Ultrafast Photophysics Of Bismuth Oxide Matrix Composite Films Of Metal Nanoparticles

In this thesis, the energy relaxation dynamics and third-order nonlinear optical susceptibilities of noble metal nanoparticles embedded in Bi₂O₃ thin films were investigated by using femtosecond (fs) pump-probe, time-resolved optical Kerr effect (OKE) and z-scan techniques. The effects of metal concentration, excitation intensity, the size of nanoparticles and the structure of films on the ultrafast photophysical dynamics of this kind of materials were investigated.The main innovative results and conclusions are as follows:1. The energy relaxation dynamics of silver/bismuth oxide (Ag:Bi₂O₃) nanocomposite films with different Ag concentration fabricated by magnetron co-sputtering were investigated by utilizing fs pump-probe technique. In the moderateexcitation regime, the films with relatively low Ag concentration ( < 40%) show fast and weak power-dependent energy relaxation behavior, while the films with relatively high Ag concentration (45-60%) possess slow and strong power-dependent energy relaxation. The different relaxation characteristics are suggested to be resulted from the differences of the electron-phonon coupling and the electron-surface coupling among these Ag:Bi₂O₃ samples with different Ag concentration, closely related to the structural and thermal properties of the surrounding Bi₂O₃ matrix and the Ag nanoparticles.2. The ultrafast nonlinear optical properties of Ag:Bi₂O₃ nanocomposite films were investigated by fs OKE techniques. We found that the third-order nonlinearoptical susceptibilities (X³) of the films increase with the increase of Ag concentration in Ag concentration domain of 13.2-35.7%. We attribute this to the increase of the density of Ag particles and the enhancement of the local field caused by the electromagnetic dipole-dipole interaction. When the Ag concentration is higherthan 35.7%, the local field will be averaged out by the coalescence of the nanoparticles, leading to the decrease of the nonlinear optical susceptibility. The research results also show that the values of X³ increase with the increase of annealing temperature. Basing on the calculation of the local field inside the particles by applying Mie theory, we suggest that these results are caused by the size-dependent local field enhancement effect. By analyzing the time-resolved OKE response, we conclude that in sub-picosecond region, the third-order nonlinearity of the films mainly comes from the contribution of photo-induced hot electrons.3. The ultrafast nonlinear optical properties of Cu:Bi₂O₃ nanocomposite films were also studied utilizing fs pump-probe, z-scan and OKE techniques. In high excitation region, the as-grown Cu:Bi₂O₃ films (without annealing) showinstantaneous response, with dominant contribution from the real part of x³ This reveals that the nonlinear optical response of as-grown composite films mainly comes from the induced polarization of electrons in amorphous Cu/Cu-Bi-O and the contribution from the Bi₂O₃ matrix. The morphologic structure and the nonlinear optical response of Cu:Bi₂O₃ composites were greatly improved via annealing at 600℃. The relaxation process in picosecond region in OKE response emerges in the annealed films, indicating the important contribution of hot electrons in these films, in agreement with the increase of the imaginary part of X³.In conclusion, the ultrafast dynamics and the nonlinear optical response of metal nanoparticles (including Ag and Cu) embedded in Bi₂O₃ matrix were investigated. The effects of the metal concentration, excitation intensity, the size of nanoparticles and the structure of films on the ultrafast response and optical nonlinearity of these nanocomposites were thoroughly analyzed. The thermal annealing was found useful in improving the optical properties of the nanocomposites. Our research results are meaningful in accumulating the information of the optical properties of metalcomposite nanoparticles and provide new knowledge to this interesting field.