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We report the fabrication of waveguide amplifiers produced by RF-sputtering, using a PbO-GeO2 glass (PGO glass) film codoped with Er3+/Yb3+. RIB waveguides were obtained from PGO thin films using optical lithography followed by reactive ion etching process. The optical losses in the waveguide were ≈1.0 dB/cm and the maximum internal gain at 1.53 µm, with excitation at 980 nm, was 3 dB/cm. Nanostructured gold films deposited on the waveguides enhanced the Er3+ ions photoluminescence (PL) by ≈400% in the red region and ≈30% in the infrared, under 980 nm pumping. The optical gain was enhanced and reached 6.5 dB/cm. The results demonstrate that the PGO waveguides, with or without gold nanoparticles, are promising for integrated photonics.
© 2014 Optical Society of America
1. Introduction
Rare-earth (RE) doped glasses containing metallic nanoparticles (NPs) have been extensively investigated [1–14]. The studies are motivated mainly by the application of these materials in optical amplifiers, lasers and displays. It was observed that the RE ions photoluminescence (PL) as well as the glass nonlinear optical (NLO) response are increased due to the presence of the NPs. The enhanced behaviors are due to the electromagnetic field growth in the vicinity of the NPs and/or energy transfer from the excited NPs to the RE ions.
Among the photonic glasses of interest the heavy metal oxides present various advantages, such as their high refractive index (≈2.0), low cutoff phonon energies (≈800 cm−1), low optical absorption in the visible and near-infrared regions and a large NLO susceptibility [6–13]. PL and NLO enhancements were observed for tellurite and lead-germanate glasses doped with different RE ions and containing metallic NPs [6–13] as well as for other systems [14–21].
Aiming the future exploitation in integrated optics, we developed a procedure to obtain thin films of lead-germanate glasses with metallic NPs [11] and demonstrated the feasibility of obtaining RIB waveguides [22] which are attractive due to their high refractive index that enables the optical beam confinement. Radio frequency (RF) sputtering proved to be an appropriate technique to obtain films containing metallic NPs, since it is possible to achieve control and reproducibility of concentration and size distribution of the NPs by adjusting the RF power applied to the targets, the gas work pressure and the deposition time.
Although recent works reported waveguides based on high refractive index materials [23–25] for near-infrared optical amplifiers based in the 4I13/2→4I15/2 transition of erbium (Er3+) ions, studies concerning the influence of metallic NPs on the gain characteristics of Er3+ doped waveguides were not reported. Recent studies [26, 27] are mainly related to spectroscopic properties of RE doped thin films containing metallic NPs.
This work presents a study of RIB-waveguides based on lead-germanate amorphous thin films co-doped with Er3+ and Yb3+. PL and optical gain were characterized in the waveguides, and the influence of gold NPs deposited on the waveguides is presented for the first time.
2. Experimental procedures
Amorphous thin films were produced by RF-sputtering. Two kinds of targets were sputtered simultaneously: a Yb2O3-Er2O3 target and a glass target with composition (in wt%): 60PbO-40GeO2 – the PGO target. The oxide powders were mixed and then submitted to 8 tons uniaxial pressure, followed by sinterization at 750 °C for 10 h. Three Yb2O3-Er2O3 targets composed by Er2O3 pellets (diameter: 1.0 cm) positioned on Yb2O3 targets (diameter: 2.0 in) were used to produce films co-doped with Er3+ and Yb3+.
The RE doped PGO films were deposited on a silica layer formed on previously oxidized silicon wafers that were placed at 15 cm from the targets. Before the PGO film deposition, the base pressure in the chamber was 1 x 10−5 Torr to minimize the presence of contaminants. Argon plasma at 5 mTorr was sputtered during 12 h. The RF power at 13.56 MHz was set to 60 W for the glass target and 20 W for the Yb2O3-Er2O3 target. After the sputtering process, the films were heat-treated under different temperatures and time intervals in order to become transparent. Films with high adherence to the substrates and high mechanical strength were obtained.
Conventional optical lithographic processing, followed by Reactive Ion Etching (RIE), was performed to obtain RIB-waveguides. The PGO film was covered by a positive photoresist (AZ1518) sensitive to ultraviolet (UV) light. After deposition, the photoresist was pre-baked at 105 °C for 90 s, in order to evaporate the solvent. A mask with the lines-pattern was aligned with the wafer for UV exposure. The photoresist regions that were UV exposed were removed by a developer solution (MIF300). For etching, the samples were submitted to a plasma formed by argon and hydrogen, with flowing rates of 33 and 41.7 sccm, respectively. The chamber pressure was set to 50 mTorr, and the RF power applied was 100 W during 5 min. The core thickness and the rib height for the waveguides obtained in the end of the process were 1.6 µm and 70 nm, respectively.
Some RIB-waveguides were covered by layers of gold NPs obtained by RF sputtering with 5–7 W applied to a gold target using 5 mTorr of argon during 15 min. The films’ thicknesses were measured with a profilometer. Rutherford Backscattering Spectroscopy (RBS) and Particle Induced X-Ray Emission (PIXE) techniques were used to investigate the chemical composition of the films. Optical absorption measurements were performed to characterize the films’ behavior as a function of the heat-treatment and to verify the absorption bands due to the localized surface plasmon resonance (LSPR) in the NPs and the Er3+/Yb3+ transitions.
Scanning Electron Microscopy (SEM) was employed for inspection of the waveguides structure and Atomic Force Microscopy (AFM) was used to verify the presence of the gold NPs on the waveguides. The propagation losses at 633 and 1050 nm were measured using the top-view technique [28]. The near-field profiles were obtained by collecting the light at the end of the waveguide with a 10x microscope objective lens (numerical aperture: 0.4) and focusing on a CCD.
A 4 µm wide waveguide with thickness of 1.6 µm and length of 1.5 cm was fabricated and used for the gain measurements. The signal and pump wavelength were combined using a 2x1 multiplexer optimized for the specific wavelengths used. The pump power was supplied by a 980 nm laser diode coupled to a fiber optics with a maximum output power at the end of fiber of 100 mW. A 1.53 µm diode laser supplied the signal wavelength; the power coupled to the waveguides was kept constant at 1 µW, to prevent gain saturation. The input fiber delivering both pump and signal terminates in a fiber lens with a mode field diameter of 3 µm. After propagation through the amplifier, the pump and signal beams are coupled to a spectrometer through a second lensed fiber focused on the output of the guide.
3. Results and discussions
The concentration of RE ions in the films was determined by RBS and PIXE analysis, following ref. [29]. The measured concentrations of Er3+ and Yb3+ were 3.3 x 1019 ions/cm3 and 2.0 x 1021 ions/cm3, respectively. The PL spectra of the films under 980 nm excitation, are shown in Fig. 1.
Fig. 1 Emission spectra of Er3+/Yb3+ PGO thin films under excitation at 980nm in (a) visible and (b) near infrared regions.
PL bands associated to the Er3+ ions transitions centered at 405 (2H9/2→4I15/2), 480 (4F7/2→4I15/2), 520 (2H11/2→4I15/2), 550 (4S3/2→4I15/2), 670 (4F9/2→4I15/2), 800 (4I11/2→4I15/2), 850 (4F9/2→4I13/2) and 1550 nm (4I13/2→4I15/2) were observed as in [30]. An enhancement factor larger than 400%, for the band at ≈550 nm, was observed in the sample with the gold layer. Moreover, it was observed enhancement of ≈30% in the infrared, despite the large difference between the emission and LSPR frequencies.
The temporal behavior of the 4F9/2→4I15/2 transition is shown in Fig. 2. The decay rate is expected to be influenced by the interaction among the RE ions [31] and by the gold NPs [8, 10–13]. The measured lifetimes are shown in Table 1 together with the radiative lifetimes determined using the Judd-Ofelt theory [32]. The lifetimes measured were smaller than the theoretical values, as expected.
Fig. 2 PL decay curves for emission centered at 670 nm, under excitation at 980nm. Power applied to Er2O3/Yb2O3 target: (a) 30 W and (b) 15 W.
Table 1. Lifetimes measured for the RE doped films (4F9/2→4I15/2 transition). The theoretical radiative lifetime obtained using the Judd Ofelt theory is also shown.
From Fig. 2(a), the contribution of the gold NPs is not clear. Hence, other films were produced with smaller RE ions concentrations; in this case the power applied to the Er2O3-Yb2O3 target was 15 W. This was made to reduce the contribution due to the interaction among the Er3+ ions. Figure 2(b) shows the decay curves for the films with and without the gold cover layer. Notice that the signal decay without the gold layer is not exponential that indicates that even in the sample with smaller Er3+ concentration the interaction among the Er3+ ions is relevant. We remark that the measured lifetime at 670 nm is much smaller than the radiative one, no matter if gold is present or not. This can be attributed to the presence of clustering erbium ions. Indeed, with basis on the Inokuti-Hirayama theory [31], we verified that the time behavior is characteristic of dipole-dipole interaction among the active Er3+. However, it is possible to observe that in the presence of gold NPs the decay time of the 670 nm emission decreases.
The films with larger concentrations of RE ions (30W) were used for fabrication of RIB waveguides, since the gain increases with the concentration of Er3+ ions [33]. The measured radiative lifetimes at 1530 nm (4I13/2→4I15/2) were ~2.5 ms for the samples with and without gold NPs (30W), which are also smaller than the theoretical lifetime (~6 ms) [32]. The experimental lifetimes at 1530nm for the samples with smaller RE ions concentrations, obtained under 15W deposition power, were not obtained, due to PL low intensity in these samples.
Figure 3 shows the propagation losses in the Er3+/Yb3+ codoped waveguides at 633 and 1050 nm as a function of the waveguides width; the minimum losses were ≈5.0 and ≈1.0 dB/cm for 633 and 1050 nm, respectively.
Fig. 3 (a) Propagation losses for Er3+/Yb3+ codoped PGO waveguides as a function of waveguides width. Insets show the near-field profiles for four waveguides. (b) Optical gain as a function of pump power. The pump and signal wavelengths were 980 and 1530 nm, respectively. The inset shows the near field profile at 1050 nm, at the end of the waveguide with the gold nano layer.
Figure 3(b) presents optical gain curves as a function of the excitation power coupled in the waveguide. The pump power coupled to the waveguide was estimated taking into account the difference in mode shape between the fiber and the waveguide as reported in ref [34]. The maximum internal gain observed for a waveguide without the gold layer was ≈3 dB (2 dB/cm). With the gold layer deposited the gain reached ≈6.5 dB (4.3 dB/cm). Considering losses due to coupling and propagation, net gain is achieved only in the sample with the gold layer (~0.9 dB). However, due to limitations of our setup, pump power was limited to 100 mW, but it was not observed saturation of the gain. This means that the maximum internal gain achievable in these waveguides could be even higher, if pump power could be increased. The AFM analysis revealed that the gold layer is actually a nanostructured thin film composed of islands having diameter of ≈20 nm and height of ≈10 nm. This is in agreement with the Volmer-Weber growth mechanism for thin films that predicts columnar structures, mainly in multilayered structures, as is the case for the samples produced in this work [35]. After the gold layer deposition, the optical guiding was observed only in the infrared region. This is understood because the LSPR for the gold NPs is centered at ≈600 nm. The propagation losses are higher for optical frequencies near the LSPR, due to the high optical absorption coefficient and ohmic dissipation [36]. Figure 3(b) also shows the near-field profiles obtained at 1050 nm for waveguides with and without gold layer. The waveguides covered with the gold layer show that part of the propagation mode is deviated to the interface between the dielectric film and the gold layer because of the interaction between the gold layer and the optical guided modes within the waveguide. The gain enhancement is correlated to the PL enhancement being attributed to the local field growth in the vicinity of the NPs. According to [17] the optimum distance RE-NP for PL enhancement is ≈15 nm while quenching occurs when the RE ions are closer to the NPs.
Figure 4(a) presents a schematic of the waveguides. Considering that the range of influence of the gold layer is ≈20 nm, only 1.3% of the PGO thin film is under influence of the gold NPs layer. The results indicate that most of the optical field was confined to the PGO core and the losses due to ohmic dissipation in the gold layer were small [36, 37]. Figure 4(b) shows a SEM image of the RIB waveguide covered with the gold layer and Fig. 4(c) shows details of the nanostructured gold film. Further studies should be performed, in order to investigate the behavior of the waveguides optical gain under higher influence of the NPs. In this work, the gain enhancement observed in samples with the gold NPs was higher than 100%, but it could be even higher, for different waveguides structure, where distance between rare earth ions and the NPs could be controlled, for example.
Fig. 4 (a) Schematics showing the details of the waveguide with the gold nanostructured thin film. (b) SEM micrograph of the waveguide with the gold nanolayer. (c) AFM analysis of the waveguide, showing the details of the nanostructured gold layer deposited on the waveguide core.
In summary we observed enhancement of the photoluminescence and optical gain at 1.53 µm due to the presence of gold NPs deposited on the RIB waveguide. As far as we know this is the first observation of this effect and the results show the relevance of the developed procedure for future applications in integrated optics.
Acknowledgments
This work was supported by the National Institute of Photonics (INCT de Fotônica) project granted by the Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq. The authors also acknowledge the Laboratório de Feixes Iônicos (LAMFI - IFUSP) for RBS and PIXE analysis and the Laboratório de Filmes Finos (LFF - IFUSP) for AFM measurements.
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