Lanthanide-doped upconversion nanocrystals (UCNCs) have recently found great potential in the applications of near-infrared bioimaging and nonlinear optoelectronic devices due to their tunable spectral characteristics and excellent photostability. In particular, their near-infrared excitation bands at 808 and 980 nm are within the first biological transparency window, indicating high penetration depth and low photothermal damage. However, the low quantum efficiency of such UCNCs (typically less than 1%) constitutes an intrinsic obstacle to practical use. To overcome this limitation, many physical and chemical methods have been developed to improve the absorption and emission efficiencies, including surface passivation, energy transfer management, and host lattice manipulation. Recently, the plasmonic enhancement effect in subwavelength metallic nanostructures has been used to tackle this long-standing issue through enlarging the absorption cross-section of lanthanide ions and accelerating their radiative decay rate. In addition, plasmonic nanostructures can also influence the polarization state of the upconversion luminescence of nearby UCNCs, which cannot be achieved with the other above-mentioned approaches and has so far remained relatively unexplored.
In this work, we propose to employ the double plasmon resonances of a silica-coated gold nanorod to enhance the upconversion luminescence intensity of CaF2:Yb3+,Er3+ nanocrystals and simultaneously modulate the polarization state of the red and green emissions. We have successfully synthesized hybrid plasmonic upconversion nanostructures consisting of sub-10 nm CaF2:Yb3+,Er3+ UCNCs attached on silica-coated gold nanorods in a core-shell-satellite geometry. By precisely tuning the thickness of silica shell, we have achieved a maximum luminescence enhancement factor of ~7-fold for the red emission band and ~3-fold for the green emission band. With the enhanced upconversion emissions, such hybrid plasmonic upconversion nanostructures exhibit better multiphoton imaging contrast of HeLa cells in both red and green imaging channels, demonstrating their use as a promising nonlinear fluorescent probe for high-contrast bioimaging applications. More interestingly, we have observed that the two emissions from single hybrid nanostructures are highly polarized with distinctive polarization response, with the red emission polarization along the long axis of the gold nanorod and the green emission polarization along the incidence polarization. We have analyzed the physical origin responsible for the polarized upconversion emissions by combining Förster resonance energy transfer theory and full-wave electrodynamic simulations. We have found that the coherent interaction between the emission dipoles of UCNCs and the plasmonic dipoles of the GNR determines the emission polarization state in various situations and thus open the way to the accurate control of the UC emission anisotropy for a wide range of bioimaging and biosensing applications as well as polarized illuminators in spectrometers and polarization-sensitive nanoscale photodetectors.
The related work has been published in Light: Science & Applications.