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Abstract
In this work, we demonstrate that whispering gallery mode (WGM) resonances can be observed in small ZnO microspheres embedded with Au nanoparticles (AuNPs). In general, one can enhance the WGM resonances by decreasing air fraction in porous ZnO microsphere resonators grown by the hydrothermal method. We demonstrate that embedding AuNPs in a porous microsphere can achieve this effect. Moreover, as the size of such a hybrid microsphere resonator shrinks, the presence of AuNPs near the surface can further enhance the WGM resonances due to the plasmonic effect.
© 2017 Optical Society of America
1. Introduction
Whispering gallery mode (WGM) is a type of resonance related to a standing wave traveling around a concave surface. A good resonator should provide a strong confinement of light, i.e. high quality factor (Q) and high mode stability. Due to the constraint of the wavelength, the size of such resonator must be big enough to be considered as good resonator. Smaller resonators have more geometrical dispersion, which is manifested in unequal spectral separation between adjacent modes [1]. However, there is still a need of submicron scale whispering gallery mode resonators (WGMRs) for applications in solar cells or lasers, where both small scale and strong light confinement are required [2, 3]. There are a few theoretical works [4–6] suggesting that hyperbolic-metamaterial based resonators have the capability of confining electric fields in subwavelength scale. In such resonators, where alternating layers of dielectric and metallic materials are stacked, it is predicted that the Q factor can be higher than predicted in the electrostatic limit [4], whereas the lasing threshold is low [5]. In its simplest form, core-shell metal-dielectric structures were proposed theoretically that they would have a significant enhancement in Q factor [6].
In our previous works [7], we have successfully synthesized ZnO WGMRs by hydrothermal techniques. We also showed that [8], our hydrothermal based ZnO resonators possess a uniform porosity and such porosity has certain effects on WGM. In this work, by using similar techniques, hybrid Au-ZnO micro spheres (h-MS) were formed. The WGM established by such h-MS showed significant enhancement comparing to the porous one. Detail analyses based on X-ray photoemission (XPS), scanning electron microscope (SEM), and theoretical simulation (with effective medium theory) reveal that the embedding of Au nanoparticles (AuNPs) plays an important role in modifying the WGM behavior in h-MSs.
2. Experimental and theoretical analysis
2.1 Synthesis of Au-ZnO MS and characterization
The porous ZnO and hybrid Au-ZnO MSs were synthesized by hydrothermal techniques. Generally, the synthesis process of h-MS is similar to the one of ZnO porous MS (p-MS) [7, 8] except the use of AuNPs of 300 nm diameter (purchased from Sigma-Aldrich) as seed particles. The SEM (Nano Nova), micro-photoluminescence (μ-PL) spectroscopy using a Horiba Jobin Yvon HR-800 UV setup with a He-Cd laser source (325 nm line) and XPS (ULVAC-PHI 5000 Versaprobe spectrometer with a monochromatic Al Kα X-ray source (1486.6 eV)) were utilized to characterize the morphology, optical properties, and content.
2.2 Effective-medium theory for WGM analysis
The fraction of air and/or metal inside p-MS or h-MS is calculated via the Maxwell Garnet approximation in the effective-medium theory (EMT) with [9]
where the subscripts med and inc represent medium and inclusion, respectively, and δinc denotes the volume fraction of inclusion.
Here, we consider p-MS being the mixture of ZnO and air, whereas h-MS being the mixture of ZnO, air, and Au. In this case the inclusion inside p-MS resonators is air. The medium of h-MS is the air/ZnO mixture, while Au is considered as the inclusion. Previous studies based on MRI model adopted the concept of group index after utilizing Schiller’s asymptotic function and solving the characteristic equation [7, 8]. In the current model, instead of re-calculate these two steps to obtain Δn, we applied directly the value of εeff obtained for a given value of δinc in the characteristic equation to calculate xtheory. For each resonance mode, such optimized δinc should give us the best value of εeff that give us xtheory xexpriment. As an intrinsic property of the cavity, δinc should be a frequency-independent quantity; thus, calculated values of δinc for all resonance modes should be approximately the same. Such modification of refraction index by using the effective-medium theory is more physical than the one introduced by the group index concept. Illustration of optimizing process for p-MS of 1.87 μm is shown in Fig. 1(a) as the EMT was used to calculate the air fraction in the MS for different mode-number assignments. For example, the black curve corresponds to δinc values needed to produce peak positions that match the experiment with the assignment of polar mode number, l from 10 to 21. Only the blue curve shows a nearly constant value of air fraction, which indicates that the mode assignment of l from 12 to 23 is the most physical one. The deviation of the blue curve from the flat line observed in UV range is caused by the polariton-exciton coupling [10]. The best-fit air fraction of 17% and mode assignments derived from the blue curve were then employed to obtain calculated mode frequencies, which are in good agreement with experiment, as shown in Fig. 1(b).
Fig. 1 (a) Porosity fraction for the 1.87μm ZnO resonator associated with different mode assignments for the peak positions: mode number l from 10 to 21 (black), 11-22 (red), 12-23 (blue), and 13-24 (pink). (b) Comparison between theory and experiment at optimized air fraction of 17% for the 1.87 μm ZnO resonator with mode assignment given by the blue curve in Fig. 1(a). (c) SEM pictures of h-MSs of sizes 1.77 μm, 4.95 μm, 5.13 μm, and 6.27 μm.
2.3 Core-shell model for WGM analysis
We model the system by a core-shell structure, where the electromagnetic fields are given by
for TE mode, and the boundary conditions at r = b and r = R lead to [11]in (2) are vector spherical harmonics. Determinant of (4) as a function of k for a given b gives us the characteristic equation that is similar to the one for conventional spherical resonators [7, 8]. Here, only the principal (n = 1) radial mode was considered.3. Results and discussions
3.1 Hybrid Au-ZnO MS – morphology and characteristics
In order to study the structural property of Au-ZnO MS, we employed SEM and XPS. The SEM picture in the Fig. 1(c) shows that the MS has a similar morphology of our porous ZnO resonators considered in [7, 8] with size ranging from sub-micron to 7 μm. It also suggested that Au NPs are embedded inside the MS with no particles observed outside.
To confirm the presence of AuNPs, the core level XPS spectra of Au-ZnO MS were analyzed in Fig. 2. The specific values of all peaks are listed in Table 1. There are some interesting facts which can be deduced after analyzing spectra in Fig. 2 carefully. The weak intensity of both Au 4f5/2 and 4f7/2 peaks indicates that AuNPs are inside the ZnO matrix, similar to those observed in Au@ZnO nanocomposites [12] and Au@ZnO yolk-shell nanostructures [13]. However, there is a negative shift comparing to the bulk value of Au 4f5/2 and similar to those in Au decorated ZnO nanostructures, indicating the formation of negatively charged AuNPs [14–16]. Moreover, Figs. 2(c) and 2(d) show the decreasing of OV peak intensity as well as a positive shift of Zn 2p3/2 and O 1s (OL and OS) peaks comparing to p-MS. Therefore, one can say that, AuNPs are preferred to sit on the high-energy defect sites [13]. There could be electron transfer from ZnO to Au, leading to the increasing of charge density on AuNPs’ surface [14].
Fig. 2 XPS spectra of hybrid Au@ZnO MS (a) survey spectrum, (b) Zn 3p – Au 4f region, (c) Zn 2p3/2 region, (d) O 1s region.
Table 1. Binding energy in hybrid Au-ZnO MS
3.2 Tailoring of WGM in hybrid Au-ZnO MS
Figure 3 shows μ-PL spectra of hybrid Au-ZnO MS for sizes of 2.76 μm, 1.77 μm, and 1.28 μm. The WGM resonances in these microspheres are clearly seen and exhibit better quality (in terms of Q-factor and resonance profile) in comparison to p-MSs of similar sizes [Figs. 3(a) and 3(b)]. In Fig. 3(b), the WGM behavior of 1.77 μm h-MS is significantly improved over the 1.87 μm p-MS even though its size is smaller, thanks to the reduced air volume fraction due to the presence of AuNPs. We can also observe the secondary peaks clearly, which is less obvious in p-MS. Moreover, as the size approach to 1μm range, our h-MS still exhibits clear resonance peaks [Fig. 3(c)], while it is not the case for p-MS.
Fig. 3 WGM spectra in Au-ZnO MS of various sizes (a) 2.76 μm (b) 1.77 μm, and (c) 1.28 μm (in red). Spectra in black and purple are for porous ZnO MS.
It is easy to recognize from Figs. 3(a) and 3(b) that the air fraction in MS plays an important role in tailoring WGM characteristics. We also analyzed a 4.75μm p-MS and found the air fraction is around 24%. It is seen that MS with smaller air fraction has better WGM behavior. Table 2 presents detail properties of h-MS of various sizes and different numbers of embedded AuNPs (all with diameter of 300nm). We can see the air fraction of h-MS in general is around 10% and as the size of MS increases (and the number of AuNPs increases in proportion), the air fraction is decreased. Hence, from the air fraction view point, one can easy understand why h-MSs possess better WGM quality than p-MSs with size > 3 μm. However, this view point might not be appropriate for small-size h-MS, as the air fraction is rather high. In that case, the enhancement in WGM resonance in h-MS could be due to the plasmonic effect. Thus, it is useful to learn how NPs distribute inside the dielectric MS. Hence, we employed a core-shell model in which we assumed the h-MS composed of two parts. The core, with unknown radius b, is a mixture of air and ZnO, while the shell layer consists of air, ZnO, and AuNPs. For a given air fraction and number of AuNPs, the core radius is treated as a fitting parameter to fit the WGM peak positions, i.e. xtheory xexperiment. After fitting the experimental results for various sizes of h-MS and different number of AuNPs, we obtained a best-fit value of b ~0.5 R. From this analysis, we obtain the distribution of AuNPs as described in Fig. 4, assuming AuNPs are evenly spaced in the shell.
Table 2. Component characteristics in hybrid Au-ZnO MS
Fig. 4 (From left to right) Au NPs distribution for different sizes (1.28, 1.40, 1.69, 1.77, 2.03 and 2.76 μm).
We can see that the smaller the MS size, the closer AuNPs to the surface, where the average distance between the center of AuNPs to the MS surface, d ~160, 175, 211, 221, 254, and 345 nm respectively. It is suggested in [20] that WGM is sensitive only to the structural morphology near the MS surface, where the WGM related EM field has a localization depth given in [21, 22]. In our case, the calculated value for the principal radial mode is 160 nm - 270 nm, depending on the polar mode number considered. We hence conclude that in small size h-MS the WGM is more influenced by the plasmonic effect due to AuNPs near the surface, whereas in large size ones, the WGM enhancement is mostly due to the reduced air fraction when AuNPs are embedded, although the plasmonic effect may also help somewhat.
4. Conclusions
In summary, we demonstrate how we could obtain WGM in small ZnO MS by using hybrid Au@ZnO MS. Because the MS size is in micron scale, it is difficult to use TEM or other advance techniques to characterize its inner morphology. We show that by using the combination of XPS, PL and theoretical analysis based on effective-medium theory, we can understand how component characteristics affect the WGM behavior. In micron scale, the quality of the resonator is the main factor whereas the location of AuNPs will become more important as the resonator approach to nanoscale. Our studies provide a useful guide for synthesizing submicron WGM resonators.
Funding
Ministry of Science and Technology, Taiwan (MOST 103-2221-E-001-011-MY3 and 104-2112-M-001-009-MY2).
Acknowledgments
We thank C. H. Chien for fruitful discussions and S. H. Wu for XPS measurements.
References and links
1. A. B. Matsko, A. A. Savchenkov, D. Strekalov, V. S IIchenko, and L. Maleki, “Review of applications of whispering gallery mode resonators in photonics and nonlinear optics,” IPN Progress Report 42, 162 (2005).
2. R. B. Wehrspohn and J. Upping, “3D photonic crystals for photon management in solar cells,” J. Opt. 14(2), 024003 (2012). [CrossRef]
3. G. E. Jonsson, H. Fredriksson, R. Sellappan, and D. Chakarov, “Nanostructures for enhanced light absorption in solar energy devices,” Int. J. Photoenergy 2011, 939807 (2011).
4. C. Wu, A. Salandrino, X. Ni, and X. Zhang, “Electrodynamical light trapping using whispering gallery resonances in hyperbolic cavities,” Phys. Rev. X 4(2), 021015 (2014). [CrossRef]
5. M. Wan, P. Gu, W. Liu, Zh. Chen, and Zh. Wang, “Low threshold spaser based on deep-subwavelength spherical hyperbolic metamaterial cavities,” Appl. Phys. Lett. 110(3), 031103 (2017). [CrossRef]
6. W. Liu, B. Lei, and A. E. Miroshnichenko, “Q-factor and absorption enhancement for plasmonic anisotropic nanoparticles,” Opt. Lett. 41(15), 3563–3566 (2016). [CrossRef] [PubMed]
7. R. S. Moirangthem, P. J. Cheng, P. C. Chien, B. T. Ngo, S. W. Chang, C. H. Tien, and Y. C. Chang, “Optical cavity modes of a single crystalline zinc oxide microsphere,” Opt. Express 21(3), 3010–3020 (2013). [CrossRef] [PubMed]
8. T. H. Ngo, C. H. Chien, S. H. Wu, and Y. C. Chang, “Size and morphology dependent evolution of resonant modes in ZnO microspheres grown by hydrothermal synthesis,” Opt. Express 24(14), 16010–16015 (2016). [CrossRef] [PubMed]
9. V. A. Markel, “Introduction to the Maxwell Garnett approximation: tutorial,” J. Opt. Soc. Am. A 33(7), 1244–1256 (2016). [CrossRef] [PubMed]
10. J. R. Chen, T. C. Lu, Y. C. Wu, S. C. Lin, W. F. Hsieh, S. C. Wang, and H. Deng, “Characteristics of exciton-polaritons in ZnO-based hybrid microcavities,” Opt. Express 19(5), 4101–4112 (2011). [CrossRef] [PubMed]
11. A. Julien and P. Guillon, “Electromagnetic analysis of spherical dielectric shielded resonators,” IEEE Trans. Microw. Theory Tech. 34(6), 723–729 (1986). [CrossRef]
12. L. Sun, G. Wei, Y. Song, Zh. Liu, L. Wang, and Z. Li, “Solution-phase synthesis of Au@ZnO core-shell composites,” Mater. Lett. 60(9-10), 1291–1295 (2006). [CrossRef]
13. F. Gu, H. Chen, D. Han, and Z. Wang, “Metal-organic framework derived Au@ZnO yolk-shell nanostructures and their higly sensitive detection of acetone,” RSC Advances 6(35), 29727–29733 (2016). [CrossRef]
14. N. Gogurla, A. K. Sinha, S. Santra, S. Manna, and S. K. Ray, “Multifunctional Au-ZnO plasmonic nanostructures for enhanced UV photodetector and room temperature NO sensing devices,” Sci. Rep. 4(1), 6483 (2015). [CrossRef] [PubMed]
15. T. I. Chanu, T. Muthukumar, and P. T. Manoharan, “Fuel mediated solution combustion synthesis of ZnO supported gold clusters and nanoparticles and their catalytic activity and in vitro cytotoxicity,” Phys. Chem. Chem. Phys. 16(43), 23686–23698 (2014). [CrossRef] [PubMed]
16. P. K. Chen, G. J. Lee, S. H. Davies, S. J. Masten, R. Amutha, and J. J. Wu, “Hydrothermal synthesis of coral-like Au/ZnO catalyst and photocatalytic degradation of Orange II dye,” Mater. Res. Bull. 48(6), 2375–2382 (2013). [CrossRef]
17. R. K. Biroju, P. K. Giri, S. Dhara, K. Imakita, and M. Fujii, “Graphene-assisted controlled growth of highly aligned ZnO nanorods and nanoribbons: growth mechanism and photoluminescence properties,” ACS Appl. Mater. Interfaces 6(1), 377–387 (2014). [CrossRef] [PubMed]
18. F. J. Sheini, R. Yousefi, and K. R. Patil, “Surface characterization of Au-ZnO nanowire films,” Ceram. Int. 38(8), 6665–6670 (2012). [CrossRef]
19. L. K. Ono and B. R. Cuenya, “Formation and thermal stability of Au2O3 on gold nanoparticles: size and support effects,” J. Phys. Chem. C 112(12), 4676–4686 (2008). [CrossRef]
20. Y. Minowa, Y. Oguni, and M. Ashida, “Inner structure of ZnO microspheres fabricated via laser ablation in superfluid helium,” Opt. Express 25(9), 10449–10455 (2017). [CrossRef] [PubMed]
21. V. V. Datsyuk, “Some characteristics of resonant electromagnetic modes in a dielectric sphere,” Appl. Phys. B 54(2), 184–187 (1992). [CrossRef]
22. V. V. Datsyuk and I. A. Izmailov, “Optics of microdroplets,” Phys.- Usp. 44(10), 1061–1073 (2001). [CrossRef]
