Shape-dependent nonlinear absorption and relaxation in $CuInS2$ nanocrystals

We report the observation of nonlinear-optical response and relaxation in copper indium sulfide $(CuInS2)$ crystalline nanoparticles and nanorods with femtosecond Z-scans and transient absorption measurements. Dependent on the size and shape, two-photon absorption and saturation in the one-photon absorption manifest themselves in these measurements as laser excitation wavelength of 780 nm is in the proximity of the lowest-lying excitonic energy, particularly for the nanorods. The transient absorption measurements by both one-photon and two-photon excitation reveal that electron-hole recombination is faster in smaller nanoparticles, or in nanorods with smaller aspect ratio. However, the decay to trapping states is independent of the size and aspect-ratio.