Development of Optical Resonance Dynamic Spectroscopy for Single Microparticles

微颗粒，如有机微晶体、半导体粉末、金属珠、液滴等，具有不同于块状材料的特殊物理和化学性质。尽管各种各样的光谱技术已经被应用于阐明发生在微粒子上的反应过程，但这些研究都是通过传统的时间分辨而空间分辨的光谱进行的，因此实验结果提供了具有不同性质的许多粒子的总和的信息。单个微粒的行为预计会因大小和形状的不同而不同。在这项研究中，我们提出了一种新的动态光谱学方法，用于分析单个微粒的光物理和光化学现象，具有高精度和高灵敏度。该方法基于微球内部的光学共振。与单路通过粒子造成的吸收损失相比，球形微粒子的光共振极大地放大了发射强度的变化。即实现了对微粒中受激分子的腔内吸收光谱分析。此外，由于微米大小的短腔具有单次泵浦产生皮秒激光脉冲的能力，因此可以基于泵浦-探针方法测量粒子中的超快瞬态吸收过程。我们成功地观察到与自由空间相比，聚合物微球内的自发发射增强，即腔量子电动力学效应。荧光衰减率随粒径的减小而增大，2.4 μm乳胶球的增强因子为-17。在芳香族分子掺杂的聚合物微球中，偶极子-偶极子相互作用引起的能量传递过程的微球腔增强也被观察到。

Microparticles, such as organic microcrystals, semiconductor powders, metallic beads, and liquid droplets, possess characteristic physical and chemical properties different from bulk materials. Although various kinds of spectroscopic techniques have been applied for elucidating reaction processes occurring in/on microparticles, these studies were performed by conventional time-resolved but space-unresolved spectroscopy so that the experimental results provided information on the sum of many particles with different properties. Behaviors of individual microparticles are expected to be different from each other depending on the sizes and shapes. In this research, we propose a new dynamics spectroscopy method for analyzing the photophysical and photochemical phenomena of a single microparticle with high accuracy and high sensitivity. This method is based on the optical resonance within the microsphere. Compared with the absorption loss caused by a single path through the particle, the change in the emission intensity was extremely amplified by the optical resonance in a spherical microparticle. Namely, the intracavity absorption spectroscopy is realized for the analysis of excited molecules in a microparticle. In addition, since a micrometer-sized short cavity has the ability to produce a picosecond lasing pulse with a single shot pumping, the ultrafast transient absorption process in the particle can be measured based on the pump-probe method. We succeeded in observing enhancement of spontaneous emission within polymer microspheres compared to that in free space, i.e., cavity quantum electrodynamic effect. Fluorescence decay rate increased with reducing the particle size, and the enhancement factor was determined to be -17 for a 2.4-μm latex sphere. The microspherical cavity enhancement of energy transfer processes caused by dipole-dipole interactions was also observed for aromatic-molecules-doped polymer microspheres.