Two-photon absorption of (n = 3–10) clusters compared with Si bulk

The two-photon absorption (TPA) properties of the Si n (n = 3–10) clusters were investigated by using the density functional response approach. The TPA coefficients of the Si clusters are systematically smaller than those of Si bulk for 800–2200 nm. The TPA coefficients of the Si clusters as well as Si bulk increase with a decrease of the input light wavelength. The analysis of the TPA enhancement shows that the channel of double resonance makes a significant contribution to the large TPA coefficients of the Si clusters in the visible range.