Two-photon absorption in penicillamine capped CdS tetrapods

Abstract

The two-photon absorption of water soluble CdS tetrapods, stabilized with a racemic mixture of chiral ligands, was measured using two techniques: open-aperture Z-scan and two-photon excited emission. The investigated CdS tetrapods showed two-photon absorption in the wavelength range 575-875 nm, with the maximum value of 7900 GM at 625 nm. Thanks to the high luminescence quantum yield, and relatively strong two-photon absorption, the CdS tetrapods showed intense two-photon excited emission upon near-infrared excitation with two-photon excitation action cross-section equal to 3200 GM and 420 GM at 650 nm and 700 nm, respectively. An evident blue shift of the two-photon absorption maxima with respect to the one-photon spectra at the wavelengths multiplied by two was observed, and discussed based on the reported theoretical calculation for CdS quantum dots with tetrahedral symmetry. Since CdS tetrapod arms show low electron and hole densities, they do not contribute significantly to the values of the two-photon absorption cross-section. The performed investigation of the optical properties of these water soluble colloidal nanoparticles indicates their potential applications in nonlinear bioimaging.