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Abstract
Metallic films are widely used as electrodes in micro-cavities. However, the quenching effect of metal films is generally considered fatal for lasing, and presents a major obstacle to the development of electrically pumped lasers. In this article, we report on the dramatic performance enhancement of random lasing assisted by a plasmonic hybrid structure composed of (Au core)-(Ag shell) nanorods deposited on Ag film (Au@Ag NRs-Ag film). This study reveals that the Au@Ag NRs-Ag film hybrid structure can more effectively enhance the lasing properties than independent Au@Ag NRs or Ag film. Besides, compared with hybrid structures composed of Ag film with Au nanorods or Au nanospheres, the gain medium deposited on Au@Ag NRs-Ag film has the lowest lasing threshold: only 12.5% of that of the neat gain medium. As a unique plasmonic hybrid nanostructure, Au@Ag NRs-Ag film exhibits a stronger localized electrical field and scattering effect than the hybrid structures composed of Ag film with regular Au nanoparticles. This is attributed to the broader and stronger plasmonic absorption of Au@Ag NRs, as well as to the stronger plasmonic coupling between the localized surface plasmons of Au@Ag NRs and the delocalized surface plasmon polariton of Ag film. Our results could provide a simple way to effectively avoid the negative effects of metal films and realize a lower pumped threshold.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optically pumped organic semiconductor lasers have been investigated extensively owing to their unique advantages, such as broad spectral emission range, ease of processing, and low cost of organic materials [1–5]. At present, the development of an electrically pumped laser is the ultimate goal [6]. Typically, a metallic electrode is an essential part of electrically pumped devices. However, the quenching effect of metallic electrodes on the lasing of dye molecules remains a major obstacle for electrically pumped lasers as it inhibits lasing [7].
In the drive towards developing an electrically driven polymeric laser, it is important to demonstrate that an optically pumped laser can work in the presence of the metal film. Considerable works had been done to reduce the negative effects of the metal film and lower the lasing threshold. Reufer et al. adopted a distributed feedback (DFB) structure to allow lasing action in an organic thin-film in the presence of the metal electrode [8]. Villers et al. used a thin spacer layer of polystyrene or oxidized Ca between a MEH-PPV film and Ca or Al electrode [9]. Ma et al. studied the effect of gain medium film thickness on the amplified spontaneous emission of Ag-backed red-fluorescent-dye-doped polymer film [10]. Hou et al. found that Ag cladding produces lower loss than Al for gain medium with metal cladding [7]. Zhai et al. overcame the Joule loss of the metal film by decreasing the contact area between the active layer and DFB metal film [11]. However, despite these attempts, in which the quenching effect of metal films was reduced and the lasing thresholds of devices with metal films were lowered, further reductions in the lasing threshold are needed for devices with a metal film.
Presently, plasmonic metal nanoparticles, such as Ag nanoparticles, Au nanoparticles, and core-shell nanostructures, have been used to enhance lasing properties via the plasmonic effect [12–15]. Localized surface plasmon resonance (LSPR) can be excited on metal nanoparticles and surface plasmon polariton (SPP) can be excited on metal films, which all exhibit an enhanced field [16,17]. Furthermore, the plasmonic coupling interaction between the LSPR of metal nanoparticles and the SPP of metal films could lead to a more intense local electric field [18,19]. However, few publications have considered the depositing of metal nanoparticles close to a metal film in a random laser.
In this study, (Au core)-(Ag shell) nanorods deposited close to Ag film (Au@Ag NRs-Ag film), a special plasmonic hybrid structure, was developed to enhance random lasing. Compared with gain media deposited on independent Ag film or Au@Ag NRs, a low lasing threshold was found for the gain medium deposited on the Au@Ag NRs-Ag film hybrid structure. This is because of plasmon coupling between the LSPR of Au@Ag NRs and the SPP of the Ag film. Furthermore, to demonstrate the superiority and uniqueness of the Au@Ag NRs-Ag film on lasing, devices based on the hybrid structures of Au nanorods-Ag film or Au nanospheres-Ag film were prepared instead of an Au@Ag NRs-Ag film. The results revealed that the Au@Ag NRs-Ag film could better lower the lasing threshold than other plasmonic hybrid structures. This is because that the Au@Ag NRs exhibit a much stronger plasmonic effect compared with pure Au nanostructures; furthermore, a broader LSPR spectrum for Au@Ag NRs, comprising with four strong LSPR peaks, ensures a complete overlap with absorption and emission spectra of the gain medium. As such, stronger plasmon coupling between the LSPR of Au@Ag NRs and the SPP of Ag film leads to the more intense local electric field and scattering effect, which significantly increase the light absorption and quantum efficiency of the dyes. The results of this study provide a simple approach to effectively enhance the random lasing properties of gain media in the presence of a metal film.
2. Experimental section
2.1 Synthesis of (Au core)-(Ag shell) nanorods
Uncoated Au nanorods (Au NRs) were prepared via a seed-mediated method [20]. The as-synthesized Au NRs solution was centrifuged at 10000 rpm for 30 min, and then redispersed in an aqueous cetyltrimethylammonium chloride (CTAC) solution (0.08 M) at the same volume. A total of 0.3 mL AgNO3 (0.01 M) was then added into the 2 mL Au nanorods solution, followed by the addition of 0.15 mL ascorbic acid solutions (0.1 M). The solution was kept in an isothermal oven at 65 ℃ for 4.5 h to obtain Au@Ag NRs [21]. Figure 1(a) shows a transmission electron microscopy (TEM) image of Au@Ag NRs. The average length and diameter of the Au cores are 48 ± 3 nm and 16 ± 2 nm, respectively. The average thicknesses of the Ag shells are 8 and 4 nm at the side and end, respectively. For comparison, the Au NRs (similar dimensions as the Au@Ag NRs) and Au nanospheres (diameter ∼50 nm) were prepared as shown in Fig. 1(b) and (c). The Au NRs were synthesized via a seed-mediated method [20]. The Au nanospheres (Au NSs) were synthesized using the method of Liz-Marzan [22].
2.2 Fabrication of a random laser based on an Au@Ag NRs-Ag film hybrid structure
To investigate the effect of the Au@Ag NRs-Ag film hybrid structure on lasing, we fabricated devices for which the gain medium is deposited on glass, on Ag film, on Au@Ag NRs island film, and on Au@Ag NRs-Ag film hybrid structures with 10 and 100 nm thick SiO2 spacers. The device structures are as follow (Fig. 2):
- Device A: Glass/ LiF (10 nm)/ Gain medium
- Device B: Glass/ Ag film (50 nm)/ SiO2 (10 nm)/ LiF (10 nm)/ Gain medium
- Device C: Glass/ Au@Ag NRs/ LiF (10 nm)/ Gain medium
- Device D: Glass/ Ag film (50 nm)/ SiO2 (10 nm)/ Au@Ag NRs/ LiF (10 nm)/ Gain medium
- Device E: Glass/ Ag film (50 nm)/ SiO2 (100 nm)/ Au@Ag NRs/ LiF (10 nm)/ Gain medium
The Au@Ag NRs island films were obtained by spin-coating at 450 rpm and dried at 80 ℃. The density of the Au@Ag NRs is 1.1 × 109 cm−2, as shown in the atomic force microscopy (AFM) image of Fig. 3(a). The 50 nm-thick Ag films were deposited by thermal evaporation under a vacuum of 1×10−5 Pa at a rate of 0.3 nm s−1. The 10 nm-thick SiO2 layers were deposited by radio frequency sputtering. And the 10 nm-thick LiF layers were prepared by thermal evaporation for reducing the absorption loss and the quenching effect of the dye molecules caused by direct contact between Au@Ag NRs and gain medium. The gain medium was prepared with polystyrene (PS), Alq3, and DCJTB in chloroform solution (PS: Alq3: DCJTB=300:100:3.5, wt.%), and was spin-coated at 3000 rpm, then the gain medium films were annealed at 110 °C for 10 min. The thicknesses of gain medium layers were approximately 250 nm.
Fig. 3. Atomic force microscope (AFM) images of (a) Au@Ag NRs, (b) Au NRs, and (c) Au nanospheres island films.
To further present the advantages of the Au@Ag NRs-Ag film hybrid structure on the enhancement of lasing properties, related devices based on hybrid structures composed of Ag film with Au NRs (similar dimensions to the Au@Ag NRs) or Au nanospheres (diameter ∼50 nm), were also prepared; the device structures were the same as that shown in Fig. 2(d). To allow for direct comparison, particle number concentrations of the three different nanostructures needed to be consistent. The density ratio of Au@Ag NRs, Au NRs, and Au NSs was 1:1.18:1.21, as shown in Fig. 3.
2.3 Characterization
Surface particle densities were measured using an AFM system (NT-MDT). The thicknesses of gain medium films were measured using a Stylus Profiler (Dektak 6M). The absorption and photoluminescence (PL) spectra were obtained using a UV-Vis spectrophotometer (HITACHI U-3010) and fluorescence spectrometer (fluoromax-4), respectively. The devices were pumped at normal incidence with an pulsed Nd: YAG laser (355 nm, 10 Hz repetition rate, and 5.5 ns pulse duration). An adjustable slit and cylindrical lens were used to shape the beam into a stripe of 7 mm × 1 mm. When the device is pumped, the light is partially confined into the waveguide and amplified by the gain medium as it is reflected (or scattered) by the internal surfaces of waveguide and propagates along the path of optical gain in waveguide. As a result, the light is waveguided along the length of the excitation stripe and emits from the end of the strip. The edge emission spectra were collected from the edge of the device with a fiber optic spectrometer (Ocean Optics SpectraSuite, USB2000). The lasing thresholds, peak intensities, and full width at half maximum (FWHM) were measured.
3. Results and discussion
3.1 LSPR of the Au@Ag NRs
It is challenging to prepare elongated anisotropic Ag nanostructures [23]. Au nanorods can act as excellent supports for the formation of Ag shells owing to the advantages of a tunable-longitudinal plasmon wavelength and chemical stability [24]. The plasmon wavelength can be tuned by altering the size of the Au core and Ag shell [25,26]. Thus, in this work, the Au@Ag NRs were expected to exhibit a specific plasmonic spectrum to match the gain medium.
As shown in Fig. 4(a), the absorption spectrum of Au@Ag NRs was measured. We can find that the ‘rod-like’ Au@Ag NRs have transverse and longitudinal plasmon resonance bands; there are four strong LSPR peaks (at 350, 409, 474, and 596 nm). To illustrate the evolution of the plasmon spectrum after coating the Ag shells, the LSPR spectrum of the initial Au NRs before coating the Ag shells (average length and diameter of 48 ± 3 nm and 16 ± 2 nm, respectively) was also measured. Figure 4(a) shows that the initial Au nanorods exhibit longitudinal and transverse plasmon resonance peaks of 772 and 520 nm. After coating the Ag shells, these two plasmon peaks blue-shift to 596 and 474 nm, respectively. The two emerging plasmon peaks (409 and 350 nm) are the plasmon mode of Ag. In addition, to further determine the LSPR spectrum of Au@Ag NRs, we simulated the absorption spectrum of Au@Ag NRs with the finite difference time domain (FDTD) method. The average size of the Au@Ag NRs shown in Fig. 1(a) was chosen. There is a good agreement between simulated and measured absorption spectra of Au@Ag NRs. The slight difference can be ascribed to that the experimental spectrum is measured from the ensemble samples while the calculated spectrum is based on the average size.
Fig. 4. (a) Measured absorption spectra of initial pure Au NRs before and after coating of Ag shells, and the simulated absorption spectrum of Au@Ag NRs, the spectra have been normalized against the maximum of the absorption of Au@Ag NRs. (b) measured absorption spectra of Au@Ag NRs, Au NRs with similar dimensions to the Au@Ag NRs, Au nanospheres; the absorption and emission spectra of Alq3 and DCJTB.
Meanwhile, the plasmon spectra of Au NRs (similar dimensions to the Au@Ag NRs) and Au nanospheres (diameter ∼50 nm) with the same particle number concentration are shown in Fig. 4(b). It is noted that the performance of the Au@Ag NRs is distinctive. Compared with the Au NRs and Au nanospheres, broader and stronger plasmonic resonance is observed for Au@Ag NRs. Figure 4(b) also shows the absorption and emission spectra of Alq3 and DCJTB, an excellent donor-acceptor system [27]. Alq3 is the donor and DCJTB is the acceptor, and the Förster resonance energy transfer system is formed. Thus, the broad plasmonic resonance band and multiple plasmonic peaks of the Au@Ag NRs promise considerable overlap with both absorption and emission spectra of the gain medium and guarantee an effective interaction to enhance lasing.
3.2 Lasing properties based on the Au@Ag NRs-Ag film hybrid structure
To investigate the effect of the Au@Ag NRs-Ag film hybrid structure on lasing, we fabricated devices for which the gain medium is deposited on glass (device A), Ag film (device B), Au@Ag NRs (device C), and Au@Ag NRs-Ag film hybrid structures with 10 nm (device D) and 100 nm (device E) thick SiO2 spacers. Figure 5(a) shows the emission spectra of the neat gain medium of device A with different pump energies. At first, the emission spectrum is broad and the emission intensity increases slowly with increasing pump energy; when the pump energy exceeds the threshold, the emission spectrum becomes narrow and the emission intensity sharply increases. The changes in emission intensities and FWHMs are shown in the inset of Fig. 5(a), where the threshold of the neat gain medium is 32.9 µJ cm−2. Figure 5(b) –(e) show the emission spectra of gain media based on the Ag film (Device B), Au@Ag NRs island film (Device C), and Au@Ag NRs-Ag film hybrid structures with 10 nm (Device D) and 100 nm (Device E) thick SiO2 spacers. The emission spectra of Device C–E show a coherent random lasing phenomenon as indicated by the emergence of sharp spikes in the emission spectra due to the introduction of Au@Ag NRs [28–30]. Figure 5(f) shows the emission intensities with different pump energies for each device, and the lasing thresholds are shown in Table 1. The threshold of Device D (hybrid structure with 10 nm-thick SiO2) is 4.1 µJ cm−2, which is lower than that of the other devices. Therefore, compared with Au@Ag NRs or Ag film, the hybrid structure can more effectively lower the lasing threshold.
Fig. 5. Emission spectra of devices for which the gain medium is deposited on (a) glass (Device A), (b) Ag film (Device B), (c) Au@Ag NRs (Device C), and Au@Ag NRs-Ag film hybrid structures with a (d) 10 nm-thick SiO2 spacer (Device D) and (e) 100 nm-thick SiO2 spacer (Device E). Insets show the emission intensities and FWHMs of the emission spectra. (f) Emission intensities on the pump energy intensities for devices with different structures.
Table 1. Characteristics of devices for which the gain medium is deposited on glass, Ag film, Au@Ag NRs, and Au@Ag NRs-Ag film hybrid structures with 10 and 100 nm thick SiO2 spacers.
In general, regular metal nanoparticles, such as metal nanospheres or nanorods, are used to enhance lasing. In our experiment, compared with regular metal nanoparticles, the Au@Ag NRs exhibit a stronger plasmonic absorption and multiple plasmonic peaks, as shown in Fig. 4. To further determine the uniqueness of the Au@Ag NRs-Ag film hybrid structure on lasing, devices based on plasmonic hybrid structures composed of Ag film with Au nanorods (similar dimensions to the Au@Ag NRs) or Au nanospheres (diameter ∼50 nm) were prepared. The SiO2 spacer thicknesses between nanoparticles and Ag films were 10 nm, and the device structures are the same as that for the device with Au@Ag NRs-Ag film. Figure 6(a) and (b) depict the emission spectra of the devices based on the Au NRs-Ag film and on the Au NSs-Ag film hybrid structure, respectively. The corresponding emission intensities and FWHMs are shown in the insets; the lasing thresholds are 8.2 µJ cm−2 and 16.4 µJ cm−2, respectively. According to Fig. 5 and Fig. 6, compared with devices based on Au NRs-Ag film or Au NSs-Ag film, the device based on Au@Ag NRs-Ag film has the lowest lasing threshold, which is 12.5% of that of the neat gain medium. This comparison demonstrates that the hybrid structure of Au@Ag NRs and Ag film with proper distance can enhance stimulated emission to a much higher degree.
Fig. 6. Emission spectra of devices for which the gain medium is deposited on the (a) Au NRs-Ag film and (b) Au NSs-Ag film hybrid structures. Insets show the dependence of the emission intensities and FWHMs of the emission spectra on the pump intensities.
3.3 Enhanced localized electric field properties
Metal nanostructures are always used for lasing because of the enhanced localized electric field and scattering [14,31]. When the LSPR spectrum of metal nanostructures overlap well with absorption and emission spectra of the gain medium, the enhanced electric field can increase the excitation rate and quantum yield of dye molecules. Moreover, metal nanostructures can scatter the energy of emitters with greater scattering strength when they are excited; as a result, the optical path is increased and the threshold is reduced.
Firstly, for the enhanced electric field effect, the LSPR of metal nanoparticles could be directly excited, but the SPP of the metal film could not; this is because there is an inherent wave vector mismatch between the SPP and the excitation photons. However, in the Au@Ag NRs-Ag film hybrid structure, the SPP can be excited according to the diffraction effect of nanoparticles [18,32].
For investigating the plasmonic coupling effect between the LSPR of Au@Ag NRs and the SPP of Ag film, we simulated the electric field intensity distributions of hybrid structures using the FDTD method. Figure 7 shows the electric field intensity distributions of the Au@Ag NR, and the Au@Ag NR-Ag film hybrid structures with 10 and 100 nm thick SiO2 spacers; the incident wavelength of 628 nm is the wavelength of emission light. According to Fig. 7, in the region where the electric field is easily accessible by the molecules, the Au@Ag NR-Ag film hybrid structure with the 10 nm-thick SiO2 spacer exhibits the stronger electric field than others, and the random laser with the corresponding hybrid structure presents the lowest lasing threshold (Fig. 5). However, when the SiO2 spacer thickness becomes 100 nm, the corresponding electric field is similar to that of Au@Ag NR, as shown in Fig. 7(a) and (c). This suggests that plasmon coupling disappears when the distance between the Au@Ag NR and the Ag film is too large. As such, the lasing threshold of Device E is higher than that of Device D, owing to that the hybrid structure with 10 nm-thick SiO2 exhibits the stronger electric field which could affect the gain medium than that with 100 nm-thick SiO2. According to this, we identify the existence of strong plasmon coupling between LSPR and SPP when there is a suitable distance between the Au@Ag NR and the Ag film; this plasmon coupling could significantly enhance the electric field.
Fig. 7. Electric-field distribution of (a) Au@Ag NR on SiO2 without an Ag film, and the electric-field distributions of the Au@Ag NR-Ag film with (b) 10 nm and (c) 100 nm thick SiO2 spacers.
In addition, to further demonstrate the superiority and uniqueness of the Au@Ag NRs-Ag film hybrid structure, models using Au NR (similar dimension to the Au@Ag NR) or Au NS (diameter ∼50 nm) instead of Au@Ag NR in the hybrid structure were built with the SiO2 spacer thicknesses between nanoparticles and the Ag films at 10 nm. Figure 8 shows the FDTD simulation results of the field intensity distributions of the Au NR and Au nanosphere with and without the Ag film under the emission wavelength of the gain medium. According to Fig. 7 and Fig. 8, the electric field intensity of the Au@Ag NR-Ag film is higher than that of the Au NR-Ag film or Au NS-Ag film. This is attributed to the fact that the Au@Ag NRs exhibit higher electric field intensity than do the Au NRs or Au NSs (Fig. 7 and Fig. 8); this is due to that the coupling between Ag and Au, and could promote a stronger localized field than for pure Au nanostructures [33]. Meanwhile, the broader plasmonic resonance band of the Au@Ag NRs overlaps well with both absorption and emission of the gain medium, and this could guarantee an effective interaction between them. Then a stronger plasmon coupling between the Au@Ag NRs and the Ag film leads to the more intense local electric field. The remarkably enhanced localized electric field of the Au@Ag NRs-Ag film improves the light absorption, increases the excitation rate, and increases the quantum yield of dye molecules. Thus, the simulation confirms the unique local field enhancement effect of the Au@Ag NRs-Ag film.
Fig. 8. Electric-field distributions of (a) Ag NS on SiO2 without an Ag film, (b) Ag NS and Ag film with a 10 nm-thick SiO2 spacer, (c) Au NR on SiO2 without an Ag film, and (d) Au NR and Ag film with a 10 nm-thick SiO2 spacer.
3.4 Scattering properties
For the scattering effect, at an emission wavelength of 628 nm, the scattering cross-section σs of Au@Ag NR is 1.2×10−15 m2, and the σs of Au NR and Au NS are 8.7×10−16 and 2.4×10−16 m2, respectively. The scattering mean free path can be estimated via the Mie theory ls=1/ρσs, where ρ is number density of nanostructure [13,31]. Then the ls of devices based on hybrid structure composed of Ag film with Au@Ag NRs, Au NRs, or Au NSs are 18.9, 22.1, and 78.3 µm, respectively. Thus, the Au@Ag NRs have stronger scattering than the Au NRs and Au NSs. Moreover, compared with Au@Ag NRs, the Au@Ag NRs-Ag film hybrid structure has a stronger scattering effect owing to external feedback of the metal film [34–36]. Figure 5 shows that Device E (hybrid structure with a 100 nm-thick SiO2 spacer) exhibits a lower lasing threshold than Device C (independent Au@Ag NRs). As these two plasmonic nanostructures exhibit similar electric field intensity (Fig. 7(a) and (c)), compared with Device C, the lower threshold of Device E is mainly attributed to stronger scattering due to the existence of the Ag film. Therefore, compared with the hybrid structures of the Au NRs-Ag film or Au NSs-Ag film, the Au@Ag NRs-Ag film hybrid structure presents a stronger localized electrical field and scattering effect, which can enhance lasing.
4. Conclusions
In this study, a unique core-shell Au@Ag NRs-Ag film hybrid structure was developed to provide a new geometry for random laser. This plasmonic hybrid structure displays a wealth of interesting plasmonic properties caused by the plasmonic interaction between the LSPR of the Au@Ag NRs and the SPP of Ag film. The Au@Ag NRs-Ag film presents a superior plasmonic effect compared with independent Au@Ag NRs or an Ag film, which could better lower the lasing threshold. Furthermore, compared with hybrid structures composed of Ag film with Au nanospheres or Au nanorods, the device based on Au@Ag NRs-Ag film has the lowest lasing threshold, which is 12.5% of that of the neat gain medium. This is attributed to the broader and stronger plasmonic absorption of the Au@Ag NRs, and then the Au@Ag NRs-Ag film hybrid structure exhibits a stronger localized electric field and scattering effect. The results of this study could provide a new way to reduce the lasing threshold of gain media in the presence of a metal film. And this plasmonic approach has huge potential for use in optoelectronic devices.
Funding
National Natural Science Foundation of China (51807159, 61605105); Natural Science Basic Research Plan in Shaanxi Province of China (2019JM-308, 2019JQ-229); China Postdoctoral Science Foundation (2017M620456, 2019M653635); Fundamental Research Funds for the Central Universities (xjj2018232).
Disclosures
The authors declare no conflicts of interest.
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