电子动态调控超快激光微纳制造

Ultrafast Laser Micro/nano Fabrication Based on Electrons Dynamics Control

超快激光是指脉宽短于10 ps的激光,在制造过程中其作用时间、功率密度等趋于极端,具有超强(也就是非线性)、超快(也就是非平衡态)的独特优势。本研究提出了电子动态调控的核心思想。电子动态决定了材料的所有特性,包括它的光学、热力学、磁学、化学、电学特性等。超快激光的加工决定于光子与电子相互作用过程。超快激光脉冲可以激发、电离电子,从而改变辐照过程中的局部瞬时电子动态。在超快激光辐照过程中,电子密度的变化可以达到几个甚至上十个数量级,如此巨大的电子密度的变化使材料的瞬时局部特性也发生了巨大的变化。在超快激光辐照非金属过程中,辐照区域的非金属可呈很强的金属态,它的反射率从原来的接近于零变成了零点九几。这样强烈的材料瞬时局部特性的变化对激光光场进行了显著重整:激光在达到自由电子临界密度之前是高斯分布;当自由电子密度达到临界密度之后,多数光都被反射了。所以,我们通过设计激光光场时空分布以调节光子与电子的相互作用过程。当我们将一个超快激光脉冲分成两束来调节子脉冲间的延迟,就可以调节电子电离过程,从而改变瞬时局部状态。此时仅仅调节一个简单的参数——子脉冲间的延迟,就可以使自由电子的密度分布产生巨大变化。在不同延迟下,峰值自由电子密度变化非常的剧烈。相应的,在不同延迟下材料的瞬时局部特性变化也非常大。同样的,通过调整延迟,激光能量的吸收情况分布变化也非常大。所以,通过改变脉冲延迟,可使自由电子密度、材料的瞬时局部特性、激光透射能量分布等产生巨大变化。通过超快脉冲时空整形,调控电子-电子相互作用过程,进而局部调控电子瞬时状态(密度温度激发态分布等),从而调控材料瞬时局部特性,进而调控材料相变过程,实现全新的目标制造方法。我们建立了超快激光与材料相互作用的多尺度量子理论模型,提出了电子动态调控超快激光微纳制造新方法,通过设计超快激光能量时域及空域分布,调控加工过程中的能量吸收、传递及材料相变过程,进而提高加工质量、精度。此外,搭建了多时间尺度电子动态实时观测系统,观测超快激光微纳加工过程中的材料瞬时局部折射率、等离子体强度等,优化加工参数,实现了对局部瞬时电子动态的主动调控,并应用于国家重大需求关键核心构件的加工工艺中,开启了电子层面调控的新机理和新方法研究。

Ultrafast lasers are the lasers whose pulse durations are shorter than 10 ps. With ultrahigh power intensities and ultrashort irradiation periods, uhrafast laser fabrication presents unique advantages of nonlinear nonequilibrium processing. In this research, we propose the core idea of electrons dynamics control （EDC）. The electrons dynamics determine most of the properties of the material, including optical, thermal, magnetic, chemical, and electrical properties. The ultrafast laser fabrication process is determined by photon-electron interactions. An uhrafast pulse can excite/ionize electrons, which then determines the localized transient material properties. During ultrafast laser irradiation, the electron density can be increased by several to ten orders of magnitude. Such a huge change in electron density also induces great changes in localized transient material properties. In the area being irradiated by femtosecond pulses, a dielectric material presents strong metal-like properties. Its ref/ectivity can be changed from almost zero to more than 0.9. Such tremendous changes in localized transient material properties strongly reshape the ultrafast laser field. In this example, the laser pulse distribution remains as a Gaussian profile until the critical free electron density is created. After the critical free electron density is reached, most of the laser energy is reflected. Hence, we can design spatial and temporal energy distributions of ultrashort laser pulses to control photon-electron interactions. We can split a single fem- tosecond pulse into double subpulses and adjust the delay between the two subpulses to control ionizations and then change the localized transient electrons dynamics. By adjusting a simple parameter, the pulse delay, the distribution of free electron density can be greatly changed. The electron density distribution is being sig- nificantly changed. Correspondingly, the localized transient material properties, and energy distribution of a transmitted laser can also be greatly changed by adjusting the pulse delay. Therefore, by shaping an ultrafast pulse in temporal and spatial domains, we are able to control electron-electron interactions, and then to control the localized transient electrons dynamics （including electron density, temperature, and excited state distribution）, and further to modify localized transient naterials properties, and then to adjust material phase change, and eventually to implement the novel fabricaion method. We established a multiscale model and proposed a novel miero/nano fabrication methodology based on ultrafast laser electrons dynamics control. For he first time, localized transient electrons dynamics and orresponding material properties were actively conolled in manufacturing. Fabrieation throughput, qualiry, and precision were significantly improved. The proased fabrication method has been selected to fabricate some core structures for the Chinese national key projects.