Impurity mediated large three photon absorption in ZnS:Cu nanophosphors

Copper doped ZnS nanophosphors with the cubic zinc blende (Sphalerite) structure have been prepared through the wet chemical route in aqueous media. The ZnS:Cu nanoparticles display a widening in the band gap with increase in Cu dopant concentration. The observed green photoluminescence (PL) in ZnS:Cu originates from the transition between the conduction band and the copper acceptor levels. The open aperture Z-scan traces of the ZnS:Cu nanoparticles embedded in PVA matrix, excited at 800 nm using 100 fs laser pulses, show an optical limiting type nonlinearity which is attributed to three photon absorption. An increase in the value of the three photon absorption coefficient is observed with increase in the Cu doping concentration. The increase in the density of intermediate defect levels plays a significant role in the enhancement of three photon absorption in Cu doped ZnS. These studies reveal that Cu doped ZnS nanoparticles in polymer matrices are potential candidates for ultra fast optical limiting applications.