-focusing collapse distance in ultrashort pulses and measurement of nonlinear index

We analyze the method of moving focus to determine the critical power for self-focusing by means of numerical simulation and a semianalytical model. It is shown that the original interpretation of a moving focus experiment does not hold in general and that inclusion of defocusing effects due to free electrons is necessary to relate the measured data to critical power.

Pulse duration dependent nonlinear propagation of a focused femtosecond laser pulse in fused silica

The nonlinear propagation of a single focused femtosecond laser pulse in fused silica has been investigated both experimentally and by numerical simulations. In particular, the filamentation behavior was systematically studied by varying pulse duration. At low pulse energy, the peak plasma density inside the filament first increases to a maximum value with increasing pulse duration and then begins to decrease. At relatively high pulse energy, denser plasma can be induced around the geometrical focus with a certain longer pulse duration, where the peak power is already below the self-focusing critical power and no filament is formed. This pulse duration dependent behavior can be explained by different ionization mechanisms.

Propagation dynamics of an ultrashort, high energy laser pulse via self-modulation in gas medium with atmospheric pressure for laser compression

Self-compression of 20-mJ laser pulses has been demonstrated in a free-space argon-filled cell. A 130-fs pulse was compressed to less than 60 fs (full width at half maximum) with an output energy of 17 mj at an argon gas pressure of 25 kPa, corresponding to an input peak power of 3.65 times the self-focusing critical power through a single filament in a 10-mJ energy region.

Self-action of a high-power 10-μm laser radiation in gases: control of the pulse duration and generation of hot electrons

The propagation of ultrashort 10-μm laser pulses of power exceeding the critical self-focusing power in xenon and air is numerically simulated. It is shown that the pulse duration in certain regimes in xenon can be decreased by 3–4 times simultaneously with the increase in the pulse power by 2–3 times. It is found that the average energy of electrons in a filament upon filamentation of 10-μm laser pulses in air can exceed 200 eV. The features of the third harmonic and terahertz radiation generation upon filamentation are discussed.

Control of filamentation of femtosecond laser pulses in a turbulent atmosphere

Various methods for controlling the onset of filamentation of a high-power laser pulse in extended vertical atmospheric paths are analysed. It is shown that the increase in the structure constant of the atmospheric turbulence on average leads to the earlier formation of 'hot' spots when the initial pulse power exceeds the critical self-focusing power by an order of magnitude and more. It is found in the numerical experiments that the use of broad focused beams is preferable for achieving the minimal standard deviation of a distance for the filament onset.

Femtosecond filaments as a new type of laser guide stars for astronomical adaptive optics

A method for creating laser guide stars based on filaments of high-power femtosecond radiation at altitudes more than 10 km is proposed. The parameters of femtosecond laser pulses optimal for the formation of laser guide stars are determined. An analytic expression for the critical self-focusing power on a vertical path is obtained.

Single-focusing regime of propagation of femtosecond light packets when self-focused in a condensed medium

Numerical simulations are used to investigate the propagation of tightly focused megawatt femtosecond light packets in a nonlinear condensed medium in the single-focusing regime. A plasma is shown to be generated in a small region whose length and diameter increase with power in the single-focusing regime of propagation of a femtosecond light packet in a transparent dielectric. The limiting electron density in the microplasma formation region is virtually independent of the laser pulse power but increases with numerical aperture of the focusing optics. At a fixed femtosecond pulse power exceeding the critical self-focusing power, the transmission of light by a dielectric layer decreases with increasing numerical aperture of the focusing optics.

Peakon profiles and collapse-bounce cycles in self-focusing spatial beams

We study the over-focusing of spatial light beams due to self-focusing nonlinearity, in both local and nonlocal nonlinear media. Numerical simulation of both cases reveals a peaked profile, with a near-cusp at the center surrounded by exponentially-decaying tails, at a critical self-focusing power. The profile is a local effect, occurring as diffraction counteracts nonlinearity. Nonlocality, however, is needed to prevent modulation instability of the initial beam and to prevent catastrophic collapse in 2D. The peaked profile remains for weak nonlocality but disappears for wide nonlocal responses. Beyond the critical power for a peaked solution, or for longer propagation distances, competition between nonlinearity and diffraction causes oscillatory collapse-bounce behavior. The numerical results are confirmed by observing these dynamics in a self-focusing glass with a nonlocal, thermal response.

Reducing the self-focusing efficiency of a femtosecond pulse in a transparent medium with dispersion when the number of light vibrations in it is decreased

This paper shows that, for light pulses composed of a small number of vibrations with the same maximum electric field and transverse size, i.e., with the same ratio of the radiation power to the critical self-focusing power, the self-focusing efficiency in a transparent disperse medium decreases when the number of vibrations in the pulse is decreased. For shorter pulses, because of the larger dispersion blurring, the size of the transverse narrowing at the focus turns out to be greater, while the radiation intensity is less.

Electron Acceleration Based on an Elongated Plasma Channel

Experiments for the laser guiding studies has been carried out with the 30-fs 100-TW Ti:sapphire laser pulse interaction with the underdense plasma produced by a long-slab (1.2 times 10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) gas jet and an ablative laser-triggered discharged capillary. Formation of an extremely long plasma channel with a length (~10 mm) 10 times above the Rayleigh length is 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 the electron acceleration with a low transverse emittance (< 0.87pi mm mrad) and high electric current (~10 nC/shot), as well as the generation of a quasi- monoenergetic electron bunch with an energy of ~80 MeV. In order to continuously elongate the plasma channel, a 4-cm- scale discharged capillary was used. We successfully demonstrated laser-plasma acceleration of high-quality electron beams up to nearly semigigaelectronvolts. Our results exactly verified the prediction of laser wakefield acceleration through a centimeter- scale plasma channel in the "blowout bubble" regime, where a microscale plasma cavity produced through the ultrarelativistic laser-plasma interactions plays an essential role in the self- injection and acceleration of electrons.

Effect of intensity clamping on laser ablation by intense femtosecond laser pulses

We have experimentally demonstrated that because of intensity clamping, when the laser peak power is higher than the critical power for self-focusing, further increase of the laser power cannot result in corresponding increase of the laser ablation rate of a metallic sample placed in gases. The ablation rate will finally approach a stabilized value. Also, the experimental technique implemented in our work could be potentially used to measure the self-focusing critical power and the nonlinear refractive index.

Spot-size evolution of laser beam propagating in plasma embedded in axial magnetic field

In this paper, evolution of the spot size of an intense laser beam propagating in axially magnetized, cold, underdense plasma has been studied. The effect of longitudinal magnetization on the laser spot for a left as well as a right circularly polarized laser beam has been considered. Critical power for nonlinear self-focusing of the beam in magnetized plasma has been obtained.

Wideband modulation instability at combination frequencies in wave beams

We study theoretically wideband modulation instability at combination frequencies in media having cubic nonlinearity of self-focusing type along with the higher-order defocusing nonlinearity. It is assumed that in a medium with a purely cubic nonlinearity, the medium dispersion does not permit modulation instability. In this case, a collapse of the wave field exists if the beam power is higher than the critical power of self-focusing. The higher-order nonlinearity limits the field at the nonlinear focus, and the instability at combination frequencies becomes possible. It turns out that the field at the nonlinear focus increases with increasing excess of the beam power over the critical power of self-focusing. The obtained values of the nonlinear dielectric permittivity are used for determination of the growth rates of instability at combination frequencies. These growth rates ensure an increase in the combination fields from noise levels up to values comparable with the field of the high-power beam. Such an increase takes place if the beam power is severalfold higher than the critical one. The developed theory can be used for explanation of spectrum superbroadening during self-focusing of sufficiently short laser pulses and high-harmonic generation.

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.