Influence of pulse duration on mechanical effects after laser-induced breakdown in water

Abstract

The influence of the pulse duration on the mechanical effects following laser-induced breakdown in water was studied at pulse durations between 100 fs and 100 ns. Breakdown was generated by focusing laser pulses into a cuvette containing distilled water. The pulse energy corresponded to 6-times breakdown threshold energy. Plasma formation and shock wave emission were studied photographically. The plasma photographs show a strong influence of self-focusing on the plasma geometry for femtosecond pulses. Streak photographic recording of the shock propagation in the immediate vicinity of the breakdown region allowed the measurement of the near-field shock pressure. At the plasma rim, shock pressures between 3 and 9 GPa were observed for most pulse durations. The shock pressure rapidly decays proportionally to r−(2⋯3) with increasing distance r from the optical axis. At a 6 mm distance of the shock pressure has dropped to (8.5±0.6) MPa for 76 ns and to <0.1 MPa for femtosecond pulses. The radius of the cavitation bubble is reduced from 2.5 mm (76 ns pulses) to less than 50 μm for femtosecond pulses. Mechanical effects such as shock wave emission and cavitation bubble expansion are greatly reduced for shorter laser pulses, because the energy required to produce breakdown decreases with decreasing pulse duration, and because a larger fraction of energy is required to overcome the heat of vaporization with femtosecond pulses.

Pulse energy dependence of subcellular dissection by femtosecond laser pulses

“…For pulse energies above 10-15 nJ, a much larger part of the cell is ablated (not shown). This phenomenon is most likely due to cavitation, which has been observed during laser irradiation of water, soft materials, and biological tissues [14,22]. Part of the energy delivered to the sample cannot be dissipated through thermal diffusion, producing a rapid, local increase in material temperature, leading to an explosive expansion of the material and, thus, damage far from the laser focus.…”

“…The nonlinear nature of the optical absorption makes it possible to treat any transparent sample, regardless of its linear absorption coefficient. Second, with a pulse duration of about 100 fs, only a few nanojoules of energy are necessary for material dissection [5,9] In contrast, green picosecond laser systems [10,11] and UV laser microscissors [12,13] require two to three orders of magnitude higher energy [14]. The low energy minimizes collateral damage in the vicinity of the laser focus and reduces the likelihood that the cell will be injured or killed.…”

Effects of cavitation bubble interaction with temporally separated fs-laser pulses

“…12,18,30 At the breakdown threshold, the cavitation bubble radius is well below 1 μm, 30 which is below the optical resolution of 2.32 μm (minimum separability) of the microscope. A more exact measurement of LIOB threshold would be to measure the onset of scattered light by the cavitation instead of its shadow.…”

“…[12][13][14][15][16][17][18][19][20]. Briefly, the photodisruption process can be described as follows: by tightly focusing an ultrashort laser pulse into an aqueous medium like biological tissue, nonlinear absorption processes, such as multiphoton, tunnel, and cascade ionization, are initiated within the focal volume due to the very high intensities.…”

Towards clinical use of a laser-induced microjet system aimed at reliable and safe drug delivery

“…[1][2][3] One possible remedy is a system based on the microjet, [4][5][6][7][8][9] whereby the generation of needle-free injection relies on the bubble dynamics within the injector. 10,11 Both jet characteristics and bubble behavior determined by the laser beam properties such as wavelength and pulse duration have been studied. 9 For general use of a laser-induced microjet injector as a clinical device, the delivery of drugs into the human tissue must be assured by a zero side effect associated with any type of drug damage done by strong thermal and inertial effects of the laser ablation-based mechanism for ejecting a microjet.…”