Abstract
Upconversion of near infrared (NIR) into ultraviolet (UV) radiation could lead to a number of applications in bio-imaging, diagnostics and drug delivery. However, for bare nanoparticles, the conversion efficiency is extremely low. In this work, we experimentally demonstrate strongly enhanced upconversion emission from an ensemble of β-NaYF4:Gd3+/Yb3+/Tm3+ @NaLuF4 core-shell nanoparticles trapped in judiciously designed plasmonic nanocavities. In doing so, different metal platforms and nanostructures are systematically investigated. Our results indicate that using a cross-shape silver nanocavity, a record high enhancement of 170-fold can be obtained in the UV band centered at a wavelength of 345 nm. The observed upconversion efficiency improvement may be attributed to the increased absorption at NIR, the tailored photonic local density of states, and the light out-coupling characteristics of the cavity.
Highlights
Photon upconversion (UC) has been the focus of considerable attention in recent years [1,2]
The remaining bare PMMA is removed along with the metallic film deposited on its top surface; leaving behind empty metallic cavities that are subsequently filled by upconverting nanoparticles (UCNPs) enriched PMMA using a spin-coating process (200 nm thickness)
In addition to the samples containing nanocavities, two types of control samples are fabricated: (i) glass substrate covered by UCNP enriched PMMA thin film, (ii) glass substrate with a 200 nm thick metallic film covered by the UCNP-PMMA thin film
Summary
Photon upconversion (UC) has been the focus of considerable attention in recent years [1,2]. As indicated in a number of studies, it is possible to further boost the UCNPs’ luminescence by appropriately engineering the electromagnetic environment they are embedded in This can be accomplished for example using photonic crystals, mixing UCNPs with quantum dots, and accompanying UCNPs with dye antennas [16,17,18,19,20]. In some applications, those aiming towards targeted drug delivery, such extreme reductions in size down to a single nanoparticle may not be advantageous [27] At this point, the question naturally arises as to whether more versatile and relatively larger plasmonic configurations such as metallic nanocavities can be used to improve the conversion efficiencies
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