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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 14 — Jul. 2, 2012
  • pp: 15418–15426
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Pumping-power-dependent photoluminescence angular distribution from an opal photonic crystal composed of monodisperse Eu3+/SiO2 core/shell nanospheres

Le Dac Tuyen, Jian Hung Lin, Cheng Yi Wu, Po-Tse Tai, Jau Tang, Le Quoc Minh, Hung-Chih Kan, and Chia Chen Hsu  »View Author Affiliations


Optics Express, Vol. 20, Issue 14, pp. 15418-15426 (2012)
http://dx.doi.org/10.1364/OE.20.015418


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Abstract

High quality opal photonic crystals (PhCs) were successfully fabricated by self-assembling of monodisperse Eu3+/SiO2 core/shell nanospheres. Angular resolved photoluminescence (PL) spectra of a PhC sample were measured with different pumping powers, and its PL emission strongly depended on spectroscopic position of the photonic stop band and the optical pumping power. Suppression of the PL occurred in the directions where the emission lines aligned with the center of the photonic stop band. Suppression and enhancement of the PL were observed at low- and high-pumping powers, respectively, in the directions where the emission lines were located at the edges of the photonic stop band. When pumping power exceeded 6 µJ/pulse, a super-linear dependence was found between the pumping power and PL intensity. The dramatic enhancement of PL was attributed to the amplification of spontaneous emission resulted from the creation of large population inversion and the slow group velocity of the emitted light inside the PhC. The opal PhC provided highly angular-selective quasi-monochromatic PL output, which can be useful for a variety of optical applications.

© 2012 OSA

1. Introduction

The modification of PL of light emitters has been demonstrated in a variety of PhC structures, including 1D multilayer dielectrics [5

5. K. Kuroda, T. Sawada, T. Kuroda, K. Watanabe, and K. Sakoda, “Doubly enhanced spontaneous emission due to increased photon density of states at photonic band edge frequencies,” Opt. Express 17(15), 13168–13177 (2009). [CrossRef] [PubMed]

,6

6. L. Frezza, M. Patrini, M. Liscidini, and D. Comoretto, “Directional enhancement of spontaneous emission in polymer flexible microcavities,” J. Phys. Chem. C 115(40), 19939–19946 (2011). [CrossRef]

], 1D cholesteric liquid crystals [7

7. S. Furumi, S. Yokoyama, A. Otomo, and S. Mashiko, “Electrical control of the structure and lasing in chiral photonic band-gap liquid crystals,” Appl. Phys. Lett. 82(1), 16–18 (2003). [CrossRef]

], 2D PhC slabs [8

8. M. Fujita, S. Takahashi, Y. Tanaka, T. Asano, and S. Noda, “Simultaneous inhibition and redistribution of spontaneous light emission in photonic crystals,” Science 308(5726), 1296–1298 (2005). [CrossRef] [PubMed]

], 3D woodpile PhCs [9

9. S. Takahashi, K. Suzuki, M. Okano, M. Imada, T. Nakamori, Y. Ota, K. Ishizaki, and S. Noda, “Direct creation of three-dimensional photonic crystals by a top-down approach,” Nat. Mater. 8(9), 721–725 (2009). [CrossRef] [PubMed]

,10

10. J. Li, B. Jia, G. Zhou, C. Bullen, J. Serbin, and M. Gu, “Spectral redistribution in spontaneous emission from quantum-dot-infiltrated 3D woodpile photonic crystals for telecommunications,” Adv. Mater. (Deerfield Beach Fla.) 19(20), 3276–3280 (2007). [CrossRef]

], 3D liquid crystal blue phase PhCs [11

11. W. Cao, A. Muñoz, P. Palffy-Muhoray, and B. Taheri, “Lasing in a three-dimensional photonic crystal of the liquid crystal blue phase II,” Nat. Mater. 1(2), 111–113 (2002). [CrossRef] [PubMed]

], and 3D opal and inverse opal PhCs [12

12. F. Di Stasio, L. Berti, M. Burger, F. Marabelli, S. Gardin, T. Dainese, R. Signorini, R. Bozio, and D. Comoretto, “Amplified spontaneous emission from opal photonic crystals engineered with structural defects,” Phys. Chem. Chem. Phys. 11(48), 11515–11519 (2009). [CrossRef] [PubMed]

17

17. H. Y. Lin, H. K. Fu, C. L. Cheng, Y. F. Chen, Y. S. Lin, Y. Hung, and C. Y. Mou, “Laser action in Tb(OH)3/SiO2 photonic crystals,” Opt. Express 16(21), 16697–16703 (2008). [CrossRef] [PubMed]

]. Among them 3D opal and inverse opal PhCs are the most promising structures due to their cost-effective and the ease of fabrication via self-assembly techniques. Moreover, their photonic stop bands can be easily tuned by varying the diameter of spherical colloids [1

1. C. López, “Materials aspects of photonic crystals,” Adv. Mater. (Deerfield Beach Fla.) 15(20), 1679–1704 (2003). [CrossRef]

,20

20. L. D. Tuyen, C. Y. Wu, T. K. Anh, L. Q. Minh, H. C. Kan, and C. C. Hsu, “Fabrication and optical characterization of SiO2 opal and SU-8 inverse opal photonic crystals,” J. Exp. Nanosci. 7(2), 198–204 (2012). [CrossRef]

].

Organic dyes, quantum dots, and rare-earth ions are commonly used as luminescent sources incorporated with a PhC. Among these luminescent sources, rare-earth ions are the most favorable because their emission bands derived from intrinsic 4f transitions are narrower than the bandwidth of a photonic stop band, whereas those of organic dyes and quantum dots are often much broader and results in a partial matching between the photonic stop band and emission band. Furthermore, rare-earth ions provide strong luminescence and do not exhibit photo-bleaching effect, which usually occurs to organic dyes [21

21. J. Hung, J. Castillo, and A. M. Olaizola, “Fluorescence spectra of Rhodamine 6G for high fluence excitation laser radiation,” J. Lumin. 101(4), 263–268 (2003). [CrossRef]

,22

22. C. Graf, S. Dembski, A. Hofmann, and E. Rühl, “A general method for the controlled embedding of nanoparticles in silica colloids,” Langmuir 22(13), 5604–5610 (2006). [CrossRef] [PubMed]

].

Several works have employed the infiltration method to incorporate rare-earth ions into opal or inverse opal PhCs [13

13. S. G. Romanov, A. V. Fokin, and R. M. De La Rue, “Eu3+ emission in an anisotropic photonic band gap environment,” Appl. Phys. Lett. 76(13), 1656–1658 (2000). [CrossRef]

,23

23. H. S. Yoo, J. Y. Han, S. W. Kim, D. Y. Jeon, and B. S. Bae, “Self-assembled SiO2 photonic crystal infiltrated by Ormosil:Eu(DBM)3 phen phosphor and its enhanced photoluminescence,” Opt. Express 17(5), 3732–3740 (2009). [CrossRef] [PubMed]

]. However, this method is not easy for precise control of the distribution of rare-earth ions. In addition, the infiltrated PhC structures are easily distorted after the thermal annealing process which is used for enhancing the PL output of rare-earth ions. These problems can be solved through synthesis and self-assembly of monodisperse rare-earth core/shell nanospheres. Core/shell nanospheres synthesized with silica cores covered with a layer of rare-earth doped material have been reported [24

24. H. Wang, C. K. Lin, X. M. Liu, J. Lin, and M. Yu, “Monodisperse spherical core-shell-structured phosphors obtained by functionalization of silica spheres with Y2O3:Eu3+ layers for field emission displays,” Appl. Phys. Lett. 87(18), 181907 (2005). [CrossRef]

,25

25. C. Lin, D. Kong, X. Liu, H. Wang, M. Yu, and J. Lin, “Monodisperse and core-shell-structured SiO2@YBO3:Eu3+ spherical particles: synthesis and characterization,” Inorg. Chem. 46(7), 2674–2681 (2007). [CrossRef] [PubMed]

]. Although these nanospheres exhibited good PL properties, their rough surfaces, induced by the indispensable acid-based reaction for rare-earth shell coating, made them difficult to form opals by the self- assembly process. In contrast, monodisperse and smooth surface of Tb(OH)3/SiO2 core/shell nanospheres were successfully synthesized under basic conditions and opal PhCs were obtained based on self-assembly of those nanospheres [17

17. H. Y. Lin, H. K. Fu, C. L. Cheng, Y. F. Chen, Y. S. Lin, Y. Hung, and C. Y. Mou, “Laser action in Tb(OH)3/SiO2 photonic crystals,” Opt. Express 16(21), 16697–16703 (2008). [CrossRef] [PubMed]

,26

26. Y. S. Lin, Y. Hung, H. Y. Lin, Y. H. Tseng, Y. F. Chen, and C. Y. Mou, “Photonic crystals from monodisperse Lanthanide-Hydroxide-at-Silica core/shell colloidal spheres,” Adv. Mater. (Deerfield Beach Fla.) 19(4), 577–580 (2007). [CrossRef]

]. It has been demonstrated that the Tb(OH)3/SiO2 opal PhC has the capability to generate laser emission by overlapping the emission wavelength of Tb3+ ions to the band edge of the photonic stop band of the opal PhC [17

17. H. Y. Lin, H. K. Fu, C. L. Cheng, Y. F. Chen, Y. S. Lin, Y. Hung, and C. Y. Mou, “Laser action in Tb(OH)3/SiO2 photonic crystals,” Opt. Express 16(21), 16697–16703 (2008). [CrossRef] [PubMed]

]. Europium ion (Eu3+) is a well-known red luminescent phosphor and is wildly used for display and lighting applications [24

24. H. Wang, C. K. Lin, X. M. Liu, J. Lin, and M. Yu, “Monodisperse spherical core-shell-structured phosphors obtained by functionalization of silica spheres with Y2O3:Eu3+ layers for field emission displays,” Appl. Phys. Lett. 87(18), 181907 (2005). [CrossRef]

,27

27. G. Wakefield, E. Holland, P. J. Dobson, and J. L. Hutchison, “Luminescence properties of nanocrystalline Y2O3: Eu,” Adv. Mater. (Deerfield Beach Fla.) 13(20), 1557–1560 (2001).

]. Eu3+/SiO2 core/shell nanospheres have great potential to serve as good building blocks for a high quality light-emitting PhC, whose PL properties can be manipulated via the photonic stop band effect.

Until now, most investigations are focused on PL properties of opal and inverse opal PhCs infiltrated with rare-earth ions, and only a few works have reported PL properties of opal PhCs composed of rare-earth ions/SiO2 core/shell nanospheres [17

17. H. Y. Lin, H. K. Fu, C. L. Cheng, Y. F. Chen, Y. S. Lin, Y. Hung, and C. Y. Mou, “Laser action in Tb(OH)3/SiO2 photonic crystals,” Opt. Express 16(21), 16697–16703 (2008). [CrossRef] [PubMed]

,26

26. Y. S. Lin, Y. Hung, H. Y. Lin, Y. H. Tseng, Y. F. Chen, and C. Y. Mou, “Photonic crystals from monodisperse Lanthanide-Hydroxide-at-Silica core/shell colloidal spheres,” Adv. Mater. (Deerfield Beach Fla.) 19(4), 577–580 (2007). [CrossRef]

]. Especially, no report has covered the fabrication and PL properties of opal PhCs composed of Eu3+/SiO2 core/shell nanospheres. Furthermore, most researches only focused on angular dependence of PL for a fixed pumping power and still lacks of investigation about how the pumping power affects PL angular dependence of a light emitting opal PhC. In this work, we present the successful synthesis of monodisperse Eu3+/SiO2 core/shell nanospheres and the fabrication of opal PhCs based on self-assembly of these monodisperse core/shell nanospheres. Angular resolved PL spectra of a suitable PhC were measured at different pumping powers to deduce the effects of photonic stop band and pumping power on PL properties. Suppression of PL was found as the emission wavelength of Eu3+ ions matched with the center of the photonic stop band independent of the pumping power. When the emission wavelength of Eu3+ ions was located at the edges of the photonic stop band, the PL output was suppressed during weak pumping excitation stage. As the pumping power was higher than a threshold value, the PL outputs were dramatically enhanced, i.e., much larger than those of a powder sample.

2. Experiments

Monodisperse Eu3+/SiO2 core/shell nanospheres with the particle size in the range of 200-500 nm were synthesized following the modified Stöber method [20

20. L. D. Tuyen, C. Y. Wu, T. K. Anh, L. Q. Minh, H. C. Kan, and C. C. Hsu, “Fabrication and optical characterization of SiO2 opal and SU-8 inverse opal photonic crystals,” J. Exp. Nanosci. 7(2), 198–204 (2012). [CrossRef]

,28

28. W. Stöber, A. Fink, and E. Bohn, “Controlled growth of monodisperse silica spheres in the micron size range,” J. Colloid Interface Sci. 26(1), 62–69 (1968). [CrossRef]

]. First, europium chloride hexahydrate (EuCl3.6H2O, 99.99%, Acrôs) and sodium citrate (C6H5Na3O7.2H2O, 99.0%, Panreac) were mixed with the Eu/citrate molar ratio of 1:2, and aqueous ammonia (NH4OH, 29%, Fisher Scientific) was added excess to make europium citrate complex as the seed solution. Citric acid is well-known as a protective agent to synthesize nanoparticles [29

29. G. Frens, “Controlled nucleation for the regulation of the particle size in monodisperse gold suspensions,” Nat. Phys. Sci (Lond.) 241, 20–22 (1973).

,30

30. B. Tang, J. Ge, and L. Zhuo, “The fabrication of La(OH)3 nanospheres by a controllable-hydrothermal method with citric acid as a protective agent,” Nanotechnology 15(12), 1749–1751 (2004). [CrossRef]

]. When the aqueous solution of europium chloride was mixed with citric acid, stable europium citrate acid complex was formed in the solution. Second, the seed solution, tetraethyl orthosilicate (TEOS, 98%, Acrôs), deionized water (18.2 MΩ.cm), and NH4OH were added in sequence in ethanol (C2H5OH, 99.8%, Hy Biocare Chem) and stirred at temperature 30 ± 1 οC for 4 h to obtain a white turbid colloidal suspension. Using the seed of europium citrate complexes as the core, the SiO2 shell was grown during the hydrolysis and condensation of TEOS in a mixture of NH4OH, C2H5OH, and H2O, forming the core/shell nanospheres. The diameter of the Eu3+ core can be controlled by varying the Eu/citrate molar ratio and the amount of seed solution. Similarly, the size of SiO2 shell can be controlled by the amount of TEOS. To prepare nanospheres with a core diameter of 25 nm and particle diameter of 315 nm, the following parameters were used: 10 ml of H2O; 160 µl of seed solution; 1.6 ml of TEOS; 6 ml of NH4OH; and 80 ml C2H5OH.

The nanospheres were separated from mother liquid and purified by using centrifugation (3000 rpm) with deionized water or ethanol for three times. Then, the colloidal solution was used to fabricate opal PhC by dispersing of Eu3+/SiO2 core/shell nanospheres in deionized water with a weight ratio of 2%. Opal PhCs were fabricated by self-assembly process via thermal-assisted gravity sedimentation method. A glass slide (1 x 1 cm2) was kept nearly horizontal into the bottom of a small vase filled with the core/shell colloidal solution. The small vase was put on the top of a heat plate at 50 οC for 2 days and the environmental humidity was kept at 55%. When water was slowly evaporated, the nanospheres self-assembled into opal structure on the substrate. The mechanical stability of the opal films was improved by heating at 100 οC for 30 min. Then, they were carefully removed from the substrate and the free-standing Eu3+/SiO2 core/shell nanosphere opal was further annealed for 2 h in air at 900 οC, which was achieved with a temperature raising rate of 2 οC/min from room temperature.

Transmission electron microscopy (TEM) (JEOL 2100), and field emission scanning electron microscopy (FESEM) (Hitachi S4800) were used to characterize and inspect core/shell nanospheres and the structure of the opal films, respectively. The reflection spectra of the opal films were measured using a fiber optics spectrometer (Ocean S2000) and a halogen tungsten light source.

Figure 1
Fig. 1 Schematic of angular-resolved PL measurement setup. Excitation beam was focused onto the sample by lens L1 (f = 10 cm). The PL was collected by lens L2 (f = 3 cm) and then focused to an optical fiber connected to the spectrometer. θ is the angle between the normal line of the sample and detection line.
shows the setup for measuring angle-resolved PL spectra of the opal films. The excitation light source used in the measurement was the fourth-harmonic of the single output of an optical parametric amplifier (TOPAS-C), which was pumped by the output of a Ti:sapphire femtosecond laser amplifier (Spitfire Pro amplifier). The excitation laser was with pulse duration 100 fs, repetition rate 1 kHz, and its wavelength was chosen at 395 nm to match the absorption of Eu3+ ions. The average excitation power was varied from 0.5 to 12 mW. A lens with a focal length of 10 cm was employed to focus the excitation laser on the sample with a spot size about 100 µm. The sample was mounted on a rotating stage, which allows PL signal to be measured at different angles. The angle between the excitation beam and the normal direction of the detector was fixed at 20ο to avoid the input of the detector being flooded by the excitation light during the experiment. A small lens was used to collect and focus the PL signal onto a fiber bundle connected to a MicroHR spectrometer (Jobin Yvon) equipped with a low temperature CCD detector. An aperture with 5 mm of opening diameter was put in front of the lens with a distance about 8 cm to the sample to limit the PL signal collection solid angle about 0.003 str (~1.8ο). Both reflection and PL experiments were measured in air atmosphere and at room temperature.

3. Results and discussions

Figure 2(a)
Fig. 2 (a) Transmission electron microscopy (TEM) image of Eu3+/SiO2 core/shell nanospheres. (b) Top view scanning electron microscopy (SEM) image of an Eu3+/SiO2 core/shell opal film after thermal annealing at 900 οC for 2 h.
shows a TEM image of Eu3+/SiO2 core-shell nanospheres. As shown in the TEM image, small dark cores (~25 nm diameter) contain Eu3+ ions appearing at the center of each nanosphere and each core is coated with silica shell with a smooth surface. Figure 2(b) shows a SEM top-view image of an opal PhC formed by self-assembly of Eu3+/SiO2 core/shell nanospheres after thermal annealing treatment at 900 οC for 2 h. As illustrated in the SEM image, the opal film exhibited highly ordered close-packed face-centered-cubic (fcc) structure. The thickness of the opal film was about 500 µm and its (111) plane was parallel to the substrate.

Both SEM and TEM images show that Eu3+/SiO2 core/shell nanospheres were highly monodispersed, and their narrow size distribution paved the way for fabricating high quality opal PhCs. Although the diameter of the core/shell nanospheres shrank about 8% from the original (the diameter of spheres decreased from original 315 to 290 nm) after the thermal annealing treatment process, the opal PhC structures still remained good periodicity. The thermal annealing treatment not only improved the mechanical stability of the opal thin films but also increased their PL intensity due to reduction of quenching effect caused by OH vibration [13

13. S. G. Romanov, A. V. Fokin, and R. M. De La Rue, “Eu3+ emission in an anisotropic photonic band gap environment,” Appl. Phys. Lett. 76(13), 1656–1658 (2000). [CrossRef]

,24

24. H. Wang, C. K. Lin, X. M. Liu, J. Lin, and M. Yu, “Monodisperse spherical core-shell-structured phosphors obtained by functionalization of silica spheres with Y2O3:Eu3+ layers for field emission displays,” Appl. Phys. Lett. 87(18), 181907 (2005). [CrossRef]

]. All the opal PhCs presented in this work have gone through the thermal annealing treatment.

Several opal PhCs with different diameters of core/shell nanospheres were fabricated. Figure 3
Fig. 3 Normal incident reflection spectra of opal PhCs composed of different diameters of Eu3+/SiO2 core/shell nanospheres.
shows the reflection spectra of these opal PhCs at normal incidence. The reflection peak shifted from 468 to 850 nm as the diameters of nanospheres increased from 215 to 390 nm. It represents the photonic stop band of the PhC can be tuned by varying the diameter of core/shell nanospheres. The quality of the opal PhCs can be judged by the band-width to peak wavelength ratio (Δλ/λ0) of the reflection peak, where λ0 is the wavelength of the reflection peak, and Δλ is the full width at half maximum (FWHM) of the reflection peak. The band-width to peak wavelength ratio of the opal PhCs fabricated in this work ranges from 5.5 to 6.3%, close to the theoretical value of 5% in the Γ-L direction [20

20. L. D. Tuyen, C. Y. Wu, T. K. Anh, L. Q. Minh, H. C. Kan, and C. C. Hsu, “Fabrication and optical characterization of SiO2 opal and SU-8 inverse opal photonic crystals,” J. Exp. Nanosci. 7(2), 198–204 (2012). [CrossRef]

,31

31. C. Y. Wu, N. D. Lai, and C. C. Hsu, “Rapidly self-assembling three-dimensional opal photonic crystals,” J. Korean Phys. Soc. 52(5), 1585–1588 (2008). [CrossRef]

].

To investigate the photonic stop band effect on PL, the opal PhC formed by 290 nm diameter of core/shell nanospheres was chosen, because its photonic stop band was close to the emission wavelengths of Eu3+ ions. Figure 4(a)
Fig. 4 (a) PL spectra of Eu3+/SiO2 core/shell nanosphere powders after the thermal annealing treatment. Inset: photograph of the PL emission of the sample pumped by the excitation laser at 395 nm. (b) Angular dependent specular reflection spectra of an opal PhC composed of Eu3+/SiO2 core/shell nanospheres with the diameter of 290 nm.
shows the PL spectrum of Eu3+/SiO2 core/shell nanosphere powders pumped by the excitation laser source at 395 nm. The PL spectrum mainly consist of narrow emission lines corresponding to the well-known Eu3+ emission from intra 4f transition 5D0 - 7Fj (j = 0, 1, 2, 3) [13

13. S. G. Romanov, A. V. Fokin, and R. M. De La Rue, “Eu3+ emission in an anisotropic photonic band gap environment,” Appl. Phys. Lett. 76(13), 1656–1658 (2000). [CrossRef]

,24

24. H. Wang, C. K. Lin, X. M. Liu, J. Lin, and M. Yu, “Monodisperse spherical core-shell-structured phosphors obtained by functionalization of silica spheres with Y2O3:Eu3+ layers for field emission displays,” Appl. Phys. Lett. 87(18), 181907 (2005). [CrossRef]

]. The inset shows the PL photograph of the Eu3+/SiO2 core/shell nanosphere powders exhibiting strong red luminescence of Eu3+ ions. The strongest emission peak was the hypersensitive transition 5D0 - 7F2 at 612 nm, and other peaks were at 579 nm (5D0 - 7F0), 592 nm (5D0 - 7F1), and 655 nm (5D0 - 7F3). The FWHM of the emission peak at 612 nm is 9.5 nm. Figure 4(b) shows the specular reflection spectra of the opal PhC obtained with various incident angles, ranging from 0ο to 40ο with respect to the surface normal direction. The photonic stop band blue-shifts with the increase of incident angle.

The angular-dependence of the reflection spectra in Fig. 4(b) indicates that in the setup shown in Fig. 1 by rotating the sample from θ = 0ο to 40ο, the photonic stop band sweeps through the spectral range of peaks in Fig. 4(a) for the PL signal collected by the detector. The interaction between the photonic stop band and the Eu3+ emission leads to strong angular-dependence of PL spectra. Figure 5(a)
Fig. 5 PL spectra of an opal PhC composed of Eu3+/SiO2 core/shell nanospheres with a diameter of 290 nm measured at different detected angles for excitation powers of 1 µJ/pulse (a) and 12 µJ/pulse (b). Insets zoom in PL spectra of emission peak at 592 nm. PL angular-distributions obtained at two pumping powers for the emission peaks at 612 nm (c) and 592 nm (d).
shows the PL spectra 1, 2, and 3 obtained by rotating the sample to three different angles θ = 0ο, 20ο, and 40ο, respectively. The sample was excited with a low pulse pumping power of 1 µJ/pulse. As illustrated, the over-all PL intensities vary strongly with θ i.e. the PL emission from the opal is anisotropic. Furthermore, the θ dependence varies with peak wavelength: for peak intensity at 612 nm (5D0 - 7F2), the θ dependence gives I3 > I1 > I2. However, for peak intensity at 592 nm (5D0 - 7F1) (inset of Fig. 5(a)), the trend becomes I1 > I3 > I2. Figure 5(b) shows the PL spectra obtained with a higher pulse pumping power of 12 µJ/pulse. The trend of the peak intensity at 612 nm becomes I1 >> I3 > I2, and that for the peak intensity at 592 nm (see inset of Fig. 5(b)) is I2 > I1 » I3. It is clear that angular-dependence of the PL emission also depends on the pumping power.

Figures 5(c) and 5(d) display the θ dependence of the PL emission peak intensities at 612 and 592 nm, respectively, obtained with two different pumping powers. In Fig. 5(c), a minimum occurs at 20ο in spite of the pumping power due to the fact that of the photonic stop band (see Figs. 4(a) and 4(b)) centers at 612 nm for θ = 20ο, which leads to maximum suppression of PL by the photonic stop band. For the 1 µJ/pulse low pumping power, the PL intensity increases as θ moves away from 20ο and reaches normal level at 40ο, where the photonic stop band center is out of the emission peak wavelength at 612 nm (see Fig. 4) and thus no suppression of the PL signal by the photonic stop band. This is consistent with the fact that the PL intensity detected at 40ο was equal to that of a powder sample composed of the same density of core/shell nanospheres. At low pumping power, the PL signals were mainly from spontaneous emission. The PL signals, detected in 0ο ≤ θ < 40ο, were suppressed due to the reduction of LDOS caused by the photonic stop band effect, even including the edges (0ο and 30ο directions) of the photonic stop band. Considering angles like 0ο and 30ο, in which the emission wavelength was located at the edges of their photonic stop bands, the PL intensities detected from these angles were smaller than that of 40ο for low pumping power. However, as the pumping power increased to 12 µJ/pulse, the θ dependence changed most dramatically where the 612 nm peak aligned with the edge of the stop band, i.e. θ = 0ο and θ = 30ο. The PL intensities were much larger than that of 40° at the same pumping power. This large amplification of spontaneous emission resulted from the creation of large population inversion and small group velocity of spontaneous emission light inside the PhC [5

5. K. Kuroda, T. Sawada, T. Kuroda, K. Watanabe, and K. Sakoda, “Doubly enhanced spontaneous emission due to increased photon density of states at photonic band edge frequencies,” Opt. Express 17(15), 13168–13177 (2009). [CrossRef] [PubMed]

,32

32. Y. Liu, C. Jiang, Y. Lin, and W. Xu, “Slow-light enhancement of stimulated emission of atomic systems in photonic crystals,” J. Opt. Soc. Am. B 27(3), 442–446 (2010). [CrossRef]

35

35. X. Xu, B. Cheng, and D. Zhang, “The enhancement of stimulated emission near a photonic band edge,” J. Phys. Condens. Matter 15(44), 7455–7461 (2003). [CrossRef]

]. Since the line-width of the 612 nm peak did not sharpen as it should have for the case of lasing (see Fig. 5(b)). Therefore the laser action (coherent stimulated emission) had not occur yet, The PL emission was probably still in the mode of amplified spontaneous emission stage (incoherent stimulated emission).

Similar pumping power dependence was also observed for the PL intensities at 592 nm peak wavelength. At θ = 20ο, where the photonic stop band edge aligned with PL emission peak, the PL intensity was significantly enhanced at high pumping power, and suppressed at low pumping power. At θ = 30ο a minimum occur for both high and low pumping powers due to the fact that the emission wavelength overlapped with the center of the photonic stop band (see Figs. 4(a) and 4(b)). Maximum suppression of PL occurred in this direction for the 592 nm emission. For θ = 0ο and θ = 40ο, the 592 nm emission line was outside of the stop band (see Figs. 4(a) and 4(b)), and the PL intensities detected in these directions reached normal level.

Figure 6
Fig. 6 Double logarithmic plot of PL intensity at 612 nm of the opal PhC versus the pumping power measured at different θ angles. The result of a powder sample is included for comparison.
shows how the relation between the pumping power and PL intensity at 612 nm varies with θ. For comparison, the result of a powder sample is also included. As illustrated, the PL intensity with θ = 40ο coincides with that of the powder sample, which confirms the photonic stop band was insignificant to the PL output in this direction. The PL intensities detected at θ = 20ο were always smaller than those with θ = 40ο. This is consistent with the picture of strong suppression of PL resulted from the photonic stop band. For θ = 0ο and θ = 30ο, the PL intensities were lower than those of θ = 40ο with pumping powers below 6 µJ/pulse. During this stage, the same suppression of PL occurred caused by the reduction of LDOS from the photonic stop band effect. For pumping power higher than 6 µJ/pulse, their PL intensity followed a super-linear dependence and significantly exceeded those of θ = 40ο. This kind of dramatic growth of PL output only occurred when the emission wavelength matched with the edges of photonic stop band. Small group velocity and large population inversion brought a strong interaction between radiation field and light emitters and produced high efficient PL output. This kind of highly angular-selective quasi-monochromatic PL output can be useful for a variety of applications such as light projection, display, and optical excitation and illumination, etc.

4. Conclusions

Monodisperse Eu3+/SiO2 core/shell nanospheres were successfully synthesized following the modified Stöber method. High quality opal PhCs were fabricated by self-assembling those monodisperse core/shell nanospheres, and their photonic stop bands can be tuned by varying the diameter of nanospheres. By overlapping of the photonic stop band and the emission peaks of Eu3+ ions, the PL emission from the opal PhC was strongly modified. As the emission line overlapped with the center of the photonic stop band, PL was suppressed independent of the pumping power. As the emission line was at the edges of the photonic stop band, two-step dependence was found between the pumping power and the PL intensity. At low pumping power, suppression of PL was observed. As the pumping power was over a threshold value 6 µJ/pulse, PL was dramatically increased due to large amplification of spontaneous emission contributed from the formation of large population inversion and slow group velocity of spontaneous emission light inside the PhC. The opal PhC provided highly angular-selective quasi-monochromatic PL output, which can be useful for a variety of optical applications.

Acknowledgments

The authors gratefully acknowledge the financial support from the National Science Council (NSC), Taiwan, under grant Nos. NSC 98-2112-M194-008-MY3, NSC 99-2112-M194-008-MY3, and NSC 98-2811-M194-007. L. D. Tuyen acknowledges the support of an NSC Taiwan Scholarship. L. Q. Minh and L. D. Tuyen acknowledge support from the National Foundation for Science and Technology Development (NAFOSTED), Vietnam, project grant 103.06.46.09.

References and links

1.

C. López, “Materials aspects of photonic crystals,” Adv. Mater. (Deerfield Beach Fla.) 15(20), 1679–1704 (2003). [CrossRef]

2.

K. Sakoda, Optical properties of photonic crystals, 2nd ed. (Springer, 2004).

3.

E. Yablonovitch, “Inhibited spontaneous emission in solid-state physics and electronics,” Phys. Rev. Lett. 58(20), 2059–2062 (1987). [CrossRef] [PubMed]

4.

S. John, “Strong localization of photons in certain disordered dielectric superlattices,” Phys. Rev. Lett. 58(23), 2486–2489 (1987). [CrossRef] [PubMed]

5.

K. Kuroda, T. Sawada, T. Kuroda, K. Watanabe, and K. Sakoda, “Doubly enhanced spontaneous emission due to increased photon density of states at photonic band edge frequencies,” Opt. Express 17(15), 13168–13177 (2009). [CrossRef] [PubMed]

6.

L. Frezza, M. Patrini, M. Liscidini, and D. Comoretto, “Directional enhancement of spontaneous emission in polymer flexible microcavities,” J. Phys. Chem. C 115(40), 19939–19946 (2011). [CrossRef]

7.

S. Furumi, S. Yokoyama, A. Otomo, and S. Mashiko, “Electrical control of the structure and lasing in chiral photonic band-gap liquid crystals,” Appl. Phys. Lett. 82(1), 16–18 (2003). [CrossRef]

8.

M. Fujita, S. Takahashi, Y. Tanaka, T. Asano, and S. Noda, “Simultaneous inhibition and redistribution of spontaneous light emission in photonic crystals,” Science 308(5726), 1296–1298 (2005). [CrossRef] [PubMed]

9.

S. Takahashi, K. Suzuki, M. Okano, M. Imada, T. Nakamori, Y. Ota, K. Ishizaki, and S. Noda, “Direct creation of three-dimensional photonic crystals by a top-down approach,” Nat. Mater. 8(9), 721–725 (2009). [CrossRef] [PubMed]

10.

J. Li, B. Jia, G. Zhou, C. Bullen, J. Serbin, and M. Gu, “Spectral redistribution in spontaneous emission from quantum-dot-infiltrated 3D woodpile photonic crystals for telecommunications,” Adv. Mater. (Deerfield Beach Fla.) 19(20), 3276–3280 (2007). [CrossRef]

11.

W. Cao, A. Muñoz, P. Palffy-Muhoray, and B. Taheri, “Lasing in a three-dimensional photonic crystal of the liquid crystal blue phase II,” Nat. Mater. 1(2), 111–113 (2002). [CrossRef] [PubMed]

12.

F. Di Stasio, L. Berti, M. Burger, F. Marabelli, S. Gardin, T. Dainese, R. Signorini, R. Bozio, and D. Comoretto, “Amplified spontaneous emission from opal photonic crystals engineered with structural defects,” Phys. Chem. Chem. Phys. 11(48), 11515–11519 (2009). [CrossRef] [PubMed]

13.

S. G. Romanov, A. V. Fokin, and R. M. De La Rue, “Eu3+ emission in an anisotropic photonic band gap environment,” Appl. Phys. Lett. 76(13), 1656–1658 (2000). [CrossRef]

14.

R. C. Schroden, M. Al-Daous, and A. Stein, “Self-modification of spontaneous emission by inverse opal silica photonic crystals,” Chem. Mater. 13(9), 2945–2950 (2001). [CrossRef]

15.

P. Lodahl, A. Floris Van Driel, I. S. Nikolaev, A. Irman, K. Overgaag, D. Vanmaekelbergh, and W. L. Vos, “Controlling the dynamics of spontaneous emission from quantum dots by photonic crystals,” Nature 430(7000), 654–657 (2004). [CrossRef] [PubMed]

16.

M. Aloshyna, S. Sivakumar, M. Venkataramanan, A. G. Brolo, and F. C. J. M. van Veggel, “Significant suppression of spontaneous emission in SiO2 photonic crystals made with Tb3+-doped LaF3 nanoparticles,” J. Phys. Chem. C 111(10), 4047–4051 (2007). [CrossRef]

17.

H. Y. Lin, H. K. Fu, C. L. Cheng, Y. F. Chen, Y. S. Lin, Y. Hung, and C. Y. Mou, “Laser action in Tb(OH)3/SiO2 photonic crystals,” Opt. Express 16(21), 16697–16703 (2008). [CrossRef] [PubMed]

18.

E. M. Purcell, “Spontaneous emission probabilities at radio frequencies,” Phys. Rev. 69, 681 (1946).

19.

M. Barth, A. Gruber, and F. Cichos, “Spectral and angular redistribution of photoluminescence near a photonic stop band,” Phys. Rev. B 72(8), 085129 (2005). [CrossRef]

20.

L. D. Tuyen, C. Y. Wu, T. K. Anh, L. Q. Minh, H. C. Kan, and C. C. Hsu, “Fabrication and optical characterization of SiO2 opal and SU-8 inverse opal photonic crystals,” J. Exp. Nanosci. 7(2), 198–204 (2012). [CrossRef]

21.

J. Hung, J. Castillo, and A. M. Olaizola, “Fluorescence spectra of Rhodamine 6G for high fluence excitation laser radiation,” J. Lumin. 101(4), 263–268 (2003). [CrossRef]

22.

C. Graf, S. Dembski, A. Hofmann, and E. Rühl, “A general method for the controlled embedding of nanoparticles in silica colloids,” Langmuir 22(13), 5604–5610 (2006). [CrossRef] [PubMed]

23.

H. S. Yoo, J. Y. Han, S. W. Kim, D. Y. Jeon, and B. S. Bae, “Self-assembled SiO2 photonic crystal infiltrated by Ormosil:Eu(DBM)3 phen phosphor and its enhanced photoluminescence,” Opt. Express 17(5), 3732–3740 (2009). [CrossRef] [PubMed]

24.

H. Wang, C. K. Lin, X. M. Liu, J. Lin, and M. Yu, “Monodisperse spherical core-shell-structured phosphors obtained by functionalization of silica spheres with Y2O3:Eu3+ layers for field emission displays,” Appl. Phys. Lett. 87(18), 181907 (2005). [CrossRef]

25.

C. Lin, D. Kong, X. Liu, H. Wang, M. Yu, and J. Lin, “Monodisperse and core-shell-structured SiO2@YBO3:Eu3+ spherical particles: synthesis and characterization,” Inorg. Chem. 46(7), 2674–2681 (2007). [CrossRef] [PubMed]

26.

Y. S. Lin, Y. Hung, H. Y. Lin, Y. H. Tseng, Y. F. Chen, and C. Y. Mou, “Photonic crystals from monodisperse Lanthanide-Hydroxide-at-Silica core/shell colloidal spheres,” Adv. Mater. (Deerfield Beach Fla.) 19(4), 577–580 (2007). [CrossRef]

27.

G. Wakefield, E. Holland, P. J. Dobson, and J. L. Hutchison, “Luminescence properties of nanocrystalline Y2O3: Eu,” Adv. Mater. (Deerfield Beach Fla.) 13(20), 1557–1560 (2001).

28.

W. Stöber, A. Fink, and E. Bohn, “Controlled growth of monodisperse silica spheres in the micron size range,” J. Colloid Interface Sci. 26(1), 62–69 (1968). [CrossRef]

29.

G. Frens, “Controlled nucleation for the regulation of the particle size in monodisperse gold suspensions,” Nat. Phys. Sci (Lond.) 241, 20–22 (1973).

30.

B. Tang, J. Ge, and L. Zhuo, “The fabrication of La(OH)3 nanospheres by a controllable-hydrothermal method with citric acid as a protective agent,” Nanotechnology 15(12), 1749–1751 (2004). [CrossRef]

31.

C. Y. Wu, N. D. Lai, and C. C. Hsu, “Rapidly self-assembling three-dimensional opal photonic crystals,” J. Korean Phys. Soc. 52(5), 1585–1588 (2008). [CrossRef]

32.

Y. Liu, C. Jiang, Y. Lin, and W. Xu, “Slow-light enhancement of stimulated emission of atomic systems in photonic crystals,” J. Opt. Soc. Am. B 27(3), 442–446 (2010). [CrossRef]

33.

J. P. Dowling, M. Scalora, M. J. Bloemer, and C. M. Bowden, “The photonic band edge laser: A new approach to gain enhancement,” J. Appl. Phys. 75(4), 1896–1899 (1994). [CrossRef]

34.

S. Nojima, “Enhancement of optical gain in two-dimensional photonic crystals with active lattice points,” Jpn. J. Appl. Phys. Lett. 37(Part 2, No. 5B), L565–L567 (1998). [CrossRef]

35.

X. Xu, B. Cheng, and D. Zhang, “The enhancement of stimulated emission near a photonic band edge,” J. Phys. Condens. Matter 15(44), 7455–7461 (2003). [CrossRef]

OCIS Codes
(160.5690) Materials : Rare-earth-doped materials
(250.4480) Optoelectronics : Optical amplifiers
(250.5230) Optoelectronics : Photoluminescence
(160.5298) Materials : Photonic crystals

ToC Category:
Photonic Crystals

History
Original Manuscript: May 14, 2012
Manuscript Accepted: June 15, 2012
Published: June 25, 2012

Citation
Le Dac Tuyen, Jian Hung Lin, Cheng Yi Wu, Po-Tse Tai, Jau Tang, Le Quoc Minh, Hung-Chih Kan, and Chia Chen Hsu, "Pumping-power-dependent photoluminescence angular distribution from an opal photonic crystal composed of monodisperse Eu3+/SiO2 core/shell nanospheres," Opt. Express 20, 15418-15426 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-14-15418


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References

  1. C. López, “Materials aspects of photonic crystals,” Adv. Mater. (Deerfield Beach Fla.)15(20), 1679–1704 (2003). [CrossRef]
  2. K. Sakoda, Optical properties of photonic crystals, 2nd ed. (Springer, 2004).
  3. E. Yablonovitch, “Inhibited spontaneous emission in solid-state physics and electronics,” Phys. Rev. Lett.58(20), 2059–2062 (1987). [CrossRef] [PubMed]
  4. S. John, “Strong localization of photons in certain disordered dielectric superlattices,” Phys. Rev. Lett.58(23), 2486–2489 (1987). [CrossRef] [PubMed]
  5. K. Kuroda, T. Sawada, T. Kuroda, K. Watanabe, and K. Sakoda, “Doubly enhanced spontaneous emission due to increased photon density of states at photonic band edge frequencies,” Opt. Express17(15), 13168–13177 (2009). [CrossRef] [PubMed]
  6. L. Frezza, M. Patrini, M. Liscidini, and D. Comoretto, “Directional enhancement of spontaneous emission in polymer flexible microcavities,” J. Phys. Chem. C115(40), 19939–19946 (2011). [CrossRef]
  7. S. Furumi, S. Yokoyama, A. Otomo, and S. Mashiko, “Electrical control of the structure and lasing in chiral photonic band-gap liquid crystals,” Appl. Phys. Lett.82(1), 16–18 (2003). [CrossRef]
  8. M. Fujita, S. Takahashi, Y. Tanaka, T. Asano, and S. Noda, “Simultaneous inhibition and redistribution of spontaneous light emission in photonic crystals,” Science308(5726), 1296–1298 (2005). [CrossRef] [PubMed]
  9. S. Takahashi, K. Suzuki, M. Okano, M. Imada, T. Nakamori, Y. Ota, K. Ishizaki, and S. Noda, “Direct creation of three-dimensional photonic crystals by a top-down approach,” Nat. Mater.8(9), 721–725 (2009). [CrossRef] [PubMed]
  10. J. Li, B. Jia, G. Zhou, C. Bullen, J. Serbin, and M. Gu, “Spectral redistribution in spontaneous emission from quantum-dot-infiltrated 3D woodpile photonic crystals for telecommunications,” Adv. Mater. (Deerfield Beach Fla.)19(20), 3276–3280 (2007). [CrossRef]
  11. W. Cao, A. Muñoz, P. Palffy-Muhoray, and B. Taheri, “Lasing in a three-dimensional photonic crystal of the liquid crystal blue phase II,” Nat. Mater.1(2), 111–113 (2002). [CrossRef] [PubMed]
  12. F. Di Stasio, L. Berti, M. Burger, F. Marabelli, S. Gardin, T. Dainese, R. Signorini, R. Bozio, and D. Comoretto, “Amplified spontaneous emission from opal photonic crystals engineered with structural defects,” Phys. Chem. Chem. Phys.11(48), 11515–11519 (2009). [CrossRef] [PubMed]
  13. S. G. Romanov, A. V. Fokin, and R. M. De La Rue, “Eu3+ emission in an anisotropic photonic band gap environment,” Appl. Phys. Lett.76(13), 1656–1658 (2000). [CrossRef]
  14. R. C. Schroden, M. Al-Daous, and A. Stein, “Self-modification of spontaneous emission by inverse opal silica photonic crystals,” Chem. Mater.13(9), 2945–2950 (2001). [CrossRef]
  15. P. Lodahl, A. Floris Van Driel, I. S. Nikolaev, A. Irman, K. Overgaag, D. Vanmaekelbergh, and W. L. Vos, “Controlling the dynamics of spontaneous emission from quantum dots by photonic crystals,” Nature430(7000), 654–657 (2004). [CrossRef] [PubMed]
  16. M. Aloshyna, S. Sivakumar, M. Venkataramanan, A. G. Brolo, and F. C. J. M. van Veggel, “Significant suppression of spontaneous emission in SiO2 photonic crystals made with Tb3+-doped LaF3 nanoparticles,” J. Phys. Chem. C111(10), 4047–4051 (2007). [CrossRef]
  17. H. Y. Lin, H. K. Fu, C. L. Cheng, Y. F. Chen, Y. S. Lin, Y. Hung, and C. Y. Mou, “Laser action in Tb(OH)3/SiO2 photonic crystals,” Opt. Express16(21), 16697–16703 (2008). [CrossRef] [PubMed]
  18. E. M. Purcell, “Spontaneous emission probabilities at radio frequencies,” Phys. Rev.69, 681 (1946).
  19. M. Barth, A. Gruber, and F. Cichos, “Spectral and angular redistribution of photoluminescence near a photonic stop band,” Phys. Rev. B72(8), 085129 (2005). [CrossRef]
  20. L. D. Tuyen, C. Y. Wu, T. K. Anh, L. Q. Minh, H. C. Kan, and C. C. Hsu, “Fabrication and optical characterization of SiO2 opal and SU-8 inverse opal photonic crystals,” J. Exp. Nanosci.7(2), 198–204 (2012). [CrossRef]
  21. J. Hung, J. Castillo, and A. M. Olaizola, “Fluorescence spectra of Rhodamine 6G for high fluence excitation laser radiation,” J. Lumin.101(4), 263–268 (2003). [CrossRef]
  22. C. Graf, S. Dembski, A. Hofmann, and E. Rühl, “A general method for the controlled embedding of nanoparticles in silica colloids,” Langmuir22(13), 5604–5610 (2006). [CrossRef] [PubMed]
  23. H. S. Yoo, J. Y. Han, S. W. Kim, D. Y. Jeon, and B. S. Bae, “Self-assembled SiO2 photonic crystal infiltrated by Ormosil:Eu(DBM)3 phen phosphor and its enhanced photoluminescence,” Opt. Express17(5), 3732–3740 (2009). [CrossRef] [PubMed]
  24. H. Wang, C. K. Lin, X. M. Liu, J. Lin, and M. Yu, “Monodisperse spherical core-shell-structured phosphors obtained by functionalization of silica spheres with Y2O3:Eu3+ layers for field emission displays,” Appl. Phys. Lett.87(18), 181907 (2005). [CrossRef]
  25. C. Lin, D. Kong, X. Liu, H. Wang, M. Yu, and J. Lin, “Monodisperse and core-shell-structured SiO2@YBO3:Eu3+ spherical particles: synthesis and characterization,” Inorg. Chem.46(7), 2674–2681 (2007). [CrossRef] [PubMed]
  26. Y. S. Lin, Y. Hung, H. Y. Lin, Y. H. Tseng, Y. F. Chen, and C. Y. Mou, “Photonic crystals from monodisperse Lanthanide-Hydroxide-at-Silica core/shell colloidal spheres,” Adv. Mater. (Deerfield Beach Fla.)19(4), 577–580 (2007). [CrossRef]
  27. G. Wakefield, E. Holland, P. J. Dobson, and J. L. Hutchison, “Luminescence properties of nanocrystalline Y2O3: Eu,” Adv. Mater. (Deerfield Beach Fla.)13(20), 1557–1560 (2001).
  28. W. Stöber, A. Fink, and E. Bohn, “Controlled growth of monodisperse silica spheres in the micron size range,” J. Colloid Interface Sci.26(1), 62–69 (1968). [CrossRef]
  29. G. Frens, “Controlled nucleation for the regulation of the particle size in monodisperse gold suspensions,” Nat. Phys. Sci (Lond.)241, 20–22 (1973).
  30. B. Tang, J. Ge, and L. Zhuo, “The fabrication of La(OH)3 nanospheres by a controllable-hydrothermal method with citric acid as a protective agent,” Nanotechnology15(12), 1749–1751 (2004). [CrossRef]
  31. C. Y. Wu, N. D. Lai, and C. C. Hsu, “Rapidly self-assembling three-dimensional opal photonic crystals,” J. Korean Phys. Soc.52(5), 1585–1588 (2008). [CrossRef]
  32. Y. Liu, C. Jiang, Y. Lin, and W. Xu, “Slow-light enhancement of stimulated emission of atomic systems in photonic crystals,” J. Opt. Soc. Am. B27(3), 442–446 (2010). [CrossRef]
  33. J. P. Dowling, M. Scalora, M. J. Bloemer, and C. M. Bowden, “The photonic band edge laser: A new approach to gain enhancement,” J. Appl. Phys.75(4), 1896–1899 (1994). [CrossRef]
  34. S. Nojima, “Enhancement of optical gain in two-dimensional photonic crystals with active lattice points,” Jpn. J. Appl. Phys. Lett.37(Part 2, No. 5B), L565–L567 (1998). [CrossRef]
  35. X. Xu, B. Cheng, and D. Zhang, “The enhancement of stimulated emission near a photonic band edge,” J. Phys. Condens. Matter15(44), 7455–7461 (2003). [CrossRef]

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