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Abstract
The organic semiconductor lasers (OSLs) have been seen as a promising light source for future applications. Achieving organic semiconductors with low amplified spontaneous emission (ASE) threshold is a key progress toward the electrically pumped OSLs. In this paper, the ASE properties of CBP: 2wt% BUBD-1 blend films were optimized using buffer layers containing silver nanoparticles (Ag NPs) with different ratios. Both photoluminescence intensity and ASE properties of blend films were optimized when the buffer layer with 25 vol% Ag NPs was introduced. The lowest ASE threshold is 0.47 µJ/Pulse (6.71 µJ/cm2), which reduces 67.6%, and the highest gain factor is 20.14 cm−1, which enhances 47.8% compared with that without buffer layers. The enhancement of ASE properties of blend films was ascribed to the four functions of the Ag NPs doped buffer layers, including the low refractive index of PMMA and the triple localized surface plasmon resonance (LSPR) effects of Ag NPs in buffer layers. The results show that the buffer layer modified by metal nanoparticles has great application potential in improving the lasing performance of organic small molecules.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Since the first optically pumped organic semiconductor laser (OSLs) came out in 1996, more and more attention has been paid to OSLs because of its flexibility, compact ystem and easy fabrication [1–3]. In the past few decades, although various optically pumped lasers based on organic semiconductors have been demonstrated, it is still impossible to obtain stimulated emission under electrical pumping [4–6]. In order to realize electrically pumped OSLs, people's current research mainly focuses on two aspects, organic gain materials and device structures [7,8]. To reduce the ASE threshold and increase the net gain of organic gain material is an important part for realizing the electrically pumped OSLs [9–12].
Precious metal nanoparticles (NPs) are widely used to enhance the performance of optoelectronic devices due to their unique localized surface plasmon resonance (LSPR) properties [13–16]. For instance, X. Meng et al. achieved the enhanced emission of coherent random lasing in polymer films by introducing Ag NPs [17]. Maet. reported using gold nanospheres (Au NSs) to improve the ASE performance of red emitting polymers by increasing the density of excited states (gain) and reducing energy losses (loss) [18]. To improve the performance, the LSPR extinction spectrum of the nanoparticles should overlap with the absorption or emission spectrum of the luminescent molecule. If the LSPR extinction spectrum overlaps with the emission spectrum of the luminescent molecule, the excited state of the molecule is coupled with the extinction of the LSPR, resulting in the enhanced emission. If the extinction spectrum of LSPR overlaps with the absorption spectrum of the luminescent molecule, the enhancement of the local electric field around it will increase the absorption probability of light and lead to the increasement of excited states density and light emission [19–21]. However, direct contact between the metal structure and the luminescent layer can cause emission quenching. It is found that the buffer layer can keep the fluorophore and noble metal nanoparticles at a suitable distance, which can form a controllable medium environment and avoid emission quenching. According to the refraction law, using a buffer layer with a low refractive index (such as PMMA) to cover the active layer is an effective means to optimize the planar waveguides. All of these will lead to the improved ASE performance of organic gain medium [22,23].
In this paper, the buffer layer with Ag NPs was used to improve the ASE performance of organic small molecule BUBD-1, which is a sky-blue organic gain medium. We chose the CBP: 2wt% BUBD-1 blend films as the gain medium based on our previous work [24]. The ASE threshold of these blend films was reduced further and the energy transfer mechanism was analyzed. Our studies show that the multiple LSPR effect of Ag NPs in the buffer layer lead to the enhancement of ASE properties of organic small molecules.
2. Materials and methods
The Ag NPs were synthesized by using the method mentioned in Ref. [25,26]. Firstly, 5 ml of 10 mM silver nitrate (AgNO3) solution was mixed with 15 ml of 10 mM trisodium citrate dihydrate (Na3C6H5O7) and stirred vigorously for 10 min. Then 5 ml of 10 mM sodium borohydride (NaBH4) was added to this mixed solution and stirred vigorously again for 2 min. The solution color turned to yellow which signified the formation of Ag NPs, and then the nanoparticles were purified by centrifugation. Figure 1 shows the SEM image of the prepared Ag NPs on silicon substrate. The samples presented spherical and a mean particle size of approximate 20 nm.
The organic material of 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) and N,N'-(4,4'-(1E, 1′E)-2,2'-(1,4-phenylene)bis(ethene-2,1-diyl)bis(4,1-phenylene))-bis(2-ethyl-6-methyl-phenylaniline) (BUBD-1) were purchased commercially from Xi‘an Paulette photoelectric technology co. Ltd. and e-Ray Optoelectronics Technology Co. Ltd, respectively. The chloroform was used as the solvent to prepare CBP (10 mg/ml) and BUBD-1 (10 mg/ml) solutions, respectively. And the above two solutions were mixed in proportion to obtain CBP: 2wt% BUBD-1 blend films. PMMA was also dissolved in chloroform to form a 6 mg/mL solution. Then the PMMA/ x vol% Ag NPs (x=10%, 15%, 20%, 25%, 30%, 50%) were obtained by mixing the prepared Ag NPs and PMMA solutions with different proportions. We noted that the Ag NPS was hydrophilic and immiscible with n-butyl acetate, then the mixtures were stirred until they were used for spin coating.
The glass substrates used in our work was cleaned by sonication in detergent followed by sonication in alcohol with each step for 20 min. Then it was dried by nitrogen. The samples were prepared with two steps. Firstly, the PMMA/ x vol% Ag NPs films were spin coated at a rotation speed of 2000rpm and then annealed at 200°C for 30 min to form a buffer layer of about 50 nm thickness which can wrap the Ag NPs fully. Secondly, the CBP: 2wt% BUBD-1 solution was spin-coated on the cross-linked buffer layer to obtain a blend film with 150 nm thickness, and then annealed at 80°C to remove residual solvent. All experiments were carried out in air at room temperature without any further treatment. The molecular structures of the host material CBP and the guest material BUBD-1 are shown in Fig. 2.
In our work, the absorption spectrum was recorded by UV–Vis spectrometer (UV-3310), the thickness of the film was measured by M2000 spectroscopic ellipsometer. The photoluminescence spectra and the decay kinetics of samples were recorded by Edinburgh FLS920 spectrophotometer system. PLQY were measured with Fluorescence Spectrometer (FLS920) combined with an integrating sphere (IS). An OPO laser (Vihrant 355LD) was used as the pump source with a repetition rate of 20 Hz, a pulse width of 7 ns and a wavelength of 355 nm. The laser then went through the aperture, a slit, and a cylindrical mirror in sequence to form a striped spot (0.7 cm × 0.1 cm) on the surface of the samples. The pump energy was measured by a laser power meter. The fluorescence data were recorded by an Ocean fiber optic spectrometer from the edge of the films [24]. The ASE measurement equipment is shown in Fig. 3.
3. Results and discussion
The optical and ASE properties of BUBD-1 pure film and CBP: x wt% BUBD-1 blend films were studied in our previous work [24]. The CBP: 2wt% BUBD-1 blend film has the best luminescence properties. In order to further enhance the luminescence and ASE properties of the CBP: 2wt% BUBD-1 blend film, the inert material PMMA and Ag NPs was introduced as a buffer layer to study the effect of the buffer layer on luminescence and ASE properties of the blend film. The normalized steady absorption spectra of CBP, BUBD-1 films and the colloidal Ag solution as well as the normalized PL spectra of CBP film, CBP: 2wt%BUBD-1 blend films are shown in Fig. 4. It can be found that Ag NPs shows an absorption band between 350 nm to 450 nm, the peak locates at about 390 nm, which has a large overlap with the absorption spectrum of BUBD-1 film and the PL spectrum of CBP film. Then the localized surface plasmon resonance (LSPR) effects induced by Ag NPs will enhance the light absorption in BUBD-1 films and the light emission in CBP films. The energy transfer efficiency between CBP and BUBD-1 molecule will be influenced by the LSPR effects.
Fig. 4. Normalized absorption spectra of Ag NPs solution, BUBD-1 and CBP films and emission spectra of CBP, CBP: 2wt% BUBD-1films.
In this paper, the effect of PMMA: x vol% Ag NPs buffer layer on luminescence characteristics of CBP: 2wt%BUBD-1 blend film was systematically studied. The PMMA: x vol% Ag NPs buffer layers (x=10%, 15%, 20%, 25%, 30% and 50%) were prepared and introduced between the substrate and the CBP: 2wt%BUBD-1 blend film. Within the studied Ag NPs doping ratio (10%−50%), the existence of buffer layer increases the luminescence intensity of the blend film, as shown in Fig. 5(a) and 5(b). The luminescence intensity of CBP: 2wt% BUBD-1 blend film increases significantly with increasing doping ratio of Ag NPs in the buffer layer. When the doping ratio of Ag NPs increased to 25%, the luminescence intensity of CBP: 2wt% BUBD-1 blend film reaches the maximum. However, the luminescence intensity begins to decrease when the ratio of Ag NPs increases further.
Fig. 5. (a) The PL spectra of blend films with/without buffer layers. (b) The PL integral intensity of blend films with buffer layers of different ratios of Ag NPs.
The surface morphology of CBP: 2wt% BUBD-1 blend films with buffer layers of different ratios of Ag NPs were measured and the atomic force microscopy (AFM) images were shown in Fig. 6. The low root-mean-square (RMS) roughness manifested that the Ag NPs were wrapped well and showed a smooth surface morphology, which was benefit to form a light waveguide. The lowest RMS roughness 0.294 nm was recorded for the buffer layer doping with 25% Ag NPs.
The ASE properties of CBP: 2wt% BUBD-1 blend films with/without buffer layers were also studied using the equipment shown in Fig. 3. The measured output emission spectra from the edge of the blend films as a function of increasing pump energy are shown in Figs. 7 and 8. When the sufficient pump energy was provided, the spectra narrowing became dominant, the emission intensity increased sharply and the FWHM of the spectrum declined dramatically. Figure 7 and Fig. 8 shows an observed abrupt change in slope efficiency, and the turning point was defined as the ASE threshold. Besides the ASE threshold, the net gain factors were measured by the variable-stripe-length method which observed laser emission from the film edge as a function of excitation length. The output emission I(λ) should obey the following formula [27–29].
where A (λ) is a constant related to the emission cross section, Ip is the intensity of pumping light, and L is the length of the narrow strip. Therefore, the intensity variation of the emitted light from the edge of sample is obtained by changing the length of the narrow strip, and then the net gain factor g (λ) of the material can be obtained through the fitting process of the above formula.Fig. 7. (a) The evolution of emission spectra of CBP: 2wt%BUBD-1 with the pump energy. Insert: Dependence of FWHM (solid dot) and the output intensity (solid square) on the pump energy. (b) The relationship between the peak spectral output intensity and the pump stripe length.
Fig. 8. The evolution of emission spectra of PMMA/CBP: 2wt%BUBD-1 (a) PMMA: 25vol% Ag NPs/ CBP: 2wt% BUBD-1 (c) with the pump energy. Insert: Dependence of FWHM (solid dot) and the output intensity (solid square) on the pump energy. The relationship between the peak spectral output intensity and the pump stripe length of PMMA/CBP: 2wt% BUBD-1 (b) PMMA: 25vol % Ag NPs/ CBP: 2wt% BUBD-1 (d).
The measured CBP: 2wt% BUBD-1 blend film on glass has the ASE threshold value of 1.45 µJ/pulse (20.71 µJ/cm2) and the net gain factor of 13.63 cm-1. When the PMMA: x vol% Ag NPs buffer layer were introduced, the ASE properties of CBP: 2wt% BUBD-1 films were enhanced. As an example, the ASE characteristics of PMMA/ CBP: 2wt% BUBD-1 and PMMA: 25 vol% Ag NPs/ CBP: 2wt% BUBD-1 are shown in Fig. 8. The ASE characteristics of other samples are shown in Supplement 1 Fig. S1. Figure 9 clearly shows the variation of ASE threshold and FWHM of the blend films when the buffer layer contains Ag NPs with different ratios. The ASE threshold and net gain factor of the blend film are listed in Table 1. As can be seen from Fig. 9 and Table 1, the ASE threshold of blend film reduced from 1.45 µJ/pulse (20.71 µJ/cm2) to 1.14 µJ/pulse (16.28 µJ/cm2) when the PMMA buffer layer introduced. As the doping ratios of Ag NPs in the buffer layer increases, the ASE performance of the blend film firstly improves and then decreases. When the doping ratio is 25%, the optimized ASE performance reaches. Compared with the blend film without a buffer layer, the optimized ASE threshold is 0.47 µJ/Pulse (6.71 µJ/cm2), which has a decrease of 67.6%, and the net gain factor is 20.14 cm-1, which has an increase of 47.8%.
Table 1. ASE threshold and gain coefficient, fluorescence average lifetime and PLQY of CBP: 2wt% BUBD-1 film doped with different ratios of Ag NPs.
The addition of the buffer layer with Ag NPs not only increased the luminous intensity of the CBP: 2wt% BUBD-1 film, but also improved its ASE performance. The optimized ASE threshold is only 0.47 µJ/pulse, which is lower than that of blend films with PMMA buffer layer. It indicates that both the PMMA and Ag NPs have positive effects on the ASE properties of blend films. The enhanced ASE properties can be partially attributed to the low refractive index of buffer layers and partially to the LSPR effects of Ag NPs in the buffer layers. On one hand, the inert material PMMA has a lower refractive index, and which will change little when a low ratio of Ag NPs introduced [18]. The low refractive of buffer layer will enhance the total internal reflective of light and the effect of light waveguide, which is benefit for the light emission from the edge of films. On the other hand, the Ag NPs in the buffer layer will lead to a LSPR effect. As can be seen in Fig. 4, there is a well overlap between the absorption of Ag NPs and the absorption of BUBD-1 film. The local enhancement of electromagnetic fields near Ag NPs will activate the light-emitting molecule and more molecule be excited to the excited state. The increased excitation efficiency results in an enhanced density of singlet exciton of BUBD-1, which is benefit for the construction of population inversion. Figure 4 also illustrates a well overlap between the absorption of Ag NPs and the PL of CBP film, which will resonant increase the PL of CBP film. In the blend films, the Forster resonance energy transfer (FRET) occurs from CBP to BUBD-1 molecule [24]. The enhanced PL intensity of CBP means more energy can be transferred from CBP to BUBD-1. In order to test it, the energy transfer efficiency and radiation attenuation rate were investigated.
The transient fluorescence spectrum of CBP: 2wt% BUBD-1 blend films with or without the PMMA: x vol% Ag NPs buffer layer were measured by time-correlated single photon counting technique excited by the nanosecond laser of 371 nm and pulse width of 75.8 ps. The PL decay were monitored at 500 nm in all cases. The fluorescence lifetime was fitted by biexponential model, ($R(t )= {B_1}{e^{( - t/\tau }}{_1^)} + {B_2}{e^{( - t/}}{^\tau _2}^).$). χ2 indicated how well the fit is and 1 means the best fit. The average lifetime was got by $\mathrm{\bar{\tau }\ =\ }{\mathrm{\tau }_1}x\%+ {\mathrm{\tau }_2}y\%.$Fig. 10 shows the PL decay of CBP: 2wt% BUBD-1 blend films with or without Ag NPs. The average lifetimes are listed in Table 1. When the pure PMMA buffer layer introduced, the decay curve has no difference. The decay time is about 4.73 ns. However, when the Ag NPs was introduced, the decay time reduced and the minimum decay time of 3.9 ns was obtained when the ratio of Ag NPs is 25%. It is basically consistent with the change of photoluminescence intensity.
Fig. 10. The PL decay of CBP: 2wt% BUBD-1 blend films with/without buffer layers. Insert(a): The PL decay of CBP films.
The photoluminescence quantum yield (PLQY) were measured and listed in Table 1, when the ratio of Ag NPs increased to 25%, the PLQY achieve the maximum value of 85±5%. The higher PLQY indicates that the decay rate of non-radiative transitions is much smaller than that of radiative transitions.
As we know, the radiative (ΓRE) and non-radiative (ΓNRE) decay rates can be expressed by the measurement of fluorescence lifetime (τ0) and PLQY (q0) as [30]:
and then both of the radiative decay rate and the nonradiative decay rate are correlated with the stimulated emission phenomenon. The decreased lifetime means a larger radiative and nonradiative decay rate. Then the increased PLQY means the larger radiative decay rate. According to the formula (4), the radiative decay rate is enhanced in the blend film with the doping ratio of 25% in the buffer layer. Moreover, non-radiative decay rate is decreased. The radiative decay rate is intrinsic to the nature of ASE performance, the ASE threshold (Eth) is inversely proportional to ΓRE, Eth ∝ 1 + ΓNRE /ΓRE [31], which is consistent with our experimental results in ASE properties.The variation of lifetime also leads to a variation of energy transfer efficiency. In the blend film, the energy transfer efficiency between the host and guest molecules can be expressed as [32]:
here, τD, τDA represent the fluorescence lifetime of the host material and guest material doped with the host material. The energy transfer efficiency was calculated by using the average lifetime, listed in Table 1. Here, ${\mathrm{\bar{\tau }}_D}$ is 12 ns (CBP). The energy transfer efficiency of the blend film without buffer layer was 60.5% (${\mathrm{\bar{\tau }}_{DA}}$= 4.74 ns), which increased to 67.5% (${\mathrm{\bar{\tau }}_{DA}}$= 3.90 ns) when the buffer layer of PMMA: 25 vol% Ag NPs was introduced. The enhanced energy transfer efficiency may be ascribed to the occurrence of surface plasmon excited on the surface of Ag NPs. The energy of excited surface plasmon is about 3.17 eV (peak wavelength 390 nm), which is between the S1 of CBP molecule (3.36 eV) and S1 of BUBD-1 molecule (2.6 eV) [33]. The surface plasmon likes a bridge to help transferring the energy of CBP to BUBD-1 and lead to a higher energy transfer efficiency.A schematic diagram of energy transfer and ASE emission in the blend film with or without the PMMA: x vol% Ag NPs buffer layer are shown in Fig. 11. At first, the LSPR effect resonant enhanced the PL intensity of CBP and the absorption intensity of BUBD-1 molecule. Next, the Forster energy transfer from CBP to BUBD-1 molecule is enhanced by the bridge of surface plasmons excited near the Ag NPs. All of these enhanced the population of S1 of BUBD-1 molecules and promoted the building of population inversion, which increased the PL intensity and ASE properties of the blend films.
Fig. 11. The energy transfer process between the host material (CBP) and the organic laser material (BUBD-1) (a) without Ag NPs (b) with Ag NPs.
4. Conclusion
In summary, we have demonstrated the buffer layer enhanced PL and ASE properties in CBP: 2wt% BUBD-1 blend films by incorporating Ag NPs into the inert material PMMA. A combination of steady and transient PL spectra, AFM images, ASE properties and optical gain factor of these blend films has provided. The lowest ASE threshold of 0.47 µJ/pulse (6.71 µJ/cm2) and highest gain factor of 20.14 cm-1 were obtained when the buffer layer with 25% Ag NPs was added. The boost of optical property can be ascribed to lower refractive index of PMMA and the trifunctional of Ag NPs, which includes the resonant enhancement of PL of CBP, the resonant enhancement of the absorption of BUBD-1, and the bridge of local surface plasmon to transfer the energy of CBP to BUBD-1. This study points out that the buffer layer with noble metal nanoparticles has great potential applications to improve the lasing performance of organic small molecules.
Funding
National Natural Science Foundation of China (61775089); Natural Science Foundation of Shandong Province (ZR2020KB018); Project of Liaocheng University (318011904, 318051650).
Acknowledgments
The authors acknowledge financial support from the National Natural Science Foundation of China, Natural Science Foundation of Shandong Province and Project of Liaocheng University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. Y. Jiang, Y. Y. Liu, X. Liu, H. Lin, K. Gao, W. Y. Lai, and W. Huang, “Organic solid-state lasers: a materials view and future development,” Chem. Soc. Rev. 49(16), 5885–5944 (2020). [CrossRef]
2. V. T. N. Mai, A. Shukla, M. Mamada, S. Maedera, P. E. Shaw, J. Sobus, I. Allison, C. Adachi, E. B. Namdas, and S.-C. Lo, “Low Amplified Spontaneous Emission Threshold and Efficient Electroluminescence from a Carbazole Derivatized Excited-State Intramolecular Proton Transfer Dye,” ACS Photonics 5(11), 4447–4455 (2018). [CrossRef]
3. M. Mamada, T. Fukunaga, F. Bencheikh, A. S. D. Sandanayaka, and C. Adachi, “Low Amplified Spontaneous Emission Threshold from Organic Dyes Based on Bis-stilbene,” Adv. Funct. Mater. 28(32), 1802130 (2018). [CrossRef]
4. R. Muñoz-Mármol, N. Zink-Lorre, J. M. Villalvilla, P. G. Boj, J. A. Quintana, C. Vázquez, A. Anderson, M. J. Gordon, A. Sastre-Santos, F. Fernández-Lázaro, and M. A. Díaz-García, “Influence of Blending Ratio and Polymer Matrix on the Lasing Properties of Perylenediimide Dyes,” J. Phys. Chem. C 122(43), 24896–24906 (2018). [CrossRef]
5. S. Z. Bisri, T. Takenobu, and Y. Iwasa, “The pursuit of electrically-driven organic semiconductor lasers,” J. Mater. Chem. C 2(16), 2827–2836 (2014). [CrossRef]
6. A. J. Kuehne and M. C. Gather, “Organic Lasers: Recent Developments on Materials, Device Geometries, and Fabrication Techniques,” Chem. Rev. 116(21), 12823–12864 (2016). [CrossRef]
7. C. Grivas, “Optically pumped planar waveguide lasers, Part I: Fundamentals and fabrication techniques,” Prog. Quantum Electron. 35(6), 159–239 (2011). [CrossRef]
8. C. Grivas, “Optically pumped planar waveguide lasers: Part II: Gain media, laser systems, and applications,” Prog. Quantum Electron. 45-46, 3–160 (2016). [CrossRef]
9. X. Wu, Y. Li, W. Li, L. Wu, B. Fu, W. Wang, G. Liu, D. Zhang, J. Zhao, and P. Chen, “Enhancing Optically Pumped Organic-Inorganic Hybrid Perovskite Amplified Spontaneous Emission via Compound Surface Plasmon Resonance,” Cryst. 8(3), 124 (2018). [CrossRef]
10. S. A. Veldhuis, P. P. Boix, N. Yantara, M. Li, T. C. Sum, N. Mathews, and S. G. Mhaisalkar, “Perovskite Materials for Light-Emitting Diodes and Lasers,” Adv. Mater. 28(32), 6804–6834 (2016). [CrossRef]
11. J. Yi, J. Huang, Y. Lin, C.-F. Liu, T. Cheng, Y. Jiang, W. Tang, W.-Y. Lai, and W. Huang, “Improved amplified spontaneous emission of organic gain media with metallic electrodes by introducing a low-loss solution-processed organic interfacial layer,” RSC Adv. 6(55), 49903–49909 (2016). [CrossRef]
12. M. L. De Giorgi and M. Anni, “Amplified Spontaneous Emission and Lasing in Lead Halide Perovskites: State of the Art and Perspectives,” Appl. Sci. 9(21), 4591 (2019). [CrossRef]
13. J. Yang, D. Song, S. Zhao, B. Qiao, Z. Xu, P. Wang, and P. Wei, “Highly efficient and bright blue organic light-emitting devices based on solvent engineered, solution-processed thermally activated delayed fluorescent emission layer,” Org. Electron. 71, 1–6 (2019). [CrossRef]
14. X. Wu, X. F. Jiang, X. Hu, D. F. Zhang, S. Li, X. Yao, W. Liu, H. L. Yip, Z. Tang, and Q. H. Xu, “Highly stable enhanced near-infrared amplified spontaneous emission in solution-processed perovskite films by employing polymer and gold nanorods,” Nanoscale 11(4), 1959–1967 (2019). [CrossRef]
15. C. Li, Z. Liu, Q. Shang, and Q. Zhang, “Surface-Plasmon-Assisted Metal Halide Perovskite Small Lasers,” Adv. Opt. Mater. 7(17), 1900279 (2019). [CrossRef]
16. P. Chen, Z. Xiong, X. Wu, M. Shao, Y. Meng, Z. H. Xiong, and C. Gao, “Nearly 100% Efficiency Enhancement of CH3NH3PbBr3 Perovskite Light-Emitting Diodes by Utilizing Plasmonic Au Nanoparticles,” J. Phys. Chem. Lett. 8(17), 3961–3969 (2017). [CrossRef]
17. X. Meng, K. Fujita, S. Murai, and K. Tanaka, “Coherent random lasers in weakly scattering polymer films containing silver nanoparticles,” Phys. Rev. A 79(5), 053817 (2009). [CrossRef]
18. X. Zhou, L. Liu, X. Wu, Y. Yang, X.-F. Jiang, X. Chen, Q.-H. Xu, Z. Xie, and Y. Ma, “An Au NP doped buffer layer in a slab waveguide for enhancement of organic amplified spontaneous emission,” J. Mater. Chem. C 5(6), 1356–1362 (2017). [CrossRef]
19. N. M. Kha, C. H. Chen, W. N. Su, J. Rick, and B. J. Hwang, “Improved Raman and photoluminescence sensitivity achieved using bifunctional Ag@SiO(2) nanocubes,” Phys. Chem. Chem. Phys. 17(33), 21226–21235 (2015). [CrossRef]
20. S. Ning, Z. Wu, H. Dong, L. Ma, B. Jiao, L. Ding, L. Ding, and F. Zhang, “The enhanced random lasing from dye-doped polymer films with different-sized silver nanoparticles,” Org. Electron. 30, 165–170 (2016). [CrossRef]
21. Y. Wan, Y. An, and L. Deng, “Plasmonic enhanced low-threshold random lasing from dye-doped nematic liquid crystals with TiN nanoparticles in capillary tubes,” Sci. Rep. 7(1), 16185 (2017). [CrossRef]
22. T. Jiang, Y. Du, Y. Ma, J. Zhou, C. Gu, and S. Tang, “Decrease of amplified spontaneous emission threshold achieved by core–shell Ag nanocube@SiO2with ultrasmall shell thicknesses,” Mater. Res. Express 4(11), 115030 (2017). [CrossRef]
23. M. Signoretto, N. Zink-Lorre, I. Suárez, E. Font-Sanchis, Á Sastre-Santos, V. S. Chirvony, F. Fernández-Lázaro, and J. P. Martínez-Pastor, “Efficient Optical Amplification in a Sandwich-Type Active-Passive Polymer Waveguide Containing Perylenediimides,” ACS Photonics 4(1), 114–120 (2017). [CrossRef]
24. T. Liu, S. Li, W. Wang, Y. Liu, H. Du, Y. Guo, Q. Shi, D. Zhang, L. Zhao, and Q. Fan, “Reduced optically pumped amplified spontaneous emission threshold of BUBD-1 thin films by thermally activated delayed fluorescent materials,” J. Lumin. 212, 76–82 (2019). [CrossRef]
25. V. V. Mokashi, L. S. Walekar, P. V. Anbhule, S. H. Lee, S. R. Patil, and G. B. Kolekar, “Study of energy transfer between riboflavin (vitamin B2) and AgNPs,” J. Nanopart. Res. 16(3), 2291 (2014). [CrossRef]
26. M. Voicescu, D. G. Angelescu, S. Ionescu, and V. S. Teodorescu, “Spectroscopic analysis of the riboflavin—serum albumins interaction on silver nanoparticles,” J. Nanopart. Res. 15(4), 1555 (2013). [CrossRef]
27. M. McGehee, S. C. Veenstra, E. Miller, M. Díaz-García, and A. Heeger, “Amplified spontaneous emission from photopumped films of a conjugated polymer,” Phys. Rev. 58(11), 7035–7039 (1998). [CrossRef]
28. S. Ning, Z. Wu, H. Dong, L. Ma, X. Hou, and F. Zhang, “Enhancement of lasing in organic gain media assisted by the metallic nanoparticles–metallic film plasmonic hybrid structure,” J. Mater. Chem. C 4(24), 5717–5724 (2016). [CrossRef]
29. E. B. Namdas, M. Tong, P. Ledochowitsch, S. R. Mednick, J. D. Yuen, D. Moses, and A. J. Heeger, “Low Thresholds in Polymer Lasers on Conductive Substrates by Distributed Feedback Nanoimprinting: Progress Toward Electrically Pumped Plastic Lasers,” Adv. Mater. 21(7), 799–802 (2009). [CrossRef]
30. M. Inoue, T. Matsushima, and C. Adachi, “Low amplified spontaneous emission threshold and suppression of electroluminescence efficiency roll-off in layers doped with ter(9,9′-spirobifluorene),” Appl. Phys. Lett. 108(13), 133302 (2016). [CrossRef]
31. L. Ma, Y. Yu, B. Jiao, X. Hou, and Z. Wu, “Theoretical evidence of low-threshold amplified spontaneous emission in organic emitters: transition density and intramolecular vibrational mode analysis,” Phys. Chem. Chem. Phys. 20(29), 19515–19524 (2018). [CrossRef]
32. T. Furukawa, H. Nakanotani, M. Inoue, and C. Adachi, “Dual enhancement of electroluminescence efficiency and operational stability by rapid upconversion of triplet excitons in OLEDs,” Sci. Rep. 5(1), 8429 (2015). [CrossRef]
33. C.-L. Ho, M.-F. Lin, W.-Y. Wong, W.-K. Wong, and C. H. Chen, “High-efficiency and color-stable white organic light-emitting devices based on sky blue electrofluorescence and orange electrophosphorescence,” Appl. Phys. Lett. 92(8), 083301 (2008). [CrossRef]
