上转换荧光现象（upconversion fluorescence）学习

青草

封面图片是

Example of normal Stokes emission through fluorescence (left, red) and anti-Stokes emission (right, blue) through sensitized triplet-triplet annihilation based photon upconversion, samples excited with green light.

定义
Photon upconversion is the sequential absorption of two or more long-wavelength photons leading to the emission of a photon with a shorter wavelength.
下面这个实验视频现象挺明显的
NIRtoviolet photon upconversion_1080p


https://www.zhihu.com/video/1582762797703569411
看个视频具象地理解一下
Upconversion process of NaYF4 Nano Crystals Doped with Ytterbium and Erbium HD


https://www.zhihu.com/video/1582755485987659776
前言
根据要求




正文
photon upconversion 是指某个材料（？这里不是很精确）吸收了sequential的两个或多个长波长的光子，然后emit一个短波长的光子，换言之多个低能量的光子转化一个高能量的光子。这是和fluorescence and phosphorescence不同的，后者是emit一个更长波长的光，这里要引入一个概念叫Stokes Shift

The Stokes Shift is named after Irish physicist George Gabriel Stokes and is the spectral shift to lower energy between the incident light and the scattered or emitted light after interaction with a sample.

这张图展示了stokes shift在fluoresence当中

我们看到这个upconversion luminance和上图是反着的，所以又被称为anti-stokes fluorescence。
在非有机材料中，特别是rare-earth doped matrices，有三种upconversion mechanism，我将在下文介绍
Excited State Absorption Upconversion（下文简称ESA）



Excited state absorption upconversion.

ESA的mechanism如上图hypothetical three level system所示，当emission center从ground state 1吸收一个光子能量提升到intermediate excited state 2，然后如果在这个能级在没有跌落到ground state之前它又吸收了第二个光子，就能进入a higher-lying exciting state。然后从这个3->1这个过程会radiate一个能量更高的光子。要完成这个效应的必要条件是state 2存在的时间要足够长，长到可以再吸收一个光子。
Energy Transfer Upconversion（下文简称ETU)




Sequential energy transfer upconversion (left) and energy transfer followed by excited state absorption upconversion (right).

ETU需要a sensitizer and emitter (typically two different types of rare-earth ions)。比较简单的一个ETU如左图，sensitiser吸收一个光子，然后经过一个energy transfer process给emitter，然后再重复这部，然后再emit一个短波长的光子。或者像右边这张图，中间经过了一个类似ESA的过程。ETU出现的必要条件ETU to be efficient the sensitizer and emitter must be spatially close so that ET can occur, and the energy of the intermediate excited state of the emitter must be lower than the excited state of the sensitizer to provide an energetic driving force.
Photon Avalanche (PA) Upconversion（下文简称PA)



Photon avalanche upconversion.

PA并不是一个常见的mechanism，主要发生于laser cavity的里面。PA是基于在两个closely spaced ions中，发生的cross relaxation energy transfer (CR ET)效应。这个反应过程还是蛮神奇的，非常符合“雪崩”这个名称，首先所有的ion都在ground state，有一个ion先自发到state 2，然后经过一个ESA到state 3，从 state 3掉到state 2发生CR ET，这个能量导致另一个ion从ground state到state 2， state 2就有两个ion，然后再ESA一直重复下去，2变4，4变8，直到所有的ion都到state 2，然后再ESA，到state 3，最后全部emit出来。所以特点是Upconversion then proceeds directly through excited state absorption from state2to3 without any ground state absorption occurring first.
Triplet-Triplet Annihilation (TTA) Upconversion



Schematic of sensitised triplet-triplet annihilation upconversion.

结合我之前的讲解，上面这张图应该可以很轻松的看懂了。
Example of Upconversion – Erbium & Ytterbium Doped Sodium Yttrium Fluoride






这个案例看图说话应该比较容易了。
再看一个视频，也是一个上转换非常明显的案例：
NIRtoblue photon upconversion based on ST absorption and triplettriplet annihilation_1080p


https://www.zhihu.com/video/1582838431364849664

原文地址：https://zhuanlan.zhihu.com/p/588845064