Multicale Theoretical Study Of Ultrafast Laser Micro/Nano Fabrication Based On Electron Dynamics Control

Ultrafast laser micro/nano fabrication is a frontier field for advanced processing.It provides a new revolutionary technology for micro-nano processing because of the non-contact,non-polluting processing model and high-precision,high-quality processing results.The interaction mechanism of ultrafast laser with material is totally different from that for Continuous laser or long pulse laser,because of its ultrashort pules duration and ultrahigh instantaneous power.Therefore,ultrafast laser can be used to fabricate,with micro/nano precision,electromechanical system,sensor,photoelectric device,energy device,functional surface and so on.The duration of ultrafast laser pulse is much shorter than relation time of electron-lattice.The energy deposition processing has been completed before lattice become heat.Electron and lattice are nonequilibrium.The interaction process of ultrafast laser with material including phase change and fabrication result is determined by the interaction process of laser with electron.Based on such conception,our group propose the ultrafast laser micro/nano fabrication based localized transient electron dynamics control(EDC):because of the unique advantage of ultrashort pulse duration and ultrahigh transient power,By shaping energy distribution of femtosecond pulses in temporal and spatial domains,we can control photon-electron interactions,and then to control the localized transient electron dynamics(including electron density,temperature,and excited state distribution),and further to modify localized transient materials properties,and then to adjust material phase change,and finally to implement the novel fabrication method with high-precision,high-quality.Based on the core idea of EDC,the thesis present multi-scale theoretical study for electron dynamics control in the interaction process of ultrafast laser with material:Firstly,a multi-scale theoretical model for the interaction between ultrafast lasers and materials is established,which can realize the multi-scale description for the processes of electron excitation,laser energy transmission,and plasma expand,and can fully reveal the ultrafast laser micro/nano process.It provides a theoretical basis for development of ultrafast laser micro/nano process in more extreme directions.we investigated the excitation process in linear carbon chains with three different structure irradiated by femtosecond laser pulses within the framework of time dependent density functional theory.Meanwhile,we investigated ultrafast response of dielectric properties of monolayer phosphorene to femtosecond laser pulse.We verified theoretically the feasibility for ultrafast control of dielectric properties by femtosecond laser pulse.We studied femtosecond laser processing of metals by hydrodynamic model including radiation transport,which achieved the multiscale descript of interaction process of ultrafast laser with material.The main innovations of the thesis are as followed:1.revealing the nonlinear electron excitation process in finite system irredarated by femtosecond laser pules.For the linear carbon chains passivated by hydrogen atoms,it is noted that the excited polarization currents were reversible.For the femtosecond laser with wavelength of 200 nm,the excited current could not follow the laser pulse in shape and frequency any more.Otherwise,the amplitude of the current was about one order smaller than that for 600 nm and 400 nm.it is because that the femtosecond laser pulse induced the transient break of pi bonds.Due to the different bond type,the polarization current in the odd chain was larger(about 8%)than that in the even chain2.by choosing suitable laser intensities,realizing effectively manipulate the conductance of linear carbon chain in femtosecond time scale and without damage.For infinite linear carbon chain irradiated by femtosecond laser,there are double“threshold”for excitation and breakdown in infinite linear carbon chain,respectively.The different response of excited currents in infinite linear carbon chain for femtosecond lasers with different wavelengths was also observed.Plasma-like oscillation was observed in the infinite linear carbon chain for the situation of 200 nm.Such oscillations will promote energy absorption for sub-pulse because of the formation of a resonance with the electric field of the femtosecond laser pulse.3.Verifing theoretically the feasibility for ultrafast control of dielectric properties by femtosecond laser pulse,which provides theoretical basis for EDC in ultrafast laser micro/nano fabrication.Our work demonstrated that due to nonlinear ionization induced by femtosecond laser,the dielectric response showed noteworthy features:negative divergence for the real part of the dielectric function at low frequencies and a remarkable“quasi-exciton”absorption peak for the imaginary part of the dielectric function.A new absorption peak appeared at nearly 2.8 e V for the dielectric function along the armchair direction in the case of 10¹¹W/cm²,which indicated the form of unfilled states and the shift of bands induced by the femtosecond laser.The absorption energy of monolayer phosphorene for femtosecond laser polarization along the Y direction is almost two times as much as that along the X direction.Therefore,more absorption energy induced more electron-hole pairs resulting in the stronger dielectric response.For femtosecond laser with different wavelength,the photon with higher energy can excited valence electron from deeper valence band to conduction band and induce dielectric response at higher frequency.4.proposeing a semiempirical method based on the Drude–Sommerfeld model to revise the absorption ratio.In our method,the collision frequency between electrons and ions can be modified for a wide range of electron temperatures from solid-state to high-temperature plasma.Additionally,a series of HD simulations illustrated that the modified electron–ion collision frequency can exactly describe the transient properties of electrons on the metal surface and the thermodynamic process of the metal film.A simple function was identified to describe the relationship between laser intensity and peak electron temperature on the surface,it was expected to be beneficial for fabrication.We investigated the relaxation process between electron and lattice and energy transport process in Al film irradiated by femtosecond laser.And predicted the form of shock wave in Al film.