Low-threshold anti-Stokes frequency conversion in Zn0.6Cd0.4S microcrystals with adsorbed metal-organic nanoclusters

We demonstrate the feasibility of low-threshold (10^-4 to 10^-2 W cm^-2) anti-Stokes conversion of photons in the range 1.72 — 2.00 eV to luminescence at 2.14 — 2.38eV at temperatures from 77 to 300 K in Zn_0.6Cd_0.4S crystals with nanoscale complexes of dye molecules and silver subnanoclusters adsorbed on their surface. The spectral characteristics of the centres involved in anti-Stokes frequency conversion indicate that this process in the crystals studied is due to sequential two-photon interband optical transitions. The absorption of a photon by a dye molecule and electron excitation transfer to an adsorbed silver subnanocluster produce a free hole in the valence band. Subsequent photoionisation of the subnanocluster produces a free electron. The last step of the anti-Stokes frequency conversion process is radiative recombination of the electron with a hole localised at an emission centre.