Third-Order Nonlinear Optical Response near the Plasmon Resonance Band of Cu_2-xSe Nanocrystals

The third-order nonlinear optical properties of water-soluble Cu_2-xSe nanocrystals are studied in the near infrared range of 700–980 nm using a femtosecond pulsed laser by the Z-scan technique. It is observed that the nonlinear optical response of Cu_2-xSe nanocrystals is sensitively dependent on the excitation wavelength and exhibits the enhanced nonlinearity compared with other selenides such as ZnSe and CdSe. The W-shaped Z-scan trace, a mixture of the reversed saturated absorption and saturated absorption, is observed near the plasmon resonance band of Cu_2-xSe nanocrystals, which is attributed to the state-filling of free carriers generated by copper vacancies (self-doping effect) of Cu_2-xSe nanocrystals as well as the hot carrier thermal effect upon intense femtosecond laser excitation. The large nonlinear optical response and tunable plasmonic band make Cu_2-xSe nanocrystals promising materials for applications in ultra-fast all-optical switching devices as well as nonlinear nanosensors.