Abstract
During femtosecond laser fabrication, photons are mainly absorbed by electrons, and the subsequent energy transfer from electrons to ions is of picosecond order. Hence, lattice motion is negligible within the femtosecond pulse duration, whereas femtosecond photon-electron interactions dominate the entire fabrication process. Therefore, femtosecond laser fabrication must be improved by controlling localized transient electron dynamics, which poses a challenge for measuring and controlling at the electron level during fabrication processes. Pump-probe spectroscopy presents a viable solution, which can be used to observe electron dynamics during a chemical reaction. In fact, femtosecond pulse durations are shorter than many physical/chemical characteristic times, which permits manipulating, adjusting, or interfering with electron dynamics. Hence, we proposed to control localized transient electron dynamics by temporally or spatially shaping femtosecond pulses, and further to modify localized transient materials properties, and then to adjust material phase change, and eventually to implement a novel fabrication method. This review covers our progresses over the past decade regarding electrons dynamics control (EDC) by shaping femtosecond laser pulses in micro/nanomanufacturing: (1) Theoretical models were developed to prove EDC feasibility and reveal its mechanisms; (2) on the basis of the theoretical predictions, many experiments are conducted to validate our EDC-based femtosecond laser fabrication method. Seven examples are reported, which proves that the proposed method can significantly improve fabrication precision, quality, throughput and repeatability and effectively control micro/nanoscale structures; (3) a multiscale measurement system was proposed and developed to study the fundamentals of EDC from the femtosecond scale to the nanosecond scale and to the millisecond scale; and (4) As an example of practical applications, our method was employed to fabricate some key structures in one of the 16 Chinese National S&T Major Projects, for which electron dynamics were measured using our multiscale measurement system.
Highlights
Because of their ultrashort irradiation periods and ultrahigh intensities, femtosecond laser pulses in some aspects fundamentally change the laser-material interactions mechanisms compared with long laser pulses, which has created wide-range and exciting new possibilities in micro/nanoscale fabrication[1,2,3]
Ionization mechanism can be adjusted by changing femtosecond laser energy and its temporal/spatial distribution to control laser-material interactions[24]
This review summarizes our recent progresses based on EDC by shaping femtosecond pulses in micro/nanofabrication: 1) Theoretical modeling for EDC feasibility: four models were developed, which consists of the ab initio calculations for electron dynamics[98,99,100,101,102,103,104,105,106,107], a revised molecular dynamics simulation for phase change[108,109], a plasma model for ionization processes[110,111,112,113,114,115] and an improved two-temperature model for energy transport[116,117,118]
Summary
Because of their ultrashort irradiation periods and ultrahigh intensities, femtosecond laser pulses in some aspects fundamentally change the laser-material interactions mechanisms compared with long laser pulses, which has created wide-range and exciting new possibilities in micro/nanoscale fabrication[1,2,3]. The ultrahigh intensity makes femtosecond laser-material interactions a strongly nonlinear process[4,5]. Ionization mechanism can be adjusted by changing femtosecond laser energy and its temporal/spatial distribution to control laser-material interactions[24]. The ultrashort irradiation period of a femtosecond laser makes femtosecond laser-material interactions a strongly nonequilibrium process[3,25]. V te 1⁄4 2p3ffi2pffipffimðffiZffieffiÃðÞk2BnTeÞe34=l2nL f1 þ exp1⁄2Àmðne; T Þ=kBT gF 1=2 ke 1⁄4 13n2e teCe εðt ; r;
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