Third-Order Nonlinear Optical Response near the Plasmon Resonance Band of Cu2-x Se Nanocrystals
Chinese Physics Letters
The third-order nonlinear optical properties of water-soluble Cu2− Se nanocrystals are studied in the near infrared range of 700-980 nm using a femtosecond pulsed laser by the -scan technique. It is observed that the nonlinear optical response of Cu2− Se 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 -scan trace, a mixture of the reversed saturated absorption and
... rption and saturated absorption, is observed near the plasmon resonance band of Cu2− Se nanocrystals, which is attributed to the state-filling of free carriers generated by copper vacancies (self-doping effect) of Cu2− Se nanocrystals as well as the hot carrier thermal effect upon intense femtosecond laser excitation. The large nonlinear optical response and tunable plasmonic band make Cu2− Se nanocrystals promising materials for applications in ultra-fast all-optical switching devices as well as nonlinear nanosensors. Semiconductor nanostructures of transition metal chalcogenides are known to exhibit a variety of fascinating properties and show promising applications as building blocks in electronics, optoelectronics, sensors, bio-imaging, and so on. [1, 2] Moreover, semiconductor nanostructures of transition metal chalcogenides have also exhibited attractive nonlinear optical properties such as nonlinear absorption,  nonlinear refraction  and optical limiting, [5, 6] benefiting from the strong confinement of excitons. More importantly, extensive research efforts have been devoted recently to the coupled hybrid nanostructures of semiconductor nanomaterials with noble metal nanocrystals such as Ag and Au, [7−13] which have been demonstrated to possess unique linear and nonlinear optical properties compared with their individual counterparts. [8,9,11,14−21] Recently, the heavily-doped semiconductor nanocrystals of copper chalcogenides such as Cu 2− S and Cu 2− Se have emerged to draw great attention since they exhibit metallic behavior due to 'self-doping' by the existence of a large number of copper vacancies, and eventually a tunable plasmonic response in the near infrared (IR) wavelength range which is not easily accessed by noble metal nanocrystals. [22−29] In 2011, Dorfs et al. reported detailed studies of self-doping on the plasmonic properties of Cu 2− Se nanocrystals,  after successful preparation and plasmonic property studies of Cu 2− S. [31, 32] The silica sol-gel glasses embedded with copper selenides have also revealed both semiconductor-like absorption band and especially the near-IR absorption of plasmon re-sonance band which is associated with the band structure of Cu 2− Se. [33−36] To the best of our knowledge, the third-order nonlinear optical response of copper selenide has been rarely studied so far, though a series of selenides such as ZnSe, CdSe, PbSe have been investigated extensively to show many interesting nonlinear optical properties. [3−6,37−39] In this Letter, we report on a detailed experimental investigation of the nonlinear response of Cu 2− Se nanocrystals under femtosecond laser excitation in the near-infrared regime (700-980 nm), by performing the -scan technique. The maximum values of the nonlinear absorption coefficient and nonlinear refractive index are estimated to be 7.38×10 −9 m/W and 2.64×10 −15 m 2 /W, respectively. The free carriers generated by copper vacancy (self-doping effect) of Cu 2− Se nanocrystals as well as the hot carrier thermal effect upon intense femtosecond laser excitation has been discussed to attribute to the enhanced third-order nonlinear response of Cu 2− Se nanocrystals near its plasmon resonance band. The nanocrystals can be synthesized using the simple and rapid assisted-microwave method. [40, 41] Here the powder Cu 2− Se nanocrystals are prepared based on the assisted-microwave method as follows: 0.397 g CuCl 2 ·2H 2 O, 0.201 g Na 2 SeO 3 , 0.467 g polyvinylpyrrolidone (PVP) and 0.249 g NaOH were placed into a three-necked flask, and ethylene glycol (EG) of 60 ml was added. The mixed solution was magnetically stirred at room temperature to obtain the homogeneous solution.