Three-photon excited band edge and trap emission of CdS semiconductor nanocrystals

The three-photon excited band edge and trap emission of CdS semiconductor nanocrystals was studied. The band edge emission intensity was found to show a cubic dependence on excitation energy, that demonstrated a three photon absorption process, while the trap emission did not show such a cubic dependence. Using a simple theoretical model, it was suggested that the number of trap states played an important role in their emission intensity dependence of multiphoton excitation. The three photon absorption cross section of CdS nanocrystals was also measured, and found to be three to four orders of magnitude higher than those of the previously reported common UV fluorescent dyes.