Spectrally resolved two-photon absorption properties and switching of the multi-modal luminescence of NaYF4:Yb,Er/CdSe hybrid nanostructures

Abstract

Spectrally resolved femtosecond nonlinear optical study in the range of 950–1350 nm has been conducted for three colloidal NaYF₄:Yb,Er/CdSe hybrid nanostructures, which differ in the size of the surface decorating CdSe quantum dots (6.6, 6.8 and 7.0 nm). The two-photon absorption cross sections σ₂ of the hybrid nanoconstructs are as large as 2.55 × 10⁵ GM in the near-infrared region (∼1150 nm); their very good nonlinear absorption properties being imparted by the CdSe nanoparticles. Further, we demonstrate that by merging NaYF₄:Yb,Er and the CdSe nanoparticles into hybrid materials, it is possible to combine their mechanistically distinct two-photon-induced luminescence properties. Indeed, we report the switching of multi-modal luminescence of the colloidal NaYF₄:Yb,Er/CdSe hybrid nanomaterials, in which both constituents can be excited separately or simultaneously, depending on the selection of the excitation source. The emissive modes include visible up-conversion and near-infrared emission of erbium(III) ions doping the NaYF₄ core, as well as one- or two-photon induced band-gap luminescence of the outer layer, comprising the CdSe quantum dots. Toggling between the emission modes is achieved by single or dual beam excitation of the respective components, using a continuous wave 980 nm laser diode for NaYF₄:Yb,Er nanoparticle emission, and femtosecond laser near-infrared pumping for CdSe quantum dot band-gap emission. The Förster resonance energy transfer between both optically active components of the synthesized hybrid nanostructures, i.e. up-converting nanoparticles (donor) → quantum dots (acceptor), has been determined to have 2.5% to 6% efficiency, depending on the size of the participating CdSe nanoparticles. Finally, we show that the multimodal luminescence properties of these hybrid nanomaterials can be potentially employed for 2D anti-counterfeiting applications.