NaYF4: Er3+, Yb3+ nanoparticles (NPs) were synthesized by using a high temperature thermal decomposition approach. Under a 980 nm laser excitation, intense emission at 1.53 μm from NaYF4: Er3+,Yb3+ NPs was obtained. NaYF4: Er3+, Yb3+ NPs-PMMA covalent-linking nanocomposite was prepared by copolymerization of the oleic acid-modified NaYF4: Er3+, Yb3+ NPs and methylmethacrylate(MMA). In this case, the doping mass concentration of NPs in the polymer matrix could be up to 1%, which is 10 times larger than previously published results. We constructed optical waveguide amplifiers with a structure of embedded waveguide using the NPs-PMMA nanocomposite as the core material. For an input signal power of 0.14 mW and a pump power of 400 mW, a relative optical gain of 7.6 dB was obtained at 1540 nm in a 15 mm-long waveguide.
© 2015 Optical Society of America
Erbium doped waveguide amplifiers (EDWAs) have attracted much attention in recent years, due to its potential application for the development of optical communication technology in the low loss telecommunication windows at 1535 nm wavelength [1–3]. In comparison with the erbium doped fiber amplifier (EDFAs), EDWAs could afford high gain in a much smaller device size and be integrated with photonic devices based on silicon substrates, such as switches, couplers and splitters . So far a lot of efforts have been made to develop the workable hosts for fabricating EDWAs, which can be inorganic materials (e.g. phosphate glasses [5,6], LiNbO3 crystal [7,8], Al2O3 , Ta2O5 ) or polymeric materials [11–16]. Initially, the erbium ions were doped into the inorganic materials as the EDWA workable hosts. However, the inorganic materials were not easy to be compatible with the silicon substrate during the device fabricating process. In contrast to inorganic hosts, polymeric materials have excellent properties, such as simple processing, high bandwidth and potentially low cost. Especially, Er3+ doped inorganic nanoparticles (NPs) can be dispersed into polymer matrices and used to construct polymer-based EDWAs [17–20]. In this NPs-doped-polymer technique, the gain of the EDWA devices was related to not only the emission efficiency of Er3+ at 1.53 μm, but also the dispersibility of NPs in polymer hosts. In order to achieve EDWAs with high gain, there are some reports to improve the emission efficiency of Er3+doped inorganic NPs at 1.53 μm. For Er3+-doped inorganic NPs, their luminescence quantum yield is generally low due to nonradiative energy losses caused by the surface defects as well as from vibrational deactivation ascribed to solvent molecules and ligands adsorbed on the NPs. To solve this problem, one effective strategy to improve the luminescence of NPs is the construction of core-shell architectures, where a shell is grown around the luminescent core with similar lattice constants. Zhen et al. synthesized the oleic acid-modified LaF3: Er,Yb-LaF3 core-shell nanocrystals and obtained a bigger optical gain for EDWA using LaF3 core-shell NPs than that using LaF3 core-only NPs. However, the shell is inert and the optical gain is merely about 3.5 dB . Zhai et al. reported a strategy to improve the intensity of the 1.53 μm fluorescent band in BaYF5:Yb3+, Er3+ NPs with the use of an active-shell (containing Yb3+) and constructed an EDWA based on BaYF5:Yb3+, Er3+ active-core–active-shell NPs with an enhanced optical gain of ~6.3 dB . Good dispersibility is also necessary for obtaining high gain EDWAs because it can increase the doping concentration of NPs in the polymer hosts. Up to now, some methods have been developed to improve the dispersibility of NPs in polymer matrix. The coating agents, such as ammonium di-n-octadecyldithiophosphate, pyridinium di-n-hexaoctyldithiophosphate, oleic acid and so on, were used to stabilize NPs against agglomeration in a polymer matrix. For examples, the surface-coated NaYF4 and LaF3 NPs with good dispersion in organic solvents have been successfully prepared for constructing EDFAs recently [19–23]. However, in all these previous works, erbium-doped NPs were just dispersed into polymer matrix by a physical doping method [17,18,24,25]. In this case, the doping concentration of NPs in polymer matrix could not be high in order to avoid aggregation of NPs. Furthermore, long time stability of EDWAs based on the physical doping method was not so good owing to the aging of NPs in polymer matrix. Therefore, it is necessary to explore new approach to disperse NPs into polymer matrix for improving the performances of EDWAs.
In this paper, we reported a new strategy to improve the dispersibility of NPs in polymer matrix, in which the NPs were linked with the polymer matrix by the covalent bands. NaYF4:Er3+,Yb3+ NPs coated with oleic acid (OA) were synthesized by using a high temperature thermal decomposition approach. Therefore, the surface of NPs was modified by the unsaturated functional groups. Then these NaYF4: Er, Yb NPs were copolymerized with methyl methacrylate (MMA) and the NPs-PMMA covalent-linking nanocomposites were synthesized. Using the NPs-PMMA covalent-linking nanocomposites as the core material, we constructed optical waveguide amplifiers with a structure of embedded waveguide. For an input signal power of 0.14 mW and a pump power of 400 mW, a relative optical gain of 7.6 dB was obtained at 1540 nm in a 15 mm-long waveguide. This strategy not only increases the inorganic NPs doping concentration in polymer matric, but also avoids the aggregation of the NPs, and consequently improves the properties of the EDWAs.
2.1 Synthesis of NaYF4: Er3+, Yb3+NPs-PMMA
All chemicals were of analytical grade and used without further purification. YCl3•6H2O (99.999%), YbCl3•6H2O (99.999%), ErCl3•6H2O (99.999%), NaOH (98%), NH4F (98%) were supplied by Shanghai Chemical Reagent Company. 1-Octade-cene (ODE, 90%) and oleic acid (OA, 90%) were supplied by Alfa Aesar. Methylmethacrylate (MMA) was supplied by Beijing Chemical Reagent Company.
In a typical procedure for the synthesis of β-NaYF4: 18%Yb, 2%Er nanoparticles, a mixture of 1mmol RECl3•6H2O (RE = 0.8Y, 0.18Yb, 0.02Er), 15 mL ODE and 6 mL OA were added to a 100 mL four-neck round-bottom flask. After 6 mL of OA and 15 mL of ODE were added, the solution was heated to 100 °C for 10 min and then to 150 °C for 30 min and then cooled to room temperature with a gentle flow of argon gas through the reaction flask. Meanwhile, a solution of NH4F (4 mmol) and NaOH (2.5 mmol) dissolved in methanol (10 mL) was added, and then the solution was then kept at 50 °C for 1 h. After methanol was evaporated, the solution was heated to 310°C in an argon atmosphere for 60 min and then naturally cooled to room temperature. The resultant solid products were precipitated by the addition of ethanol, collected by centrifugation, washed with ethanol three times, and finally redispersed in cyclohexane for further experiments. Then covalent-linking nanocomposite was synthesized by copolymerization of β-NaYF4: 18%Yb, 2%Er NPs and MMA. MMA were prepolymerized for 30 min at 60 °C in a single neck flask. Then the redispersed NPs solution was added to the flask. The mixture was muddy, but became clear as the copolymerization of NPs and MMA proceeded. The final product was transparent paste-like liquid. The concentration of NPs in covalent-linking nanocomposite was about 1wt%. When the nanocomposite was coating on the substrate of waveguide devices, a transparent solid film was obtained. From the synthesis process of covalent-linking nanocomposite, we know that the Er3+ doped NPs were used as active dopants. It means that Er3+ ions were not directly incorporated into the polymer but doped into the inorganic matrix, thus reducing the effects of environmental organic bands, such as –OH band, on the luminescence of Er3+. In our experiments, these NPs were synthesized using hydrophobic OA as capping ligands. Therefore, the hydrophobic surface can also effectively protect the NPs from the –OH group quenching. In addition, in the copolymerization process of NPs and MMA, all reagents are hydrophobic, thus resulting rare –OH groups in the final nanocomposites. We used the nanocomposites as the core material to fabricate the EDWA device and the influence of the –OH band on the device can be greatly reduced. This is an advantage of the nanocomposites used in EDWA devices. Figure 1 is the schematic image of the NaYF4: Er, Yb NPs-PMMA covalent-linking nanocomposite. The NPs are linked with the PMMA molecular chains by covalent bonds. Therefore, the NPs can be well dispersed and against the aggregation.
The size of the samples was further characterized by a JEM-2100F electron microscope at 200 kV. Figures 2(a) and Fig. 2(b) show the transmission electron microscope (TEM) image of the as synthesized NPs and the corresponding histogram of size distribution, respectively. The particle sizes range from 20 to 26 nm and the average size is determined to be approximately 24 nm. The above results show that the NaYF4: Er3+, Yb3+ samples have a good uniformity of size. The phase structure of the products were examined by X-ray diffraction (XRD) (Model Rigaku Ru-200b), using a nickel-filtered Cu Kα radiation (λ = 1.5406 Å) in the range of 10° ≤ 2θ ≤ 70°. The diffraction peaks of the sample can be indexed as a hexagonal-phase NaYF4, the locations and relative intensity of the diffraction peaks coincide well with the literature values (JCPDS NO. 16-0334).
The concentration of Er3+ in NPs has a great effect on the emssion intensity at 1.53 μm. In our experiments, the concentration of Er3+ in NPs was optimized to be about 2%. In this Er3+ doping concentration, the NPs exhibited the most intensive luminescence at 1.53μm. These results showed that we could control the clustering effects  by optimizing the concentration of Er3+ in NPs. Figure 3 shows infrared emission spectrum of NaYF4: 2%Er, 18%Yb NPs at room temperature with the excitation of a 980 nm laser diode. In the emission spectra of Er3+ and Yb3+ codoped NaYF4 nanoparticles, there are two main peaks. Since the luminescent property of Er3+ is related to the phase structure of the host, the appearance of those two emission peaks may be caused by Er3+ in different sites. In this paper, we mainly focused on the emission peaked at 1535 nm, which is assigned to the 4I13/2→4I15/2 transition of Er3+. The full width at half maximum of the emission is about 62 nm.
The NaYF4: Er3+, Yb3+ NPs-PMMA films were prepared on silicon wafers by spin coating at 3000 rpm for 30s. The polymer film was cured at 100°C for 2.5 hours. The atomic force microscopy (AFM) micrograph of the polymer film is shown in Fig. 4.Transparent films with a mean roughness of 0.9 nm were obtained. It indicated that the surface of the NaYF4: Er3+, Yb3+ NPs-PMMA film was smooth and NPs were well dispersed into PMMA.
2.2 Design and fabrication
Because the concentrations of Er3+ and Yb3+ in the NaYF4: Er3+, Yb3+ NPs-PMMA films are very high, it is very difficult to directly etch the film to form rib or rectangular waveguides. Therefore, we design an embedded waveguide to avoid this difficulty. Figure 5(a) shows the designed cross section of the embedded rib waveguide. The NaYF4: Er3+, Yb3+ NPs-PMMA was used as a core and the PMMA was used as a cladding. The refractive index of the core and upper cladding were measured using an ellipsometry method (J. A. Woollam., Co. M2000). The measured values were 1.501 and 1.477 at 1535 nm, respectively. In order to confine the optical power into the core region of the inverted-rib waveguide effectively, the device was operated under TM mode. And the TM mode field distribution of the waveguide was calculated by the beam propagation method (BPM, RSoft software), which is shown in Fig. 5(b).
The optical waveguide amplifier was fabricated by spin-coating, standard photolithography and ICP (inductively coupled plasma) etching technology. Figure 6 shows a complete fabrication process diagram. Firstly, a 5 μm-thick PMMA film as bottom cladding layer was first spin-coated onto a 2 μm thick silicon dioxide layer based on silicon substrates, and baked at 120°C for 2 h. On the cladding layer, a waveguide pattern was formed by standard photolithography and ICP etching technique using oxygen. Then the NaYF4: Er3+, Yb3+ NPs-PMMA was embedded into the grooves to form the core waveguides using spin-coating, and the device was cured at 100°C for 2.5 h. Finally, a 5 μm-thick PMMA film was spin-coated as the upper cladding and baked at 120 °C for 2 h. In comparison with the pure PMMA, the degree of polymerization and curing temperature of the NPs-PMMA covalent-linking nanocomposites were low. Lower temperature and longer curing time can make the un-reacted MMA and solvent molecules gradually evaporate. In that way, a polymer film with high quality can be obtained. If the curing temperature was high, there were some very small bubbles sometimes. Figure 7(a) is the scanning electron microscopy (SEM) micrograph of the cross-section of a groove. The groove has a width of 9 μm and a depth of 4 μm, the ridge wall is smooth and almost vertical. Figure 7(b) is the SEM micrograph of the cross-section of the waveguide after embedding the NaYF4: Er3+, Yb3+ NPs-PMMA into the groove. It indicates the groove was embedded by core nanocomposites. There was no any solubility phenomenon between the core layer and cladding.
3. Results and discussion
Figure 8 shows the schematic of the experimental setup for the optical gain measurement of the polymer waveguide amplifier. A tunable laser source (Santec TSL-210) with a wavelength range of 1510 nm to 1590 nm was used as the signal source and a 976 nm laser diode with a maximum output power of 400 mW was used as the pump source. Signal and pump light were launched into the channel waveguides by a 980/1535 nm wavelength division multiplexing (WDM) coupler. The output light from the device was collected and coupled to an optical spectrum analyzer (OSA, ANDO AQ-6315A). We measured the transmission loss of the waveguide by using a cut-back method and the measured value is about 5 dB/cm.
The relative gain was calculated using the formula ,
where and were the output signal powers in cases with and without pump light, respectively. In fact, “relative gain” represents the internal gain properties of EDWAs despite the transmission loss of the waveguide and the performance of the measurement system. It is commonly used to characterize the performance of EDWAs. In this work, we also used the term “relative gain” to characterize the performance of our EDWA and the measured gain values were exactly correct. Figure 9 shows the dependence of the output signal power on the signal wavelength when the pump power is set as 0 mW and 400 mW, respectively. The maximum relative optical gain was obtained at 1540 nm, which agreed well with the emission spectrum. The gain bandwidth is about 22 nm. Figure 10 shows the relative gain as a function of pump power for different signal wavelengths. For an input signal power of 0.14 mW, the relative gain of the waveguide amplifier increased with the increasing the pump power. When pump power was 400 mW, the maximum relative optical gain at 1540 nm was ~7.6 dB.
Then we measured the relative gain as a function of pump power for various signal power levels when the signal wavelength was fixed at 1540nm. The measured results were shown in Fig. 11.For the fixed pump power, a higher gain can be obtained when the input signal power is smaller. The stability of the EDWA was also measured. The results show that such an EDWA based on NaYF4: Er3+, Yb3+ NPs-PMMA covalent-cross-linking nanocomposites has long time stability.
In conclusion, we demonstrated an EDWA with a structure of embedded waveguide using the NaYF4: Er3+, Yb3+ NPs-PMMA covalent-linking nanocomposites as the core material. For an input signal power of 0.14 mW and a pump power of 400 mW, a relative optical gain of 7.6 dB was obtained at 1540 nm in a 15 mm-long waveguide.
This work was supported by the National Natural Science Foundation of China (Nos. 61475061, 61261130586, 61177027, 11274139, 61378004, 51072065, 61275189, 61405070 and 61077033) and the Science and Technology Development Plan of Jilin Province (No. 20140519006JH).
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