Abstract
The photo-reduction of metal ions in solution induced by femtosecond laser is an important and novel method for fabricating three-dimensional metal microstructures. However, the nonlinear absorption cross section of metal ions remains unknown because its measurement is difficult. In the present study, a method based on Two-Photon Excited Sedimentation (TPES) is proposed to measure the two-photon absorption cross section (TPACS) of metal ions in solution. The power-squared dependence of the amount of sediment on the excitation intensity was confirmed, revealing that 800 nm femtosecond laser induced reduction of metal ions was a two photon absorption process. We believe that the proposed method may be applied to measure the TPACS of several metal ions, thereby opening a new avenue towards future analysis of two-photon absorption materials.
Highlights
Since the phenomenon of two-photon absorption (TPA) was first experimentally observed in 19611, it has attracted much research interest because of its potential application in three-dimensional optical data storage[2,3,4,5], two-photon fluorescence imaging[6,7,8], and microfabrication[9,10,11,12,13]
We propose a new method based on mass sedimentation for measuring the two-photon absorption cross section (TPACS) of metal ions at the excitation wavelength of 800 nm pulsed laser
In our experiment, if the concentration of silver ions in two identical solutions changes by the same amount under conditions of SPA and TPA, the number of photons absorbed in TPA (NTPA) will be twice that in SPA (NSPA)
Summary
Since the phenomenon of two-photon absorption (TPA) was first experimentally observed in 19611, it has attracted much research interest because of its potential application in three-dimensional optical data storage[2,3,4,5], two-photon fluorescence imaging[6,7,8], and microfabrication[9,10,11,12,13]. Z-scan measurement[30,31] is widely used to measure non-linear refractive index and non-linear absorption coefficient via the “closed” and “open” methods respectively. In this measurement, the sample is typically placed at the focus of a lens, and moved along the z axis. Z-scan technique cannot be applied to measure the TPACS of metal ions because of the following reasons: (1) Metal ions in solution would be photo-reduced to neutral metal atoms upon laser irradiation and aggregate into metal particles, which lead to severe scattering. We tested the power squared dependence of two-photon excited sedimentation on the laser intensity for metal ions, demonstrating that the reduction of metal ions induced by 800 nm femtosecond laser was a TPA process
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