金属-电介质-金属柔性结构增强荧光发射藏红花真假
曹文静 孙李泽童 郭付周 宋健彤 刘啸 陈智辉 杨毅彪 孙非
Enhancing the fluorescence emission by flexible metal-dielectric-metal structures
CAO Wen-jing, SUN Li-ze-tong, GUO Fu-zhou, SONG Jian-tong, LIU Xiao, CHEN Zhi-hui, YANG Yi-biao, SUN Fei
引用本文:
积极乐观的诗句曹文静，孙李泽童，郭付周，宋健彤，刘啸，陈智辉，杨毅彪，孙非. 金属-电介质-金属柔性结构增强荧光发射[J]. 中国光学, 2022, 15(1): 144-160. doi: 10.37188/CO.2021-0084
CAO Wen-jing, SUN Li-ze-tong, GUO Fu-zhou, SONG Jian-tong, LIU Xiao, CHEN Zhi-hui, YANG Yi-biao, SUN Fei. Enhancing the fluorescence emission by flexible metal-dielectric-metal structures[J]. Ch
ine Optics, 2022, 15(1): 144-160. doi: 10.37188/CO.2021-0084
在线阅读 View online: doi/10.37188/CO.2021-0084
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牛蛙店
第 15 卷 第 1 期中国光学Vol. 15 No. 1 2022年1月Chine Optics Jan. 2022文章编号 2095-1531（2022）01-0144-17
Enhancing the fluorescence emission by flexible
metal-dielectric-metal structures
CAO Wen-jing1,2，SUN Li-ze-tong2，GUO Fu-zhou1,2，SONG Jian-tong1,2，LIU Xiao1,2，CHEN Zhi-hui1,2 *，
YANG Yi-biao1,2，SUN Fei1,2
全真模拟试卷（1. Key Laboratory of Advanced Transducer and Intelligent Control System, Ministry of Education汽车车衣
and Shanxi Province, Taiyuan 030024, China；
2. College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China）
* Corresponding author，E-mail:
Abstract: The technology of enhancing fluorescence emission can increa the nsitivity of fluorescence de-tection and the brightness of Light Emitting Diodes (LEDs), and is of great significance in improving the per-formance of light-emitting devices. Since the metal structure has a good effect in enhancing the local field and fluorescence emission, and the flexible dielectric material has flexible bendability characteristics, on the basis of above, we propo a flexible structure compod of Metal-Dielectric-Metal (MDM) to enhance the fluorescence emission. The influence of the structure on the directional emission enhancement of quantum dots is systematically studied by using the finite difference time domain method. Theoretical calculations show that the local undulations and arcs of the flexible MDM structure can promote fluorescence enhance-ment and increa the quantum efficiency of the quantum dots located at the center of the structure by about 7 times. They can alao change the refractive index and thickness of the dielectric to achieve the tunability of the target wavelength. At the same time, the experimental results shows that the flexible MDM structure does have a positive effect on the fluorescence enhancement. This discovery is valuable for future display techno-logies and flexible light-emitting devices. It is of certain guiding significance for the development and applic-ation of high-efficiency flexible devices.
Key words: fluorescence enhancement; flexible structure; directional emission; tunable wavelength
经典语录网
收稿日期：2021-04-19；修订日期：2021-05-11
基金项目：国家自然科学基金资助项目（No. 62175178，No. 11674239）；中央引导地方科技发展资金项目（No.
YDZJSX2021A013）；山西省青年拔尖人才支持计划；三晋英才支持计划
Supported by National Natural Science Foundation of China (No. 62175178, No. 11674239); the Central Guid-
ance on Local Science and Technology Development Fund of Shanxi Province (No. YDZJSX2021A013); Pro-
萤之光gram for the Top Young Talents of Shanxi Province; Program for the Sanjin Outstanding Talents of China
金属-电介质-金属柔性结构增强荧光发射曹文静1,2，孙李泽童2，郭付周1,2，宋健彤1,2，刘 啸1,2，陈智辉1,2 *，杨毅彪1,2，孙 非1,2
（1. 太原理工大学 新型传感器与智能控制教育部/山西省重点实验室, 山西 太原 030024；
2. 太原理工大学 物理与光电工程学院, 山西 太原 030024）
摘要：增强荧光发射可以提高荧光检测灵敏度、提高LED的亮度，在提高发光器件性能方面具有重要意义。由于金属结构在增强局域场、增强荧光发射方面具有很好效果，而柔性电介质材料具有灵活的可弯曲性特性，本文提出一种由金属-电介质-金属（MDM）组成的柔性结构以增强荧光发射。利用时域有限差分方法系统研究了该结构对量子点定向发射增强的影响。理论计算表明柔性MDM结构局部起伏和弧度对荧光增强起促进作用，且可以使位于结构中心位置量子点的量子效率增强约7倍。此外，还可以改变电介质的折射率和厚度从而实现目标波长的可调谐性。实验结果表明该柔性MDM结构对荧光增强有一定的促进作用，这一发现对未来的显示技术和柔性发光器件都有很大的价值，对高效柔性器件的开发应用具有一定的指导意义。
关键词：荧光增强；柔性结构；定向发射；波长可调
中图分类号：TN815 文献标志码：A doi：10.37188/CO.2021-0084
1 Introduction
With the well-known advantages such as high nsitivity and adjustable spectrum, fluorescence emission has made rapid development in optical imaging, bionsing, LED display and other fields. I
n particular, flexible LED has been widely ud in display devices (such as foldable mobile phones, curved televisions and flexible e-books), flexible light sources and wearable devices due to its bend-ability, low structural cost, light weight, conveni-ence and good performance. For example, in 2011, Wang et al. propod an efficient OLED bad on a flexible substrate, achieving an external quantum ef-ficiency of up to 60% for green fluorescence[1]. In 2013, Kim et al. propod an OLED that could be ud in wearable displays and could still have cer-tain stability even with a bending radius of 5 mm after 1 000 bending cycles[2]. In 2020, Shan et al. propod a wearable and tonable perovskite lumin-escence/detection fiber with the narrowest lumines-cence spectrum of ~19 nm, which could simultan-eously transmit and receive signals[3].
However, traditional fluorescence emission still has some limitations. For example, the fluores-cence dependent on spontaneous photon emission is isotropic in all directions, which means that the fluorescence property is basically independent of the obrvation direction, resulting in a low quantum yield of fluorescence emission. For the fluorophore with low quantum yield, further enhancing the fluor-escence emission can significantly improve the per-formance of relevant optical system (such as ns-ing nsitivity, imaging quality, luminance and sta-bility)[4-8]. Therefore, in order to improve the fluores-cence emission efficiency in practical applications and meet the miniatur
ization requirement for mod-ern fluorescence devices, it is very important to con-trol the emission direction in a cost-effective way and convert the original isotropic emission into dir-ectional emission. This rearch has also attracted considerable attention in recent years and is of great value to optical nsors, displays and light-emitting devices[9].
Previous work has proved that the fluores-cence coupled with metal nanostructures [10], metal films[4] and photonic crystals[11-12] can enhance direc-tional fluorescence emission. The fluorescence coupled with plasma substrate is enhanced by strong local field enhancement and surface plasmon reson-
青少年科学调查体验活动
第 1 期CAO Wen-jing, et al. : Enhancing the fluorescence emission by flexible (145)
ance. The fluorescence coupled with photonic crys-tals is enhanced due to photonic band structure ef-fect. The radiation of a fluorophore coupled with surface plasma resonance can be enhanced by ap-plying a grating or fishnet structure on the metal layer or using a metal bilayer (silver-gold). In 2014, Jiang et al.[13] designed a subwavelength Ag-PMMA-Ag cavity structure with a 1D-period Ag grating at the top. By using the coupling effect in the structure and changing the dielectric thickness, grating period, groove width and depth and other structural parameters, the Full Width at Half Max-i
mum (FWHM) of the fluorescence emission spec-trum of the dye molecule became the narrowest and the fluorescence intensity became the maximum. In 2018, Ren et al.[14] designed and studied the fluores-cence emission process of metal-dielectric-metal (MDM) fishnet metasurface structure, using the magnetic plasmons generated by the coupling effect between metal elements and arrays at the nanomet-er scale to control the wavelength of enhanced fluor-escence and achieve the color-controlled wavelength tunability. However, the fabrication process of the structures is more complicated. It is still worth fur-ther rearch to obtain high directional fluorescence emission enhancement bad on a simple fabrica-tion process.
The MDM structure can effectively change the fluorescence emission characteristics by changing the quantum yield and directivity of fluorescence emission[15]. For a specific dielectric layer thickness, the coupling of fluorescence with Fabry-Perot cav-ity can cau fluorescence to be emitted in a direc-tion perpendicular to the MDM structure[4, 16]. In 2016, Shiekh et al.[17] propod a planar MDM struc-ture that ud Surface Plasmon Coupled Emission (SPCE) to enhance single-molecule luminescence and incread the peak intensity and power of SPCE. In 2015, Sharmistha et al.[18] designed a planar MDM structure to control the fluorescence wavelength, angle dependence and emission polariz-ation by changing the thickness of metal layer and dielectric medi
um. However, the structures are planar structures and are not applicable to flexible displays or light-emitting devices. Bad on this, it is of great significance to proposing a relatively simple structure that can be applied to flexible dis-plays or light-emitting devices.
Since the planar MDM structure can obtain the fluorescence emission perpendicular to its surface, this paper propos a flexible MDM structure, in which the interaction between Fabry-Perot cavity and fluorescence can also produce the beam emis-sion perpendicular to the structural surface to en-hance the directional transmission of flexible light-emitting devices. In this work, the effects of differ-ent structural parameters on the fluorescence emis-sion of quantum dots were studied to obtain the structural parameters that could achieve good coup-ling. Then, the structural parameters were com-pared with tho of metal-dielectric structure and monolayer metal film structure. The results show that the local undulations and arcs of MDM struc-ture can promote fluorescence enhancement in two ways, namely enhancing the quantum efficiency and obtaining highly directional fluorescence emission. Finally, the applicability of the structure in flexible fluorescence enhancement was verified by experi-ments.
2 Model and methodology
In order to verify the applicability of flexible MDM structure in flexible light-emitting devices, we propod a flexible MDM structure, who 3D front view is shown in Figure 1 (Color online). The minimum internal radius of the structure is defined as R, the thickness of the silver film in the upper and lower layers are both d1, the thickness of the dielectric in the middle layer is d2, and the central angle corresponding to the structure is θ. The complex refractive index of silver comes from Palik Hand-book[19], and the fixed refractive index of polyvinyl alcohol (PVA) material is t as 1.52. The geomet-
146中国光学第 15 卷
ric center of the dielectric layer in the structure is t as the origin O , through which the horizontal axis is x and the vertical axis is y . The whole structure is placed in an air background (n =1).
The Finite Difference Time Domain (FDTD)method was ud to simulate the MDM structure and calculate the fluorescence enhancement when the structure was coupled with a dipole light source (which could reprent fluorescent molecules or quantum dots). The dipole light source is located at the origin O , the simulation region is [x , y ]=[−2.1∶2.1,−1.6∶5.0] μm, and the boundary condi-tions in both x and y directions are Perfect Match-ing Layers (PML). One of the i
mportant factors af-fecting the quantum efficiency of fluorescent sub-stances is radiation attenuation rate, which is posit-ively correlated with the quantum efficiency of fluorescent molecules. The higher the radiation at-tenuation rate is, the higher the quantum yield of fluorescent molecules will be. In order to analyze the influence of MDM structure on fluorescence emission, the Purcell factor F is introduced to quant-itatively reprent the radiation attenuation rate of fluorescent molecules. Its mathematical definition is shown in Formula (1)[20-21]:
n =1
y
θ
R
Ag
PVA
d 1
d 1
d 2
x
Fig. 1 Schematic diagram of the MDM structure model
compod of silver and PVA, in which: the orange area reprents the silver film with a thickness of d 1;the blue area reprents the dielectric PVA with a thickness of d 2, the inner radius of the upper silver film is R , and the central angle corresponding to the structure is θ
图 1 银和PVA 组成的MDM 结构模型示意图，其中橙色
区域代表银膜，其厚度为d 1；蓝色区域代表电介质PVA ，其厚度为d 2，上层银膜内半径为R ，结构所对应圆心角为θ
Γrad P rad Γ0
rad
P 0rad where reprents the radiation attenuation rate in the prence of the Hexible MDM structure, reprents the power radiated to the far field in the
prence of the Hexible MDM structure, and and reprent the radiation attenuation rate and the power radiated to the far field respectively in the abnce of flexible MDM structure.
3 Results and discussion
3.1 Enhancement of quantum dot emission in
different oscillation directions by MDM structure
In a uniform medium, the luminescence of quantum dots is isotropic. Several typical polariza-tion states are usually lected for theoretical analys-is. In this paper, we first studied the effect
of quantum dots in three polarization states (x , y and z )on the fluorescence emission enhancement of quantum dots in a MDM structure. The minimum internal radius of the MDM structure is R =450 nm,the thickness of Ag film is d 1=50 nm, the thickness of dielectric PVA layer is 120 nm, and the central angle corresponding to the structure is θ=60°. The
fluorescence enhancement curves of the MDM structure coupled with the quantum dots in different polarization states were obtained by simulation cal-culation. As can be en from the power curves
shown in Fig. 2(a), the far-field fluorescence emis-sion power of quantum dots in the y -polarization state is small and its curve has no significant change, indicating that this structure has little influ-ence on the fluorescence emission of quantum dots in the y -polarization state. At the same time, com-pared with the oscillation of quantum dots in the y direction, the oscillation in the x and z directions can achieve higher fluorescence enhancement and an obvious fluorescence emission peak. And when the quantum dots are in the x -polarization state, the fluorescence emission peak is the maximum. Be-
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