Size-dependent nonlinear absorption properties and ultrafast dynamics of Ag nanoparticles

The nonlinear absorption properties of spherical silver nanoparticles (AgNPs) with average sizes of 15nm and 30 nm were studied by Z-scan technique. The experimental results show that under the same excitation conditions, both samples show the switch behavior from saturable absorption (SA) to reverse saturable absorption (RSA). The research results show that, SA is caused by plasma bleaching of ground state, and the RSA results from free-carrier absorption. And nonlinear properties of AgNPs is size-dependent. The estimated values of 𝐼𝑆 were 1.62 × 1011𝑊⁄𝑚2 and 2.40 × 1011𝑊⁄𝑚2 for AgNPs with size 15nm and 30nm, respectively, where as the corresponding β values were 0.56 × 10−10 𝑚⁄𝑊 and 1.17 × 10−10 𝑚⁄𝑊. Besides, the ultrafast dynamics of the AgNPs was studied using white-light pump-probe technology. The experiments show that the process includes a slow decay and fast decay process. The theoretical values of slow decay were 5.5ps and 3.1ps for 15nm AgNPs and 30nm AgNPs, respectively, and the corresponding fast decay process values were 20ps and 55ps. The research results show that the ultrafast dynamics of AgNPs is size-dependent. The slow process was due to the electron-phonon coupling, and the fast process is due to the phonon-phonon coupling.

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