1. Introduction

Erbium (Er³⁺)-doped fiber amplifiers (EDFAs) have been widely employed to overcome losses in long haul silica fiber transmission systems [1]. This is due to the ⁴I_13/2 → ⁴I_15/2 transition (~1.53 µm) of Er³⁺ ions, which matches one of the low loss windows of optical fibers in optical communication networks [2–4]. However, for access and home network applications, EDFAs are incompatible with miniature and integrated optical devices. Compared to EDFAs, Er³⁺-doped waveguide amplifiers (EDWAs) could afford high gain in a much smaller device size and be integrated with photonic devices based on silicon substrate [5–11]. In terms of waveguide materials and fabrication processes, polymers have attracted a great deal of attention because they exhibit many advantages over inorganic glasses or crystals, such as easy processing, permitting fabrication of devices with virtually any shape, and potential low cost [6,7]. Especially, Er³⁺-doped inorganic nanocrystals (NCs) can be dispersed into polymer matrices and used to construct Er³⁺-doped polymer-based optical waveguide amplifiers (EDPOWAs) [8–11].

To obtain high gain EDPOWAs, Er³⁺-doped inorganic NCs should not only have highly efficient 1.53 µm emission but also good dispersibility in polymer matrices. To date, researchers have developed many approaches to obtain Er³⁺-doped inorganic NCs with highly efficient 1.53 µm emission and good dispersibility. For example, van Veggel et al. used ammonium di-n-octadecyldithiophosphate as a surface modification agent to obtain 1.53 µm emission of redispersible LaF₃:Er³⁺ and LaF₃:Er³⁺,Yb³⁺ NCs [8,9]. The dispersibility of NCs depends on the nature of ligands. Zhen et al. reported oleic acid-modified LaF₃:Er³⁺,Yb³⁺ NCs with excellent dispersibility in common organic solvents and a PMMA matrix, and obtained an optical signal enhancement of 1.77 dB cm⁻¹ from the polymer waveguide [1,10]. Roy et al. synthesized oleic acid-coated NaYF₄:Er³⁺,Yb³⁺ NCs and constructed EDPOWAs with a optical gain of 4.81 dB cm⁻¹ [11]. It is known that NaYF₄ is one of the most suitable nanocrystalline hosts for Er³⁺ ion doping for photoluminescent applications. In general, when Er³⁺ ion is excited by 980 nm photon, a considerable amount of electrons are populated at the ⁴I_11/2 level, further promoted to higher excited states, and the transition from the higher excited state to the lower state gives upconversion (UC) emissions. Upconversion (UC) NCs, one kind of Ln³⁺-doped NCs, can convert low energy radiation to high energy radiation and have been found numerous applications in biological imaging, labeling, therapeutics, and solar cells [12–20]. However, the upconversion processes lead a dramatic decrease of the population of ⁴I_13/2 and reduce the emission intensity around 1.53 µm. Previous studies on fluoride glasses clearly indicated that the emission intensity around 1.53 µm could be improved by suppressing UC processes [21–25]. This can be realized by doping Ce³⁺ ions into Er³⁺-doped inorganic NCs because of the existence of the phonon-assisted energy transfer (ET) between Er³⁺ and Ce³⁺, ⁴I_11/2 → ⁴I_13/2 (Er³⁺): ²F_5/2 → ²F_7/2 (Ce³⁺). To the best of our knowledge, Ce³⁺ and Er³⁺ codoped NCs have not yet been investigated, and the effects of Ce³⁺ ion addition on the emission intensity around 1.53 µm of Er³⁺ in NCs have not been clarified.

In this work, we describe the synthesis of NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NCs coated with oleic acid, which are readily dispersible in organic solvent [26]. With the addition of a suitable amount of Ce³⁺ ions, the UV and visible UC emissions of Er³⁺ in the NCs have been reduced greatly, and the NIR emission around 1.53 µm has dramatically enhanced upon the excitation with a 980 nm laser diode. Furthermore, the influence of Ce³⁺ ions on the emissions of Er³⁺ in the NaYF₄ NCs is discussed in detail here. In addition, we constructed EDPOWAs by using NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NCs as the gain medium. The measured optical gain of the EDPOWA doped with NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NCs was enhanced by ~2dB in comparison with that doped with NaYF₄:Er³⁺,Yb³⁺ NCs.

2. Results and discussions

The characterizations of double-doped NaYF₄:2%Er³⁺,20%Yb³⁺ are summarized in Fig. 1 . From the TEM image (Fig. 1A), it is easily seen that the monodisperse NCs has a spherical shape. The samples codoped with Ce³⁺ ions at different concentrations possess a similar morphology to the double-doped ones. From the corresponding HRTEM image (Fig. 1B), the lattice fringes were indicative of the high crystallinity of these particles, and the measured lattice spacing is 0.275 nm, which corresponds to the (200) planes of α-NaYF₄ NCs. The fast Fourier transformation (FFT) pattern (Fig. 1C) reveals that the sample is cubic phase. A histogram of the particle size distribution from TEM images is given in Fig. 1D. The particle sizes range from 14 to 27 nm, the average size is determined to be approximately 20 nm. Moreover, the XRD data is shown in Fig. 1E, which reveals that the sample is pure cubic phase, all the strong peaks are consistent with the calculated pattern for cubic NaYF₄ crystal (JCPDS files No. 77-2042), and no other impurity peaks can be detected from the XRD patterns. In addition, FTIR spectrum of the as-prepared NCs was recorded, as shown in Fig. 2 . The band at around 3450 cm⁻¹ can be assigned as O–H stretching. The strong absorption peaks at 2850 and 2917 cm⁻¹ are attributable to the symmetric and asymmetric C–H stretching of the oleic acid coating, respectively. In addition, bands at 1553 and 1462 cm⁻¹ can be assigned to be the asymmetric and symmetric stretching of the carboxylate group (COO⁻) of the oleic acid coating, respectively. The results indicate that the NCs have been coated with oleic acid.

 

Fig. 1 Characterization data for double-doped NaYF₄:Er³⁺,Yb³⁺ NCs. (A) TEM image; (B) HRTEM image; (C) FFT pattern of a single nanocrystal; (D) histogram of the particle sizes obtained from TEM images of 400 NCs; (E) XRD pattern.

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Fig. 2 The FTIR spectrum of the oleic acid-coated NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NCs.

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With the excitation of a 980 nm laser diode, the UC emission spectra of the triple-doped NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NCs with different Ce³⁺ concentrations (0 mol%, 2 mol%, 4 mol%, and 10 mol%) were recorded at room temperature, as shown in Fig. 3A . The Yb³⁺ ions act as sensitizers due to their high absorption coefficient at 980 nm and transfer the absorbed energy to neighboring Er³⁺ ions. The spectrum of the double-doped NaYF₄:Er³⁺,Yb³⁺ NCs exhibits several intense UC emission peaks, which are attributed to the ⁴G_11/2 → ⁴I_15/2 (~378 nm), ²H_9/2 → ⁴I_15/2 (~411 nm), ²H_11/2 → ⁴I_15/2 (~525 nm), ⁴S_3/2 → ⁴I_15/2 (~544 nm), and ⁴F_9/2 → ⁴I_15/2 (~658 nm) transitions of Er³⁺ ions, respectively. As shown in Fig. 3A, the UC luminescence of the double-doped NaYF₄ sample (without Ce³⁺ ions) is the strongest one among the four samples. Upon doping small concentration of Ce³⁺ ions (2%) into the double-doped NaYF₄ NCs, the intensities of UV and visible UC emission were all drastically quenched. It is obvious that with the increase of Ce³⁺ concentration from 2% to 10%, the UC emission intensities of Er³⁺ are gradually reduced, and when the concentration of Ce³⁺ reaches 10%, the UC emissions of Er³⁺ are almost extinguished entirely.

 

Fig. 3 (A) UC emission spectra of the NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NCs with 0%, 2%, 4%, and 10% Ce³⁺. (B) Plot of relative UC emission intensities vs. codoped Ce3 + ion concentrations. (C) DC emission spectra of the NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NCs with 0%, 2%, 4%, and 10% Ce3 + . (D) Energy level diagrams of Yb³⁺, Er³⁺, and Ce³⁺ ions, and possible processes of populations and emissions.

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In addition, the dependence of the UC emission intensity on the doping concentration of Ce³⁺ ions in the NaYF₄ sample was investigated, as shown in Fig. 3B. It is clear that the UC emission intensities decrease obviously with adding 2% Ce³⁺ ions into the double-doped NaYF₄ NCs, and the least affected one (~544 nm) has been reduced to ~50% compared to that of the sample without Ce³⁺ ions, indicating that the energy transfer from Er³⁺ to Ce³⁺ ions is highly efficient here since several emission bands of Er³⁺ ions match well with the f-f absorptions of Ce³⁺ ions. It obviously exhibits that the quenching of red emission (~658 nm) is stronger than those of the green emissions (~525 nm and ~544 nm) by adding Ce³⁺ ions and increasing their concentration. When the Ce³⁺ concentration is 2%, the emission intensities of the green emissions peaked at 525 and 544 nm reduce by 63% and 52%, respectively, while the intensity of the red emission peaked at 658 nm decreases by 78%, compared to those of the sample without Ce³⁺ ions. The above results show that the UC emissions of Er³⁺ ions have been suppressed efficiently by codoping Ce³⁺ ions.

To investigate the effects of codoped Ce³⁺ ions on the 1.53 µm emission of Er³⁺ ions, we recorded the infrared emission spectra of the triple-doped NaYF₄ NCs with different Ce³⁺ concentrations (0 mol%, 2 mol%, 4 mol%, and 10 mol%) under 980 nm excitation, as shown in Fig. 3C. The emission of 1535 nm is assigned to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺, and the addition of Ce³⁺ ions influenced the emission intensity of 1535 nm band evidently. By adding 2% Ce³⁺ ions into the NaYF₄:Er³⁺,Yb³⁺ NCs, the integrated emission intensity of 1535 nm is about 6 times stronger than that of the sample without Ce³⁺ ions. However, when a higher concentration (~10%) of Ce³⁺ ions was used, the intensity of 1535 nm emission decreased relatively, even lower than that of the sample without Ce³⁺. It could be attribute to the occurrence of the phonon-assisted ET from Er³⁺ to Ce³⁺, ⁴I_13/2 → ⁴I_15/2: ²F_5/2 → ²F_7/2 when the concentration of Ce³⁺ ions is too high [27,28]. It is obvious that the near-infrared emission around 1535 nm of Er³⁺ ions has been drastically enhanced by codoping a certain amount (~2%) of Ce³⁺ ions into the NaYF₄:Er³⁺,Yb³⁺ NCs, which means that the performance of EDPOWAs could be improved remarkably by this approach.

The possible UC and DC mechanisms for the triple-doped NaYF₄ NCs are discussed on the basis of the energy level diagram presented in Fig. 3D. Under 980 nm excitation, the Yb³⁺ ions are excited from the ²F_7/2 level to the ²F_5/2 level and then transfer their energies to the nearby Er³⁺ ions. The upper levels ⁴G_11/2, ⁴F_7/2, and ⁴F_9/2 of Er³⁺ ions are mainly populated by the following processes: ETs between Yb³⁺ and Er³⁺ (²F_5/2 → ²F_7/2: ⁴I_15/2 → ⁴I_11/2, ⁴I_13/2 → ⁴F_9/2, and ⁴I_11/2 → ⁴F_7/2), the excited state absorption of the pump radiation from the ⁴I_11/2 and ⁴I_13/2 levels, and the cross relaxation (CR) between Er³⁺ ions, as depicted in Fig. 3D. Additionally, it can be seen from Fig. 3D that the UC emissions are all related to the population of the ⁴I_11/2 level of Er³⁺ ions. With the addition of Ce³⁺ ions, the population of the ⁴I_11/2 level decreases due to the phonon-assisted ET from Er³⁺ to Ce³⁺, ⁴I_11/2 → ⁴I_13/2: ²F_5/2 → ²F_7/2; on the other hand, the ETs from Er³⁺ to Ce³⁺ (⁴S_3/2 → ⁴F_9/2, ⁴F_9/2 → ⁴I_9/2, ⁴I_9/2 → ⁴I_11/2, and ⁴I_11/2 → ⁴I_13/2: ²F_5/2 → ²F_7/2) make the ⁴S_3/2, ⁴F_9/2, ⁴I_9/2, and ⁴I_11/2 states depopulate to the ⁴I_13/2 state. As a result, the emission intensity around 1.53 µm is enhanced and the UC processes are suppressed by the addition of Ce³⁺ ions. In addition, since the energy mismatch between ⁴F_9/2 → ⁴I_9/2 (Er³⁺) and ²F_5/2 → ²F_7/2 (Ce³⁺) is smaller than that between ⁴S_3/2 → ⁴F_9/2 (Er³⁺) and ²F_5/2 → ²F_7/2 (Ce³⁺), we deduce that the ET (⁴F_9/2 → ⁴I_9/2 (Er³⁺): ²F_5/2 → ²F_7/2 (Ce³⁺)) is more effective, which causes the red emission from the ⁴F_9/2 level quenched heavier than that of green emissions from the ²H_11/2 and ⁴S_3/2 levels, shown in Fig. 3B.

To further investigate the effects of codoped Ce³⁺ ions on the UC and near-infrared emissions of Er³⁺, we measured the lifetimes of Er³⁺ ions in the triple-doped NaYF₄ NCs with different Ce³⁺ concentrations, as shown in Fig. 4 . Each decay curve can be well fitted by using a single exponential function I(t) = I₀exp(−t/τ), where I₀ is an intensity parameter for t = 0, and τ is the excited state lifetime. Obviously, with increasing the concentration of Ce³⁺ ions, the lifetimes of ⁴S_3/2, and ⁴F_9/2 states of Er³⁺ ions in NaYF₄ NCs decrease gradually owing to the occurrence of the ETs of, ⁴S_3/2 → ⁴F_9/2 (Er³⁺): ²F_5/2 → ²F_7/2 (Ce³⁺) and ⁴F_9/2 → ⁴I_9/2 (Er³⁺): ²F_5/2 → ²F_7/2 (Ce³⁺). Additionally, the lifetime reduction of the ⁴F_9/2 level is more than that of the ⁴S_3/2 level, indicating that the ET ⁴F_9/2 → ⁴I_9/2 (Er³⁺): ²F_5/2 → ²F_7/2 (Ce³⁺) is more effective than the ET ⁴S_3/2 → ⁴F_9/2 (Er³⁺): ²F_5/2 → ²F_7/2 (Ce³⁺), which is in good agreement with the measured results of the UC emission intensities.

 

Fig. 4 Emission decay curves of Er³⁺ in the NaYF₄: Er³⁺,Yb³⁺,Ce³⁺ NCs (excited at 980 nm, monitored at 544 nm and 658 nm corresponding to the ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions, respectively).

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By dispersing NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NCs into SU-8 2005 polymer matrix, we constructed EDPOWA and measured their performances. The SU-8 2005 polymer was diluted in toluene. NaYF₄ NCs (1 wt %) were added into the above solution, and it was dissolved at room temperature by ultrasonic treatment for 20 min. Waveguides of dimensions, 4 μm height by 8 μm width were fabricated by standard photolithography and wet etching technology of a thin silicon dioxide layer based on a silicon substrate. The SU-8 2005 polymer dispersed with NCs was spin-coated on the silicon dioxide layer and pre-annealed at 90 °C for 20 min. Then, the waveguide channels were cured by the photo mask using UV light at a 365 nm for 8 s and then baked at 95 °C for 10 min. Finally, a thin PMMA-GMA was used as the top cladding. The refractive indices of the materials were measured using ellipsometry method (J. A. Woollam., Co. M2000). The refractive indices of the core and cladding were 1.578 and 1.495 at 1535 nm wavelength, respectively. Figure 5A is a SEM image of the NaYF₄ NCs dispersed SU-8 2005 polymer rectangular waveguide amplifier (without top cladding), indicating that the size of the waveguide amplifier is 4 μm high and 8 μm wide.

 

Fig. 5 (A) Experimental setup for measuring the optical gain of the waveguide amplifier; (B) SEM image of the NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NCs-doped polymer waveguide amplifier; (C) Relative gain as a function of pump power (980 nm) with 0.1 mW input signal powers (1535 nm) in an 4 μm high, 8 μm wide and 1.3 cm long NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ and NaYF₄:Er³⁺,Yb³⁺ NCs-dispersed polymer waveguide.

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Figure 5B shows the schematics of the experimental setup for the optical gain measurement. Gain measurement of the NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ and NaYF₄:Er³⁺,Yb³⁺ NCs dispersed polymer waveguide amplifier was carried out as the signal light source in the wavelength range of 1510-1590 nm (Santec TSL-210) as the signal source and a 976 nm laser diode as the pump source. Both the pump and signal sources were coupled into a single output optical fiber by a 980/1535 nm wavelength division multiplexer and together butt coupled to a single output fiber. The relative gain was determined from the ratio of the out-put signal observed on the optical spectrum analyzer (OSA, Ando AQ-6315A) when both the pump and signal beams are coupled to the polymer waveguide to the signal power without the pump.

Figure 5C shows the measured relative gain as a function of pump power in a 1.3 cm long waveguide. The relative gain gradually increases with the increase of pump power. When the pump power is 210 mW and the input signal power is 0.1 mW, the maximum gain of about 4.0 dB (3.08 dB cm⁻¹) at 1535 nm was obtained. In addition, the optical gain of the EDPOWA doped with NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NCs was enhanced by ~2dB in comparison with that doped with NaYF₄:Er³⁺,Yb³⁺ NCs. This result shows that the improved 1.53 µm emission is very important for the EDPOWA.

3. Conclusion

In summary, we synthesized NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NCs via a solvothermal method, and the NCs could be easily dispersed in organic solvents and polymer matrices. The UC and DC emission spectra of the NCs with different concentration of Ce³⁺ ions were investigated in detail. The results showed that the addition of a certain amount of Ce³⁺ ions enhanced the 1.53 µm emission of Er³⁺ by ~6 times via the phonon-assisted ET (⁴I_11/2 → ⁴I_13/2 (Er³⁺): ²F_5/2 → ²F_7/2 (Ce³⁺)) while the UC emissions of Er³⁺ were quenched effectively. Using NaYF₄:Er³⁺,Yb³⁺,Ce³⁺NCs doped SU-8 2005 polymer as gain medium, we constructed an EDPOWA and measured its performance. A relative optical gain of the EDPOWA doped with NaYF₄:Er³⁺,Yb³⁺,Ce³⁺ NCs was enhanced by ~2dB in comparison with that doped with NaYF₄:Er³⁺,Yb³⁺ NCs in 1.3 cm long waveguide. This work would provide a new strategy to improve the performance of EDPOWAs effectively.