Ultrafast Optical Mapping of Nonlinear Plasmon Dynamics in Cu2–xSe Nanoparticles

We report on the experimental investigation and theoretical modeling of the ultrafast nonlinear optical response exhibited by Cu2–xSe nanoparticles in a broad range of wavelengths, from the red to the near-infrared, under excitation with intense femtosecond-laser pulses. The ultrafast dynamics of the localized plasmon resonance of the system is mapped by pump–probe differential transmission measurements. Deviations from theoretical predictions based on standard two-temperature model (TTM) are revealed in the subpicosecond time-scale where the nonlinear phenomenon is more pronounced. The key-role played by nonthermalized carriers is investigated in detail on the basis of an extended TTM.