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Abstract
This work reports on the analysis of near-IR and mid-IR luminescence spectra and their decays in fluoroindate glasses co-doped with Er3+/Tm3+. In particular, the energy transfer processes between rare earth ions in fluoroindate glasses pumped by 796 nm and 980 nm laser diode have been examined. Owing to donor-acceptor energy transfer and superposition of 1.45 µm (Tm3+: 3H4 → 3F4) and 1.55 µm (Er3+: 4I13/2 → 4I15/2) radiative transitions in fluoride glass co-doped with 0.1ErF3/0.3TmF3, a broadband near-IR luminescence in the range of third telecommunication window (FWHM = 155 nm, λexc = 796 nm) was obtained. Further analysis in the mid-IR spectral range (2.77 µm, λexc = 980 nm) showed that fluoroindate glass with 0.1ErF3/0.3TmF3 enables enhancement of luminescence intensity by c.a 14%.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years fluoroindate glasses and glass fibers have attracted wide interest due to their potential application in visible as well as an eye-safe spectral range such as optical amplifiers, sources of radiation and sensors. Nowadays there is a great interest in compact sources of radiation (ASE and lasers) operating in the near-IR (1.5 - 2 µm) and mid-IR (2.8 µm) spectral ranges. The demand for this type of sources results from the huge application potential, i.e. telemetry, optical laser systems, microsurgery, medical diagnostics and monitoring of industrial and environmental pollutants [1–9]. Fluoride glasses present indeed wide infrared transmission window and low phonon energy which makes them attractive as a potential glass-hosts for Rare Earths (RE) ions. In particular fluoroindate fibers [10,11] have been used successfully to propose supercontinuum sources [12–15] and lasers operating up to 5.4 µm [12–17]. Fluoroindate glasses due to their lower phonon energy (510 cm-1) than ZBLAN (600 cm-1) and extended infrared transmission are currently proposed as promising materials for MID-IR applications. It should be also noted that fluoride glasses and fibers belong to the advance materials which require a special conditions for manufacture as they has tendency to crystallization in the OH- and O2 presence. Also for that reason the longest wavelength from a ZBLAN fiber laser to date was based on holmium doping, with emission at 3.9 µm, albeit with cryogenic cooling as a necessity [16,17]. Development of fiber laser sources operating in the entire 3–5 µm window thus requires the focus to shift from ZBLAN to glasses with further reduced phonon energy.
In general, fluoroindate glasses show better thermal stability in comparison to the well-known commercial fluorozirconate glasses (ZBLAN). Besides, with respect to other non-oxide glasses, i.e. fluorozirconate and chalcogenide glasses, they are characterized by better mechanical properties and resistance to chemical corrosion. The experimental results suggest that fluoroindate glass fiber presents better chemical stability in water than ZBLAN [18]. These properties are essential to conclude that RE-doped fluoroindate glasses belonging to non-oxide glass family [19] are promising fluoride materials for numerous luminescent applications [20,21].
In fact fluoroindate RE- doped glasses have been mainly proposed as low-phonon materials for up-conversion luminescence applications [22–29] while the studies in the Near-Mid IR range are still limited to singly doped materials [30]. In this aspect direction into the optimization of glass compositions and their sensitization by rare earth co-doping (content and donor/acceptor ratio) to obtain efficient seems to be justified when we consider results in other fluoride glasses. A special attention has been paid to Er3+/Tm3+ co-doped glasses emitting near-IR and mid-IR luminescence. The systematic spectroscopic investigations for non-oxide glasses such as chalcohalide [31] and fluoride (ZBLAN) glasses [32] as well as low-phonon oxide (tellurite) glasses [33] revealed that Er3+/Tm3+ co-doped glass systems are very attractive candidates for near-IR and mid-IR emitting sources due to the presence of luminescence bands originated to Tm3+:3H4→3F4 (1.45 µm), Er3+:4I13/2→4I15/2 (1.55 µm), Tm3+:3F4→3H6 (1.8 µm) and Er3+:4I11/2→4I13/2 (2.77 µm) electronic transitions of trivalent rare earths. The luminescence features in the range of 1.4 µm to 1.9 µm, reported in particular for antimony-germanate and tellurite glass systems containing Er3+/Tm3+ ions with extremely large bandwidths of 420 nm [34] and 473 nm [35], make these glasses as an attractive candidates for solid-state laser sources and broadband optical fiber amplifiers in the third optical communication window. These aspects have not been examined for fluoroindate glasses co-doped with Er3+/Tm3+ yet.
In this paper, Er3+/Tm3+ co-doped fluoroindate glasses were fabricated and their luminescence properties with a special regard to the energy transfer processes between rare earth ions have been examined for the first time. In particular, the effects of optical pumping by laser diodes (@796 nm, @980 nm) on spectroscopic properties of multicomponent InF3–ZnF2–BaF2–SrF2–GaF3–LaF3 glasses co-doped with Er3+/Tm3+ are presented and discussed in details. Our investigations concern on the influence of acceptor concentration on the energy transfer mechanisms in fluoroindate glasses leading to the near-IR and mid-IR luminescence.
2. Experimental
Fluoroindate glasses doped with rare earth ions were prepared according to the following molar compositions: (38-x)InF3-20ZnF2-20SrF2-16BaF2-4GaF3-2LaF3-xLnF3 (x = 0.1; Ln = Er, Tm) and (38-x-y)InF3-20ZnF2-20SrF2-16BaF2-4GaF3-2LaF3-xErF3–yTmF3, (x = 0.1; y = 0.1, 0.2, 0.3) by melting (platinum crucible) and quenching method in glove box in a nitrogen atmosphere. To fluorinate oxide impurities in the batch, ammonium bifluoride (NH4HF2) was added to the batch before melting. The glass batches were firstly fluorinated at 270°C for 2 hours and then melted at 900°C for 1 hour. Finally, the glass was cast into a stainless steel plate and then annealed at 290°C for 2 hours slowly cooled to room temperature to minimize residual internal stress during the quenching process. Glasses were cut (size 10 × 10 × 2 mm) and prepared to perform optical measurements. FTIR spectra were recorded with a Bruker Company Vertex 70v spectrometer. Spectroscopic measurements in the wide range of 1100–2900 nm were carried out using Acton 2300i monochromator equipped with PbS detector in lock-in detection setup and a high power Roithner laser diodes (λexc = 980 nm, λexc = 796 nm, Popt(max)=1W). Luminescence decay measurements were performed using a system PTI QuantaMaster QM40 coupled with a tunable pulsed optical parametric oscillator (OPO), pumped by a third harmonic of a Nd:YAG laser (OpotekOpolette 355 LD). The laser system was equipped with a double 200 monochromator, a multimode UV-VIS photomultiplier tube (PMT) (R928) and Hamamatsu H10330B-75 detectors controlled by a computer. Luminescence decay curves were recorded and stored by a PTI ASOC-10 [USB-2500] oscilloscope.
3. Results and discussion
3.1 Luminescence properties
Fluoroindate glasses co-doped with Er3+/Tm3+ were synthesized and then studied using luminescence spectroscopy. Transmittance of the fabricated glasses was c.a 90%. In order to (a) compare luminescence spectra and their decays as well as (b) to determine the energy transfer processes between rare earth ions (Ln3+), fluoroindate glasses singly doped with Er3+ and Tm3+ ions were prepared. The previously published work indicates that thermal stability, physicochemical properties and crystallization processes in fluoroindate glass depend critically on activator concentration [36]. In this case, the molar concentrations of trivalent rare earth ions are quite low (0.1÷0.3%) and Ln3+-Ln3+ interactions are relatively small. Thus, luminescence quenching due to increasing activator concentration is practically not observed in contrast to Ln3+ highly concentrated glass samples (above 1 mol%). All received glass samples are transparent and fully amorphous, which was verified by X-ray diffraction measurements. The near-IR emission spectra of fluoroindate glasses containing 0.1ErF3 and 0.1ErF3/xTmF3 under laser diode excitation at 796 nm are presented in Fig. 1. For Er3+ singly doped glass, the emission peak centered at 1548 nm with a full width at half maximum (FWHM) of ∼69 nm is attributed to the well-known Er3+:4I13/2→4I15/2 near-IR transition. The addition of the TmF3 caused a significant broadening of the emission spectra, starting from 1350 nm to 2100 nm with deep at 1600 nm. For Er3+/Tm3+ co-doped glass samples with various concentrations of thulium ions, the near-IR emission spectra are broader resulting from the superposition of the following transitions Er3+:4I13/2→4I15/2 and Tm3+:3H4→3F4, Tm3+:3F4→3H6 ions, respectively. The 796 nm laser diode excites both the Tm3+:3H4 and Er3+:4I9/2 levels of rare earth ions. Thus, the near-IR emission band centered at 1450 nm is due to 3H4 → 3F4 transition of Tm3+ which partially overlaps with the emission of Er3+ near 1548 nm. Consequently, the FWHM value for fluoride glass with 0.1ErF3/0.3TmF3 is expanded to 155 nm (inset of Fig. 1) and covers optical telecommunication bands S (1440-1530 nm), C + L (1530-1600 nm) and U (1600-1675 nm), respectively. It should be also noted that FHWM value is much larger than that one (90 nm) obtained for Er3+/Tm3+ co-doped silica fiber [35] and comparable with experimental results for low-phonon oxide glasses [36–39]. The results are given in Table 1.
Fig. 1. Near-IR luminescence spectra of fluoroindate glasses with 0.1ErF3 and 0.1ErF3/(0.1-0.3)TmF3. Inset shows zoom of the 1550 nm region.
Table 1. Selected spectroscopic parameters for Er3+ singly doped and Er3+/Tm3+ co-doped fluoroindate glasses.
Further spectroscopic investigations indicate that co-doping with thulium increases intensity of near-IR emission band at 1800nm (Fig. 2) corresponding to Tm3+:3F4→3H6 transition. Thus, the luminescence intensity ratio I1800nm/I1550nm is close to 1.8 for glass sample co-doped with 0.1ErF3/0.3TmF3. This phenomenon is due to direct excitation of Tm3+ ions at 796 nm and the energy transfer processes Er3+ → Tm3+ referred as ET1, ET2 and ET4 on the simplified energy diagram (up-conversion luminescence wasn’t included) shown in Fig. 3.
Fig. 3. Simplified energy level diagram of Er3+/Tm3+ co-doped fluoroindate glass. The energy transfer mechanisms ET1, ET2, ET3 and ET4 under excitation at 796 nm are also indicated.
The following energy transfer processes ET1, ET2, ET3 and ET4 between Er3+ and Tm3+ ions are observed in fluoroindate glasses. Firstly, the Er3+: 4I9/2 and Tm3+: 3H4 excited states are well populated due to ground state absorption (GSA) under efficient pumping at 796 nm. Then, the excitation energy relaxes non-radiatively from 4I9/2 state to the lower-lying 4I11/2 state of erbium ions and part of the energy is transferred to the Tm3+: 3H5 state (ET2). Consequently, the lower-lying emitting Tm3+: 3F4 state is well populated by multi-phonon relaxation process due to relatively small energy gaps between these excited states of thulium ions. Depopulation mechanisms of the Er3+: 4I11/2 excited state are as follows: a) 4I11/2 → 4I13/2 radiative transition (2.77 µm), b) 4I11/2 → 4I15/2 radiative transition (0.98 µm), c) 4I11/2 → 4I13/2 non-radiative transition, and d) 4I13/2→ 4I15/2 radiative transition (1.55 µm). It is also important to notice that part of the excitation energy is transferred from the Er3+: 4I13/2 state to the Tm3+: 3F4 state (ET1), giving 3F4 → 3H6 radiative transition (1.8 µm) of thulium ions. Simultaneously, part of the energy gives important contribution to the population of the higher-lying Er3+: 2H11/2 state by excited state absorption process (ESA). The excitation energy relaxes to the Er3+: 4F9/2 state very fast and then is well transferred to the Tm3+: 2F2,3 states (ET4). The energy transfer process from Tm3+ to Er3+ ions is also observed. The Tm3+: 3H4 excited state is quite well depopulated by ET3 energy transfer to Er3+ ions and 3H4 → 3F4 radiative transition (1.45 µm) of thulium ions. According to Forster-Dexter theory [40], higher absorption cross-section of Tm3+ ions (than Er3+) at 796 nm leads to the direct energy transfer exist between Tm3+→Er3+ (ET3). This mechanism (ET3) is responsible for the population of 4I11/2 trough fast multiphonon relaxation from 4I9/2. Simultaneously, depopulation of 3H4 is observed which is proofed by the lifetime reduction. (Fig. 4). These two phenomena confirm that Tm3+ is a donor ions in this system. At the same time, cross-relaxation process between (CR) Er3+ and Tm3+ occurs. It should be noted that CR mechanism is more efficient for higher concentration of Tm3+ ions [41]. However, due to small difference between 4I13/2 (Er3+) and 3F4(Tm3+) energy levels the quasi-resonance energy transfer occurred (ET1). Hence, both analyzed mechanisms confirm that population of 3F4 level of Tm3+ increase and can promote emission at 1.8 um. The energy transfer mechanisms (λexc=796 nm) are schematically described as follows [26,27]:
Above results enable to state that optimized Er3+-Tm3+ co-doped fluoroindate fibers can be used for the construction of the compact fiber amplifier for WDM system operating in the S + C+L bands (1460-1625 nm) as well as a broadband source of radiation operating in the 1.8 µm spectral range. Analysis of luminescence decay measurements in glass samples doped with Tm3+ and co-doped with Er3+/Tm3+ presented in Fig. 4 enable to calculate the efficiency of Tm3+→Er3+ energy transfer (ET3) according to the equation:
where: $\tau _{\textrm{Tm}}^{\textrm{Tm} - \textrm{Er}}$ is the lifetime of Tm3+:3H4 in the presence of Er3+, ${\tau _{\textrm{Tm}}}$ is the lifetime of Tm3+:3H4 in thulium-singly doped fluoroindate glass. Dependence of Tm3+: 3H4 lifetime and the energy transfer efficiency varying with thulium content is presented in Fig. 5. The decay curve of Tm3+:3H4 (singly-doped glass) is characterized by single-exponential behaviour. The luminescence decay of the Er3+/Tm3+ co-doped glasses have been characterized by double-exponential behaviour and have been fitted by using the sum of two exponential decay components. It can be result of cross-relaxation (CR) process between Er3+ and Tm3+ as well as ET3 energy transfer [32,42, 44].Fig. 5. The 3H4 luminescence lifetime and the energy transfer efficiency as a function of Tm3+ content.
According to Eq. (3), the average lifetimes of 3F4 energy level of Tm3+ in the fabricated germanate glasses were calculated.
The experimental results indicate that the 3H4 lifetime of Tm3+ ions in fluoroindate glasses is reduced from 1.9 ms (0.1TmF3) to 1.4 ms (0.1ErF3/0.3TmF3). Thus, the efficiency of energy transfer Tm3+ → Er3+ increases with increasing thulium concentration as the result of smaller distance between the interacting lanthanide ions. Maximum efficiency of quasi-resonant energy transfer process Tm3+ → Er3+ (ET3) seems to be 25.5% (Fig. 5).
Luminescence decay curves for the 4I13/2 state of erbium ions in fluoroindate glasses were also examined (Fig. 6(a)). For erbium-singly doped fluoroindate glass, the measured 4I13/2 luminescence lifetime τmeas is equal to 8.6 ms and its value is quite well consistent with the radiative lifetime (τrad = 8.62 ms) calculated from the Judd-Ofelt framework [40]. Thus, the quantum efficiency of the upper laser 4I13/2 excited state of Er3+ ions seems to be nearly 100% [45]. For glass sample co-doped with Er3+/Tm3+, the 4I13/2 luminescence lifetime is reduced from 8.6 ms (0.1ErF3) to 6.1 ms (0.1ErF3/0.3TmF3) indicating Er3+ → Tm3+ energy transfer (ET1) with efficiency of 29% (Fig. 6(b)). It is in a good agreement with the value ηET (31%) obtained for the energy transfer process Tm3+ → Er3+ (ET3), discussed above. However, it is interesting to see that the efficiency for the Er3+ → Tm3+ energy transfer process (ET1) in fluoroindate glass is relatively low (29%) compared to the results for Er3+/Tm3+ co-doped oxide based glasses (ηET = 77-80%). On the other hand, the measured 4I13/2 luminescence lifetime of Er3+ ions in fluoroindate glass in nearly twice higher than that obtained for oxide glass-host matrices (see Table 1). This behaviour is mainly due to the very low phonon energy of the fluoroindate glass-host (510 cm-1). Also, low-phonon energy of fluoroindate glass enables obtaining mid-IR luminescence. It should be mentioned OH- content in fabricated glasses is below 1 ppm (confirmed by FTIR measurements). Thus 2.77 µm emission is not quenched by OH- ions. Figure 7 presents comparison of mid-IR luminescence spectra centered at 2.77 µm, which were measured for 0.1ErF3 singly doped and 0.1ErF3/0.3TmF3 co-doped fluoroindate glasses under 980 nm laser diode excitation. The energy transfer mechanisms between Er3+ and Tm3+ ions in fluoroindate glass can be written as follows:
Fig. 6. (a) Luminescence decay curves from Er3+: 4I13/2 state, (b) the energy transfer efficiency as a function of Tm3+ content (inset), λexc = 796 nm.
Fig. 7. Mid-IR luminescence spectra of 0.1ErF3 doped and 0.1ErF3/0.3TmF3 co-doped fluoroindate glasses. All transitions are indicated on the simplified energy scheme (inset), λexc = 980 nm.
Moreover it is also important to note that up-conversion luminescence processes in Er3+/Tm3+ co-doped glass systems play the important role and cannot be ignored. The up-conversion luminescence spectra corresponding to 2H11/2,4S3/2 → 4I15/2 (green) and 4F9/2 → 4I15/2 (red) transitions of Er3+ ions are changed significantly with increasing Tm3+ concentration. Thus, several Tm3+ ↔ Er3+ energy transfer processes in glass systems occur. They have been well presented and discussed in the excellent papers published recently [38,39,43]. The presence of energy transfer between Tm3+ and Er3+ ions give also important contribution to depopulation of the 4I11/2 and 4I13/2 states and corresponding near-IR and mid-IR emissions of erbium. However, these aspects concerning up-conversion luminescence processes and their mechanisms in Er3+/Tm3+ co-doped fluoroindate glasses will be examined in our future work.
4. Conclusions
In conclusion, near-IR and mid-IR luminescence properties and energy transfer processes in Tm3+/Er3+ co-doped fluoroindate glasses under 796 nm and 980 nm laser pumping have been analyzed. The measured luminescence correspond to the following bands: Tm3+:3H4→3F4 (1.45 µm), Er3+:4I13/2→4I15/2 (1.55 µm), Tm3+:3F4→3H6 (1.8 µm) and Er3+:4I11/2→4I13/2 (2.77 µm) of electronic transitions of rare earth ions. Broadband near-IR luminescence near 1.55 µm with the highest value of FWHM = 155 nm resulting from the partial energy transfer process Tm3+→Er3+ and the superposition of Er3+:4I13/2→4I15/2 and Tm3+:3H4→3F4, Tm3+: 3F4→3H6 transitions was successfully obtained for fluoroindate glass co-doped with 0.1ErF3/0.3TmF3. The obtained ultra-broad emission band covers the third telecommunication window at 1440−1530 nm, and so-called C + L (1530−1600 nm) and U (1600−1675 nm) bands, implying the promising application of the present glass as a novel material for the broadband near-IR optical amplifier in the WDM transmission systems. Based on decay curve measurements, the 3H4 (Tm3+) and 4I13/2 (Er3+) luminescence lifetimes were determined and the energy transfer efficiencies Tm3+→Er3+ and Er3+→Tm3+ were calculated. Independently on direction of the energy transfer process Tm3+↔Er3+, the values of ηET are close nearly to 30 ± 1%. Further spectroscopic analysis of mid-IR luminescence spectra related to Er3+:4I11/2→4I13/2 transition at 2.77 µm showed an enhancement of emission intensity by c.a 14%. It is due to depopulation of the 4I13/2 state through the energy transfer process from Er3+ (4I13/2) to Tm3+ (3F4) ions in fluoroindate glass. Our results demonstrate that fluoroindate glasses co-doped with Tm3+/Er3+ open the potential application in construction of novel broadband optical fiber amplifiers, ASE sources and solid-state lasers operating in the near-IR and mid-IR spectral ranges.
Funding
Narodowe Centrum Nauki (Decision No. 2017/25/B/ST8/02530.).
Disclosures
The authors declare no conflicts of interest.
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