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A simple and general approach has been developed for the enhancement of random lasing based on the composite consisting of nanospheres and nanorods array. Due to the inherent nature of high refractive index, the selected nanorods act efficiently as scattering feedback centers, which can promote the formation of closed loop paths of the emission arising from nanospheres. To illustrate our working principle, the composite consisting of Tb(OH)3/SiO2 nanospheres and ZnO nanorods was chosen as an example. Quite interestingly, it is found that the random lasing behavior can be easily achieved for the composite system, while it is absent in pure Tb(OH)3/SiO2 nanospheres. The strategy demonstrated here should be very useful for the future development of coherent light emission sources and many other optoelectronic devices.
©2009 Optical Society of America
1. Introduction
In the past decade, one dimensional semiconductor nanostructures and luminescent nanoparticles have shown their unique fundamental physics and potential applications for nanoelectronic and nanophotonic devices [1–3]. Many efforts have been concentrated on their novel optical properties of lasing effect or stimulated emission in these low-dimensional nanostructures such as semiconductor powders [4], nanowires or nanorods [5,6], and nanoribbons [7]. Compared with the laser structures needing high-reflecting end mirrors, such as permanent distributed feedback [8], wavesguiding traveling arrangements [9], micro-disk, and distributed Bragg reflectors [10], the mirror-less random laser structure is relatively simple, in which the end mirrors are replaced by random scattering feedback centers. The idea of random lasing was originally proposed by Letokhov [11] and has been studied and applied widely in semiconductor nano-materials such as ZnO nanorod arrays [12], powders [4] or in organic systems [13,14]. The key factor for the occurrence of random lasing is the existence of a high-gain medium and efficient light scattering centers to provide sufficient coherent feedback. Lawandy et al. had demonstrated random lasing with microparticles suspended in laser dye [15], and Dongge Ma et al. had shown the random lasing emission from a fluorescent dye with polystyrene nanoparticles [16]. However, compared with the suspending solution or organic system, an inorganic and solid state random lasing system is expected to be more stable and convenient for practical applications. Here, we provide an alternative approach for the enhancement of random lasing behavior arising from inorganic nanospheres. We will demonstrate that by embedding nanospheres in a nanorod template, the random laser action of the emission arising from nanospheres can be easily achieved, in which the nanorods can serve as efficient scattering centers. To illustrate our working principle, the composite consisting of Tb(OH)3/SiO2 core-shell nanospheres and ZnO nanorods was chosen as an example. The main reason for our choice is that Tb(OH)3/SiO2 nanoparticles have been demonstrated to possess a strong and stable visible emission [17], and the growth of aligned ZnO nanorods has also been well established [18]. Besides, the separation between the fixed nanorods provides an excellent space for infiltrating nanoparticles. It is founded that the random lasing behavior of Tb(OH)3/SiO2 nanospheres can be effectively enhanced by the assistance of ZnO nanorods template, which is consistent with our prediction. Therefore, the composite consisting of nanoparticles and nanorods shown here offers a simple and general route for the future development of nano-laser devices and light emitting sources.
2. Experiment
The hexagonal ZnO nanorods were fabricated by the Chemical Vapor Deposition (CVD) method [19]. The high purity (99.99 %) Zn foil (2×0.5×0.2 cm3) as precursor was placed in the alumina boat at the center of a tube furnace. The reaction chamber was sealed and kept at a pressure of 200 torr throughout the experiment. Zinc acetylacetonate hydrate (Zn(C5H7O2)2) was vaporized at 130–140°C in a furnace. Ar (purity: 99.9 %) was introduced into the reaction chamber at a flow rate of 200 sccm as a carrier gas. Subsequently, the temperature of furnace was raised to 450°C and maintained at this temperature for half hour. ZnO nanorods with a diameter in the range of 300~400 nm were formed uniformly over the entire substrate.
Tb(OH)3/SiO2 core/shell nanospheres were synthesized by a one-pot method and deposited on a quartz or silicon (100) substrate by a slow evaporation method [20]. The particle size of the nanospheres ranged from 200 nm to 300 nm was examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM, JEOL JSM-6500). These nanospheres were then deposited on ZnO nanorods obtained above. For the study of random lasing behavior, the composite consisting of Tb(OH)3/SiO2 nanospheres and ZnO nanorods were optically excited by a Q-switched Nd: YAG laser (266 nm, 3–5 ns pulse, 10 Hz) focused to a beam diameter about 0.5 mm. All measurements were performed at room temperature.
3. Results and discussion
The SEM image of Tb(OH)3/SiO2 nanospheres is shown in Fig. 1(a), which exhibits a disordered distribution. However, at the first glance it may look like an ordered structure in a small scale. The corresponding emission spectra with various optical pumping intensities are shown in Fig. 1(b). These luminescence spectra reveal two main peaks at 490 nm and 545 nm corresponding to the 5D4→7F6 and 5D4→7F5 transition of Tb ions [21], respectively. With increasing the pumping intensity, several small and sharp peaks emerge with wavelength around 545 nm. They may be attributed to the random lasing behavior of light scattering among randomly distributed nanospheres, but the evidence is not very clear. Figure 2(a) shows the SEM image of ZnO nanorods. These nanorods exhibit a more pronounced random lasing effect as shown in Fig. 2(b), according to the appearance of the sharp peaks around 390 nm. This unique fine structure is consistent with the previous reports about the random lasing in ZnO nanorods [5,6]. It therefore confirms the fact that the ZnO nanorods studied in this experiment can serve as efficient scattering centers. Intuitively, one can envisage that ZnO nanorods may act as an excellent template to enhance the lasing behavior for the emission arising from the nano materials embedded inside ZnO nanorod arrays.
Fig. 1. (a) Scanning electron microscopy of Tb(OH)3/SiO2 core/shell nanospheres. (b) Photoluminescence spectra of Tb(OH)3/SiO2 core/shell nanospheres with different pumping pulse energy.
Fig. 2. (a) Scanning electron microscopy of random-growth ZnO naorods. (b) Photoluminescence spectra of ZnO nanorods with different pumping pulse energy.
Along the guideline based on the above intuitive concept, in this study, we have fabricated the composite consisting of ZnO nanorods and Tb(OH)3/SiO2 nanospheres as shown in Fig. 3(a). In addition to the advantages of efficient scattering centers played by ZnO nanorods, the luminescence emitted from ZnO nanorods with higher photon energy could be used to excite the transition of Tb ions, which will further enhance the emission from Tb(OH)3/SiO2 nanoparticles. Quite interestingly, it is found that the random lasing behavior of Tb(OH)3/SiO2 nanospheres can be greatly enhanced as evidenced by the fact that several sharp peaks were observed around 545 nm as shown in Fig. 3(b). It is worth noting that the peaks around 390 nm from the band edge emission of ZnO nanorods also exhibit the feature of random lasing effect. But, it is less pronounced than that of pure ZnO nanords because part of these UV light is now absorbed by Tb ions. A more clear comparison of the random lasing behaviors between Tb(OH)3/SiO2 nanospheres with and without ZnO nanorods template is shown in Fig. 4. We can clearly see that the luminescence spectrum of Tb(OH)3/SiO2 nanospheres with ZnO nanorods obviously reveals more sharp peaks and stronger intensity with narrower FWHM(<0.5 nm) compared with that without ZnO nanorods. With increasing the pumping intensity, as shown in Fig. 5, the peak intensity of the emission from Tb(OH)3/SiO2 nanospheres with ZnO nanorods shows a threshold pumping behavior around 70µJ/pulse. In contrast, there is no observable threshold pumping behavior for the sample without ZnO nanorods. Therefore, ZnO nanorods can actually play the role of scattering feedback centers to provide sufficient coherent feedback and contribute to the formation of closed loop path, which is essential to a random lasing system. This result is thus consistent with the prediction based on our intuitive concept.
Fig. 3. (a) Scanning electron microscopy of Tb(OH)3/SiO2 core/shell nanospheres embedded in ZnO nanorods. (b) Photoluminescence spectra of Tb(OH)3/SiO2 core/shell nanospheres embedded in ZnO nanorods with different pumping pulse energy.
Fig. 4. Comparison of photoluminescence spectra of Tb(OH)3/SiO2 core/shell nanospheres without ZnO nanorods (a) and with ZnO nanorods (b), under the same pumping energy of 110 µJ/pulse.
Fig. 5. Plot of emission peak intensity of Tb(OH)3/SiO2
core/shell nanospheres vs pumping energy. (▪ denoted nanospheres with ZnO
nanorods, 
Fig. 6. The polarization curve of light emission from Tb(OH)3/SiO2 nanospheres embedded in ZnO nanorods, the measured photon intensity (square point) was fit to polarizer rotated angle cos2(θ) (solid line).
The closed loop length L of a scattered light in a random system could serve as a ring cavity for light, which can be obtained by the following equation [22]:
where n is the effective refractive index and Δλ is the wavelength difference between the nearest neighbor of the sharp peaks. According to the above equation, the estimated closed loop length for the fundamental resonant loop is 59.4µm, which is consistent with that of the random lasing observed in ZnO nanorods having the similar structure as shown here reported previously [6]. Besides, the emission from Tb(OH)3/SiO2 nanospheres embedded in ZnO nanorods exhibits notable polarization behavior as shown in Fig. 6. The solid line represents the cos2θ rule. The polarization degree is about 10.3% which can be attributed to the result from the light scattering by ZnO nanorods due to its well aligned geometry. In contrast, the pure Tb(OH)3/SiO2 nano particles do not exhibit significant polarization. This indicates that light emission from Tb(OH)3/SiO2 nanospheres is strongly scattered by ZnO nanorods. Therefore, we further confirm the fact that ZnO nanorods can indeed contribute to the formation of closed loop path for the emission arising from Tb(OH)3/SiO2 nanospheres.
4. Conclusion
In summary, we have developed a composite consisting of nanoparticles and nanorods, which can be used to enhance the random lasing behavior of the emission arising from nanoparticles. The enhancement can be attributed to the result of the high refractive index of nanorods, which enables them form efficient scattering feedback centers and assist the formation of closed loop path for the emission arising from nanoparticles. In this study, we have successfully demonstrated our working principle based on the composite consisting of Tb(OH)3/SiO2 nanospheres and ZnO nanorods. Our strategy shown here should be very useful for the design of coherent light emission sources and many other optoelectronic devices.
Acknowledgment
This work was supported by National Science Council and Ministry of Education of the Republic of China.
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denoted nanospheres without ZnO
nanorods)