First-principle Simulations Of Electron Dynamics And Optical Properties Modulation Of Materials By Femtosecond Laser Pulse

Laser micro/nano fabrication has broad application prospect and research value in a number of manufacturing fields,such as biology,electronics,optics,and material,because of its characteristics of high precision,no pollution,repeatablility in large area,and so on.Femtosecond laser has ultrashort pulse duration time(10^-15s scale),ultrahigh power intensity(10²² W/cm² scale)and controllable temporal and spatial shape,fundamentally changing the laser-material interactions mechanism compared with long-pulse laser.With the development of ultra-fast laser technology and the extensive and in-depth study of femtosecond laser processing technology,higher requirements have been put forward for the understanding of the interaction process between femtosecond laser and materials.How to theoretically describe the physical quantities in the interaction process of femtosecond lasers and materials has become one of the important challenges in laser micro/nano processing.Femtosecond laser-material interaction is a nonlinear and nonequilibrium complex process,including the absorption of laser energy byelectrons,energy transfer from electrons to lattices,plasma generation,phase change,and material modification,which range from nanometers to millimeters spatially and from femtoseconds to microseconds temporally.In the interaction process between femtosecond laser and material,the electron excitation effect is the most important.This process determines the entire process of subsequent electron-phonon energy transmission,plasma formation,and material phase transitions,and affects the final processing results.Based on this,the research group proposed a new ultra-fast laser manufacturing method based on electron dynamics control.In order to verify the feasibility of this method,provide a basis for further understanding of the mechanism of laser-material interaction and promote the application of ultra-fast laser manufacturing technology to the processing and application field,it is necessary to understand the changes of the transient local electron dynamics and optical properties during laser processing.In this study,the transient localized electron dynamics and corresponding dielectric function changes of molecule,cluster,two dimension and bulk materials under femtosecond laser irradiation are analyzed using time-dependent density functional theory(TDDFT).The main works and innovations of this thesis are as followed:1.The quantum model for the interaction between femtosecond laser and isolated system have been established based on time-dependent density functional theory.The optical properties and the influence of femtosecond laser irradiation of isolated system were investigated to validate the feasibility of electron dynamics control(EDC)on isolated system.It is validated that photoabsorption spectrum of molecule in excited state,charactered by frequency difference of absorption peaks in ground state,can be obtained by selecting the appropriate laser frequency.The values and positions of photoabsorption peaks of molecule in excited state can be controlled by selecting the appropriate laser intensity,and finally the desired absorption spectrum can be achieved.The effects of resonance on laser-material interactions are investigated.Dipole oscillation is no longer follow laser waveform for resonant condition and energy absorption is strongly enhanced compared with nonresonant condition.2.The quantum models to calculate transient optical properties of crystal materials under femtosecond laser based on induced field have been established.The dielectric responses of monolayer Mo S₂,a two-dimensional semiconduct,to few-cycle femtosecond laser pulse with vary wavelengths and intensities are demonstrated by using the models for the first time to validate the feasibility of EDC on 2D materials.The results reveal the feasibility of manipulating transient electronic and optical properties of monolayer Mo S₂within femtosecond time scale by ultrashort laser irradiation.The desired transient dielectric properties can be achieved by adjusting polarization direction,intensity,and wavelength of laser,extending the range of functionality of existing optoelectronic devices.The polarization direction of the laser has a marked effect on the energy absorption because of anisotropy.Change in the polarization direction changed the absorbed energy by a factor of three orders of magnitude.The frequency-dependent changes of the dielectric functions are in agreement with prediction of the Drude model.3.The quantum models to calculate transient optical properties of crystal materials under femtosecond laser have been used to research the optical properties of bulk diamond and modulation of these properties by femtosecond laser,revealing the ability to tune the dielectric functions of diamond through electron dynamics control by using tunable femtosecond laser pulse and validating the feasibility of EDC on 3D materials.The reliability of calculated results is verified according to sum rule and comparison of simulated and experimental values.It is hard for optical breakdown of diamond under few-cycle femtosecond laser pulse,making it possible to reversibly give rise to deep damage-free changes of dielectric materials in the optical properties.The scenario of the optical breakdown was clarified through breakdown study.The delay time between pump and probe pulse and double pulses are investigated to further understand the laser-material interactions mechanism.4.The multiscale quantum models for the interaction between femtosecond laser and film materials based on coupled dynamics of time-dependent density functional theory and Maxwell equations have been established.The method employs Maxwell equations to describe laser pulse propagation and time-dependent density functional theory to describe the generation of conduction band electrons in an optical medium.According to the propagation of femtosecond laser pulses,the reflectivities of diamond and graphene thin film at different intensities were simulated.By calculating the spatial distribution of the conduction band electrons inside the medium,its dominant effect on the reflectivity change of the material was validated.The ablation threshold of the material is obtained by using the critical energy criterion.The predicted value of the theory is close to the experimental value.A highly unusual behavior of optical properties of graphene films as a function of femtosecond laser intensity from 1×10¹⁰ to 5×10¹⁵ W/cm² was predicted.Graphene film exhibited the metallic high reflectivity and resonance absorption at low intensity due to very low effective electronic quality.Both the reflectivity and absorptivity dramatically decrease and the phenomenon of electromagnetically induced transparency occurred at moderate intensity.The reflectivity and absorptivity increased at high intensity due to bond breaking and the skin effect.The present study demonstrated that the electron dynamics controlled by laser pulse played a dominantly role in determining the optical responses.The behaviors of optical properties as a function of intensity suggest potential applications of graphene film for optical-field effect devices with low cost-effectiveness ratio.