Laser Plasma Filaments Research Articles

Hollow cylindrical plasma filament waveguide with discontinuous finite thickness cladding

We have explored here a hollow cylindrical laser plasma multifilament waveguide with discontinuous finite thickness cladding, in which the separation between individual filaments is in the range of several millimeters and the waveguide cladding thickness is in the order of the microwave penetration depth. Such parameters give a closer representation of a realistic laser filament waveguide sustained by a long stable propagation of femtosecond (fs) laser pulses. We report how the waveguide losses depend on structural parameters like normalized plasma filament spacing, filament to filament distance or pitch, normal spatial frequency, and radius of the plasma filament. We found that for typical plasma parameters, the proposed waveguide can support guided modes of microwaves in extremely high frequency even with a cladding consisting of only one ring of plasma filaments. The loss of the microwave radiation is mainly caused by tunneling through the discontinuous finite cladding, i.e., confinement loss, and is weakly dependent on the plasma absorption. In addition, the analysis indicates that the propagation loss is fairly large compared with the loss of a plasma waveguide with a continuous infinite thickness cladding, while they are comparable when using a cladding contains more than one ring. Compared to free space propagation, this waveguide still presents a superior microwave transmission to some distance in the order of the filamentation length; thus, the laser plasma filaments waveguide may be a potential channel for transporting pulsed-modulated microwaves if ensuring a long and stable propagation of fs laser pulses.

Analysis of microwave leaky modes propagating through laser plasma filaments column waveguide

A plasma column waveguide formed by a bundle of closely spaced plasma filaments induced by the propagation of ultrafast laser pulses in air and revived by a longer infrared laser pulse is shown to support microwave radiation. We consider values of both the plasma electron density and microwave frequency for which the refractive index of plasma is lower than the refractive index of air; therefore, a leaky plasma waveguide can be realized in extremely high frequency band. The guiding mechanism does not require high conductance of the plasma and can be easily excited by using commercial femtosecond laser sources. A theoretical study of leaky mode characteristics of isotropic and homogeneous plasma column waveguides is investigated with several values of plasma and waveguide structure parameters. The microwave transmission loss was found to be mainly caused by the microwave leakage through the air-plasma interface and is weakly dependent on the plasma absorption. In spite of losses of microwaves caused by leakage and plasma absorption, it is shown to be much lower than both that accompanying to surface waves attaching to single conducting plasma wire and the free space propagation over distances in the order of the filament length, which opens exciting perspectives for short distance point to point wireless transmission of pulsed-modulated microwaves.

Modeling of Sommerfeld surface waves propagating on a single wire of laser plasma filaments

We investigated the possibility of Sommerfeld surface waves to propagate along a conducting plasma channel produced by the filamentation of ultrafast laser pulses in air. Using the approximation of a homogenous cylindrical wire of laser plasma filaments, the phase velocity and the propagation loss of different wire configurations are calculated. The phase velocity of the propagating wave proved to be close to the speed of laser pulses, which makes attaching to such instantaneous plasma channel feasible over distances in the order of the filament length. Wire diameter, electron density and operating frequency are appearing to influence the attaching distances and propagation loss. The attenuation of the propagating wave along the plasma wire appears to be lower than that of free space over some distances in the order of the filamentation length, which opens exciting perspectives for short distance point to point wireless transmission of pulsed-modulated microwaves.

Guiding and focusing of a nanosecond infrared laser within transient hollow plasma femtosecond filament channels

Long distance guidance and focusing of infrared (IR) nanosecond-pulsed laser light in a hollow plasma channel formed with femtosecond pulsed laser plasma filaments is examined with numerical experiments. Numerical models show that a thick hollow plasma channel can provide low loss long distance (>100 m) guidance of the IR nanosecond-pulsed laser beam. The effectiveness of guidance is determined by the electron number density, thickness of the plasma filament shell and the size of the inner radius of the hollow plasma channel. The hollow plasma channel can be thought of as an open-space optical fibre. The required thickness of the plasma channel with specified electron number density and the inner radius can be estimated with modal analysis, similar to that used for optical fibres. In addition, with a small tapering angle in the hollow plasma channel, the guided light can be moderately focused during long distance propagation. Owing to the rapid decay of electron number density of a plasma channel in free space, the delay time between the generation of the plasma channel and the guided light along with the duration of the guided light are better to be maintained within a few nanoseconds.

Guiding microwave radiation using laser-induced filaments: the hollow conducting waveguide concept

A microwave waveguide that consists of a set of laser plasma filaments produced in air by the propagation of femtosecond laser pulses is investigated according to the hollow conducting waveguide concept. The conductivity, skin depth of the electromagnetic waves in this plasma waveguide and the energy required to excite such a waveguide are calculated for different possible configurations. A hollow conducting plasma waveguide is shown to support guided modes of electromagnetic radiation from millimetre to centimetre wavelength range. Our calculations show that, under the concept of conducting waveguide, it is more suitable to use the TE01 mode rather than TE11 to achieve an extended attenuation length. The attenuation length of the low-loss mode TE01 is shown to be dependent on the geometry of the plasma waveguides, the operating frequency and the plasma effective electron density. The effect of the plasma wall density spread on TE01 propagation is evaluated. Using the hollow conducting plasma waveguide operating in TE01 mode, an enhancement of microwave transmission over both free space propagation and dielectric plasma waveguide is obtained.

“All air–plasma” terahertz spectroscopy

We demonstrate terahertz wave generation and detection capabilities up to 6 THz without the need for solid state materials, biased electrodes, or forward propagating signal collection. An "all air-plasma" terahertz system is used to encode explosive material resonant signatures into the 357 nm nitrogen fluorescence line of a bichromatic field-induced laser plasma filament. These results show the practicability to extend these measurements to remote locations where terahertz pulse information is no longer limited by water vapor absorption, phonon resonance, or signal collection directionality.

Terahertz radiation from a laser plasma filament

By the use of two-dimensional particle-in-cell simulations, we clarify the terahertz (THz) radiation mechanism from a plasma filament formed by an intense femtosecond laser pulse. The nonuniform plasma density of the filament leads to a net radiating current for THz radiation. This current is mainly located within the pulse and the first cycle of the wakefield. As the laser pulse propagates, a single-cycle and radially polarized THz pulse is constructively built up forward. The single-cycle shape is mainly due to radiation damping effect.

Revival of femtosecond laser plasma filaments in air by a nanosecond laser

Short lived plasma channels generated through filamentation of femtosecond laser pulses in air can be revived after several milliseconds by a delayed nanosecond pulse. Electrons initially ionized from oxygen molecules and subsequently captured by neutral oxygen molecules provide the long-lived reservoir of low affinity allowing this process. A Bessel-like nanosecond-duration laser beam can easily detach these weakly bound electrons and multiply them in an avalanche process. We have experimentally demonstrated such revivals over a channel length of 50 cm by focusing the nanosecond laser with an axicon.

The mechanism of nitrogen fluorescence inside a femtosecond laser filament in air

The visible and near-UV fluorescence spectroscopy of air generated inside a femtosecond laser plasma filament was studied. The primary reactions, N2++N2⇒N4+;N4++e⇒N2(C3Πu)+N2, populates the electronic excited state N2(C3Пu) of N2. The N2 fluorescence is not by direct electron–ion recombination of N2++e. Using a pump–probe method, the fluorescence lifetime of C3Пu of N2 at atmospheric pressure was determined to be about 85ps.

Effect of an Energy Reservoir on the Atmospheric Propagation of Laser-Plasma Filaments

The ability to select and stabilize a single filament during propagation of an ultrashort, high-intensity laser pulse in air makes it possible to examine the longitudinal structure of the plasma channel left in its wake. We present the first detailed measurements and numerical 3-D simulations of the longitudinal plasma density variation in a laser-plasma filament after it passes through an iris that blocks the surrounding energy reservoir. Since no compensation is available from the surrounding background energy, filament propagation is terminated after a few centimeters. For this experiment, simulations indicate that filament propagation is terminated by plasma defocusing and ionization loss, which reduces the pulse power below the effective self-focusing power. With no blockage, a plasma filament length of over a few meters was observed.

Conductivity Measurements of Femtosecond Laser–Plasma Filaments

Experiments are performed to characterize the electrical properties of plasma filaments that are generated by self- guided femtosecond laser pulses propagating in air. A single plasma filament passes through a high-voltage sphere pulsed at -100 kV to a grounded electrode, which serves as a current monitor. The experiments utilize moderate electric fields to probe the filament conductivity, thereby avoiding the strong perturbations caused by electric discharges. The measured filament current decreases as ~1/R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> as the separation R between the electrodes is increased up to 1.5 m. The pulselength of the filament current signal is 2 ns (full-width at half-maximum), but the time resolution is limited by the bandwidth of the oscilloscope. The typical plasma density in the conducting filament is 9 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , which is inferred from the conductivity measurements and the size of the optical filaments. Comparisons are made with mobility values derived from electron swarm data, where the mobility depends upon the applied electric field. The conductivity of the filament is measured as the laser pulselength is varied from 50 fs to 1.5 ps. We find that relatively long laser pulses (1 ps) produce filaments with the largest conductivity. A model is used to predict the longitudinal position where the plasma filament forms and is in reasonably good agreement with measurements.

Spectrally modulated second harmonic emission from laser plasma filaments

Spectra of forward emitted second harmonic light from laser interaction with filamentary plasmas have been experimentally studied. Rather regular modulations in the frequency domain have been observed into overall red-shifted spectra. The observed spectral features are consistent with self-phase-modulation of the intense laser light in growing filaments. A model accounts for this effect.

Forward second harmonic emission from laser plasma filaments

Experimental observations are reported on the interaction of 1-μm laser light with underdense plasmas (n ≤ 0·25 nc) from thin foil plastic targets. Nominal laser intensity on target was up to 3 × 1013 W/cm2 in a 3-ns pulse, but much higher intensity was reached due to spiky laser pulses. We studied forward-emitted second harmonic light as a diagnostic of the interaction and in particular of the occurrence of filamentation. Measurements included: energy monitoring of 2ω forward emission vs. target position and laser energy; time resolved (120-ps gate) imaging of the interaction region cross section. The second harmonic energy level was found to be sensitive to target position. In addition, the images obtained with the target in position of maximum second harmonic generation showed unstable structures whose scale length is comparable with the expected one for maximum filamentation growth. These results are shortly discussed in the framework of stationary filamentation theory and second harmonic generation in inhomogeneous media.

Field distributions and resonance absorption in a laser plasma filament

In this paper we have theoretically studied the field distributions and resonance absorption in a laser-produced plasma filament. Under the condition of the cold plasma we derive the field equations, as well as their analytical solutions, to the radial and axial components of the electric field. Then, by the numerical calculations, we find that there exists a tunnel effect along the radial direction of the filament, i.e., the electric field reaches a maximum near the radial resonance point r0. Thus, the curves of variance of the radial and axial field components are obtained.