Study of the temporal and spatial dynamics of plasmas induced in liquids by nanosecond Nd:YAG laser pulses 2: Plasma luminescence and shielding

Abstract

We describe and analyze the temporally and spatially resolved luminescence and attenuation characteristics of plasmas induced in liquids commonly accepted as models for ocular media by high-irradiance Nd:YAG laser pulses of nanosecond duration. Measurements of plasma generated in different irradiation conditions, performed with the aid of a streak camera, show that, when expansion of the plasma column toward the incoming beam occurs, it coincides with quenching of the luminescence exhibited by plasma regions located closer to the focus, where breakdown has first occurred. Evidence for this effect is given both with impurityfree media, where plasma expansion occurs in a continuous and regular way, and with impurity-rich media, where plasma columns are composed either of unevenly distributed plasma regions or of single plasmas with irregular breakdown starting patterns. Experimental findings have been analyzed using a model of spatially distributed plasma shielding, in combination with a previously developed model of moving breakdown, that well interprets, in liquids, the spatially dependent breakdown starting times.

Spatial and temporal dynamics of light attenuation and transmission by plasmas induced in liquids by nanosecond Nd:YAG laser pulses

“…In particular, the maximum length Zmax, the shape of the plasma starting curves and the space-dependent luminescence patterns vs. the suprathreshold parameter fl have been analysed. The experimental data obtained using both impurity-free and impurity-rich liquids [3,4] showed a good agreement with the model. No other time-resolved plasma streaks are available in the literature for liquids.…”

“…Following the onset of breakdown at a given z, the corresponding plasma layer of thickness Az attenuates the input beam: as a result, less power is available to the underlying layers. This competing effect between plasma layers has been defined in [4] as distributed shielding and results in the quenching of the plasma luminescence. Recombination luminescence from laser-induced plasmas has previously been found to follow the input pulse shape for the case of nanosecond pulses [6]: therefore, the amplitude of the luminescence waveform Pt (z, t) is assumed to be proportional to that of the absorbed power Pa(z, t).…”

“…In turn, the energy absorbed per unit volume of plasma is obtained by dividing dEJdz by the area =2 (z). This is justified if the additional simplifying assumption is made [4] that P(z, t) is distributed uniformly across the irradiated area.…”

“…The combined model of moving breakdown and distributed shielding could accurately predict the measured streaks of plasmas, and their dependence on the laser parameters, for both impurity-free and impurity-rich liquids [3][4]. In particular, the maximum length Zmax, the shape of the plasma starting curves and the space-dependent luminescence patterns vs. the suprathreshold parameter fl have been analysed.…”

“…In two recent papers [3,4], the streak patterns induced in liquid water by nanosecond laser pulses have been investigated in terms of i) the axial dependence of the plasma starting time and ii) the dynamics of the plasma luminescence. Different models have been discussed in relation to the experimental results.…”