Plasma Density inside Femtosecond Laser Filaments in Air

J. Bernhardt1, W. Liu1, F. Théberge1, M. Châteauneuf2, J. Dubois2, S. L. Chin1

1. Centre d’Optique, Photonique et Laser (COPL) et Département de Physique, de Génie Physique et d’Optique, Université Laval, Québec, QC, G1K 7P4, Canada

2. Defense Research and Development Canada ‒ Valcartier,

2459 Pie-XI Blvd North, Québec, QC, G3J 1X5 Canada2

Abstract: We present a spectroscopic way to measure the plasma density of femtosecond laser filaments in air by using Stark broadening of the oxygen atomic fluorescence line. This method could also be applied to multiple filamentation. ©2007 Optical Society of America OCIS codes: (300.0300), (320.0320).

1. Introduction Femtosecond laser pulse filamentation induced by the dynamic balance between self-focusing and self-generated plasma defocusing, has been associated to many exciting applications such as electric discharge control in air and producing energetic few-cycle pulses through self-compression. Basically, the plasma density is a crucial information that needs to be known from the point of view of both fundamental understanding of the physics and practical applications. Previous methods such as interferometry, plasma conductivity, shadowgraphy, or side-imaging of the calibrated nitrogen fluorescence [1-3], can only be applied to single filament. However, the needs to characterize multiple filaments in air, motivate the following spectroscopic method. 2. Experiment 1 kHz transform-limited laser pulses (800nm, 42fs) with a maximum energy of 2mJ are focused (f=100cm) in air. A small zone of the generated plasma column is side-imaged onto an optical fiber and coupled to a spectrometer (2400 grooves/mm grating) equipped with an intensified CCD camera (ICCD). The Stark broadening of atomic oxygen line (O I 772.nm) is used for evaluating of the plasma density. The plasma density distribution is obtained by scanning the optical fiber along the propagation axis. This first set of experiments is carried out under single filament condition, which is verified by recording the beam pattern with a burn paper. 3. Results In our experiment, the widths iλΔ of the studied spectral line is straightforwardly determined by the Stark broadening as given by the following relationship [4]:

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ei

Nλ ω ⎛ ⎞Δ = ⎜ ⎟⎝ ⎠

where Ne is the plasma density and ω is the electron impact parameter which could be easily obtained from the literatures. The measured longitudinal plasma density distribution is of the order of (Fig. 1a). Furthermore, Fig. 1b shows the plasma density under different focusing conditions and input laser peak powers. Our results are in agreement with previous measurements obtained by using side-imaging technique [3]. It confirms that our technique can be reliably applied to study remotely the plasma density of the filaments using a standard LIDAR spectroscopy technique.

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Fig. 1. a) Longitudinal plasma density distribution of a single filament (laser conditions: 2mJ, 42fs, f=100cm). b) Peak plasma density under different focusing and energy conditions. 4. Summary and Outlook A first set of experiments has been carried out to measure the plasma density distribution of femtosecond laser filaments in air. The presented spectroscopic technique allows for a relatively simple and direct measurement of the plasma density in case of single and multiple filaments. Besides, because it is not limited to the use of close-contact optical imaging technique, it is feasible for remote detection. This is advantageous for future studies in femtosecond laser lightning discharge control and remote detection of atmospheric pollutants.

5. References [1] X.M. Zhao et al., “Femtosecond Ultraviolet-Laser Pulse Induced Lightning Discharges in Gases,” IEEE J. of Quant. Electron. 31, 599-612 (1995). [2] H. Schillinger et al., “Electrical conductivity of long plasma channels in air generated by self-guided femtosecond laser pulses,” Appl. Phys. B 68, 753-756 (1999). [3] F. Théberge et al, “Plasma density inside a femtosecond laser filament in air: Strong dependence on external focusing,” Phys. Rev. E 74, 036406 (2006). [4] H.R. Griem, Plasma Spectroscopy (McGraw Hill, New York, 1964).

a2829_1.pdf

JWA42.pdf