Laser-induced Plasma Filaments Research Articles

Ultra-broadband detection of coherent infrared pulses by sum-frequency generation spectroscopy in reflection geometry.

We have newly developed, to the best of our knowledge, a detection method for broadband infrared pulses based on sum-frequency generation spectroscopy in reflection geometry, which can avoid a restriction of the detection bandwidth originating from the phase mismatch that is inevitable for the upconversion in transmission geometry. Using a GaAs crystal, we successfully demonstrated the ultra-broadband detection of the infrared pulses generated from a two-color laser-induced air plasma filament in a region from 300 to 3300 cm-1. With the advantage of ultra-short infrared pulses, the present detection method holds promise for application to time-resolved, ultra-broadband vibrational spectroscopy.

Two-dimensional high resolution electron properties of femtosecond laser-induced plasma filament in atmospheric pressure argon

This work reports the measurement of two-dimensional electron properties over a nanosecond scale integration time across a femtosecond laser-induced plasma filament in atmospheric pressure argon. Radial electron properties across the ∼100\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 100$$\\end{document} μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu$$\\end{document}m diameter filament are obtained at discrete axial locations at 2.5 mm steps by one-dimensional high-resolution laser Thomson scattering with a spatial resolution of 10 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu$$\\end{document}m. These measurements reveal plasma structural information in the filament. The Thomson spectral lineshapes exhibit clear spectral sidebands with an α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha$$\\end{document} parameter ∼1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 1$$\\end{document}, enabling the measurement of both electron temperature and density profiles. These measurements yield electron densities on the order of 1022\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{22}$$\\end{document}/m3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3}$$\\end{document} and electron temperatures of ∼2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 2$$\\end{document} eV. Heating from the probe laser due to inverse bremsstrahlung is taken into account to correct the Thomson scattering electron temperature measurements. Under these conditions, electron-neutral collision induced bremsstrahlung becomes the dominant laser-induced plasma heating process associated with the probe laser. The measurements reveal structural features of the filament, including an asymmetrically skewed density structure in the axial direction and reversed radial distributions of electron density and temperature.

Single-shot local measurement of terahertz correlated second harmonic generation in laser air plasma filaments.

We present a single-shot detection method of terahertz correlated second harmonic generation in plasma-based sources by directly mixing an optical probe into femtosecond laser-induced plasma filaments in air. The single-shot second harmonic trace is obtained by measuring a second harmonic generation on a conventional CCD with a spatiotemporally distorted probe beam. The system shows a spectrometer resolution of 22 fs/pixel on the CCD and a true resolution on the order of the probe pulse duration. With considerable THz peak electric field strength, this formalism can open the door to single-shot THz detection without bandwidth limitations.

Backward THz Emission from Two-Color Laser Field-Induced Air Plasma Filament.

Two-color laser field-induced plasma filaments are efficient broadband terahertz (THz) sources with intense THz waves emitted mainly in the forward direction, and they have been investigated intensively. However, investigations on the backward emission from such THz sources are rather rare. In this paper, we theoretically and experimentally investigate the backward THz wave radiation from a two-color laser field-induced plasma filament. In theory, a linear dipole array model predicts that the proportion of the backward emitted THz wave decreases with the length of the plasma filament. In our experiment, we obtain the typical waveform and spectrum of the backward THz radiation from a plasma with a length of about 5 mm. The dependence of the peak THz electric field on the pump laser pulse energy indicates that the THz generation processes of the forward and backward THz waves are essentially the same. As the laser pulse energy changes, there is a peak timing shift in the THz waveform, implying a plasma position change caused by the nonlinear-focusing effect. Our demonstration may find applications in THz imaging and remote sensing. This work also contributes to a better understanding of the THz emission process from two-color laser-induced plasma filaments.

Single-shot spatiotemporal plasma density diagnosis using an arbitrary time-wavelength-encoded biprism interferometer

Single-shot ultrafast imaging reveals the transient dynamics of ultrafast phenomena. Herein, we introduce a new approach for capturing ultrafast micron-scale dynamics using an arbitrary time-wavelength-encoded biprism interferometer (TWEBI). In TWEBI, a time-wavelength-encoded pulse train is generated using a set of cascaded frequency-doubling crystals with slightly different phase-matching angles and independent delay lines, which have the advantages of independent temporal resolution, time window, and frame interval parameters. Phase measurement of TWEBI is achieved using a plug-and-play biprism interferometer. In the experiment to capture the transient dynamic of femtosecond laser-induced plasma filament, we demonstrated that the TWEBI setup can capture a total of 12 frames in a single shot, and its effective frame rate is 5 trillion frames per second (Tfps), corresponding to a temporal resolution of 200 fs and a spatial resolution of 4 μm. Moreover, the time window of the TWEBI setup can be adjusted from sub-picosecond to nanosecond timescales. Thus, TWEBI has the advantages of high-precision temporal-spatial resolution, high frame rate, adjustable time window, and no reference arm, thus providing a practical and feasible diagnostic scheme for complex transient dynamics.

Single-Shot Multi-Frame Imaging of Femtosecond Laser-Induced Plasma Propagation.

Single-shot ultrafast multi-frame imaging technology plays a crucial role in the observation of laser-induced plasma. However, there are many challenges in the application of laser processing, such as technology fusion and imaging stability. To provide a stable and reliable observation method, we propose an ultrafast single-shot multi-frame imaging technology based on wavelength polarization multiplexing. Through the frequency doubling and birefringence effects of the BBO and the quartz crystal, the 800 nm femtosecond laser pulse was frequency doubled to 400 nm, and a sequence of probe sub-pulses with dual-wavelength and different polarization was generated. The coaxial propagation and framing imaging of multi-frequency pulses provided stable imaging quality and clarity, as well as high temporal/spatial resolution (200 fs and 228 lp/mm). In the experiments involving femtosecond laser-induced plasma propagation, the probe sub-pulses measured their time intervals by capturing the same results. Specifically, the measured time intervals were 200 fs between the same color pulses and 1 ps between the adjacent different. Finally, based on the obtained system time resolution, we observed and revealed the evolution mechanism of femtosecond laser-induced air plasma filaments, the multifilament propagation of femtosecond laser in fused silica, and the influence mechanism of air ionization on laser-induced shock waves.

Flatfield ultrafast imaging with single-shot non-synchronous array photography.

We present a method for acquiring a sequence of time-resolved images in a single shot, called single-shot non-synchronous array photography (SNAP). In SNAP, a pulsed laser beam is split by a diffractive optical element into an array of angled beamlets whose illumination fronts remain perpendicular to the optical axis. Different time delays are imparted to each beamlet by an echelon, enabling them to probe ultrafast dynamics in rapid succession. The beamlets are imaged onto different regions of a camera by a lenslet array. Because the illumination fronts remain flat (head-on) independently of beamlet angle, the exposure time in SNAP is fundamentally limited only by the laser pulse duration, akin to a "global shutter" in conventional imaging. We demonstrate SNAP by capturing the evolution of a laser induced plasma filament over 20 frames at an average rate of 4.2 trillion frames per second (Tfps) and a peak rate of 5.7 Tfps.

Enhancing terahertz radiation from femtosecond laser filaments using local gas density modulation

We present a method to enhance the terahertz (THz) wave radiation from a femtosecond laser-induced plasma filament by controlling the local gas density within the filament. We develop a theoretical model for THz generation from a laser-induced air plasma filament and the subsequent propagation process, to account for a varying local gas density. By adjusting the local gas density along the filament, the transient current distribution along the filament and the resulting coherent superposition of terahertz waves can be controlled. The location of the gas jet nozzle and the relative phase between multicolor light fields both affect the transient current distribution and thus the strength of the generated THz field. Compared with the conventional terahertz generation by a two-color filament in a homogeneous gas, a three-color filament can realize an increase by 6.12 times in the generated THz pulse energy, with optimized local gas density modulation. Our results suggest that the THz amplification via local gas density modulation can be further improved with well-designed multicolor pulses.

Simultaneous elliptically and radially polarized THz from one-color laser-induced plasma filament

THz-based technologies and research applications have seen a rapid increment in recent period together with the development of novel radiation sources based both on relativistic electrons and laser techniques. In this framework, laser-induced plasma filament plays an important role in generating intense and broadband THz radiation. Although many attentions have been paid to THz emission from two-color plasma filaments, one-color plasma emission has been scarcely investigated. In particular, the polarization state of one-color THz emission is still controversial due to the limitations of the existing THz detection techniques, which are incapable of simultaneously detecting elliptically and radially polarized THz radiation. In this manuscript, we develop a novel detection method and unambiguously demonstrate for the first time that one-color laser-induced plasma filament simultaneously emits elliptically and radially polarized THz radiation. These polarization states suggest that the generation mechanism results from electric quadrupole, showing a new route for producing more complex polarization states and THz vortex beams.

Mechanism study of terahertz radiation regulation in a multi-color laser field

By establishing a theoretical model for terahertz (THz) generation from a laser-induced air plasma filament and the subsequent propagation process, we study the physical mechanisms of THz wave radiation regulation in the multi-color femtosecond laser field. It is proved that the total THz radiation in the far field is affected by three factors: the ionized electron density, transient current distribution along the laser filament, and coherent superposition between THz waves generated by each point along the filament. The results show that the wavelength, relative phase, energy ratio, pulse width, laser frequency ratio, and frequency combination of the multi-color laser are all correlated to their radiated THz energy with different changing laws. The proper regulation of a three-color filament can realize a 4 orders of magnitude increase in the generated THz pulse energy as compared with the conventional THz generation by a two-color filament. These results provide the theoretical basis for the enhancement of THz radiation energy, and they are of great significance for clarifying the physical process of laser filament.

Formation Mechanism of Excited Neutral Nitrogen Molecules Pumped by Intense Femtosecond Laser Pulses

Backward amplified spontaneous emission of neutral nitrogen molecules has been reported from laser-induced plasma filaments. The cavity-free UV emission has great potential applications in remote atmospheric sensing. However, the formation mechanism for the excited nitrogen molecules inside filaments remains controversial. Here we study the formation mechanism of excited nitrogen molecules pumped by intense femtosecond laser pulses. After modification of the electron energy distribution by inclusion of the recollision between the electron and its parent ion as well as modification of the electron collision cross section by inclusion of the secondary electron contribution, the theoretical calculations reproduce the experimental observations very well. The results clearly demonstrate that excited nitrogen molecules are generated through collisions between energetic electrons and neutral nitrogen molecules.

Conical versus Gaussian terahertz emission from two-color laser-induced air plasma filaments.

In this Letter, we demonstrate that the far-field terahertz (THz) beam generated from a Ti:Sapphire two-color laser-induced filament can exhibit a conical or Gaussian distribution, depending on the filtering experimental conditions. Using both an incoherent Golay cell detector and a two-dimensional coherent electro-optic detection covering the 0.2-2.6 THz spectral range, in our experimental conditions, we provide evidence that the conical emission is due to photo-induced carriers in the silicon filter, typically used to block the remaining pump laser light. Moreover, the low-frequency THz beam retrieves an almost $ {{\rm TEM}_{00}} $TEM00 Gaussian spatial distribution when the silicon filter is preceded by a large bandgap ceramic filter, which stops the pump beam, thus preventing the carrier generation in the silicon filter.

Simulation of EM wave propagation along a femtosecond laser plasma filament

Abstract The propagation of electromagnetic (EM) wave along a plasma filament induced by the sufficiently intense femtosecond laser pulse is studied with the commercial simulation software XFDTD. The simulated results show that the EM wave can propagate along the plasma filament, with the electric field enhanced out of the waveguide and attenuated in the waveguide. Besides, we can conclude that the relative direction between the plasma filament and the electric field can influence the guiding performance of the plasma filament. Furthermore, a maximum EM wave intensity enhancement more than threefold the free space propagation is realized. The research results imply that the laser-induced plasma filament can provide efficient transmission of the EM wave energy.

Backward Terahertz Radiation from a Two-Color Femtosecond Laser Filament

We report the first experimental observation of backward terahertz emission from the two-color laser induced plasma filament in air. The ratio of measured forward-to-backward terahertz radiation is ∼25/1. This result agrees with numerical simulations based on interferometric model assuming 0.3 mm long plasma source.

THz Frequency Modulation by Filamentary Plasma Grating

The frequency of terahertz pulse induced by dual-color laser–induced plasma filament was measured at different positions of the filament to analyze terahertz generation processes. It was experimentally observed that the most broad frequency bandwidth was attained at the tail of laser-induced plasma filament and more higher frequency components were generated under the intervention of filamentary plasma grating, which was attributed to the increased peak electron density from inelastic collisional ionization, while another infrared pulse was focused on the plasma grating to reaccelerate preformed electrons. The results demonstrated that the electron reacceleration process played an important role in determining terahertz frequency bandwidth.

Decay of femtosecond laser-induced plasma filaments in air, nitrogen, and argon for atmospheric and subatmospheric pressures.

The temporal evolution of a plasma channel at the trail of a self-guided femtosecond laser pulse was studied experimentally and theoretically in air, nitrogen (with an admixture of ∼3% O_{2}), and argon in a wide range of gas pressures (from 2 to 760 Torr). Measurements by means of transverse optical interferometry and pulsed terahertz scattering techniques showed that plasma density in air and nitrogen at atmospheric pressure reduces by an order of magnitude within 3-4 ns and that the decay rate decreases with decreasing pressure. The argon plasma did not decay within several nanoseconds for pressures of 50-760 Torr. We extended our theoretical model previously applied for atmospheric pressure air plasma to explain the plasma decay in the gases under study and to show that allowance for plasma channel expansion affects plasma decay at low pressures.

レーザープラズマフィラメントによる有機塩素化合物の炭素粒子化

レーザープラズマフィラメントによる有機塩素化合物の炭素粒子化

Physics of the conical broadband terahertz emission from two-color laser-induced plasma filaments

We propose a comprehensive physical model explaining the conical character of broadband terahertz generation from femtosecond two-color laser-induced air plasma filaments. We show, that, in contrast to other models, emission is always conical, resulting from phase matching of the radiation produced inside the filament combined with a partial back reflection of the generated terahertz field from the filament itself due to the frequency-dependent critical plasma density. The obtained conical angle varies from 2${}^{\ensuremath{\circ}}$ to 10${}^{\ensuremath{\circ}}$, depending on the plasma density distribution and filament length. Unlike previously proposed models, our model shows good agreement with our experiments as well as a wide range of experimental findings from the literature.

Temporal evolution of femtosecond laser induced plasma filament in air and N2

We present single shot, high resolution, time-resolved measurements of the relaxation of laser induced plasma filaments in air and in N2 gas. Based on the measurements of the time dependent electromagnetic signal in a waveguide, an accurate and simple derivation of the electron density in the filament is demonstrated. This experimental method does not require prior knowledge of filament dimensions or control over its exact spatial location. The experimental results are compared to numerical simulations of air plasma chemistry. Results reveal the role of various decay mechanisms including the importance of O4+ molecular levels.

Off-Axis Phase-Matched Terahertz Emission from Two-Color Laser-Induced Plasma Filaments

We observe off-axis phase-matched terahertz generation in long air-plasma filaments produced by femtosecond two-color laser focusing. Here, phase matching naturally occurs due to off-axis constructive interference between locally generated terahertz waves, and this determines the far-field terahertz radiation profiles and yields. For a filament longer than the characteristic two-color dephasing length, it emits conical terahertz radiation in the off-axis direction, peaked at 4-7° depending on the radiation frequencies. The total terahertz yield continuously increases with the filament length, well beyond the dephasing length. The phase-matching condition observed here provides a simple method for scalable terahertz generation in elongated plasmas.