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Abstract
We report a method of using neodymium complexes as active waveguide core materials to achieve optical gain at the 1060 nm wavelength when using an LED instead of an 808 nm semiconductor laser as the pump source. Through the intramolecular energy transfer mechanism between ligands and Nd3+ ions, the photoluminescence spectrum could be obtained on a 100µm thick film of neodymium complex doped PMMA polymer under excitation of a 380 nm-450 nm LED. We also present calculations showing that the pump power required to generate optical gain (turn-on power) for a LED and 808 nm laser is 3.3 mW and 40 mW, respectively. An optical gain of about 6 dB can be obtained on a 20-mm-long waveguide when pumped by a 25 mW LED compared with that of about 1.4 dB excited by a 60 mW 808 nm semiconductor laser.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Rare earth-doped optical waveguide amplifiers (RDWAs) which are used to compensate for optical losses play a key role in integrated optical systems [1–6]. They have attracted much attention in recent years. Rare earth ions can be directly doped in inorganic matrix materials such as silicate, phosphate, alumina, lithium niobate and so on to fabricate RDWAs. Also, rare earth ions can be used to synthesize rare earth complexes with organic ligands and then doped in polymer matrix to prepare RDWAs. The synthesis of rare earth complexes can solve the problem of low solubility of rare earth ions in polymer matrix. Compared to inorganic matrix materials, polymer RDWAs have many advantages, such as simple processing methods, low cost and easy integration with other photonic devices on silicon substrate. Usually, neodymiumis of great interest in the rare earths [7–9], because it emit around 1064 nm and 1330 nm wavelength, which can be used to enhance laser’s power and compensate the losses of the second window of optical communication systems. Therefore, neodymium- doped optical waveguide amplifier (NDWAs), like erbium-doped optical waveguide amplifiers (EDWAs), which are applied to the third standard communication window at 1550 nm wavelength, have received increasing attention in the past few years.
In the process of NDWAs’ research and development, the 808 nm semiconductor lasers are usually used as pump sources because of the intrinsic absorption of Nd3+ ions at ∼800 nm. The intrinsic absorption cross section of Nd3+ ions at ∼800 nm is in the order of 10−24 m2 and 10−25 m2 [10], which often requires higher pump power (100 mW∼400 mW), thus causing the energy upconversion effect of Nd3+ ions and thermal damage of waveguides. Also, an expensive 808 nm laser will increase the commercial cost of the device. In the study of polymer NDWAs, the design of neodymium complex with organic ligands is based on the fact that organic ligands have continuous absorption bands and large absorption cross sections (10−22 m2-10−23 m2) [11] in the wavelength range of 200 nm∼450 nm. The ultraviolet and visible energy absorbed by organic ligands can be effectively transferred to the excited state energy level of neodymium ions by intramolecular energy transfer. The energy transfer efficiency can reach five times of that between rare earth ions, such as Yb3+ ion and Er3+ ion [12]. However, due to the limitation of traditional optical fiber amplifier and inorganic RDWAs’ research ideas, the above advantage of neodymium complex has not been fully utilized. The neodymium complexes are just used to solve the problem of low solubility of rare earth ions in polymer matrix in the research field of optical waveguide devices.
Based on this point, two kinds of neodymium complexes: Nd(DBM)3(DBTDPO) and Nd(DBM)3(DPEPO) doped PMMA were used as the active waveguide core materials. The absorption spectra were observed and analyzed with the Judd-Ofelt theory. Photolumine- scence spectra were measured by a low-cost blue-violet LED instead of an expensive 808 nm semiconductor laser. Using the materials’parameters from absorption and photoluminescence spectra, a system model of intramolecular energy transfer between organic ligands and central Nd3+ ions was established. The relationships between the gain with pump power, organic ligands’ absorption cross section and other parameters were simulated. The properties of organic neodymium complex doped optical waveguide amplifier based on intramolecular energy transfer effect were analyzed.
2. Experimental details
2.1. Organic neodymium complexes
Three kinds of organic ligands: dibenzoylmethane (DBM), 4,6-bis (diphenylphosphoryl) dibenzothiophene (DBTDPO) and bis[2-(diphenylphosphino) phenyl]ether oxide (DPEPO) were used to encapsulate the Nd3+ ions. Two kinds of neodymium complexes: Nd(DBM)3 (DBTDPO) and Nd(DBM)3(DPEPO) were obtained. The design and synthesis of these ligands were described in detail in our previous literature [13–14]. These organic ligands can not only serve to shield the Nd3+ ions from impurities in the surrounding matrix and increase the solubility of Nd3+ ions in polymer matrix, but also preserve high triplet energy levels.
2.2. Preparation of PMMA polymer thin films
Nd(DBM)3(DBTDPO) and Nd(DBM)3(DPEPO) were dissolved with DMF (dimethyl- formamide) and subsequently added into PMMA (polymethyl methacrylate). The Nd3+ ions doping concentration was 3.8×1025 ions/m3 and 4.0×1025 ions/m3, respectively. Then the mixed solution was baked at 120℃ to form a ∼100 µm thin film. In addition, NdCl3 · 6H2O powder and NdCl3 · 6H2O-doped PMMA thin-film were prepared as comparison materials, respectively.
The absorption spectrum was recorded with Shimdazu UV3600 UV-Vis-NIR spectrophoto- meter and photoluminescence spectrum was measured with FLS980 fluorescence spectro- meters. All the measurements were carried out at room temperature.
3. Results
3.1. Absorption properties
Figure 1 shows the absorption spectra of Nd(DBM)3(DBTDPO) doped PMMA, Nd(DBM)3 (DPEPO) doped PMMA and NdCl3 · 6H2O, respectively. NdCl3 · 6H2O is used for comparison. The absorption bands at 527 nm, 583 nm, 748 nm, 802 nm and 868 nm wavelength corresponding to the ground state 4I9/2 to 4G7/2+4G9/2+2K13/2, 4G5/2+2G7/2, 4F4/2+4S3/2, 4F5/2+2H9/2, 4F3/2 transitions of Nd3+ ions can be observed in these samples. For the Nd(DBM)3(DBTDPO) and Nd(DBM)3 (DPEPO) complexes, as shown in Fig. 1(a) and Fig. 1(b), the obvious and continuous absorption bands in range of 200 nm∼450 nm wavelength can be measured for both powder and film materials. Compared with the absorption of NdCl3 · 6H2O powder and NdCl3 · 6H2O doped PMMA film, we draw a conclusion that the large absorption cross sections between 200 nm and 450 nm are attributed to the absorption of the organic ligands DBM, DBTDPO and DPEPO. Therefore, high efficiency excitation of neodymium complex may be achieved if a low power blue-violet LED is used as pump source.
Fig. 1. Normalized absorption spectra. (a) Nd(DBM)3(DBTDPO) powder and Nd(DBM)3(DBTDPO) doped PMMA film. (b) Nd(DBM)3(DPEPO) powder and Nd(DBM)3(DPEPO) doped PMMA film. (c) NdCl3 · 6H2O powder and NdCl3 · 6H2O doped PMMA film. The absorption bands were attributed to the transitions from the ground level 4I9/2 of Nd3+ ions to the corresponding levels.
3.2. Photoluminescence properties
A 405 nm laser and LED were used to excite the above neodymium complexes, respectively. The photoluminescence properties were shown in Fig. 2. Under the excitation of 405 nm laser, characteristic PL peaks at approximately 900 nm, 1060 nm and 1330 nm wavelengths corresponding to 4F3/2→4I9/2, 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions of Nd3+ ions were observed in both Nd(DBM)3(DBTDPO) and Nd(DBM)3(DPEPO) doped PMMA thin films, as shown in Fig. 2(a) and Fig. 2(b). From the absorption spectrum of Fig. 1(c), there was almost no intrinsic absorption of Nd3+ ions at 405 nm wavelength. So no emission peak has been observed in NdCl3 · 6H2O doped PMMA film, as shown in Fig. 2(c). This proves the organic ligands DBM, DBTDPO and DPEPO can efficiently transfer energy absorbed from the 405 nm laser to Nd3+ ions and help Nd3+ ions realize the transitions from ground state to excited states.
Fig. 2. PL spectra of neodymium complexes under the excitation of 405 nm laser. (a) Nd(DBM)3(DBTDPO) doped PMMA film. (b) Nd(DBM)3(DPEPO) doped PMMA film. (c) NdCl3 · 6H2O doped PMMA film.
Considering that the organic ligands have continuous absorption bands in range of 200 nm∼450 nm wavelength, an LED (center wavelength: 405 nm, 1.2W) was used as pump source. The spectral range of LED is 370 nm∼450 nm wavelength. The PL spectra in the near infrared wavelength region were recorded with an InGaAs charge-coupled device (CCD). The test system with LED pumped was shown in Fig. 3.
Compared with the 405 nm laser, the power density of LED is much lower, but the PL characteristics of Nd(DBM)3(DBTDPO) and Nd(DBM)3(DPEPO) doped PMMA thin films were both observed, as shown in Fig. 4. The fluorescence peak near 1060 nm wavelength was due to the 4F3/2→4I11/2 transition of Nd3+ ions. The full width at half maximum (FWHM) is about 27 nm and 41 nm centered around 1064 nm. Broad photoluminescence spectrum can get a wide gain bandwidth which is significant for optical amplification. Therefore, the energy transfer effect between the organic ligands and the central Nd3+ ions can help Nd3+ ions achieve the transition from ground state to excited state under the excitation of a low power blue-violet LED. This may have the same gain effect compared with the direct excitation of Nd3+ ions by intrinsic absorption.
Fig. 4. PL spectra of neodymium complexes under the excitation of an LED. (a) Nd(DBM)3(DBTDPO) doped PMMA film.(b) Nd(DBM)3(DPEPO) doped PMMA film.
3.3. Judd-Ofelt analysis
The Judd-Ofelt theory [15, 16] was used to analyze the absorption spectrum of Nd(DBM)3 (DBTDPO) and Nd(DBM)3(DPEPO) doped PMMA polymer, respectively. The J–O intensity parameters Ωt (t=2, 4, 6) were obtained in Table 1. According to J–O theory, some material parameters, such as radiative lifetime, absorption and emission cross section parameters which were used to simulate optical gain can be calculated. The value of radiative lifetime τ from the excited state 4F3/2 to the ground state4I11/2 of Nd3+ ions in Nd(DBM)3(DBTDPO) and Nd(DBM)3(DPEPO) doped PMMA is 398.36 µs and 679.82 µs, respectively, which is on the same order as reported in other literature, as shown in Table 2. Also, the stimulated absorption cross section at 405 nm and emission cross section at ∼1060 nm wavelength were calculated from the spectrum, as shown in Table 3.
Table 1. Results of Judd-Ofelt analysis
Table 2. Radiative Lifetime of Nd3+Ions in Polymer Matrices
Table 3. Stimulated absorption and emission cross-section
4. Theoretical basis
4.1. Device structure
Figure 5 is the cross section of waveguide structure for optical gain simulation. The Nd(DBM)3(DBTDPO) doped PMMA was used as core material. The Nd(DBM)3(DBTDPO) doped PMMA film was first spin-coated on a Si substrate with a 2 µm thick SiO2 as the bottom cladding. The refractive index of the core layer is 1.490 at ∼1060 nm wavelength. The waveguide’s cross section dimension is 4×6 µm2 and the length is 2 cm.
4.2. Theoretical model
For organic rare earth complexes, it has been demonstrated that a ligand-sensitization scheme can be used to enhance the PL efficiency of the rare earth ions [21]. We established a system model of intramolecular energy transfer between organic ligands with central Nd3+ ions to simulate the optical gain. The schematic diagram of the ligand-Nd3+ system was shown in Fig. 6. The organic ligand can realize the transition from ground state S0 to high energy singlet state S1 by absorbing 405 nm pumped light energy, and then transit to triplet state T by intersystem crossing. Nd3+ ions could be excited from the ground state 4I9/2 to 4F9/2 by energy transfer from organic ligands in the triplet state T, as shown in Fig. 6. After relaxation to the 4F3/2 level, the luminescence at 1330 nm (4F3/2→4I13/2), 1060 nm (4F3/2→4I11/2), and 900 nm (4F3/2→4I9/2) can be achieved.
When we consider the optical gain characteristics at 1060 nm in Nd3+ complex doped optical waveguide, a 1060 nm signal laser is needed to realize the stimulated radiation of Nd3+ ions from 4F3/2 to 4I11/2 state. Therefore, the radiation probability of 1060 nm is much higher than that of 900 nm (4F3/2→4I9/2) and 1330 nm (4F3/2→4I13/2). Ignoring the radiation probabilities of 900 nm and 1330 nm has little influence on the simulation results, but it can simplify the process of the solution for the rate equation. So we consider a four level system of Nd3+ ions as a simplification of Fig. 6, as shown in Fig. 7.
Therefore, the fundamental rate equation can be founded from Eqs. (1) to (8):
The optical gain G(dB), was calculated unding Eq. (9):
where Ps0, Ps(L) are the signal power at the input and output of the waveguide.4.3. Optical gain calculation
According to the theoretical model we have established, the relationship between optical gain and material parameters, such as Nd3+ ions concentration, stimulated absorption cross section of Nd(DBM)3(DBTDPO) doped PMMA polymer were simulated. The value of parameters used in the theoretical model are shown in Table 4. The radiative lifetime, stimulated emission cross section at 1060 nm and absorption cross section of 405 nm are calculated by J-O theory which is discussed in section 3.3. The calculation is based on the experimental absorption spectra of a 100um-thick Nd(DBM)3(DBTDPO) doped PMMA film.
Table 4. Main parameters used in the calculation
Figure 8 shows the gain as a function of pump power at 405 nm for different concentrations of Nd3+ ions. The gain and threshold pump power increase with increasing concentration of Nd3+ ions. When the pump power is 50 mW, the gain increases from 2.8 dB to 12.8 dB as the concentration of Nd3+ ions varies from1.8×1025 ions/m3 to 7.8×1025 ions/m3. It indicates that Nd3+ ions doping concentration is an important factor affecting the gain performance. Considering the actual solubility of the Nd(DBM)3(DBTDPO) in PMMA, we use the value of 3.8×1025 ions/m3 as the following simulated concentration parameter. The theoretical gain of 6 dB can be obtained at this concentration on a 2-cm-long waveguide.
Fig. 8. The optical gain as a function of pump power at 405 nm for different concentrations of Nd3+ ions.
The relationship between the stimulated absorption cross section at 405 nm with optical gain is shown in Fig. 9. The turn-on pump power (power required when generating gain) decreases from 37 mW to 3.3 mW as the stimulated absorption cross section at 405 nm increases from 0.5×10−23m2 to 5.1×10−23m2.From the simulation results, when the stimulated absorption cross section increased 10 times, the turn-on pump power reduced nearly 10 times correspondingly. That is because the larger the stimulated absorption cross section of the ligands, the higher the absorption efficiency of the material for the pump light. Therefore, at a lower pump power of 405 nm (about several mW orders of magnitude), sufficient energy can be absorbed through energy transfer between ligands and Nd3+ ions to help Nd3+ ions achieve population inversion. In general, the absorption cross section of organic ligands at 405 nm (10−22m2-10−23m2) [11] could be about 2 orders of magnitude larger than the intrinsic absorption cross section of Nd3+ ions at 808 nm (10−24m2-10−25m2) [10], which means that the turn-on pump power required will reduce by about 2 orders of magnitude correspondingly.
Fig. 9. The optical gain as a function of pump power at 405 nm for different stimulated absorption cross sections.
Figure 10 is the relationship between radiative lifetime of Nd3+ ions with optical gain. The longer the radiative lifetime of Nd3+ ions, the stronger the ability to store particles at this level. To achieve the same gain for the lifetimes of 398 µs, 39.8 µs and 3.9 µs, the pump power must reach 3.3 mW, 64 mW and 425 mW, respectively. The Fig. 10 shows that if the lifetime of the state 4F3/2 decreases one level, there is a corresponding increase in pump power so that the gain could keep at the same level. Therefore, the radiative lifetime is also an important factor affecting optical gain.
We compared the optical gain under 808 nm intrinsic excitation with that under 405 nm excitation based on intramolecular energy transfer, as shown in Fig. 11. The fundamental rate equation used to simulate gain under 808 nm excitation is referred in Ref. [8, 9]. The stimulated absorption cross section at 808 nm is 3.9×10−24m2 according to the absorption spectrum of Nd(DBM)3(DBTDPO) doped PMMA film. Other parameters, such as Nd3+ ions concentration, radiative lifetime and waveguide dimensions take the same value in Table 4. It can be shown that the pump power required for the waveguide to generate gain is 3.3 mW when excited by a 405 nm LED. For an 808 nm laser as the pumping source, a turn-on power of 40 mW is demanded. And compared with the maximum optical gain of 1.4 dB produced by an 808 nm laser with 60 mW power, the maximum gain of 6 dB can be obtained by a 405 nm LED with 25 mW. We explained this result from the point of view of the population inversion, as shown in Fig. 12. It reveals the population of Nd3+ ions at the ground state 4I11/2 and the metastable state 4F3/2 as a function of pump power. For generating optical gain, one of the necessary conditions is the population reversion, which requires the ion concentrations Ne in the metastable state be comparable with ion concentrations in the ground state Ng. When the pump power is low, the pumping energy is not enough to excite Nd3+ ions from ground state 4I11/2 to metastable state 4F3/2, so the necessary condition of population reversion can’t be achieved. When the pump power reaches 3.3 mW under 405 nm LED excitation, the value of Ne and Ng is exactly equal and the optical gain starts to be generated. Because of the stimulated absorption cross section of the organic ligands at 405 nm is much larger than that of Nd3+ ions at 808 nm and effective energy transfer from organic ligands in the triplet state T to Nd3+ ions, the needed turn-on power under 405 nm LED excitation at 3.3 mW is much lower than that of 808 nm excitation at 40 mW. In short, we drew a conclusion that polymer NDWA can achieve high optical gain at 1060 nm wavelength under a low power LED pumping.
Fig. 11. Calculated optical gain as a function of pump power for direct excitation using an 808 nm laser and for excitation via the ligands using a 405 nm LED.
Fig. 12. The population of Nd3+ ions in the ground state and the metastable state as a function of pump power under 405 nm and 808 nm excitation, respectively. Squares and triangles represent Ng and Ne, respectively.
5. Conclusion
We investigated the absorption and PL properties of Nd(DBM)3(DBTDPO) doped PMMA and Nd(DBM)3(DPEPO) doped PMMA polymer thin flims. The PL peaks at 900 nm, 1060 nm and 1330 nm wavelengths corresponding to the 4F3/2→4I9/2, 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions of Nd3+ ions were observed. The intramolecular energy transfer from the organic ligands DBM, DBTDPO and DPEPO to the central Nd3+ ions were confirmed. The parameters, such as radiative lifetime, absorption and emission cross-section were calculated using J–O theory. A system model of intramolecular energy transfer between organic ligands with central Nd3+ ions to simulate the optical gain was established. We discussed the relationships between different parameters and the optical gain of Nd(DBM)3(DBTDPO) doped PMMA polymer optical waveguide amplifier. According to the intramolecular energy transfer effect, a small-sized polymer NDWA can achieve high optical gain at 1060 nm wavelength under a low-cost LED excitation.
Funding
Xiamen University (Principal's Fund 20720150086); National Natural Science Foundation of China (61875170).
Disclosures
The authors declare no conflicts of interest.
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