First-principles Calculations And Their Validations For Ultrafast Micro/Nanofabrication Based On Electron Dynamics Control

Ultrafast laser micro/nano fabrication is a frontier and interdisciplinary field, involvingmechanics, optics, physics, chemistry and materials. It is widely applied in the fields ofnational defense, biological science, information, medical appliances, and automobile. Anultrafast pulse laser, whose pulse duration time is short than10ps （10-11s）, can easilyachieve very high peak power density, which makes the macro/nanofabrication process tendto extreme. The intensity of ultrafast laser is high than10¹²W/cm²and nonlinear absorptiondominate, including electron avalanche ionization, multiphoton ionization, and tunnelionization. The controlled laser intensity can selectively ionize electrons and the idea ofelectron dynamics control is feasible. Femtosecond laser pulse duration is much shorterthan the electron-lattice relaxation time （10-10～10-12s）. Energy absorption is completedbefore lattice changes. Due to the significant electron-lattice nonequilibrium state,femtosecond laser material interaction including phase change and fabrication results isactually determined by laser-electron interaction. Recast, thermal damage （microcracks）,and heat-affected-zone are greatly reduced during the nonequilibrium energy transfer andthe manufacturing quality can be improved. In ultrafast laser micro/nanofabrication, themechanisms of nonequilibrium and nonlinear absorption effects are significantly differentwith those in traditional manufacturing. Therefore, ultrafast laser fabrication improvementmust be implemented by electron dynamics control. For example, by using a shaped pulsetrain, chemical reactions can be controlled; the motion of bound electrons can be controlled;the high harmonic generation process can be significantly enhanced and the overallconversion efficiency can be improved; a single quantum dot spin can be completelyquantum controlled; ionization process can be controlled; the electron emission can bespectrally and temporally tuned and coherent phonon oscillations can be enhanced orcancelled.With the continuous requirements of extremely high-quality and precision, newchallenges are represented for ultrafast laser micro/nanofabrication, such as high-efficiency,cross-scale manufacturing, selective and controllable fabrication, and so on. In theoretical aspects, when the time is short to the femtosecond and the size is small to the nanometer,many classical theories failed in the applications for ultrafast laser material interactions.The transient localized changes of optical properties and thermodynamic properties of thematerial are critical. Consequently, quantum mechanics must be considered. Thelaser-material interaction is a nonlinear and nonequilibrium complex multiscale process,which involves from the nanometer to the millimeter and from the femtosecond to themicrosecond scale. Furthermore, there is no such a comprehensive theoretical model thatcan fully describe the process, which becomes the bottleneck of ultrafast lasermicro/nanofabrication. Based on these challenges and questions, our group has proposed anew ultrafast micro/nanofabrication method based on electron dynamic control. By usingthe unique characteristics of the ultrafast laser, we design spatial/temporal ultrafast laserpulse trains and material properties to control photon-electron interactions, then to controllocalized transient nanoscale electron dynamics （density, temperature, and electrondistribution）, and phase change mechanisms to achieve high-quality and high-precisionmicro-/nano-scale manufacturing.Base on the aforementioned new method, this study presents theoretical andexperimental studies on the transient localized electron dynamics and its influence on thefollowing machining process during laser-material interactions. The main contents of thisthesis include the following four aspects:1. For isolated systems （atoms, molecules and clusters） and bulk materials, weestablish different quantum models to describe ultrafast laser-material interactionsrespectively, which are employed to theoretically validate the feasibility of the proposedidea to control localized transient nanoscale electron dynamics （excitation, ionization,energy absorption, multipole, densities, and distributions of free electrons）.2. The quantum models are applied to simulate the nonlinear electron-photoninteractions during shaped ultrafast laser ablation of different materials. Effects of the keypulse parameters on the electron dynamics are discussed.3. For a certain laser wavelength, the intensity dependence of energy absorption onmultiphoton and/or tunnel ionization mechanisms is investigated. The relationship betweenthe energy absorption and laser intensity is given. 4. According to the theoretical results, experimental research is carried out to discussthe effects of femtosecond laser pulse train parameters and resonance effect on the structuresize, morphology and recasting zones, which is applied to experimentally validate thefeasibility of the proposed new ultrafast micro/nanofabrication method based on electrondynamic control.The main innovations of the thesis are as followed:1. We establish quantum models to describe nonlinear and nonperturbative responsesof atoms, molecules and clusters induced by ultrafast laser, which provides an efficientmethod for the calculation of photoexcitation and ionization of isolated systems. Ourcalculation reproduces the relative ionization rates of a nitrogen molecule as well as thelaser intensity dependence, which are in agreement with the experimental results. Also, theresonant effects on nonlinear electron dynamics of Li₄cluster under femtosecond laserpulse irradiation are investigated. It is demonstrated that multiphoton ionization is stronglyenhanced if the laser is in resonance with the collective mode. For higher laser intensitieswhen tunnel ionization becomes significant, the relationship between the spectral crosssection and the number of ionized electrons, absorbed energy gradually disappears and theeffect of resonant enhancement on ionization and absorbed energy is not apparent. Theinnovation results were published in Journal of Applied Physics （2013,114:143105）.2. To describe the nonlinear processes during ultrafast ablation of infinite periodicsystems, the surface charge effect must be taken into consideration. In consideration oflaser field and induced potential, we establish quantum models to describe nonlinear andnonperturbative responses of bulk materials induced by ultrafast laser. In addition, thequantum model is employed for the femtosecond laser pulse processing of bulk material inthis study. The impacts of the pulse parameters on electron dynamics are also investigated.The innovation results were published in Physics Letters A （2011,375:3200-3204）, Journalof Physics: Condensed Matter （2012,24:275801, cover featured）, and Energy MaterialsNanotechnology Fall Meeting （2013, cover featured）.3. We present first-principles calculations for nonlinear photoionization of diamondinduced by the intense femtosecond laser field by the quantum model. For a certain laserwavelength, the intensity dependence of energy absorption on multiphoton and/or tunnel ionization mechanisms is first investigated, where laser intensity regions vary from10¹²W/cm²to1016W/cm². Theoretical results show that:（1） at the fixed laser wavelength, therelationship between the energy absorption and laser intensity shows a good fit ofδE=c_MI^N（N is the number of photons absorbed to free from the valence band） whenmultiphoton ionization dominates;（2） while when tunnel ionization becomes significant,the relationship coincides with the expression ofδE=c_TIⁿ（n<N）. The innovation resultswere published in Physics Letters A （2012,376:3327-3331） and Journal of Applied Physics（2013,113:143106）.4. By the quantum models for ultrafast ablation bulk material, we first give theexplanations for the ablation volume changes by adjusting pulse delay, and the periodicfluctuations phenomenon of laser induced surface structures depths with the increase ofpulse delay. The innovation results were published in International Conference onAdvanced Laser Technologies （2013）.5. We set up a femtosecond laser precision micro fabrication system. According to thetheoretical results, experiments are carried out to study the effects of femtosecond laserparameters and resonance effect on the structure size, morphology and recasting zones. Themicro/nano processing technology is obtained for high-quality, high-precision and highefficiency micro/nanofabrication. The innovation results were published in Conference onLasers and Electro-Optics （2011） and International Photonics and Opto-ElectronicsMeetings （2012）.This thesis is based on the research projects supported by the National Basic ResearchProgram of China （973Program）（Grant No.2011CB013000） and National Natural ScienceFoundation of China （NSFC）（Grant Nos.90923039and51025521）. The main innovationsof the thesis were all published in the international influential applied physical and opticaljournals, especially two articles which published in Journal of Physics: Condensed Matterand Energy Materials Nanotechnology Fall Meeting were featured as the cover articles.