1. Introduction

There is considerable interest in photonic crystals in which the refractive index changes periodically [1,2]. A photonic band gap is formed in the photonic crystals, and the propagation of electromagnetic waves can be controlled by the photonic band gap [3]. Various scientific and engineering applications, such as optical waveguides with sharp bands, optical circuits, optical signal modulators, and so on [4–6], are expected by using the photonic band gap effect. To date, the spontaneous emission modification of luminescent species embedded in the photonic crystals, including organic dyes, semiconductors and rare-earth ions, has been widely investigated [7–9]. Inhibition and enhancement phenomenon of spontaneous emission of luminescent species were realized in the photonic crystals. Additionally, redistributed spontaneous emission spectra have been observed from quantum dots and dyes infiltrated inside the photonic crystals [10,11].

Recently, the modification of photonic band gap on upconversion (UC) emission has been demonstrated. For example, a decrease of UC emission of dopants in the range of the band gap as well as sharp enhancement at the band gap edge was obtained in opal photonic crystal [12–16]. Influence of competition between excited-state absorption and spontaneous emission from intermediate excited state on upconversion emission was observed in inverse opal photonic crystals [17]. We reported the energy transfer enhancement in the UC emission in inverse opal [18]. However, an important issue remains to be solved in the UC emission photonic crystals. In particular, the influence of upconversion emission modification of rare earths on near infrared emission has not been investigated yet in the photonic crystals. Here, the CeO₂: Er³⁺, Yb³⁺ inverse opals with various photonic band gaps were designed and prepared. The influence of upconversion emission modification of rare earths on near infrared emission was investigated in the inverse opal. The infrared emission was enhanced due to the inhibition of upconversion emission in the inverse opals, which is significant for photoluminescence.

2. Experimental

Opal templates with three dimensional ordered periodicity were prepared by a self-assembly method. Cleaned quartz substrates were vertically immersed in colloidal suspensions of mono-dispersed polystyrene (PS) microspheres with the diameter of 400 nm, 460 nm and 350 nm, and placed in an incubator at approximately 50 °C. The PS colloidal spheres were slowly self-assembled into highly ordered opal templates on the quartz substrate, driven by the capillary force of the liquid in the evaporating process. The CeO₂: 0.01Er³⁺, 0.01Yb³⁺ precursor solution was prepared by using CeO₂, Er₂O₃ and Yb₂O₃ as raw materials. The Er₂O₃ (0.0191 g) and Yb₂O₃ (0.0197 g) were dissolved into the hot concentrated nitric acid, respectively. The CeO₂ (1.7211 g) was dissolved into the solution consist of concentrated nitric acid and hydrogen peroxide with the rate of 1:1 in volume. After evaporating, dying Ce(NO₃)₃, Yb(NO₃)₃ and Er(NO₃)₃ were obtained. In the preparation of the 0.1 M CeO₂: 0.01Er³⁺, 0.01Yb³⁺ precursor solution, Ce(NO₃)₃, Yb(NO₃)₃ and Er(NO₃)₃ were dissolved in absolute ethyl alcohol. The mixture was stirred for at least 0.5 h, forming a transparent solution. The prepared precursor solutions were used to infiltrate into the voids of the opal templates. The samples were finally calcined at 950 °C for 5 h with the 40 °C/h ramp ratio to remove PS spheres.

The morphology of the inverse opals was characterized with a scanning electron microscopy (SEM) system. The X-ray diffraction (XRD) patterns of the samples were obtained with a D8 ADVANCE. The transmittance spectra were measured by HITACHIU-4100 spectrophotometer. The upconversion and near infrared luminescence spectra of the inverse opals under a 980 nm infrared laser excitation were measured with HITACHIU-F-7000 and omni-λ300 spectrophotometer, respectively.

3. Results

Figure 1(a), 1(b) and 1(c) show the SEM images of the templates made of PS microspheres with the diameter of 400, 460 and 350 nm, respectively. The light parts in the SEM images were considered as the PS microspheres, while the dark parts were the interspace among microspheres. The PS microspheres formed into a highly ordered face-centered cubic (fcc) structure with (111) plane parallel to the surface of quartz substrate. Inverse opal samples prepared by opal templates constructed with PS microspheres 400, 460 and 350 nm in diameter were denoted as IPC-1, IPC-2 and IPC-3, respectively. Figure 1(e), 1(f) and 1(g) show the SEM images of IPC-1, IPC-2 and IPC-3, respectively. The IPC-1, IPC-2 and IPC-3 samples have a long-range ordered hexagonal arrangement of inverse opal. The average center-to-center distance between the air spheres in the IPC-1, IPC-2 and IPC-3 was about 340, 410 and 280 nm, respectively, which is about 10-20% smaller than the original size of the PS template due to the shrinkage of the microspheres’ diameters during calcination. The disordered template was prepared by using two kinds of polystyrene microspheres with the diameter of 220 and 400 nm, as shown in Fig. 1(d). For comparison, the CeO₂: Er³⁺, Yb³⁺ reference sample (RS) was prepared by using disordered opal template. Completely disordered structure was observed in the RS sample, as shown in Fig. 1(h).

 

Fig. 1 The SEM image of the opals template constructed with PS microspheres with diameter of 400 nm (a), 460 nm (b), 350 nm (c), the disordered template made of mixing PS microspheres with the diameter of 220 nm and 400 nm (d), the IPC-1 sample (e), the IPC-2 sample (f), IPC-3 sample (g), and the RS sample (h).

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The structures of the inverse opal samples were also characterized. Figure 2(a) shows the XRD pattern of inverse opal on quartz substrate. The broad band range from 17° to 25° in the XRD pattern was considered as the diffraction of quartz substrate. It can be seen that the XRD pattern of inverse opal is in exact agreement with the corresponding standard cards (No. 01-0800) in Fig. 2(b). No impurity peaks were observed, implying that inverse opal is CeO₂ in pure cubic phase.

 

Fig. 2 The XRD patterns of the IPC sample (a) and the corresponding standard cards No. 01-0800 (b).

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Figure 3 shows the transmittance spectra of the IPC-1, IPC-2, IPC-3 and RS sample measured in the normal direction. From the transmittance spectra, it can be seen that the photonic band gap for the IPC-1, IPC-2 and IPC-3 were located at 545, 680 and 450 nm, respectively. However, there was no photonic band gap in the RS sample due to its disordered structure. Theoretically, the position of the photonic band gap in face centered cubic (fcc) photonic crystals can be estimated by Bragg’s law of diffraction combined with Snell’s law:

 

Fig. 3 The transmittance spectra of the IPC-1, IPC-2, IPC-3 and RS sample.

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where λ is the center of the photonic band gap. ${\text{n}}_{{\text{CeO}}_{\text{2}}}$, n_air and n_eff represent refractive index of the CeO₂ (${\text{n}}_{{\text{CeO}}_{\text{2}}}$ = 2.2), air and inverse opal, respectively. ${\text{f}}_{{\text{CeO}}_{\text{2}}}$, θ and D denote the filling factor, angle between the incident light and the normal to the inverse opal surface and the center-to-center distance of the neighboring hollow spheres, respectively. Ideally, CeO₂: Er³⁺, Yb³⁺ takes 26% of the space in the inverse opal. According to the equation above, the photonic band gaps for IPC-1, IPC-2 and IPC-3 are estimated to 555 nm, 669 nm and 457 nm, respectively.

Figure 4(a), 4(b) and 4(c) show the UC emission spectra of the IPC-1, IPC-2, IPC-3 and RS sample. The green and red UC emissions are associated with 4f-f transitions of Er³⁺. In all the emission spectra of Er³⁺ ions, green emission with the main peak centered at approximately 548 nm accompany with two peaks at approximately 527 and 561 nm was observed, which were attributed to the ²H_11/2→⁴I_15/2 (527 nm), and ⁴S_3/2→⁴I_15/2 (548 and 561 nm) transition. Additionally, red emission centered at approximately 659 nm and 680 nm was observed belonging to the transition from the ⁴F_9/2 to ⁴I_15/2. As can be seen, the photonic band gap of the IPC-1 overlaps with green the UC emission. Note that the UC emission spectra of the IPC-1 and RS sample are normalized to the UC emission band of Er³⁺ ions at 659 nm, which is situated relatively far away from the photonic band gap of the IPC-1. While the UC emission spectra of the IPC-2, IPC-3 and RS sample are normalized at 548 nm emission band, which is far away from the photonic band gap of the IPC-2, IPC-3. It is obvious that green and red UC emission in the IPC-1, IPC-2 is suppressed, respectively, in contrast to the corresponding RS sample, which is a traditional phenomenon for the modulation of photonic crystals to UC emission. However, little influence was observed in the IPC-3 because its upconversion luminescence range out of the photonic band gaps region.

 

Fig. 4 The upconversion emission spectra of the IPC-1 and RS sample (a), the IPC-2 and RS sample (b) and the IPC-3 and RS sample (c).

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In order to investigate the UC emission mechanism of the inverse opal, excitation power dependence of UC emission intensity was measured. The UC emission intensity I_up is proportional to the n-th power of the pump power as I_up∝Pⁿ pump, where n is the number of the photons involved in the UC emission process. Figure 5 shows the excitation power dependence of the green and red emission of the IPC-2 sample. At low excited power (below 1.2 W), two photons process was involved in both green and red UC emission mechanisms. Figure 6 shows a typical energy level diagram for the UC emission under a 980 nm excitation. For two photons green and red UC emission processes, Er³⁺ ions are excited to the ⁴I_11/2 state by ground state absorption (GSA) and energy transfer (ET) between Yb³⁺ and Er³⁺ ions, which subsequently decay to the ⁴I_13/2 state. The radiative transitions from ⁴I_13/2 to ⁴I_15/2 state could give rise to the near infrared emission located at 1560 nm. Additionally, following ET process, Er³⁺ ions on the ⁴I_13/2 and ⁴I_11/2 states are further excited to the ⁴F_9/2 and ⁴F_7/2 states, respectively. The transition from the ⁴F_9/2 to ⁴I_15/2 results in the red UC emission. Er³⁺ ions on the ⁴F_7/2 state decays rapidly and nonradiatiely to the next ²H_11/2 and ⁴S_3/2 level. The radiative transitions from the ²H_11/2 and ⁴S_3/2 to ⁴I_15/2 could give rise to the green UC emissions. Therefore, both the green and red emissions are two photons process at low excited power. When the excitation power exceeds 1.5W, the green and red UC emission is six photons process. One new and interesting UC emission mechanism may be occurred in the CeO₂: 0.01Er³⁺, 0.01Yb³⁺ inverse opal. Both green and red UC emission at higher pumping power is generated by photon avalanche upconversion process. The avalanche process may produce by an efficient cross-relaxation energy transfer followed by an excited state absorption mechanism. The avalanche process was shown in Fig. 6. For the green emission, the cross-relaxation process, noted by the dotted line in the Fig. 6, permits an efficient population of level ⁴I_11/2, and enhances the excited state absorption from ⁴I_11/2 to ⁴F_7/2 state. Similarly, for the red UC emission, cross-relaxation energy transfer labeled by the dash line in the Fig. 6 populates the ⁴I_13/2 state. Following by an excited state absorption leads to the population of the red emitting ⁴F_9/2 level.

 

Fig. 5 Dependence of the green and red UC emission intensity on the pump power for the IPC-2 sample.

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Fig. 6 Energy level diagrams of Yb³⁺ and Er³⁺ ions and the UC emission mechanism.

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Rare earth ions doping materials with near infrared emission is desirable for the actual device application, such as infrared laser, amplifiers for optical communication and fluorescent bimolecular labeling in bioassays. Near infrared emission properties of the inverse opals was measured. Figure 7 shows the near infrared emission spectra of the IPC-1, IPC-2, IPC-3 and RS sample. Near infrared emission band at approximately 1560 nm was observed, which was attributed to the ⁴I_13/2→⁴I_15/2 transition. Significant enhancement of the infrared emission for both IPC-1 and IPC-2 were observed compared with the RS sample and IPC-3, which may be due to the photonic band gap effects. As shown in Fig. 6, the electrons at the excited state ⁴F_9/2, ²H_11/2 and ⁴S_3/2 can relax nonradiatiely to the ⁴I_13/2 level or transit to the ground state ⁴I_15/2 through the spontaneous emission. The suppression of spontaneous emission from the ⁴F_9/2 and ⁴S_3/2/²H_11/2 to ⁴I_15/2 in the inverse opals results in enhancement of nonradiative decay from ⁴F_9/2 and ⁴S_3/2/²H_11/2 to the ⁴I_13/2. Therefore, the near infrared emission for both IPC-1 and IPC-2 were enhanced compared with the RS sample. On the other hand, the photonic band gap of the IPC-3 is situated relatively far away from both green and red emission band, which has no influence on the UC emission, as shown in Fig. 4(c). So the intensity of near infrared emission at 1560 nm is similar in both IPC-3 and RS sample.

 

Fig. 7 The infrared emission spectra of the IPC-1, IPC-2, IPC-3 and RS sample.

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4. Conclusion

The CeO₂: Er³⁺, Yb³⁺ inverse opals with different photonic band gaps were prepared. The properties and mechanism of UC emission and the infrared emission of the inverse opals samples were studied. The green or red UC emissions were suppressed in the inverse opals compared with disordered reference sample, while the near infrared emission in both IPC-1 and IPC-2 were enhanced due to the inhibition of upconversion emission in the inverse opals. Additionally, the photon avalanche upconversion process is observed in the inverse opal.