纳米金属粒子在纳秒脉冲激光照射下的特异破碎过程及其动力学研究

Small metallic particles in nanometer scale have broad absorption bandwidth covering UV to near-infrared region due to the surface plasmon resonance. The absorption spectrum is correlated to their geological size and shape. When the metallic nanoparticles in liquid surroundings are irradiated by intense resonant laser pulses, the absorbed photons induce heating and melting, followed by break down and recrystallization. This process is known as laser induced fragmentation (LIF). The rate of the LIF process has been found to be related to the pulse energy and wavelength of the irradiation laser, and the volume of the irradiated region, but the dynamics of LIF are not yet understood completely. We have observed unconventional LIF in our previous experiments, where the rate of LIF is significantly altered by other conditions, such as the transverse mode of the laser, and the mean distance between neighboring particles. This research is intended to utilize techniques including absorption spectrum, time-resolved spectroscopy and SEM imaging to put these unconventional LIF processes under quantitative analysis, and to find the dynamics behind these interesting processes.

由于表面等离子体共振的影响，金属纳米颗粒在紫外到近红外波段有很宽的吸收带宽，其吸收光谱与颗粒的尺寸和形状密切相关。当金属纳米颗粒在液体环境中受到共振波段内的强脉冲激光照射时，吸收光子的能量使颗粒迅速加热熔化，并发生破裂和重结晶。这一过程被称为激光诱导碎裂(laser induced fragmentation)。在以往的研究中，LIF过程的速率被发现与脉冲激光的脉冲能量、激光波长和照射区域体积相关，但LIF的具体机理还没有释明。我们在前期实验中观察到了特异的LIF现象。在这些实验中，我们观察到LIF过程的速率受到其他因素的显著影响，这些因素包括：激光的空间模式、相邻纳米颗粒之间的平均距离等。这些尚未为人研究过的特异过程很可能是揭示LIF过程内在机理的关键，本研究旨在应用吸收光谱、时间分辨光谱、电子显微镜成像技术对这些特异的碎裂现象做出量化分析，进而探究这些过程的机理。

本项目的主要研究内容为金属纳米颗粒在纳秒激光脉冲照射下发生的破裂过程和固-液界面光热效应现象。由于表面等离子体共振的影响，金属纳米颗粒在紫外到近红外波段有很宽的吸收带宽，其吸收光谱与颗粒的尺寸和形状密切相关。当金属纳米颗粒在液体环境中受到共振波段内的强脉冲激光照射时，吸收光子的能量使颗粒迅速加热熔化，并发生破裂和重结晶。这一过程被称为激光诱导碎裂(laser induced fragmentation，简称LIF)。在实验研究中，课题组验证了特异的LIF现象，发现LIF过程的速率受到各种因素的显著影响，这些因素包括：激光的空间模式、相邻纳米颗粒之间的平均距离等。本项目采用时间分辨光谱测量、吸收截面测量等方法对这类LIF过程进行研究，并基于实验测量的结果建立理论模型，用于解释特异LIF过程的动力学机理。在本项目研究过程中，课题组搭建了纳秒激光脉冲照射纳米金属粒子的实验平台，能够可控地实现纳秒激光脉冲单次/多次照射纳米粒子悬浊液试样的实验，并基于实验结果建立了粒子破裂过程中微小气泡产生和增长的理论模型。实验和理论分析的结果表明，金属纳米颗粒因光热效应产生气泡的过程中生成的冲击波传播降低了过程中气泡的产生和生长速率，因此对LIF速率和光热效应效率产生影响。这一结果对基于量子点、贵金属纳米颗粒等纳米材料光热效应的制热、汽化等实际工艺设计有重要的指导意义。