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We report on the enhanced random lasing from an organic gain medium in a planar waveguide, with the assistance of bristled Ag/TiO2 composite nanowires. Tris(8-hydroxyquinolinato)aluminum (Alq3) and 4-(dicyanomethylene)-2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) are used as donor-acceptor lasing dyes. Comparing to the gain medium with silver nanowires, the lower lasing threshold is achieved utilizing bristled Ag/TiO2 nanowires, and the threshold is reduced by 3.7 times than that of the neat gain medium. It is attributed to the broader localized surface plasmon resonance spectrum of Ag/TiO2 nanowires which could sufficiently overlap with both absorption and emission spectra of donor-acceptor lasing dyes, the stronger localized electric field enhancement effect and scattering effect. In addition, the unique plasmonic waveguide effect could also contribute to the enhanced lasing and lead to the lower lasing threshold. This method is expected to be a potential metal-modified technology for improving the lasing performance.
© 2016 Optical Society of America
1. Introduction
Random lasers based on plasmonic metal nanostructures, have drawn considerable attention in recent years due to their unique physical properties and abundant underlying applications [1–4]. It had been demonstrated that the metal nanoparticles (NPs) could significantly enhance the lasing properties through unique localized surface plasmon resonance (LSPR). Therefore, many researchers optimized the lasing characteristics utilizing metal nanoparticles. Dice et al. demonstrated the surface plasmons enhanced incoherent random lasing from the suspension containing Rhodamine 6G and silver nanoparticles [5]. X. Meng et al. reported on the observation of enhanced emission of coherent random lasing in gain medium doping with Ag NPs [6,7]. T. Zhai demonstrated an enhanced random laser based on gold nano-island structures with a layer of dye-doped polymer [8]. And E. Heydari reported the emission enhancement for the gold NP-based waveguided random laser [9]. In those works, it can be found that the plasmonic random lasers were always based on the regular metal nanoparticles to enhance the lasing, and those metal nanoparticles could only offer narrow LSPR spectra to match the gain medium. However, the excellent overlap of the LSPR band with both absorption and emission spectrum of the gain material is vital to achieve optimal lasing enhancement [10,11].
In general, there are large stokes shifts between absorption and emission spectra of gain media, particularly for the donor-acceptor lasing dyes, which does favor the lower lasing threshold, such as 4-(dicyanomethylene)-2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)4H-pyran (DCJTB) and tris(8-hydroxyquinolinato)aluminum (Alq3) donor-acceptor lasing dyes [12,13]. However, the narrow plasmon resonance bands of regular metal NPs (Ag or Au NPs nano-spheres) are too narrow to cover both the absorption and emission of donor-acceptor lasing dyes [5–9]. Therefore, to further optimize random lasing efficiency, the metallic nanostructures with wider LSPR spectrum covering both the absorptions and emissions of donor-acceptor lasing dyes is desired, which could provide us an ideal way to achieve low threshold random lasing.
In this letter, the bristled Ag/TiO2 composite nanowires were prepared and applied into the Alq3/DCJTB donor-acceptor lasing dyes. In comparison to the general spherical metal nanostructures, the Ag/TiO2 nanowires exhibit the stronger scattering strength, higher localized electric field enhancement and the broader plasmon resonance band, which could sufficiently overlap with both the absorption and emission spectra of the donor-acceptor gain medium. And when the fluorophores is in close proximity to metallic nanostructure surface, there is extremely high absorption loss which is detrimental to lasing. The bristled TiO2 shell on Ag nanowires could reduce the absorption and avoid the quenching effect of the dyes in close contact to the metal nanostructures [14,15]. Moreover, the unique plasmonic waveguide effect of one-dimensional metal nanostructure could promote the light propagation as an excitation-plasmon-photon radiation process with low loss [16,17]. When the incident light acts on one facet end of Ag nanowire, the SPP mode can be effectively excited and propagating towards the distal end of the nanowire, finally the propagating plasmons coupled out as photon emission at the output end by scattered effect [18, 19]. Therefore, the plasmonic waveguide effect of Ag nanowire could promote the light propagation from one end to the other end with less loss. Those extra light emitting from the distal end of the nanowire could ensure the gain medium far away from the pump region to obtain the light, the light path in gain medium is effectively improved, and the more dye molecules could be excited to the higher energy levels, then excitation efficiency can be enhanced, which could lead to the higher lasing performance. Therefore, the novel Ag/TiO2 nanowires could greatly contribute to the enhanced lasing performance since the unique plasmonic properties.
2. Sample preparation and experimental setups
2.1 Synthesis of bristled Ag/TiO2 composite nanowires
The Ag/TiO2 nanowires were synthesized as follows [20]: the silver nitrate (AgNO3) (1 mmol, 170 mg) was dissolved in 6 mL of ethylene glycol (EG), and then tetrabutyl titanate (TBT) (0.5 mmol, 170 mg) was added dropwise into this mixture. After stirring for 10 min, the mixture was loaded into a 20 mL hydrothermal synthesis reactor, the hydrothermal reaction was maintained at 240 °C for 14 h. Then the resulting solution was cooled down to room temperature. The final precipitate was collected and washed with ethanol for three times. Figure 1 shows the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Ag/TiO2 nanostructures. The composites have high aspect ratio with several micrometers length, the diameter of Ag nanowire is about 80 nm.
2.2 Fabrication of the device based on Ag/TiO2 nanowires
The devices based on Ag/TiO2 nanowires were fabricated as follows: firstly, the gain medium was prepared with that Polystyrene (PS), Alq3 and DCJTB were dissolved in chloroform solution (PS: Alq3: DCJTB = 300:100:3.5, wt%). Then the different concentrations of Ag/TiO2 nanowires, varied from 2.0 × 10−5 g/cm3 to 4.18 × 10−4 g/cm3, were doped into the polymer solution. Finally, the solution doping with Ag/TiO2 nanowires was spin-coated with 3000 rpm on the glass substrates. The device structure is shown in Fig. 2. For comparison, the devices based on pure Ag nanowires of similar dimension were also fabricated, in which the Ag nanowire was prepared by hydrothermal method [21]. The thickness of gain medium film is about 400 nm.
2.3 Characterization
The absorption and photoluminescence spectra were obtained by UV-Vis spectrophotometer (HITACHI U-3010, Japan) and Fluorescence Spectrometer (Fluoromax-4 spectrofluometer). The polymer thicknesses were measured with Ellipsometer (SE MF-1000, Korea). The devices were photopumped at normal incidence with a Nd: YAG laser (355 nm, 10 Hz repetition rate, and 5.5 ns pulse duration). An adjustable slit and a cylindrical lens were used to shape the beam into a stripe with the size of 7 mm × 1 mm. The edge emission spectra were measured by Fiber Optic Spectrometer (Ocean Optics SpectraSuite, USB2000).
3. Results and discussion
Figure 3 shows the LSPR spectrum of bristled Ag/TiO2 composite nanowires. Compared with the narrow plasmonic resonance spectrum of pure Ag nanowire, which has a plasmonic resonance peak at 390 nm arising from the transverse resonance, the obvious broaden and red-shift with a stronger plasmonic resonance centered at about 467 nm is observed for Ag/TiO2 nanowires, this phenomenon is consistent with the published reports [20,22]. The bristled TiO2 shell on Ag nanowires is considered to cause the red-shift in the plasmon absorption as the result of the change of the refractive index and dielectric constant of the medium surrounding the Ag nanowire [23]. It is found that the existence of TiO2 not only reduce the absorption and prevent the quenching caused by direct contact between Ag nanowire and gain medium, but could tune the coupling effect between gain medium and plasmonic nanowire. At the same time, the absorption and the emission spectrum of Alq3 and DCJTB are also exhibited in Fig. 3. Alq3/DCJTB is a Förster resonance energy transfer system, there is a good spectral overlap between the emission of Alq3 as a donor and the absorption of DCJTB as an acceptor, which guarantees the perfect energy transfer between Alq3 and DCJTB. According to the Fig. 3, it is noteworthy that compared with pure Ag nanowires, the broader and stronger plasmonic resonance band of Ag/TiO2 nanowires could better match with both absorption and emission spectra of Alq3/DCJTB.
Fig. 3 The LSPR spectra of Ag/TiO2 nanowires (NWs) and Ag NWs with the absorption and emission of Alq3 and DCJTB.
For comparison, the device with neat gain film was fabricated as the reference. Figure 4 depicts the emission spectra of the reference device at different pump intensities. At first, the emission intensity increases slowly with the increase in pump intensity, the broad emission spectrum is shown. With further increase in the pump intensity, the emission spectrum becomes narrow with a steep increase of the emission intensity. The rapid decrease in linewidth of emission spectra above the threshold is shown in the inset of Fig. 4, which exhibits the threshold of 30.2 μJ/cm2.
Fig. 4 Emission spectra of the device glass/PS: Alq3: DCJTB. The inset shows the dependence of the intensity and FWHM of the emission spectrum on the pump energy intensity.
In order to evaluate the contributions of Ag/TiO2 nanowires on lasing properties, the devices that gain medium doping with different concentrations of Ag/TiO2 composites, varied from 2.0 × 10−5 g/cm3 to 4.18 × 10−4 g/cm3, were prepared and investigated. Figure 5(a) shows the emission spectra of gain medium containing Ag/TiO2 nanowires with concentration of 2.97 × 10−4 g/cm3 under different pump energies. Different from emission spectra of neat gain medium shown in Fig. 4, the sharp spikes with full width at half maximum (FWHM) of about 1 nm are observed, which indicates the occurrence of coherent random lasing, caused by Ag/TiO2 nanowires [24]. Figure 5(b) presents the intensity and FWHM of stimulated emission for the gain medium with Ag/TiO2 nanowires. When the pump energy is higher than the lasing threshold, the emission intensity increases rapidly and the FWHM decreases to about 1 nm. In order to further certify the functionality of Ag/TiO2 nanowires on random lasing, related experiments associated with pure Ag nanowires of the similar dimension were carried out. The concentrations of introduced Ag nanowires were varied from 3 × 10−5 g/cm3 to 4.38 × 10−4 g/cm3. Figure 5(c) shows the emission spectra for gain medium with pure Ag nanowires, the coherent random lasing is observed, which exhibits similarities to that in Ag/TiO2 nanowire based random laser. The threshold behavior of the Ag nanowire-based random laser is shown in Fig. 5(d). Figure 5(e) illustrates the effect of concentration of Ag/TiO2 nanowires on lasing threshold. The lasing thresholds for gain medium with different concentration of Ag/TiO2 and Ag nanowires are exhibited. It shows that with the increase of Ag/TiO2 nanowires (Ag nanowires) concentration, the threshold exhibits a profound reduction at first, and then turns to increase with further increment in Ag/TiO2 nanowires (Ag nanowires) concentration. The lowest lasing threshold for device with Ag/TiO2 nanowires is 8.1μJ/cm2 at the concentration of 1.65 × 10−4 g/cm3, and that with Ag nanowires is 20.2μJ/cm2 at the concentration of 1.94 × 10−4 g/cm3, respectively. We know that the dye amount is reduced since the existence of Ag/TiO2 nanowires, therefore, the dye adsorption decreases with the increase of doping concentration of the nanocomposites. The result of Fig. 5(e) indicates that plasmonic effect of the Ag/TiO2 nanowires enhanced at the expense of the lower dye amounts, so the optimized concentration reflects the balance between those two aspects. At the same time, it also suggests that Ag/TiO2 nanowires and Ag nanowires could all reduce the lasing threshold. However, comparing to the gain medium with Ag nanowires, the device that gain medium with Ag/TiO2 nanowires has the lower lasing threshold, which is reduced by 3.7 times than that of reference device. It demonstrates that the Ag/TiO2 nanowires could enhance stimulated emission to a much higher degree than the initial pure Ag nanowires.
Fig. 5 Emission spectra of the device with (a) Ag/TiO2 nanowires, at the concertration of 2.97 × 10−4 g/cm3, (c) Ag nanowires, at the concertration is 3.44 × 10−4 g/cm3, (b) and (d) the lasing intensity and FWHM of the emission spectra on the pump energy corresponding to (a) and (c). (e) Dependence of lasing threshold on concertration of Ag/TiO2 and Ag nanowires.
As we know, the metallic nanostructures have been usually used to enhance the lasing efficiency, the two mechanisms are accepted as: (a) Enhancement of scattering effect and (b) Enhancement of localized electric field effect. Firstly, for one of the mechanism of scattering effect, it found that Ag/TiO2 nanowire has the stronger scattering strength than the pure Ag nanowire. Because the scattering cross section σsc of Ag/TiO2 nanowire is larger than that of pure Ag nanowire, in which the σsc of Ag/TiO2 nanowire is 2.68 × 10−14 m2 at the emission wavelength of 630 nm, calculated by the Finite Difference Time Domain (FDTD) method, and the σsc of Ag nanowire is 1.75 × 10−14 m2 . We know the σsc is inversely proportional to the mean free path, ls, according to the relation ls = 1/ρσsc, where ρ is number density of nanostructure, then the scattering strength is proportional to σsc [14,15]. Therefore, the Ag/TiO2 nanowire exhibits the stronger scattering strength than the Ag nanowire. The coating of TiO2 increases the scattering strength.
For another mechanism of enhanced electric field effect, the good overlap between the plasmon resonance spectrum and the absorption of lasing dyes enables stronger pump light available for gain medium, the more dye molecules are excited simultaneously, which will in turn improve the absorption and pump efficiency. And the large overlap of the plasmon resonance band with the emission spectrum of lasing dyes could increase the quantum yield of the dye molecules. Therefore, a high degree of plasmon-induced emission enhancement could be achieved if both of them participate in the enhancement. According to the Fig. 3, comparing to the Ag nanowires, the Ag/TiO2 nanowires exhibit the broader plasmonic resonance spectrum, overlapping considerably with both absorption and emission of Alq3/DCJTB donor-acceptor gain medium, which is expected to the other reason that Ag/TiO2 nanowire is superior to pure Ag nanowire for stimulated emission.
In addition, Ag nanowire could serve as a very effective waveguide for plasmon propagation. It has the advantage of both localizing the electromagnetic energy in nanoscale region and propagating light by surface plasmon polariton (SPP) effect. When the incident light excites one end of Ag nanowire, the SPP mode can be excited from end of the nanowire due to an additional wave vector provided by scatter effect to match that of the SPP mode propagating along the length of Ag nanowires. But the SPP could not be excited when the incident radiation is focused on the midsection of a smooth wire, because the wave vector of incoming photon is not matched to that of the SPP. At this time, a scattering mechanism could be needed to provide an additional wave vector to excite the SPP [16,25]. For the Ag/TiO2 composite nanowire, besides the reducing the absorption and preventing the quenching, the TiO2 on the Ag nanowire could scatter the light into propagating axial plasmon modes, the additional wave vector could be provided to match that of the SPP mode. Therefore, comparing with the Ag nanowire, the SPP could be better excited by the direct incident light for the Ag/TiO2 nanowire, the plasmon propagation effect can be more effectively achieved and promote the light propagation from one end to the other end of Ag nanowire with less loss, which may contribute to the enhancement of the lasing.
To further illustrate the plasmonic behaviors of the Ag/TiO2 composite nanowire, the electric field intensity distributions of the Ag/TiO2 and Ag nanowire are calculated using the FDTD method. In simulations, the one near cusp of the nanowires were stimulated with the incident wavelength at 355 nm and 630 nm which are wavelength of pump light and emission light, respectively. Figure 6 shows the electric field intensity distributions of the pure Ag nanowire and the Ag/TiO2 nanowire on the cross section at the incident wavelength of 355 nm and 630 nm. As a result, it can be clearly seen that the local electrical fields of both Ag nanowire and Ag/TiO2 nanowire can be greatly enhanced, but the electrical field of the Ag/TiO2 nanowire are stronger than that of Ag nanowire, which is also important to trigger lower threshold lasing.
Fig. 6 Distribution of electric field at the wavelengths of (a), (c) 355 nm, (b), (d) 630 nm near the Ag nanowire and Ag/TiO2 nanowire.
4. Conclusion
In summary, the typical bristled Ag/TiO2 composite nanowire was synthesized and introduced into the Alq3/DCJTB donor-acceptor gain medium. It was observed that Ag/TiO2 nanowires could better enhance the lasing properties and lower lasing threshold than the Ag nanowires. The lasing threshold of device based on Ag/TiO2 nanowires with suitable concentration is reduced by 3.7 times compared with that of reference device. According to the experiment and FDTD simulation, it found that the broader LSPR spectrum of bristled Ag/TiO2 nanowires which could cover both the absorption and emission spectrum of donor-acceptor lasing dye, the stronger localized field enhancement and scattering effect play the important roles to achieve the higher performance random lasing. Meanwhile, the plasmonic waveguide effect of Ag/TiO2 nanowires which could be better excited by the direct incident light may also contribute to the enhanced lasing performance. Our strategy provided here is very useful for creating highly efficient optoelectronic devices.
Funding
National Natural Science Foundation of China (61605105), Natural Science Basic Research Plan in Shaanxi Province of China (2016JQ6038), Scientific Research Program Funded by Shaanxi Provincial Education Department (16JK1085), Scientific Research Fund of Shaanxi University of Science and Technology (2016BJ-02).
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