Transition from linear- to nonlinear-focusing regime in filamentation

Lim, Khan; Durand, Magali; Baudelet, Matthieu; Richardson, Martin

doi:10.1038/srep07217

Cited by 71 publications

(46 citation statements)

References 37 publications

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“…Role of the focusing geometry on the filament formation When laser radiation with power far above critical propagates through a nonlinear medium, filamentation can take place due to the dynamical balance among self-focusing, plasma defocussing and diffraction [20][21][22]. The process of filament formation can be described by the action of two main nonlinear effects.…”

Section: Resultsmentioning

confidence: 99%

“…In the experiments we changed focusing conditions to achieve different regimes of filamentation. Since filamentation is a complicated process appearing from the competition between Kerr self-focusing, diffraction, dispersion and plasma defocusing, the abrupt change in the focusing conditions could break the relations between the processes and could dramatically change the 'life' of the filament [22].…”

Section: Resultsmentioning

confidence: 99%

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Highly extended high density filaments in tight focusing geometry in water: from femtoseconds to microseconds

et al. 2015

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We report a new regime of filamentation in water in tight focusing geometry, very similar to the socalled superfilamentation seen in air. In this regime there is no observable conical emission and multiple small-scale filaments, but instead a single continuous plasma channel is formed. To achieve this specific regime the principal requirement is the usage of tight focusing and supercritical power of laser radiation. Together they guarantee extremely high intensity in the microvolume in water (∼10 14 W cm −2 ) and clamp the energy in the ultra-thin (approximately several microns) channel with a uniform plasma density distribution in it. Each point of the 'superfilament' becomes a center of spherical shock wave generation. The overlapped shock waves transform into one cylindrical shock wave. At low energies, a single spherical shock wave is generated from the laser beam waist, and its radius tends toward saturation as energy increases. At higher energies, a long stable contrast cylindrical shock wave is generated, whose length increases logarithmically with laser pulse energy. The linear absorption decreases the incoming energy delivered to the focal spot, which dramatically complicates the filament formation, especially in the case of loose focusing. Aberrations added to the optical scheme lead to multiple dotted plasma sources for shock wave formation, spaced along the axis of pulse propagation. Increasing the laser energy launches the filaments at each of the dots, whose overlapping leads to enhancing the length of the whole filament and therefore the shock impact on the material.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Highly extended high density filaments in tight focusing geometry in water: from femtoseconds to microseconds

et al. 2015

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“…Jim et al divide the filamentation into linear-focusing (high-NA) and nonlinear-focusing (low-NA) [31] regime according to the NA size. In the following, by using the lens of f = 40 cm, we investigate the variation of fluorescence intensity around 337 nm in the linear-focusing (high-NA) case, as shown in Figure 6(a).…”

Section: Resultsmentioning

confidence: 99%

Nitrogen fluorescence induced by the femtosecond intense laser pulses in air

et al. 2016

High Pow Laser Sci Eng

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Our experiments show that external focusing and initial laser energy strongly influences filament generated by the femtosecond Ti-sapphire laser in air. The experimental measurements show the filament length can be extended both by increasing the laser energy and focal length of focusing lens. On the other hand, the plasma fluorescence emission can be enhanced by increasing the laser energy with fixed focal length or decreasing the focal length. In addition, the collapse distance measured experimentally are larger than the calculated ones owing to the group-velocity-dispersion effect. In addition, we find that the line widths of the spectral lines from N 2 is independent of filament positions, laser energies and external focusing.

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“…A looser focus is found to enable a deeper extension to the mid-IR (see Fig. S5 in Supplement 1) because the spectral broadening is dominated by SPM of neutral atoms, which symmetrically extends the spectrum to both sides [25] with the minimal selfsteepening and/or plasma effect. With f 1000 mm, the supercontinuum from the mid-IR filament reaches 3500 nm, which is used in the experiment for absorption spectroscopy.…”

Section: Lettermentioning

confidence: 95%

Mid-infrared laser filaments in air at a kilohertz repetition rate

Liang¹,

Weerawarne²,

Krogen³

et al. 2016

Optica

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Laser filamentation overcomes diffraction over a highly extended distance, making itself a powerful tool for long-range stand-off detection and light detection and ranging (LIDAR) applications. Mid-infrared (mid-IR) wavelengths are optimal for detecting biochemicals and air pollutants due to molecular fingerprints. Here, we demonstrate mid-IR laser filamentations in ambient air at a kilohertz repetition rate for the first time. Laser filaments significantly longer than the linear confocal parameter are generated with a pump power exceeding the critical power in air using a kilohertz, 2.1 μm, femtosecond, multi-millijoule optical parametric chirped-pulse amplifier. Odd-harmonic generation up to the ninth order at ultraviolet and the mid-IR spectral extensions up to 3.5 μm are observed. The highest third and fifth harmonic efficiencies from ambient air are obtained, to our knowledge, thanks to the extended interaction length within the filaments. Numerical simulations reproduce the harmonic generation with good agreement and confirm that the plasma effect dominates over the higherorder Kerr effect as the main defocusing mechanism of laser filamentation in our experiment. The detection of atmospheric CO 2 is demonstrated via mid-IR absorption spectroscopy. High-flux ultrabroadband mid-IR filaments are useful for the fast and sensitive detection of multiple chemical species in air.Laser filamentation is a highly nonlinear process that occurs as the self-focusing from the optical Kerr effect is balanced by diffraction, plasma defocusing, and other nonlinear mechanisms. The laser peak power needs to be higher than the critical power that scales with λ 2 to reach the filamentation regime, where λ is the laser wavelength. Filamentation has been widely studied in solids and high-pressure gases for various applications, such as supercontinuum generation, few-cycle pulse compression, and high-harmonic generation [1,2]. Filamentation in air has especially attracted considerable attention for applications in remote sensing, LIDAR, and laser-induced breakdown spectroscopy [3,4]. Compared to the traditional LIDAR and spectroscopy based on linear laser beam propagation, filamentation overcomes diffraction over much longer distances, and, additionally, the back scattered light is much more directional [2]. This makes laser filamentation a powerful tool for long-range stand-off detection. Moreover, the broadband supercontinuum from the filamentation covers a large spectral range, enabling us to access multiple single-wavelength traces simultaneously in a single-shot measurement. However, most experimental work on filamentation in ambient air has been limited to the visible and near-infrared (near-IR) range, where high-energy, ultrafast laser sources are readily available. By extending the spectral range to the mid-IR, one could explore higher-order nonlinear processes, as the odd harmonics in the visible and near-IR regions propagate well in the atmosphere through filaments [5][6][7]. More importantly, mid-IR filamentation in air ...

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Transition from linear- to nonlinear-focusing regime in filamentation

Cited by 71 publications

References 37 publications

Highly extended high density filaments in tight focusing geometry in water: from femtoseconds to microseconds

Highly extended high density filaments in tight focusing geometry in water: from femtoseconds to microseconds

Nitrogen fluorescence induced by the femtosecond intense laser pulses in air

Mid-infrared laser filaments in air at a kilohertz repetition rate

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