  We report on large three-photon absorption (3PA) in glutathione-capped ZnS
semiconductor nanocrystals (NCs), determined by both Z-scan and transient
transmission techniques with 120-fs laser pulses. The monodispersed,
water-soluble ZnS NCs are synthesized by a modified protocol with a mean
diameter of 2.5 nm. Their 3PA cross-section is determined to be around
2.7x10^-78 cm^6s^2photon^-2 at an optimal wavelength of commercial Ti:sapphire
femtosecond lasers. This value is nearly one order of magnitude greater than
that of CdS NCs, and four to five orders of magnitude higher than those of the
previously reported common UV fluorescent dyes.
