New approach to probe quantum efficiency of single molecules in an organic matrix

Organic molecules are attractive to both physicists and chemists because molecules could have high quantum efficiencies in light emission and be chemically synthesized to have transitions at desired wavelengths. Moreover, single molecules, as isolated individual quantum systems, are actually versatile sources of single photons since a single two-level system cannot emit two photons simultaneously, as each excitation and emission cycle require a finite time. Compared to various other solid-state single photon emitters such as quantum dots, color center in diamond, defects in two-dimensional materials, single molecules embedded in crystalline organic matrix, possess several unique properties including small size of about one nanometer (suitable for high-density doping), flexibility in the synthesis, and strong and stable Fourier-limited zero-phonon lines at low temperature. In particular, dibenzoterrylene (DBT) molecules embedded in anthracene (AC) crystal have been actively studied as definitely stable single-photon emitters with non-blinking emission and lifetime-limited linewidth, the integration of single DBT molecules with planar photonic circuits has been also explored.

However, the quantum efficiency of single DBT molecules in anthracene matrix, as a critical piece of information, has not been experimentally measured. The quantum efficiency of an emitter indicates the ability to emit a photon once an excitation photon is absorbed. While the theoretical definition of quantum efficiency is crystal clear, its experimental measurement is highly nontrivial. In the past two decades, there have been several experiments reporting the measurements of absolute quantum efficiency of single emitters. The existing applied methods can be classified into two types, i.e., (i) study the emitter's decay rate with nano-controlled variation of the optical environment and (ii) measure the saturation of the emission of the emitter with pre-characterized total detection efficiency of the system. Both methods are difficult to implement for DBT molecules which possess an in-plane dipole orientation in the AC crystal and thus their quantum efficiencies have not been measured at the single-molecule level.

The research group led by Professor Xue-Wen Chen from Huazhong University of Science and Technology proposed a simple method to measure and confirm the near 100% intrinsic quantum efficiency of single DBT molecules embedded in AC microcrystal by monitoring the fluorescence lifetime change during the process of natural sublimation of the microcrystal. The intrinsic quantum efficiency could be extracted from the variation of the Purcell factor with the thickness. The research results are published inChinese Optics Letters , Volume 20, Issue 7, 2022 (P. Ren, et al, Probing fluorescence quantum efficiency of single molecules in an organic matrix by monitoring lifetime change during sublimation).

The AC microcrystal exposed in air will slowly sublimate, the decrease of the thickness of the microcrystal due to sublimation induces the change of the optical environment of the molecules, and consequently, the change of the Purcell factor or the local density of optical states (LDOS), which manifests through the modification of the fluorescence lifetime. With the help of the home-built confocal microscope combined with an atomic force microscope (AFM) shown in Figure 1 (a), researchers observed the change of fluorescence lifetime on the same molecule during the sublimation and accurately recorded the lifetimes and the corresponding crystal thicknesses at different time. By identifying the orientation of the molecule emission dipole from the radiation pattern through back focal plane (BFP) imaging which is shown in Figure 1(b), researchers established a Purcell factor distribution with a function of crystal thickness and molecule position to describe the sublimation induced lifetime change and analyze the quantum efficiency, corresponding results are shown in Figure 1(c). They finally deduced the average intrinsic quantum efficiency of the single DBT molecules embedded in AC microcrystal is 95%, which is agrees with the reported near-unity values for DBT molecules at ensemble level in low temperature.

This work innovatively utilizes the natural sublimation of AC microcrystal which induces optical environment change for embedded DBT molecules and experimentally probes fluorescence quantum efficiency of single DBT molecules by monitoring the fluorescence lifetime change due to the optical environment variation. Such simple approach also can be applied on other organic molecules sharing in-plane orientation in organic matrix host.

监控晶体自升华，测量单分子荧光量子效率

有机染料小分子具有荧光量子效率高，便于化学合成，且其辐射波长可定制等众多优点，因而物理学家和化学家都对有机分子作为量子体系感兴趣。有机单分子作为孤立的量子系统，每个激发和辐射周期均需要一定的时间，不能同时辐射两个光子，因而可作为通用的单光子源。

相比于如量子点、金刚石中的色心等其他的固态单量子辐射体，掺杂在有机结晶固体中的单分子具有尺寸小、可高密度掺杂、制备过程简单以及稳定的零声子线跃迁等特点。在众多的固态单分子体系中，掺杂在蒽晶体中的二苯并三萘嵌二苯（DBT）分子尤以突出，其具有无闪烁、线宽寿命受限、高稳性等优点。近几年，国内外科研人员在基于DBT的高稳定单光子源及其与平面光子芯片集成等前沿方向做了广泛的研究并取得了一系列突破性进展。

荧光量子效率作为单光子辐射体的重要内禀属性之一，代表了辐射体在吸收了一个光子被激发后，再次辐射出一个光子的能力。但DBT分子的荧光量子效率并没有在单分子水平上得到测量。

近十几年中，学界陆续报道了一些关于单光子辐射体量子效率实验测量的工作，但从实现手段上来说都十分繁琐和困难。这些测量方法主要概括为两类：

第一类方法：基于珀塞尔效应利用纳米操纵改变辐射体周围的光学环境，进而通过分析跃迁速率变化，提取出量子效率。该方法需要精准地操纵辐射体与光学结构的相对空间排布，往往要求达到纳米级别的精度，操作难度极高，并且比较适用于偶极取向为面外的量子辐射体。

第二类方法：通过对单光子辐射体荧光进行饱和测量，根据实验测量的荧光强度饱和曲线推算出量子效率。这种方法的难点在于需要准确地标定实验系统的探测效率，特别是需要测量进入物镜的收集效率、以及通过物镜之后各种光学元件的传输效率，然而这些效率与结构及其它诸多因素相关，往往不能得到令人满意的结果。

因此，上述两类方法都不适用于蒽晶体中掺杂具有面内辐射偶极取向的DBT分子的量子效率测量，蒽晶体中单个DBT分子的量子效率目前也并没有十分准确的数据支持。

华中科技大学陈学文教授领衔的量子纳米光学研究小组提出了一种固态单量子辐射体量子效率的简易测量方案，探测了掺杂在蒽微纳晶体中单个DBT分子的量子效率，实验结果显示DBT分子具有接近于100%的荧光量子效率。相关工作发表在Chinese Optics Letters 2022年第20卷第7期（P. Ren, et al, Probing fluorescence quantum efficiency of single molecules in an organic matrix by monitoring lifetime change during sublimation）上，并被选为当期封面。

原理：直接暴露在空气中很薄的蒽微纳晶体会缓慢升华，该过程引起的晶体厚度的减薄会影响掺杂在内部的单分子周围的光学环境，导致珀塞尔系数（）或局域光子态密度（LDOS）发生改变，从而直接影响单分子荧光寿命。新方案通过测量蒽晶体在室温自然升华减薄过程中一系列的晶体厚度和DBT单分子荧光寿命，得到两者相关联的珀塞尔系数分布，从而提取出了单分子的本征量子效率。

研究人员利用图1（a）中有原子力显微镜的倒置显微系统，观测到了同一个分子在晶体升华过程中荧光寿命的变化，并准确测量和记录下了荧光寿命和对应的晶体厚度。在通过后焦面成像确认了DBT分子的辐射偶极取向后，见图1（b），研究人员建立了与晶体厚度和辐射体位置相关的珀塞尔系数分布图来描述由升华引起的辐射体荧光寿命变化，并分析提取出辐射体的本征量子效率，相关结果见图1（c）。从不同分子的测量结果来看，平均95%的本征量子效率值与已报道的DBT分子掺杂蒽微纳晶体团聚体系在低温下接近于100%的本征量子效率数值相吻合。

该工作巧妙地利用了晶体自然升华带来的厚度减薄并改变辐射体光学环境进而影响荧光寿命的这一物理过程，实验测量了单个分子在一系列不同晶体厚度下对应的荧光寿命，与建立的珀塞尔系数分布模型比对，提取出了蒽微纳晶体中单个DBT分子接近于100%的本征量子效率。未来研究人员将继续探索该测量方法在其他具有面内偶极取向的单分子掺杂有机晶体体系中的应用。