Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application

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.

Cylindrically Focused Nonablative Femtosecond Laser Processing of Long‐Range Uniform Periodic Surface Structures with Tunable Diffraction Efficiency

“…However, these techniques have certain limitations, such as high cost, requirement of complex systems like masks and vacuums, and lack of flexibility. The formation of laser‐induced periodic surface structures (LIPSS), which is first observed by Birnbaum in 1965, provides a mask‐free, nonvacuum, and low‐cost processing procedure for fabricating periodic structures on sample surfaces and exhibits powerful potential in various applications . After decades of research, multiple works have focused on LIPSS formation on a diverse range of materials .…”

Femtosecond-laser-irradiation-induced structural organization and crystallinity of Bi2WO6

“…These periodical structures with a subwavelength-scale period hold potential in technological applications because of their unique advantages, including multiphonon absorption, a precise ablation threshold, excellent controllability, and a negligible heat-affected zone [54][55][56][57][58][59][60] . Fs irradiation allows high-level control of the modification of the morphology and properties in nanofabrication approaches, where the feature size is limited only by the spot size of the laser beam [61][62][63] . This control is due to the fact that lasers can deposit a large amount of energy per unit area over a temporal scale shorter than the phonon-electron relaxation time 64 .…”

Femtosecond Laser Micro/Nano-manufacturing: Theories, Measurements, Methods, and Applications

“…Femtosecond laser pulse shaping in the time domain is a new fabrication method that can improve and control ultrafast laser fabrication [49][50][51][52][53]. During femtosecond laser fabrication, most photon energy is initially absorbed by electrons.…”

“…Basically, microhole fabrication requires high-quality (no recast layer, no crack, no heat-affected zone, uniform density) and various materials (alloys, polymers, glass, etc.). Especially, in a few special cases, microholes must have a high aspect ratio (10:1 to 100:1 or even higher) and be nonpolluting [53,167]. Femtosecond laser has the advantages of minimizing heataffected zone, less recasting layer, non-contact in processing, and no need of liquid/acid-base assistance and it has become one of the most promising processing methods to fabricate high-aspect-ratio microholes [167,[172][173][174][175].…”

“…Unlike metal materials, femtosecond lasers can achieve high-efficiency ablation of transparent materials because of their nonlinear ionization effect, and their nonthermal phase transition mechanism can lead to high-quality processing results [53,180]. Large aspect ratio is very important for microholes.…”