The article information
 Zhenzhen Zhang, Leicheng Yin, Xiaolong Xu, Jiangying Xia, Kang Xie, Gang Zou, Xiaojuan Zhang, Zhijia Hu, Qijin Zhang
 张珍珍, 殷雷铖, 徐小龙, 夏江营, 谢康, 邹纲, 张晓军, 胡志家, 张其锦
 NearField Scattering Enhancement of Perylene Based Aggregates for Random Lasing
 苝聚集体随机激光的近场散射增强
 Chinese Journal of Chemical Physics, 2019, 32(6): 739746
 化学物理学报, 2019, 32(6): 739746
 http://dx.doi.org/10.1063/16740068/cjcp1807167

Article history
 Received on: July 12, 2018
 Accepted on: October 18, 2018
b. School of Instrument Science and Optoelectronic Engineering, Hefei University of Technology, Hefei 230009, China;
c. Aston Institute of Photonic Technologies, Aston University, Birmingham B47ET, UK;
d. State Key Laboratory of Environmentfriendly Energy Materials, Southwest University of Science and Technology, Mianyang 621000, China
Light scattering in disordered materials has attracted much interest recently. And the light scattering by groups of particles has important applications in light detection and ranging (LiDAR) experiments [1] and astrophysics [2] through its multiple scattering effects, such as enhanced backscattering [3, 4] and negative polarization branches [5], and so forth. In particle groups, according to the distance between the particles (
It is well known that the occurrence of RL in disordered medium requires a gain medium and a scattering medium. The gain medium can be a fluorescent dye, a quantum dot and so on. Perylene diimide (PDI) is a kind of fluorescent dye which has high quantum yield in dilute solutions (close to 100%) [12] and excellent performance such as high molar absorptivity, and optical, chemical and thermal stability [13]. Up to now, PDI has been widely used in the field of liquid crystal display [14] and fluorescent sensor [15].
Strong enough scattering from scatterers is another element for pumping RL. The scatterer can be divided into strong scatterer, such as ZnO [16], and weak scatterer, like polyhedral oligomeric silsesquioxanes (POSS) nanoparticles (NPs). Due to weak scattering, POSS NPs' application is limited to some extent. For obtaining RL, some researches [17, 18] need the help of waveguide effect of a fiber. In our recent work, POSS units were chemically bonded with PDI to fabricate a hybrid molecule, i.e.
In this work, enhancement in the nearfield scattering of POSS units in DPP single molecule and aggregates in CS
The commercial aminopropylisobutyl POSS was obtained by Hybrid Plastics. CS
Active hybrid molecules (DPP) were synthesized with POSS connected to both ends of PDI by covalent bond. DPP, DCP and POSS molecular structures are shown in FIG. 1(a), and synthesis process of DPP and DCP referred to previous relevant reports [12]. DPP was dissolved in CS
According to physical model in Ref.[20] which can be exerted to study our issue, a similar doubleparticle model is depicted in FIG. 2 (b) and (c). In FIG. 2(b), it is shown that two adjacent dielectric spherical nanoparticles are symmetrical along the
$ \begin{eqnarray} \overleftrightarrow{I} {\rm{e}}^{j\mathbf{k}\cdot\mathbf{r}}& = &\sum\limits_{mn}(1)^m \frac{2n+1}{n(n+1) } j^n\\ &&\left[\mathbf{C}_{mn} (\mathbf{\hat{k}} )\mathbf{F}_{1mn}^{(1)} (k\mathbf{r}) j\mathbf{B}_{mn} (\mathbf{\hat{k}})\mathbf{F}_{2mn}^{(1) } (k\mathbf{r})\right] \end{eqnarray} $  (1) 
$ \begin{eqnarray} E_s^i (\mathbf{r}, \omega)& = &\sum\limits_{mn}\left[f_{mn}^i\mathbf{F}_{1mn}^{(3)} (k\mathbf{r}^i )+g_{mn}^i\mathbf{F}_{2mn}^{(3) } (k\mathbf{r}^i )\right] \end{eqnarray} $  (2) 
$ \begin{eqnarray} E_0^i (\mathbf{r}, \omega)& = &\sum\limits_{mn}\left[p_{mn}^i\mathbf{F}_{1mn}^{(1)} (k\mathbf{r}^i )+q_{mn}^i\mathbf{F}_{2mn}^{(1)} (k\mathbf{r}^i )\right] \end{eqnarray} $  (3) 
The total extinction cross section of the cluster is [26]:
$ \begin{eqnarray} C_{ {\rm{ext}}} = \frac{4\pi}{k^2}\sum\limits_{i = 1}^{N_ {\rm{s}}}Re\left\{\sum\limits_{mn}E_{mn} \left(f_{mn}^i p_{mn}^{i*}+ g_{mn}^i q_{mn}^{i*}\right)\right\} \end{eqnarray} $  (4) 
The total absorption cross section is [26]
$ \begin{eqnarray} C_{ {\rm{abs}}}& = &\frac{4\pi}{k^2}\sum\limits_{i = 1}^{N_ {\rm{s}}}Re\left\{\sum\limits_{mn}E_{mn}\left(d_n^i  g_{mn}^i ^2+ c_n^i  f_{mn}^i^2 \right) \right\} \end{eqnarray} $  (5) 
$ \begin{eqnarray} d_n^i& = &\frac{j \varphi{_n'} (m^i x^i ) \varphi_n^* (m^i x^i ) m^{i*}}{m^i \varphi_n (m^i x^i ) \varphi{_n '} (x^i )\varphi_n (x^i ) \varphi{_n'} (m^i x^i )^2} \end{eqnarray} $  (6) 
$ \begin{eqnarray} c_n^i& = &\frac{(j\varphi{_n'} (m^i x^i ) \varphi{_n^*} (m^i x^i ) m^i)}{\varphi_n (m^i x^i ) \varphi{_n'} (x^i )m^i \varphi_n (x^i ) \varphi{_n'} (m^i x^i )^2 } \end{eqnarray} $  (7) 
$ \begin{eqnarray} E_{mn}& = &\frac{n(n+1)}{(2n+1)}\frac{(n+m)!}{(nm)!} \end{eqnarray} $  (8) 
$ \begin{eqnarray} \phi_n(x)& = &xj_n(x) \end{eqnarray} $  (9) 
Here, with the assistance of Eqs.(1
Inspired by the physical simulation results, DPP was fabricated, and a series of active DPP solutions were designed to explore NFSE. DPP contains both of fluorescent dye (PDI) and scattering groups (POSS), and has different aggregation extent at different concentrations in CS
UVVis absorption of DPP, DCP & 2POSS, and DCP is shown in FIG. 4 (a), (b), and (c) respectively. Taking the UVVis absorption of DPP for an example, the perylene imide molecule has three characteristic peaks between 400 and 600 nm (531, 492, and 462 nm) from right to left corresponding to 00, 01, 02 electronic transitions [2527] of PDI respectively. To explain the physical meaning of 02, 01, 00, we have supplied the energy level configuration for electron transition of perylene imide molecules in FIG. 5 from which we can see the 00 electron transition in the absorption spectra represents the transition from the 0 energy level of the ground state to the 0 energy level of the first electron excited state, the 01 represents the transition from the 0 energy level of the ground state to the 1 energy level of the first electron excited state, and 02 represents the transition from the 0 energy level of the ground state to the 2 energy level of the first electron excited state, which have been marked with three upward black arrows. And 01 and 02 transitions are affected by the aggregation state [25, 28], and the 00 electronic transition is not. With the increase of the concentration of DPP and DCP, DPP and DCP will aggregate, which leads to the change of the absorption peak intensity corresponding to the 01 electron transition different from that of 00, so
In order to investigate the effect of pump energies on fluorescence, a pulsed laser with an output of 532 nm was used to pump samples with and without aggregation. All tests are performed in a 10 mm cuvette where the concentrations of DPP are 10
When the dye's concentration is 10
The mechanism of the population inversion for stimulated emission of random laser is different from the traditional one, which is shown in FIG. 8(a). And the mechanism of RL is as follows. When the gain medium (or dye) is pumped by the laser, fluorescence will be emitted, and the emitted light photons will scatter in the disordered scattering medium. When the scattering is strong, a loop will be formed in which the photons will be localized. In this process, the gain medium will absorb the photons continuously and emit fluorescence, and more photons will be localized in this loop. Only when the light intensity reaches a certain intensity will they be emitted, thus completing the inversion of particle number and stimulated emission. The RL phenomena of DPP and DCP & 2POSS in CS
To summarize, in this work, through physical simulation it is found that strong NFSE exists in POSS NPs aggregates. To verify physical simulation result, we have investigated the NFSE phenomenon in aggregated DPP solution with the concentration over 10
Supplementary materials: Fluorescence emission spectra of DPP, DCP & 2POSS and DCP at different concentrations are provided, demonstrating that the three samples aggregate at concentrations above 10
This work was supported by the National Natural Science Foundation of China (No.51673178, No.51273186, No.21574120, No.11874012, No.11404087, and No.11574070), Basic Research Fund for the Central Universities (No.WK2060200012), Science and Technological Fund of Anhui Province for Outstanding Youth (No.1608085J01), Fundamental Research Funds for the Central Universities of China, Postdoctoral Science Foundation (No.2015M571918 and No.2017T100442), the European Union's Horizon 2020 Research and Innovation Programme under the Marie SkłodowskaCurie Grant Agreement (No.744817), the Project of State Key Laboratory of Environmentfriendly Energy Materials, Southwest University of Science and Technology (No.18zxhk10).
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b. 合肥工业大学光电子工程和仪器科学学院，合肥 230009;
c. 阿斯顿大学，阿斯顿光子科技研究所，伯明翰 B47ET;
d. 西南科技大学环境友好材料国家重点实验室，绵阳 621000