Liu Qingyun, Liu Haichun, Li Deyang, Qiao Wen, Chen Guanying, Ågren Hans
Department of Theoretical Chemistry and Biology, KTH Royal Institute of Technology, Stockholm, Sweden.
School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, P.R. China.
Nanoscale. 2019 Aug 7;11(29):14070-14078. doi: 10.1039/c9nr03105g. Epub 2019 Jul 17.
The dearth of high upconversion luminescence (UCL) intensity at low excitation irradiance hinders the prevalent application of lanthanide-doped upconversion nanoparticles (UCNPs) in many fields ranging from optical bioimaging to photovoltaics. In this work, we propose to use microlens arrays (MLAs) as spatial light modulators to manipulate the distribution of excitation light fields in order to increase UCL, taking advantage of its nonlinear response to the excitation irradiance. We show that multicolored UCL from NaYF:Yb,Er@NaYF:Yb,Nd and NaYF:Yb,Tm@NaYF:Yb,Nd core/shell UCNPs can be increased by more than one order of magnitude under either 980 or 808 nm excitation, by simply placing a polymeric MLA onto the top of these samples. The observed typical green (525/540 nm) and red (654 nm) UCL bands from Er and a blue (450/475 nm) UCL band from Tm exhibit distinct enhancement factors due to their different multi-photon processes. Importantly, our ray tracing simulation reveals that the MLA is able to spatially confine the excitation light (980 and 808 nm) by orders of magnitude, thus amplifying UCL by more than 225-fold (the 450 nm UCL band of Tm) at low excitation irradiance. The proposed MLA method has immediate ramifications for the improved performance of all types of UCNP-based devices, such as UCNP-enhanced dye sensitized solar cells demonstrated here.
在低激发辐照度下，高上转换发光（UCL）强度的缺乏阻碍了镧系掺杂上转换纳米颗粒（UCNPs）在从光学生物成像到光伏等许多领域的广泛应用。在这项工作中，我们建议利用微透镜阵列（MLAs）作为空间光调制器来操纵激发光场的分布，以增加UCL，这是利用了其对激发辐照度的非线性响应。我们表明，通过简单地将聚合物MLA放置在这些样品顶部，在980或808 nm激发下，来自NaYF:Yb,Er@NaYF:Yb,Nd和NaYF:Yb,Tm@NaYF:Yb,Nd核壳UCNPs的多色UCL可以增加一个多数量级。观察到的来自Er的典型绿色（525/540 nm）和红色（654 nm）UCL带以及来自Tm的蓝色（450/475 nm）UCL带由于其不同的多光子过程而表现出不同的增强因子。重要的是，我们的光线追踪模拟表明，MLA能够在空间上将激发光（980和808 nm）限制几个数量级，从而在低激发辐照度下将UCL放大超过225倍（Tm的450 nm UCL带）。所提出的MLA方法对所有类型的基于UCNP的器件的性能改进具有直接影响，例如这里展示的UCNP增强型染料敏化太阳能电池。
