Enhanced Brightness and Polarized Emission in Upconversion Photoluminescence

Lanthanide-doped upconversion nanoparticles (UCNPs) showcase a plethora of exceptional attributes, including versatile multicolor emission and prolonged emission lifetimes, rendering them highly suitable for a wide range of light-emission applications. Despite their numerous advantages, advancing the utilization and practicality of UCNPs encounters substantial hurdles.

Fluorescence, a critical aspect of UCNP behavior, encompasses polarization as a fundamental characteristic. This property furnishes orientation and structural insights in an additional dimension and has found extensive application in fluorescence polarization imaging techniques. However, the inherent upconversion emission processes associated with the 4f electronic transition in UCNPs are generally feeble and lack distinct polarization characteristics.

In a recent publication in eLight, a team of scientists led by Professors Xiangping Li, Zi-Lan Deng (Jinan University), Junjie Li (Institute of Physics, CAS), and Yuri Kivshar (Australian National University) have introduced a dielectric metasurface that amplifies upconversion emissions.

Previously, research predominantly focused on enhancing upconversion luminescence by integrating UCNPs with plasmonic nanostructures. These nanostructures generate a concentrated electromagnetic field, significantly augmenting absorption cross-sections and upconversion radiation rates. Metallic structures supporting surface plasmons effectively confine light and create potent electric fields, thereby enhancing upconversion fluorescence.

However, metallic nanostructures possess inherent losses, resulting in low-quality factor (Q factor) values. Direct contact with emission materials can cause a quenching effect. The Q factor, a pivotal parameter for resonant excitation, typically ranges up to a few tens for plasmonic resonances. Special designs like metal-insulator-metal (MIM) structures can achieve high Q factors by situating emitters close to the metallic surface. Nevertheless, this proximity leads to a rapid drop in quantum efficiency due to the quenching effect.

In contrast, all-dielectric resonant metasurfaces, constructed from high-index dielectric nanostructures with low losses at visible frequencies, support a more diverse array of multipolar Mie-resonances. These metasurfaces offer a promising alternative for fluorescence enhancement.

Collective high-Q resonances emerge through coupling Mie multipoles in periodic arrays, facilitated by breaking the symmetry of meta-atoms. This transformation converts bound states in the continuum (BICs) into a quasi-BIC, characterized by finite yet extraordinarily high Q factors. These innovative concepts have recently been effectively employed for fluorescence enhancement and low-threshold lasing applications.

Leveraging these advancements, the research team implemented high-Q collective modes of resonant metasurfaces to achieve polarization-controlled dual-band upconversion bursts. They meticulously designed and fabricated high-Q resonant metasurfaces composed of diatomic nano bricks. The design supports a quasi-BIC mode for y-polarized incidence and another high-Q Mie resonance mode for x-polarized incidence. NaYF4:Yb/Er UCNPs deposited on the metasurface exhibited ultrabright upconversion luminescence at dual bands.

By adeptly manipulating the polarization analyzer’s rotation, the team demonstrated emissions in linearly- and cross-polarized manners at dual bands, characterized by ultra-high degrees of polarization (DoPs). This demonstration marks a significant milestone towards efficient enhancements and precise polarization control of UCNP emission, holding promise for applications in low-threshold polarization upconversion lasers and hyperspectral imaging/sensing.

Source: Chinese Academy of Sciences

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