A New Unified Model In The Process Of Pulsed Laser Deposition

Pulsed laser deposition is one of the most attractive new techniques, developed in the 1980's. Then, along with the multi-component oxide high-temperature superconductor discovered, PLD technique makes a great success in depositing such film materials. The PLD technique has so many advantages, such as high pulsed laser energy density, similarity between target and prepared thin film components, the high deposition rate, and lower substrate temperature. With the development of Pulsed laser technology, the pulse width of laser output arranges from nanosecond to femtosecond in the PLD process. The development trend of PLD is the ultra short and ultra high laser.Recently, our group has studies every aspect of the PLD process. The relevant physics mechanism research of the PLD technique has flourished. This paper studies the ablation mechanism of different pulse width and set up a unified model. It can describe the thermal conduction phenomenon in the high energy laser ablation process. Specially, it is still available when laser pulse width ranges from nanosecond to femtosecond timescale. We get the ablation thresholds by solving the unified model, which agree with the experimental data well.Firstly, we analysis the characters of laser-targets interaction: the laser–electron interaction, the electron-electron scattering, the electron-ion coupling. Then it is obvious that the thermal equilibrium ablation corresponds with long pulse laser and the non-equilibrium ablation corresponds with ultrashort pulse laser. Usually, we still adopt the traditional thermal conduction model for long pulse laser and the two temperature model(TTM) for ultrashort pulse laser. The electron-phonon coupling time ( t ie) and laser pulse width ( t p) are introduced to the standard two temperature model (TTM). For long pulses, the new unified model can converted into the traditional thermal conduction model; while for ultrashort pulses, it can change into the standard two temperature model.As an example of gold target, we get the dependence of electron and ion temperature evolvement on time and position by solving the thermal conduction equation using the finite-difference time-domain (FDTD) method. It is in good agreement with experimental data. We obtain the critical temperature of the onset of ablation using the Saha equation and then the theoretical value of laser ablation threshold when laser pulse width ranges from nanosecond to femtosecond timescale which consists well with the experimental data. Otherwise, we get the ablation threshold for the different pulse width, which arrange from nanosecond to femtosecond timescale. At the same time, the theoretic result agrees with the experimental data well.Above all, the unified model can describe the phenomenon thermal conduction, which arrange from nanosecond to femtosecond timescale. However, we should consider that the absorptance changed with the waver length and the plasma shielding generated by high-power nanosecond pulsed laser ablation for the next step.