Long-distance femtosecond laser filaments in air
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This paper reviews the recent studies of filamentation of femtosecond lasers pulses in air in the Institute of Physics, Chinese Academy of Sciences. The filamentation mechanisms of free propagated femtosecond laser pulses, effect of air turbulence on the filamentation, interaction between filaments are presented.Keywords:
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Component (thermodynamics)
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Laser filamentation in transparent material has a wide range of applications, from three dimensional manufacturing to biological technologies. Various experimental results showed that femtosecond laser pulse filamentation in fused silica strongly depends on laser focusing conditions. However, the physical mechanism governing each regime has not been fully understood. For the first time, single and multiple re-focusing of the laser pulse in interaction of femtosecond laser pulse with fused silica, and consequent single and multiple damage zones (filaments) have been observed in our extensive three-dimensional, high resolution FDTD (finite-difference time-domain) simulations. We show that Kerr nonlinearity plays a crucial role loose laser focusing regime, while it is not an important factor in tight laser focusing regime, where geometrical focusing becomes important. Our simulation results agree well with existing experimental findings. In addition, the improved analytical model prediction gives a reasonable estimate of the shift from Kerr nonlinearity regime to linear geometrical focusing regime.
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Laser filamentation in transparent material has a wide range of applications, from three dimensional manufacturing to biological technologies. Various experimental results showed that femtosecond laser pulse filamentation in fused silica strongly depends on laser focusing conditions. However, the physical mechanism governing each regime has not been fully understood. For the first time, single and multiple re-focusing of the laser pulse in interaction of femtosecond laser pulse with fused silica, and consequent single and multiple damage zones (filaments) have been observed in our extensive three-dimensional, high resolution FDTD (finite-difference time-domain) simulations. We show that Kerr nonlinearity plays a crucial role loose laser focusing regime, while it is not an important factor in tight laser focusing regime, where geometrical focusing becomes important. Our simulation results agree well with existing experimental findings. In addition, the improved analytical model prediction gives a reasonable estimate of the shift from Kerr nonlinearity regime to linear geometrical focusing regime.
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Broadband and energetic terahertz (THz) pulses can be remotely generated in air through filamentation.We review such THz generation and detection in femtosecond Ti-sapphire laser induced remote filaments.New results are presented on the direct relationship between THz generation in a two color filament and induced N2 fluorescence through population trapping during molecular alignment and revival in air.This further supports the new technique of remote THz detection in air through the sensitive measurement of N2 fluorescence.
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A new method and associating system has been presented to characterize pre-pulses of femtosecond laser using laser filamentation in transparent media. Pre-pluses of the laser system has been measured experimentally and it is in good agreement with the results obtained by third order cross-correlator. This method can be used for fast detection of temporal laser intensity relatively in order to avoid formation of pre-plasmas before laser matter interaction experiments.
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Characterization
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Lithium fluoride
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Supercontinuum
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In this work, we experimentally demonstrate a 200% enhancement of terahertz (THz) wave amplitude generated by femtosecond laser filamentation in air. The experimental setup simply uses a semicircular phase plate to generate two parallel filaments. Temporally overlapped THz pulses from two filaments coherently add up, giving rise to significant enhancement of the THz pulse amplitude. It has been foreseen that further enhancement would be achieved if the design of phase plates could be optimized to generate a filament array. This simple method makes full use of the laser energy and could potentially open a new approach to remotely enhance the THz emission in air.
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Photomixing
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Femtosecond laser-induced optical breakdown in liquids results in filamentation, which involves the formation and collapse of bubbles. In the present work, we elucidate spatio-temporal evolution, interaction, and dynamics of the filamentation-induced bubbles in a liquid pool as a function of a broad spectrum of laser pulse energies (∼1 to 800 µJ), liquid media (water, ethanol, and glycerol), and the number of laser pulses. Filament attributes such as length and diameter have been demarcated and accurately measured by employing multiple laser pulses and were observed to have a logarithmic dependence on laser energy, irrespective of the medium. The size distribution of persisting microbubbles is controlled by varying the pulse energy and the number of pulses. Our experimental results reveal that introducing consecutive pulses leads to strong interaction and coalescence of the pulsating bubbles via Bjerknes force due to laser-induced acoustic field generation. The successive pulses also influence the population density and size distribution of the micro-bubbles. We also explore the size, shape, and agglomeration of bubbles near the focal region by controlling the laser energy for different liquids. The insights from this work on filamentation-induced bubble dynamics can be of importance in diverse applications such as surface cleaning, fluid mixing and emulsification, and biomedical engineering.
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