Laboratory of Nanophotonic Functional Materials and Devices, School of Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou, People's Republic of China.
Nanotechnology. 2013 Feb 22;24(7):075201. doi: 10.1088/0957-4484/24/7/075201. Epub 2013 Jan 28.
We investigate systematically the competition between the second harmonic generation (SHG) and two-photon-induced luminescence (TPL) that are simultaneously present in Au nanoparticles excited by using a femtosecond (fs) laser. For a large-sized (length ~ 800 nm, diameter ~ 200 nm) Au nanorod, the SHG appears to be much stronger than the TPL. However, the situation is completely reversed when the Au nanorod is fragmented into many Au nanoparticles by the fs laser. In sharp contrast, only the TPL is observed in small-sized (length ~ 40 nm, diameter ~ 10 nm) Au nanorods. When a number of the small-sized Au nanorods are optically trapped and fused into a large-sized Au cluster by focused fs laser light, the strong TPL is reduced while the weak SHG increases significantly. In both cases, the morphology change is characterized by scanning electron microscope. In addition, the modification of the scattering and absorption cross sections due to the morphology change is calculated by using the discrete dipole approximation method. It is revealed that SHG is dominant in the case when the scattering is much larger than the absorption. When the absorption becomes comparable to or larger than the scattering, the TPL increases dramatically and will eventually become dominant. Since the relative strengths of scattering and absorption depend strongly on the size of the Au nanoparticles, the competition between SHG and TPL is found to be size dependent.
我们系统地研究了在飞秒激光激发下同时存在于金纳米粒子中的二次谐波产生 (SHG) 和双光子诱导发光 (TPL) 之间的竞争。对于大尺寸（长度约 800nm，直径约 200nm）的金纳米棒，SHG 似乎比 TPL 强得多。然而，当金纳米棒被飞秒激光分裂成许多金纳米粒子时，情况完全相反。相比之下，只有在小尺寸（长度约 40nm，直径约 10nm）的金纳米棒中才观察到 TPL。当用聚焦飞秒激光将许多小尺寸的金纳米棒光学捕获并融合成一个大尺寸的金团簇时，强 TPL 会降低，而弱 SHG 会显著增加。在这两种情况下，形态变化都通过扫描电子显微镜进行了表征。此外，还通过离散偶极子近似方法计算了由于形态变化引起的散射和吸收截面的修饰。结果表明，当散射远大于吸收时，SHG 占主导地位。当吸收与散射相当或更大时，TPL 会显著增加并最终占主导地位。由于散射和吸收的相对强度强烈依赖于金纳米粒子的尺寸，因此发现 SHG 和 TPL 之间的竞争是尺寸依赖性的。
