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Abstract
Based on the intramolecular energy transfer mechanism between organic ligand TMHD (2, 2, 6, 6-tetramethyl-3, 5-heptanedione) and central Er3+ ions, optical gains at 1.55 µm were demonstrated in three structures of polymer waveguides using complex Er(TMHD)3-doped polymethylmethacrylate (PMMA) as the active material. With the excitation of two low-power UV light-emitting diodes (LEDs) instead of 980 or 1480 nm lasers, relative gains of 3.5 and 4.1 dB cm-1 were achieved in a 1-cm-long rectangular waveguide with an active core of Er(TMHD)3-doped PMMA polymer. Meanwhile, relative gain of 3.0 dB cm-1 was obtained in an evanescent-field waveguide with cross-section of 4 × 4 µm2 using passive SU-8 polymer as core and a ∼1-µm-thick Er(TMHD)3-doped PMMA as upper cladding. By growing a 100 nm thick aluminum mirror and active lower cladding, the optical gain was doubled to 6.7 dB cm-1 in evanescent-field waveguides because of the stimulated excitation of Er3+ ions in the upper and lower cladding and the improved absorption efficiency.
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1. Introduction
With the successful application of erbium-doped fiber amplifiers (EDFAs) in long-distance optical fiber communication in the C-band (1530–1565 nm), erbium-doped waveguide amplifiers (EDWAs), which are suitable for integrating photonic chips, have attracted extensive attention in the past two decades. EDWAs can be integrated with active or passive optical devices, such as Mach–Zehnder interferometers (MZI), arrayed waveguide gratings (AWG), and micro-ring resonators (MRR), to compensate for the 1.55 µm optical loss in an efficient on-chip optical system [1–5]. Erbium-doped inorganic materials [6–8] (e.g., silicate, phosphate, crystals, and alumina) or organic materials (e.g., organic complexes or oleic acid-modified nanoparticles) can be used to fabricate EDWAs. Recently, an optical gain of 3.6 dB cm-1 at 1.53 µm was obtained in an Er:LNOI waveguide pumped by a 980 nm semiconductor laser with a pump power of 21 mW [9]. An on-chip net gain of 1.43 dB cm-1 at 1.55 µm in an Er:Si3N4 waveguide was achieved under the excitation of a 1480 nm laser with a 245 mW pump power [10]. Compared with the above inorganic materials, polymers have the advantages of simple processing methods, low cost, easy adjustment of the refractive index, and compatibility with various substrates. The dissolution of inorganic erbium salts in polymers could be solved by encapsulating Er3+ ions into organic complexes or oleic acid-modified nanoparticles, such as Er(DBM)3phen, Er(TTA)3(TPPO)2, LaF3: Er3+,Yb3+, and NaYF4: Er3+,Yb3+ nanoparticles [11]. Poly(methyl methacrylate) (PMMA), polystyrene (PS), and polycarbonate (PC) are used as doped hosts for these complexes and nanoparticles to fabricate polymer EDWAs [12–14]. Currently, Er3+, Yb3+ codoped nanoparticles have become the mainstream materials for preparing polymer EDWAs because of their advantages of low phonon energy and the typical gain value was 6.2 dB cm-1 under the excitation of a 270 mW 980 nm laser [15]. Although ErIII complexes were proposed as early as 1997, only a few reports have been published to date [16–19]. These shortcomings include a small absorption cross-section at 980 nm, low metastable life of Er3+ ions, and inability to withstand excessive pump laser power, limiting the application and development of ErIII complexes in polymer EDWAs. For both inorganic and organic EDWAs, commercial 980 nm or 1480 nm semiconductor lasers are often selected as the pump sources to realize stimulated radiation of Er3+ ions because of the alignment of the narrow absorption spectrum of the lasers and Er3+ ions. However, high commercialization costs and the high laser pump energy would lead to the energy up-conversion effect of Er3+ ions and thermal damage of waveguides [20,21], which are bottlenecks limiting the application of EDWAs.
Organic complexes were initially synthesized for efficient energy transfer between ligands and rare-earth ions. Organic ligands have continuous absorption bands and large absorption cross sections (10−22–10−23 m2) [22,23] in the UV wave band. The ultraviolet and visible energies absorbed by organic ligands can be effectively transferred to the excited-state energy level of the central rare earth ions. The energy transfer efficiency can reach five times that between rare-earth ions, such as Yb3+ and Er3+ ions [24]. However, the advantages of organic complexes have not been completely utilized, owing to the limitations of the traditional research methods of the optical fiber amplifiers and inorganic waveguide amplifiers. They are only used to solve the problem of the low solubility of rare earth ions in polymer hosts in the research field of optical waveguide amplifiers.
Based on this fact, complex Er(TMHD)3 (TMHD:2,2,6,6-tetramethyl-3, 5-heptanedione)-doped PMMA was prepared as the active material to fabricate polymer waveguides with three different structures using standard photolithography and inductively coupled plasma (ICP) technology. The absorption and photoluminescence (PL) spectra were recorded. Optical gains were demonstrated by the intramolecular energy transfer effect between the organic ligand TMHD and Er3+ ions. Under the vertical top pumping of 365 and 405 nm light emitting diodes (LEDs), the relative gains of 3.5 and 4.1 dB cm-1 at 1.55 µm were obtained, respectively, in 1-cm-long rectangular waveguides with Er(TMHD)3-doped PMMA as an active core. A maximum relative optical gain of 3.0 dB cm-1 was achieved in waveguides based on evanescent-field coupling with passive SU-8 as channel waveguides and Er(TMHD)3-doped PMMA as the upper cladding. An aluminum (Al) reflector with Er(TMHD)3-doped PMMA as a lower cladding was added to evanescent-field waveguides to improve the absorption efficiency of the LED and gain performance. The relative gain doubled and improved to 6.7 dB cm-1 because of the stimulated excitation in both the lower and upper claddings.
The propagation loss and coupling loss of three different waveguides at 1.55 µm were evaluated using the cut-back method. The sensitization mechanism between the organic ligand TMHD and Er3+ ions is discussed. This result demonstrates the possibility of optical amplification of the ErIII complex through an intramolecular energy transfer effect with low-power LED pumping. Er(TMHD)3-doped PMMA and its proper LED pumping technology could undoubtedly greatly reduce the commercial cost of EDWA in planar photonic integration.
2. Experiment and analysis
2.1 Synthesis of complex Er(TMHD)3-doped PMMA polymer
Er(TMHD)3 (Erbium(III)-tris-(2,2,6,6-tetramethyl-3,5-heptanedione)) with a purity of > 99% was purchased from Sigma-Aldrich (Shanghai) Trading Co. Ltd. In general, a choosing the ligand is crucial for proficient ligand-to-central ion energy transfer, which requires desirable matching of the emissive energy level of Ln3+ and the ligand excited state. The organic ligand TMHD, a β-diketone ligand, is desirable for sensitizing lanthanide emissions, as it possesses a strong π–π* absorption within a large wavelength range in the ultraviolet region. It acts as an “antenna” to transfer the energy of π–π* absorption to effectively illuminate levels of central ions and can form brightly emissive and thermally improved adducts with lanthanide ions. It has been used to improve the quantum efficiency, solubility, and color purity of high-performance electroluminescent materials [25–29]. The singlet S1 and triplet T1 excited states of TMHD match the 4F7/2 energy level of the Er3+ ions. Hence, the energy absorbed by the ligand can be directly transferred to the Er3+ ions. PMMA is a polymer with excellent optical properties and has the advantages of high transmittance, flexibility, and easy processing [30]. Therefore, it was chosen as the polymer host in this study. Er(TMHD)3 complex (0.01 g) was dissolved in 0.2 mL toluene and subsequently added to a solution of 1 g PMMA (Macklin Biochemical Co. Ltd.) in 4 mL n-butyl acetate. Subsequently, the mixture was put in a glass weighing bottle and stirred at room temperature for 48 h, then heated in a vacuum oven for 6 h with a gradient heating process from 75 to 120 °C, to remove solvent and form an approximately 180-µm-thick transparent film. The doping concentration of Er(TMHD)3 in PMMA was 1 wt%, and the concentration of Er3+ ions was approximately 4.4 × 1025 ions/m3. In order to produce several decibels of optical gain in a centimeter-long waveguide, the doping concentration of Er3+ ions in polymer hosts should as high as possible. However, high concentration easily leads to poor film-formation and concentration quenching, thus increase the transmission loss and scattering loss of the waveguide, which is an unfavorable factor for optical gain. On the premise of ensuring the film with lower surface roughness, the concentration of ∼ 1.0 wt% for Er(TMHD)3 in PMMA is almost the maximum doping concentration. Additionally, inorganic ErCl3·6H2O powder and ErCl3·6H2O-doped PMMA film were prepared to compare the absorption properties of the complexes.
2.2. Absorption and photoluminescence properties
The absorption spectra of Er(TMHD)3 powder and Er(TMHD)3-doped PMMA film were recorded using a Shimadzu UV-Vis-NIR spectrophotometer at room temperature, as shown in Fig. 1. The Er(TMHD)3 and ErCl3·6H2O powders exhibited several absorption peaks at 365, 375, 405, 450, 488, 522, 544, 654, 795, and 978 nm, corresponding to the intrinsic transitions of Er3+ ions from the ground state 4I15/2 to the 4G9/2, 4G11/2, 2H9/2, 4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, and 4I11/2 excited states, respectively [31]. Compared to the absorption of ErCl3·6H2O in the UV band, the Er(TMHD)3 powder showed a significant continuous absorption band in the wavelength range of 300–350 nm, which is attributed to the absorption of the TMHD ligand [32]. Compared with the Er(TMHD)3 powder, the absorption intensity of almost all the intrinsic absorption peaks of Er3+ ions is attenuated in the Er(TMHD)3-doped PMMA film because the concentration of Er3 + ions in the polymer film is much lower than that in the powder. However, there is still a strong absorption in the ultraviolet band from 300 to 350 nm, which mainly depends on the absorption of the TMHD and PMMA ligands. The pump rate Rp is an important factor affecting the optical gain performance of the waveguide amplifier, which can be determined by the absorption intensity σabs multiplied by the photon flux density φ: Rp=σabs×φ [33]. Meanwhile, the photon flux density φ is determined by the pump power per unit area. Therefore, a large absorption intensity σabs can improve the pump rate Rp. For the traditional pumping method using semiconductor lasers to achieve the intrinsic absorption of Er3+ ions at 1480 or 980 nm, a relatively large absorption intensity σabs can be obtained by increasing the doping concentration of Er3+ ions or co-doping Yb3+ ions in the material. However, it is easy to cause a concentration quenching and up-conversion effect [34,35], which is not conducive to the down-conversion emission of Er3+ ions at 1.53 µm. A high photon flux density φ requires high laser energy, causing destructive thermal damage to the waveguide end face.
In this study, a new approach that is different from the traditional research idea was proposed, which was based on the intramolecular energy transfer mechanism between the ligand TMHD and central Er3+ ions in the UV band. Moreover, the intrinsic absorption of Er3+ ions at 365, 375, and 405 nm, corresponding to the 4G9/2, 4G11/2, and 2H9/2 energy levels, respectively, were also utilized. Depending on the large absorption intensity σabs of approximately 6.1 × 10−23 m2 at 300–400 nm in Er(TMHD)3-doped PMMA, a higher pumping rate Rp can be provided and the waveguide fabricated with Er(TMHD)3-doped PMMA is expected to achieve optical gain under excitation of low-power UV LEDs. This method can reduce the influence of concentration quenching and avoid destructive thermal damage to the waveguide pumped by a high-power laser.
Figure 2 presents the photoluminescence (PL) spectra of the Er(TMHD)3-doped PMMA film excited by 365 and 405 nm LEDs under the same 130 mW pump power, respectively. The irradiation spectra of 365 and 405 nm LEDs cover 320–400 nm and 370–420 nm, respectively. The PL peak at 1535 nm is due to the transition of Er3+ ions from the excited state 4I13/2 to the ground state 4I15/2 [36]. The fluorescence full width at half maximum (FWHM) of 365 and 405 nm are both approximately 52 nm. Compared with the FWHM value of Er3+ ions in inorganic hosts [37], the broadening of the emission is associated to the coupling of f–f electronic transition of Er3+ ions with vibrational modes of the PMMA matrix [38]. Since the absorption intensity of Er(TMHD)3-doped PMMA at 365 nm is higher than that at 405 nm, the film excited by the former has stronger PL intensity, indicating that better gain performance can be obtained by using a 365 nm LED excitation. The PL intensity linearly increases with increasing pump power, as shown in Fig. 2(b).
Fig. 2. Room temperature infrared PL spectra of the Er(TMHD)3-doped PMMA film. (a) Under the excitation of 365 nm and 405 nm LEDs (130 mW), respectively. (b) PL intensity at 1535 nm as a function of the LED pump power.
2.3. Waveguide fabrication
The doping concentration of Er3+ ions in the polymer host should reach 1025–1026 ions/m3 [39,40], which is approximately two orders of magnitude higher than that in EDFA, to produce an optical gain of several decibels in a centimeter-long waveguide. Such a high doping concentration leads to poor uniformity, poor solubility, and high propagation loss. Therefore, one of the key factors in preparing high-performance EDWA is obtaining polymer films with good film-forming qualities. Figure 4(a) displays an atomic force microscope (AFM) micrograph of the Er(TMHD)3-doped PMMA film. The root-mean-square roughness Rq was 0.35 nm for the 6-µm-thick film on the Si substrate after thermal annealing. Compared with LaF3:Er3+,Yb3+ nanocrystal-doped organic-inorganic hybrid materials [41] and NaYF4/NaLuF4:Yb,Er nanocomposite-doped PMMA [42], Er(TMHD)3-doped PMMA has better film quality, which is conducive to obtaining a lower transmission loss of the optical signal.
Three waveguide structures were designed and fabricated using Er(TMHD)3-doped PMMA as the active material to compare their optical gain performance. Their cross sections are shown in Fig. 3. For the rectangular waveguide in Fig. 3(a), Er(TMHD)3-doped PMMA polymer with a thickness of 4 µm was spin-coated as the core layer on a silicon substrate with 2-µm-thick SiO2 as the lower cladding. The refractive index of the core was 1.483 at 1550 nm wavelength. Then, an aluminum film of 50 nm thickness was deposited onto the core layer surface to form a dry etching mask by magnetron sputtering. A group of rectangular waveguides with a cross-section of 4 × 4 µm2 was fabricated using standard photolithography and inductively coupled plasma (ICP) etching. The scanning electron microscope (SEM) micrographs (SUPRA 55 SAPPHIRE) of the rectangular waveguides is shown in Fig. 4(b). Figure 3(b) displays the cross-section of the evanescent-field waveguides with passive SU-8 polymer as channel waveguides on a silicon substrate with a 2-µm-thick SiO2 as the lower cladding and a ∼1-µm-thick Er(TMHD)3-doped PMMA as the upper cladding. The refractive index of SU-8 was 1.552 at 1550 nm. When the optical signal laser travels in the passive SU-8 channel waveguides, the photon energy can enter from the passive core to the active upper cladding in the form of an evanescent wave, triggering stimulated radiation of the active material in the upper cladding under the excitation of the pump source. A SEM micrograph of the evanescent-field waveguides is shown in Fig. 4(c). A 100-nm-thick Al mirror was first sputtered on the SiO2 layer and then spin-coated with a 6-µm-thick Er(TMHD)3-doped PMMA as the lower cladding to improve the utilization efficiency of the radiation of the LED. Then, SU-8 channel waveguides were fabricated and coated with Er(TMHD)3-doped PMMA as the upper cladding. Er3+ ions contained in the lower cladding could help increase the number of particles participating in the stimulated radiation, which is expected to further improve the gain performance. The cross-section of the waveguides and the propagation path of the evanescent wave are presented in Figs. 3(c) and (d). A SEM micrograph of the evanescent-field waveguides with an Al mirror is shown in Fig. 4(d).
Fig. 3. Schematic diagram of the three structures of waveguides. (a) Rectangular waveguides with Er(TMHD)3- doped PMMA as channel waveguides. (b) Evanescent-field waveguides with SU-8 polymer as channel waveguides and Er(TMHD)3-doped PMMA as upper cladding. (c) Evanescent-field waveguides with Er(TMHD)3-doped PMMA as active cladding and an Al reflector.
Fig. 4. (a) AFM micrograph of the Er(TMHD)3-doped PMMA film. (b) Top view and cross-section (inset) SEM micrographs of active rectangular waveguides (4 × 4 µm2) with Er(TMHD)3-doped PMMA as channel waveguides. (c) Top view and cross-section (inset) SEM micrographs of evanescent-field waveguides with SU-8 polymer as channel waveguides (4 × 4 µm2) and Er(TMHD)3-doped PMMA as upper cladding (∼1-µm-thick on top surface of the waveguide). (d) Top view and cross-section (inset) SEM micrographs of evanescent-field waveguides with Er(TMHD)3-doped PMMA as active lower cladding (6-µm-thick) and a 100-nm-thick Al reflector to improve the utilization efficiency of the radiation of the LED.
3. Results and discussion
An experimental setup for optical gain measurement was established by adopting the vertical top-pumping mode of the LED, as shown in Fig. 5. A 1550 nm laser source (KG-DFB-15-10-SM-FA, Yuwei, China) was used as the signal source, and two LEDs with different central wavelengths (Yonglin Optoelectronics Co., Ltd), such as 365 nm (YL-UVAHP-6565-365) and 405 nm (YL-UVAHP-6060-405), were used as the pump source. The LED was placed approximately 2 mm above the Er (TMHD)3-doped PMMA waveguides. The viewing angle of the LED was 140°, and the total illumination area was approximately 1 cm2, which almost completely covered the area of a 1-cm-long waveguide device. The power available on the device was approximately 295 mW based on the irradiation intensity distribution when the power of the LED was 328 mW. Therefore, the radiation efficiency of the LED power collected on the device in the total power of the LED were estimated to be approximately 90% [43]. The input signal laser was coupled to the optical waveguides using a single-mode fiber with an isolator. The output signal was coupled to an optical spectrometer (Ocean Optics FLAME-NIR-INTSMA25).
The relative gain is defined as 10lg(PP + S/PS) [44], where PP + S and PS are the output signal powers measured with and without pump power, respectively. Figure 6 shows the relationship between the output optical signal intensity and the pump power under excitation of 365 and 405 nm LEDs, respectively, in a 1-cm-long rectangular waveguide. The output signal intensity linearly increased as the pump power increased. The relative gains of rectangular waveguide could reach approximately 3.5 and 4.1 dB cm-1 under the excitation of a 405 nm (328 mW) and 365 nm (176 mW), respectively, when the output power of the 1550 nm signal laser was 0.6 mW. Under the excitation of the 405 nm LED, the pump power required to achieve the same level of gain values of approximately 3.5 dB cm-1 was almost twice that of the 365 nm LED; hence, the excitation effect of the 365 nm LED was better than that of the 405 nm LED, which is consistent with the conclusion obtained from the PL spectra. At a fixed pump power, a small-signal gain phenomenon was observed. The gain value slightly decreased as the optical power of the input signal increased. The maximum gain value decreased from 4.1 to 2.1 dB cm-1 as the signal power varied from 0.6 to 0.8 mW with an excitation of the 176 mW 365 nm LED, and the gain variation trend was the same under the 405 nm excitation, as shown in the inset of Figs. 6(a) and (b).
Fig. 6. (a), (b) Relationship between the output signal intensity and relative gain at 1.55 µm in rectangular waveguides with Er(TMHD)3-doped PMMA as channel waveguides under excitation of 365 nm and 405 nm LEDs, respectively. The insets show the relationship between the relative gain and different signal power.
Relative gains can also be obtained in waveguides based on evanescent-field coupling. The waveguide structure is presented in Fig. 3(b). Figure 7(a) shows the relationship between the output optical intensity and pump power under the excitation of different LEDs based on evanescent-field waveguides with Er(TMHD)3-doped PMMA as the upper cladding. The maximum gains were 3.0 and 2.5 dB cm-1 under the excitation of a 365 nm LED (176 mW) and a 405 nm LED (328 mW), respectively, when the output power of the signal power was 0.6 mW. Similar to the gain performance of the rectangular waveguide, the gain linearly increased with the pump power. The gain value reaches 6.9 dB (2.3 dB cm-1) in a 3-cm-long evanescent-field waveguide under 365 nm LED excitation, as shown in the inset of Fig. 7(a). The longer the waveguide (1–3 cm), the higher the optical gain (3–6.9 dB), but the smaller the gain obtained in the waveguide per unit length (3–2.3 dB cm-1), which is not a fixed value. The total relative gain increases because the amount of Er3+ ions increases with increasing waveguide length. However, the light emitted by one LED cannot completely cover the entire long straight waveguide; therefore, Er3+ ions are not entirely activated. We consider this is one of the reasons for the reduction in gain per unit length. Based on the data shown in Fig. 7(b), the relative gain performance of 365 nm LED pumping is still better than that of 405 nm LED pumping. When the power of signal laser changes from 0.6 to 0.7 mW, the gain also changes from 3.0 and 2.5 dB cm-1 to 2.3 and 2.1 dB cm-1, respectively, under the excitation of a 365 nm LED (176 mW) and a 405 nm LED (328 mW). Similar to the trend of the rectangular waveguide, the gain value slightly decreased as the input signal optical power increased.
Fig. 7. (a) Relationship between the output signal intensity in evanescent-field waveguide with Er(TMHD)3-doped PMMA as upper cladding. The inset shows the gain in evanescent-field waveguides of different lengths. (b) Comparison of relative gains obtained in evanescent-field waveguides with different signal power under 365 and 405 nm LED excitations.
Er(TMHD)3-doped PMMA was added as an active lower cladding and an aluminum reflector was grown under the lower cladding to improve the gain performance of evanescent-field waveguides. The waveguide structure of evanescent-field waveguides with active lower cladding and an Al reflector is shown in Figs. 3(c) and (d). The pump light can penetrate the waveguide and reflect back to the active upper and lower cladding through the Al reflector when the LED vertically radiates the waveguide from the upper surface at a distance of 2 mm, and then excite the active materials in the cladding. Due to the presence of stimulated excitation in both the upper and lower cladding, with the improved pump efficiency, relative gains of 6.7 and 5.2 dB cm-1 at 1550 nm are observed in a 1-cm-long waveguide under 365 nm (176 mW) and 405 nm LEDs (328 mW) pumping, respectively, as shown in Figs. 8(a) and (b). In contrast to 3.0 and 2.5 dB cm-1 of evanescent-field waveguides, the maximum relative gains of 6.7 and 5.2 dB cm-1 are more than doubled and exceed the gain values of 4.1 and 3.5 dB cm-1 of the rectangular waveguides, under 365 nm (176 mW) and 405 nm LEDs (328 mW) pumping, respectively, as shown in Figs. 8(c) and (d). Otherwise, as shown in the inset of Figs. 8(a) and (b), the maximum gains increased from 6.7 and 5.2 dB to 9.6 and 7.6 dB when the length of waveguide varied from 1 to 2 cm; but the gain per unit length decreased from 6.7 and 5.2 dB cm-1 to 4.8 and 3.8 dB cm-1, under 365 nm (176 mW) and 405 nm LEDs (328 mW) pumping, respectively, which is the same trend as that of the evanescent-field waveguide. Another way to improve the gain per unit length is to use the LED array in vertical top pumping technology such that the entire long straight waveguide can be completely irradiated by LEDs. Alternatively, curved spiral waveguides can be prepared and better covered in the LED irradiation range to obtain higher gain per unit length. In addition, due to the weak absorption of complex Er(TMHD)3 at 978 nm, it is difficult to obtain the luminescence of Er3+ ions at 1550 nm in these three waveguide structures under excitation of a conventional 976 nm semiconductor laser.
Fig. 8. (a), (b) Relationship between the output signal intensity in evanescent-field waveguide with active cladding and Al reflector. The insets show the relative gain of waveguide with different lengths. (c), (d) Comparison of relative gains as a function of pump power in three different structures of the waveguide.
The relationship between the relative gain G(L)rel, internal gain G(L)int, and net gain G(L)net can be described by Eqs. (1) and (2) [45].
where αloss is the propagation loss of the waveguide and αcou is the coupling loss of the two end facets of the waveguide. Using the cut-back method, the propagation and coupling losses of three different waveguides at 1550 nm were measured, as shown in Fig. 9(a). The coupling loss is approximately 2.0 dB/facet. Compared with the propagation loss of 3.5 dB cm-1 in the active rectangular waveguide, the propagation loss of the evanescent-field waveguide using passive SU-8 as the core and Er(TMHD)3-doped PMMA as the upper cladding is approximately 1.8 dB cm-1. For the evanescent-field waveguide with an Al reflector, the propagation loss was approximately 1.7 dB cm-1. The propagation loss of the active waveguide is higher than that of the passive waveguide because of the increase in the absorption loss and scattering loss with doped active materials, as well as the roughness of the active rectangular waveguide. According to the above formula, the gain results are displayed in Figs. 9(b), (c), and (d). Under the excitation of 365 nm LED, the maximum gain G(L)rel, G(L)int, and G(L)net in the rectangular waveguide are 4.1, 0.6, and -3.4 dB cm-1. Meanwhile, G(L)rel, G(L)int, and G(L)net in the evanescent-field waveguide are 3.0, 1.2, and -2.8 dB cm-1, respectively. A negative net gain indicates that the optical loss is greater than the gain generated in the waveguide. The higher internal gain and net gain of the evanescent-field waveguide than that of the rectangular waveguide are due to the low propagation loss of the passive waveguide. For the evanescent-field waveguide with an Al reflector, the overall evaluation of gain is the best among the three waveguide structures. The maximum gain value of G(L)rel, G(L)int, and G(L)net are 6.7, 5.0, and 1.0 dB cm-1, respectively. As a comparison, under excitation of the 405 nm LED, the maximum net gain G(L)net in the rectangular waveguide, evanescent-field waveguide, and evanescent-field waveguide with the Al reflector are -4.0, -3.3, and -0.5 dB cm-1, respectively.Fig. 9. (a) Measured propagation loss and coupling loss of different waveguides at 1550 nm by cut-back method. Gains of (b) rectangular waveguide, (c) evanescent-field waveguide, and (d) evanescent-field waveguide with an Al reflector. (The red part is the propagation loss αloss, and the purple part is the coupling loss αcou.)
Based on the absorption and PL spectra and the gain measurement results of the waveguides, a ligand-sensitization scheme between the organic ligand TMHD and Er3+ ions under the excitation of the UV LEDs is presented in Fig. 10. It is generally assumed that the major energy transfer mechanism between an excited state on an organic molecule and a lanthanide ion is via the triplet state of the organic molecule; thus, photoexcited molecules must undergo intersystem crossing (ISC) [46]. The energy level of the triplet excited state (T1, 22200 cm-1) of ligand TMHD matches that of the 4F7/2 state (20400 cm-1) of Er3+ ions with an energy gap of 1800cm-1, which can inhibit reverse energy transfer and promote efficient energy transfer without additional high-energy phonon assistance. The quantum number difference ΔJ of the total angular momentum of the excited state 4F7/2 and ground state 4I15/2 is 4, satisfying the Förster resonance energy transfer (FRET) mechanism [47–49]. Therefore, under the excitation of UV LEDs, the ligand TMHD can obtain energy and transfer from the ground state S0 to the first excited state S1, and then realize energy transfer from the S1 to T1 state via the ISC process, followed by energy transfer from the T1 state of TMHD to the 4F7/2 state of Er3+ ions. Finally, Er3+ ions complete two non-radiative transition (NRT) processes from the 4F7/2 to 4I11/2 state and from the 4I11/2 to 4I13/2 state, and a radiative transition (RT) process from 4I13/2 to 4I15/2, and finally realize photon emission at 1550 nm. The energy transfer path is summarized in route 1:
Er3+ ions in the ground state 4I15/2 can also be excited to 4G9/2, 4G11/2, and 2H9/2 by ground state absorption (GSA) under the excitation of UV LEDs, and then through the NRT process to the 4F7/2 state. On the one hand, Er3+ ions in the 4F7/2 state can achieve 1550 nm luminescence via the two aforementioned NRT processes and one RT process via route 1 [50]. On the other hand, the Er3+ ions in the 4F7/2 state can transition to the 2H11/2, 4S3/2, and 4F9/2 states, respectively, in the NRT process [51], from where they radiatively decay to the ground state 4I15/2, realizing green and red emissions at 521, 547, and 658 nm, respectively, which is consistent with the PL spectrum of the Er(TMHD)3-doped PMMA film in the visible light bands with the excitation of a 365 nm LED, as shown in Fig. 10(c). The energy-level transition processes of Er3+ ions excited by UV LEDs are summarized as follows:
Fig. 10. (a) Intramolecular energy transfer mechanism of ligand-Er3+ ions and luminescence processes of Er3+ ions under a UV LED pumping. (b) Up-conversion luminescence at green and red lights under 980 nm LD pumping. (c) The visible emissions at 521, 547, 658 nm of the Er(TMHD)3-doped PMMA film.
Note that the three visible light emissions obtained here should also be considered as down-conversion, which has the same properties as the down-conversion luminescence of Er3+ ions at a wavelength of 1550 nm. This is because Er3+ ions in the ground state almost transit to the highest energy level via the GSA process with ultraviolet energy, and the pump energy is transferred from the higher excited states of Er3+ ions to the lower excited states. However, for the current mainstream pumping technology with a 980 nm laser as the pump source in the research of EDWAs, Er3+ ions are excited from the 4I15/2 to 4I11/2 state via the GSA process and then excited to the 4F7/2 state via excited state absorption (ESA) or cross-relaxation (CR) processes from neighboring Er3+ ions, as shown in Fig. 10(b). The Er3+ ions in the 4F7/2 state can transit to the 2H11/2 and 4S3/2 states via the NRT process and then transition from these two states to the 4I15/2 state to generate two green emissions. Meanwhile, the excited Er3+ ions in the 4I11/2 state can decay non-radiatively to the lower excited state 4I13/2. They can be excited to the 4F9/2 state via the ESA process with the excitation of a 980 nm laser, causing the red emission via the RT process [52]. Here, the green and red emissions are up-conversion luminescence, and the 980 nm pump energy is transferred from the lower excited states to the higher excited states.
Figure 11 presents the application prospects of Er(TMHD)3-doped PMMA as active materials to active two-stage amplification at a wavelength of 1.55 µm in planar photonic integration. The schematic diagram integrates a spiral waveguide and a 1 × 8 beam splitter [53]. Er(TMHD)3-doped PMMA was spin coated as the active upper cladding on passive waveguides. An Al reflector is added on the SiO2 layer. Passive materials, such as polymer and SiO2, and Er(TMHD)3-doped PMMA active materials can be selected for the lower cladding as required. Thus, with only one LED pumping, the synchronous secondary amplification function of the whole chip with low power consumption, low optical loss, high efficiency, and easy integration can be realized based on the method of evanescent wave coupling combining with an Al reflector according to this study.
4. Conclusions
Optical gains at a wavelength of 1.55 µm were demonstrated in polymer waveguide amplifiers based on complex Er(TMHD)3-doped PMMA with the excitation of UV LEDs, instead of 980 nm lasers. The absorption and PL spectra were recorded. The absorption intensity of Er(TMHD)3-doped PMMA at 365 nm is higher than that at 405 nm, causing the PL excited by the former to have a stronger intensity. Three different waveguide structures based on Er(TMHD)3-doped PMMA polymer were fabricated. Under vertical top pumping of 365 nm (176 mW) and 405 nm LED (328 mW), the relative gains of 4.1 and 3.5 dB cm-1 at 1.55 µm were achieved in rectangular waveguide with cross-section of 4 × 4 µm2, respectively. Based on evanescent-field coupling, the relative gain of 3.0 and 2.5 dB cm-1 were observed in a 1-cm-long evanescent-field waveguide under excitation of 365 and 405 nm LEDs respectively, when passive SU-8 was fabricated as channel waveguide and Er(TMHD)3-doped PMMA as upper cladding. The optical gain was doubled and improved to 6.7 dB cm-1 under the excitation of a 176 mW 365 nm LED by adding Er(TMHD)3-doped PMMA as the lower cladding and Al reflector in the evanescent-field waveguide because of the presence of stimulated excitation in both the upper and lower cladding and an improved absorption efficiency of the LED. The propagation loss and coupling loss of three different waveguides at 1.55 µm were evaluated using the cut-back method. The propagation loss of 3.5 dB cm-1 in rectangular waveguide with an active core was higher than that of 1.8 and 1.7 dB cm-1 in evanescent-field waveguide and evanescent-field waveguide with reflector with passive SU-8 core, respectively, because of the increase of absorption and scattering loss with doped active materials, as well as the roughness of the active rectangular waveguide. The sensitization mechanism between the organic ligand and Er3+ ions is also discussed.
These results indicate the possibility of optical amplification in EDWA based on Er(TMHD)3 complex-doped PMMA at 1.55 µm through the intramolecular energy transfer effect with low-power LED pumping. Optical loss can be compensated by conveniently integrating Er(TMHD)3-doped PMMA into the waveguide parts of optical functional devices on various commercial photonic platforms.
Funding
National Key Research and Development Program of China (2021YFB2800500); National Natural Science Foundation of China (61875170, 61107023); Natural Science Foundation of Fujian Province (2022J01063).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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