几种非球形金纳米粒子的非线性光学性质研究

【作者】 陈爽；

【导师】 高亚臣；

【作者基本信息】 黑龙江大学 ， 物理电子学， 2020， 硕士

【摘要】 在贵金属纳米粒子中,等离子体共振起源于导带中自由电子在电磁场作用下产生的集体振荡。这一效应会引起金属纳米粒子内部的局域电场增强,使其具有增强的非线性和超快的响应时间。因此,金属纳米粒子可以应用于全光超快开关、生物传感器、催化剂和药物等领域。金属纳米粒子的光学特性会受到材料的种类、形状、尺寸等因素的影响。本文研究了三种非球形金纳米粒子(金纳米笼、金纳米棒和金三角片)的非线性光学吸收特性和超快动力学过程,具体工作如下。采用宽带纳秒Z扫描研究了金纳米笼在不同能量和不同激发波长下的非线性吸收特性。结果发现,在低强度时,样品表现出饱和吸收,而在高强度时,样品发生了饱和吸收向反饱和吸收的转变。这分别归因于基态漂白和自由载流子吸收。此外,样品的非线性吸收依赖于激发波长,在共振区饱和强度最大。利用纳秒Z扫描研究了金纳米棒在532 nm处的非线性吸收特性。金纳米棒表现出饱和吸收特征,饱和吸收归因于基态等离子体的漂白。此外,利用飞秒泵浦探测技术研究了金纳米棒的超快动力学过程。结果表明,该动力学过程包括一个快衰减过程和一个慢衰变过程,对应的时间常数分别为3 ps和105 ps。理论结果表明,快衰变过程是由于电子-声子的耦合,慢衰变过程是由于声子-声子的耦合。利用宽带纳秒Z扫描研究了金三角片的非线性吸收特性。在低激发强度下表现出饱和吸收。随着入射光强的增加,发生从饱和吸收向反向饱和吸收的转换。此外利用飞秒时间分辨瞬态吸收光谱研究了金三角的超快动力学过程。光动力学过程被发现是一个双指数能量弛豫,具有快速和缓慢的衰减分量。有趣的是,当探针波长远离等离子体共振峰时,弛豫的衰减也表现出相干激励的振动模式引起的调制。三种金属纳米粒子的非线性吸收特性对比结果表明,金纳米笼表现出最强的饱和吸收,其次是金三角片,金纳米棒。泵浦探测结果表明,形状对金属纳米粒子动力学过程有一定的影响,但是这种影响并不大。金三角片和金纳米棒相对于之前报道的金纳米球具有更长电子弛豫时间因为它们具有较高的热电子初始温度。结果表明,这些金纳米材料是未来光电子和光限幅应用的候选者。更多还原

【Abstract】 In noble metal nanoparticles,the surface plasma resonance(SPR)results from collective oscillations of the free electrons in the conducting band when interacting with electromagnetic fields.The effect causes metal nanoparticles to have enhanced nonlinearity and fast response.Thus,metal nanoparticles may be used in many applications such as all-optical ultrafast switching,biological sensors,catalytic agents and medicine.The optical properties of metal nanoparticles are affected by factors such as the type,shape,and size of the material.In this paper,we have investigated the nonlinear absorption and ultrafast dynamics process of three different shapes of gold nanoparticles(gold nanocages,gold nanorods,and gold triangle nanoprisms),the specific work is as follows:The nonlinear absorption of gold nanocages were investigated using a broadband nanosecond Z-scan.It was found that at low intensities the sample shows saturation absorption(SA),while at higher intensities a switch from SA to reverse saturation absorption(RSA)occurs.Moreover,the nonlinear absorption of the sample is sensitively wavelength-dependent,and,in the resonant region,saturation intensity is the largest.The nonlinear absorption of gold nanorods were investigated by nanosecond Z-scan technique at 532 nm.It was found that the gold nanorods exhibit SA characteristics.The SA was ascribed to the bleaching of ground state plasmon.Besides,the ultrafast dynamics process of the gold nanorods were investigated by femtosecond pump-probe technology.It was found that the dynamics process includes a fast decay and a slow decay with time constant of 3ps and 105 ps,respectively.Theoretical results show that the fast decay process is due to the coupling of electron-phonon,and the slow decay process is due to the coupling of phonon-phonon.The nonlinear absorption of Au triangular nanoprisms were investigated by using broadband nanosecond Z-scan measurements.We found that Au triangular nanoprisms exhibit SA at low excitation intensities.With the increase of incident intensity,a switch from SA to RSA occurs.The ultrafast dynamics process of the Au triangular nanoprisms were investigated by femtosecond time-resolved transient absorption spectrum.Photo-dynamics process was found to be a double-exponential energy relaxation with a fast and a slow decay component.Interestingly,when probe wavelength is away from the plasma resonance peak,the decay of relaxation also shows the modulation due to the vibration mode of the coherent excitation.The comparison results of the nonlinear absorption characteristics of the three metal nanoparticles indicate that the gold nanocages exhibit the strongest SA,followed by the Au triangular nanoprisms and the gold nanorods.Ultrafast dynamics process results show that the shape has a certain effect on the dynamic process of metal nanoparticles,but this effect is not large.The relaxation times in Au triangular nanoprisms and gold nanorods are longer than that reported in Au nanospheres because they have higher initial temperatures of hot electrons.These nonlinear optical properties of these gold nanomaterials are indicative of the feasibility of their application in ultrafast optoelectronics.更多还原