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16LP-PS-10
Simplified model for estimations of threshold fluences for laser melting and evaporation of nanoparticles
V. Pustovalov1, A. Chumakov2
1Belarusian National Technical University, Polytechnical Institute, Minsk, Belarus 2Institute of physics of NAS Belarus, centre of plasma, Minsk, Belarus
Action of laser radiation pulses on nanoparticles leads to the initiation of different processes, for example, melting or evaporation of nanoparticle, and so on [1-4]. Many of these processes have thermal nature and threshold character. It means the initiation of these processes is realized under the achievement of some fixed threshold values of nanoparticle temperatures under the action of laser pulses with threshold radiation parameters. Theoretical investigations and estimations of laser threshold fluencies are carried out for melting and evaporation of solid spherical nanoparticles by laser pulses in a liquid media.
A new analytical methodology of general interest is presented for the explanation and determination of threshold parameters of various processes of laser-nanoparticles interaction. A simplified model for estimation of laser threshold fluence for melting Em and evaporation Eev of solid (metal) nanoparticles has been developed. The temporal dependencies [5] of nanoparticle temperature are used for estimations of threshold laser fluencies.
To =Tn+IKabsro[1- expf- t/xo)]/4k^
The characteristic time to is determined by ro=coporo2/3kw, co, po are the heat capacity and density of nanoparticle material accordingly, I- laser intensity, Kabs - efficiency factor of absorption of laser radiation with wavelength X by spherical nanoparticles with radius ro [6,7] , t - time, k® is the heat conduction coefficient of surrounding medium (water).
The dependencies of threshold fluencies on pulse durations, laser wavelengths and nanoparticle radii are established and discussed. The model has been validated through direct comparison with the results of laser experiments with nano, pico- and femtosecond lasers and mutual agreement of these results. Comparison of some predicted values of the threshold laser fluencies for melting and evaporation of gold spherical nanoparticle in water with experimental data is given. Estimation of maximal nanoparticle temperature at the end of laser action can provide the determination of realized processes under laser action on nanoparticles and necessary validation of experimental data.
Explanation of the existence of minimal values of Em, Eev at ro ~ 20-40 nm for various laser wavelengths has been given. It is significant, that the values of Em, Eev don't depend on pulse durations tp, that much less than characteristic time of to, tp<< to. These model and results can be used for the precise processes of nanoparticles treatment and applications in various laser technologies. This model is used analytical equations and can be applied for different experimental conditions.
The knowledge of optimum range of fluence can be translated to the requirements for a laser and nanoparticles parameters. The precise determination or estimations of threshold parameters is very important for the success of laser-nanoparticle applications in different fields of nanoscience and nanotechnology
References
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