Giant two-photon absorption of anatase TiO2 in Au/TiO2 core-shell nanoparticles

Abstract

We report on deep-to-near-UV transient absorption spectra of core-shell

Au / {SiO}_{2}

and

Au / {TiO}_{2}

nanoparticles (NPs) excited at the surface plasmon resonance of the Au core, and of UV-excited bare anatase

{TiO}_{2}

NPs. The bleaching of the first excitonic transition of anatase

{TiO}_{2}

at

\sim 3.8 eV

is a signature of the presence of electrons/holes in the conduction band (CB)/valence band (VB) of the material. We find that while in bare anatase

{TiO}_{2}

NPs, two-photon excitation does not occur up to the highest used fluences (

1.34 {mJ / cm}^{2}

), it takes place in the

{TiO}_{2}

shell at moderate fluences (

0.18 {mJ / cm}^{2}

) in

Au / {TiO}_{2}

core-shell NPs, as a result of an enhancement due to the plasmon resonance. We estimate the enhancement factor to be of the order of

\sim 10^{8} - 10^{9}

. Remarkably, we observe that the bleach of the 3.8 eV band of

{TiO}_{2}

lives significantly longer than in bare

{TiO}_{2}

, suggesting that the excess electrons/holes in the conduction/valence band are stored longer in this material.