Synergistic Upconversion Enhancement Induced by Multiple Physical Effects and an Angle-Dependent Anticounterfeit Application

Semiconductor plasmon nanoparticles are currently attracting extensive interest because of their unique double character as a semiconductor and metal. In this work, we report that Cu2–xS nanoparticles (NPs) demonstrate not only tunable localized surface plasmon resonance (LSPR) but also a two-photon absorption effect under infrared light pumping, which depends strongly on the size, composition, and band gap of the NPs. This interaction with the upconversion nanoparticles NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ was systemically studied under excitation of multiwavelengths 808, 980, and 1540 nm. For the localized electromagnetic field to be enhanced further, the Cu2–xS NPs were inlayed into the surfactant apertures of three-dimensional poly(methyl methacrylate) opals. On the basis of the synergistic interaction of the LSPR effect, the nonlinear effect and the photonic crystal effect of the hybrids, an upconversion enhancement of up to 1500-fold was achieved with an absolute brightness of 1282 cd/m2 under excitation by 1.25 W/mm2 980 nm light. The experimental results were analyzed through comparison with finite-difference time-domain calculations. Finally, on the basis of the hybrids, novel angle-dependent infrared anticounterfeiting was successfully performed, which is extremely difficult to simulate. Our discovery provides a new concept for designing and optimizing luminescent materials and highlights the novel application of plasmonic semiconductor NPs in photonics.