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Abstract
Glasses of composition xRE2O3-(17−x) La2O3-83 TiO2 were prepared by levitation melting from x = 0.1 to 9 for RE=Er and at x =0.1 and 1 for RE=Dy. The glasses have high transition temperature, exhibit low OH, and 1 mm thick discs are transparent out to 6 μm. Mid-infrared emission lineshapes and lifetimes are comparable to what is seen in tellurite glasses containing Er3+ and Dy3+. For x fixed at 1, the Er3+:4I11/2 → 4I13/2 transition at 2716 nm has a fluorescence lifetime of 254 μs and the Dy3+:6H13/2 → 6H15/2 transition at 2957 nm has a fluorescence lifetime of 9.09 μs. The results indicate that doped lanthanum titanate glasses offer the thermal stability of a ’hard’ glass with the host properties typically associated with ’soft’ glasses. Problems with the measurement of spectral features in the mid-infrared that could erroneously be assigned as resulting from rare-earth ion emissions are presented and discussed.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-power and compact mid-infrared (MIR) sources and detectors are currently in demand for industrial, directed energy, remote sensing, spectroscopic and biomedical applications [1]. Silica fiber lasers offer a choice solution for sources up to 2 $\mu$m, though they are generally impractical for wavelengths beyond 2.5 $\mu$m. Current compact mid-infrared sources that are commercially available employ fluoride fiber lasers, in which significant progress has been made in recent years [2,3]. However, fluoride glasses have a low glass transition temperature (T$_g$), roughly between 530-570 K, making them prone to damage at high pump powers [4]. Aside from low thermal stability, fluoride-based glasses display comparatively poor chemical and mechanical properties relative to oxide glasses.
Efforts to provide alternatives to fluoride glasses for MIR applications that have better physical properties focus usually on oxide glass systems, with tellurites receiving the most attention [5–8]. Other studied hosts include germanates [9], gallates [10], and ’heavy metal oxide’ glasses [11,12]. By far, the MIR transition most investigated in oxide materials is erbium’s $^4I_{11/2} \rightarrow ^4I_{13/2}$ transition at circa 2700 nm (see Fig. 1), which has only recently been made to lase in a tellurite glass fiber for the first time [13]. On the other hand, Dy$^{3+}$ is scarcely studied in oxide hosts, although Gomes et al. have indicated there is potential for Dy$^{3+}$ doped TeO$_2$ glasses to achieve population inversion beyond 3 $\mu$m at the Dy$^{3+}$:$^6H_{13/2} \rightarrow ^6H_{15/2}$ transition (see Fig. 1) [6]. This transition belonging to Dy$^{3+}$ has received a great deal of attention in fluoride hosts recently due, in part, to the large tuning bandwidth [14,15] and in-band pumping potential [16,17]. Despite continuing progress in fluoride glasses, the enhanced durability enabled by oxygen cross-linked network formers remains of interest. In addition to active optical elements and gain media, a diverse portfolio of robust materials with differing properties is desirable for the design of lens arrays, beam delivery, frequency conversion, and other optical functions.
Fig. 1. Jablonski diagrams of Er$^{3+}$ (a) and Dy$^{3+}$ (b) showing energy levels most relevant to this study.
Rare-earth titanates prepared by containerless melting techniques are promising materials for optical and photonic applications [18–20]. Rare-earth titanates have glass transition temperatures near 1070 K, which gives them excellent thermal stability compared to fluorides and the vast majority of soft glasses [18,21]. For example, tellurites typically have glass transition temperatures between 575-700 K [22,23]. The uniquely high average coordination number of the network-forming titanate polyhedra [24] produces a phonon energy comparable to that of soft oxide glasses typically studied for photonic applications [25,26]. Glass formation is restricted to the larger rare-earth elements [19], but partial substitution of several smaller elements for La has been shown [27]. Up-conversion luminescence has been demonstrated [27] but near- and mid-infrared luminescence have not been investigated. It is also notable that the refractive index and dispersion are both high, with $n_d \approx 2.3$ and $\nu _d \approx 20$ [28,29].
2. Glass preparation
In this work, we have used the levitation melting technique to prepare glasses of nominal composition $x$RE$_2$O$_3$-(17$-x$) La$_2$O$_3$-83 TiO$_2$ with $x=$ 0 to 9 for RE$=$Er ($x$ErLT) and at $x=$0.1 and 1 for RE$=$Dy ($x$DyLT). Details of the glass preparation are akin to those reported elsewhere [19,24] and a temperature versus time plot of the melting process is given in Supplement 1. Crystallization was encountered at $x=$ 11 for RE$=$Er, where the mean ionic radius of the lanthanide constituents reached 0.940 Å. This result is in good agreement with the lower limit of the lanthanide ionic radius for binary rare-earth titanates as discussed by Alderman et al. [19]. The synthesized glass spheres of about 2 mm diameter were mounted in acrylic, then ground and polished on two opposite sides to form disks of about 1 mm thickness.
3. Results and discussion
To confirm composition, glass discs were sputter-coated with 4 nm of Au/Pd and analyzed with scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS, Hitachi SU8030). EDS was collected at 12 sites across each glass disc. The TiO$_2$ content of each specimen was within 1.0 mol% of the nominal values, and the Er$_2$O$_3$ content was generally within 0.2 mol% of the nominal values. Note that the measured variations of 0.2 mol% are likely smaller than the EDS method’s precision.
The infrared transmission spectrum (Shimadzu IRSpirit FTIR Spectrophotometer) is shown in Fig. 2(a) for a glass with 4% Er$_2$O$_3$. The glasses show good transparency throughout the whole 3-5 $\mu$m atmospheric window and reach 6 dB of the original transmission at 1655 cm$^{-1} ($6 $\mu \rm {m}$). The low hydroxyl species content of the glasses is evidenced by the nearly uninterrupted transmission between 3000-3500 cm$^{-1}$. This is highlighted with the inset of Fig. 2(a), where less than 2% loss in transmission is observed in the region that OH$^{-}$ species in glasses have characteristic absorption bands. The UV cutoff, taken here where the absorbance per cm goes to 6, of the lanthanum titanate base glass (Fig. 2(b)) is at a frequency of 25751 cm$^{-1}$ (388 nm). This gives a 6 dB bandwidth of 24096 cm$^{-1}$ for glassy lanthanum titanate discs with a thickness of roughly 1 mm.
Fig. 2. (a) Infrared transmission of 4 Er$_2$O$_3$-13 La$_2$O$_3$-83 TiO$_2$ glass. Not corrected for Fresnel losses. (b) measured UV-VIS absorbance and (c) Normalized Raman gain spectrum of 17 La$_2$O$_3$-83 TiO$_2$ glass.
The spontaneous Raman scattering (Horiba LabRAM HR Evolution) was corrected for the Bose-Einstein thermal population factor to give the Raman gain lineshape [30] (Fig. 2(c)). Stretching modes of the diverse network of titanate polyhedra [24] give rise to a broad high frequency envelope with a maximum at 743 cm$^{-1}$ and a shoulder at about 850 cm$^{-1}$. Defined here as the 50% threshold, the nominal Raman gain bandwidth is 315 cm$^{-1}$, greater than silica (200 cm$^{-1}$) and more than twice that of tellurites (140 cm$^{-1}$) [1].
Near-infrared absorption measurements (Agilent Cary 7000 UV-VIS-NIR Spectrophotometer) were made in transmission mode on glasses containing 1% active ion concentrations (see Fig. 3). For 1DyLT, band maxima are recorded at 904, 1096, 1277, and 1687 nm. For 1ErLT, band maxima are recorded at 980 and 1533 nm. The corresponding upper energy state arrived at after ground state absorption are indicated by the corresponding band in Fig. 3.
Fig. 3. Baseline corrected absorbance spectra of 1RE$_2$O$_3$-16La$_2$O$_3$-83 TiO$_2$ with RE=Dy in blue (dashed) and RE=Er in red (solid).
The widely used Er$^{3+}$:$^4I_{13/2} \rightarrow ^4I_{15/2}$ emission (measured with Yokogawa AQ6370D OSA) is shown in Fig. 4(a) (blue line) for the glass with the lowest erbium concentration (0.1 mol%) so that reabsorption effects are negligible. The emission band, peaking at 1533 nm, is characterized by a mean wavelength of 1542 nm and an effective linewidth of 69 nm. The lanthanum titanate glass is shown alongside measurements of NIR erbium emission from a soda lime silicate glass (NCS, black) and a fluoride phosphate glass (FP20, red). A large degree of inhomogeneous broadening in the lanthanum titanate host can be well seen by comparison of the lineshape to that of the NCS and FP20 hosts. The ErLT spectrum is much broader than the NCS host and only slightly broader than FP20. In FP20 glasses, mixed coordination environments of the RE$^{3+}$ cation with fluorine and oxygen promote broad emission lineshapes. Large degrees of inhomogeneous broadening, such as seen here, are often suitable for broad tuning ranges.
Fig. 4. (a) Static NIR fluorescence of ErLT compared to silicate and fluoride phosphate hosts (offset for clarity). (b) Time-domain fluorescence of 1.5 $\mu$m NIR emission in ErLT. (c) Mean lifetime values for Er$^{3+}$ $^4I_{13/2} \rightarrow ^4I_{15/2}$ versus erbium concentration, x. Red dotted line is intended to guide the eye.
The lifetime of the transition is 3.0$\pm$0.2 ms for 0.1ErLT (Fig. 4(b)), determined by using the expectation value integral of normalized fluorescence intensity, I(t), $\langle \tau \rangle = \frac {\int t I(t) dt}{\int I(t) dt}$, with t$=0$ set at $I(t)=$1. This is comparable to sodium zinc lanthanum tellurite (NZLT) glasses, where intrinsic values of 3.27 ms are reported [5]. The measured lifetimes (Fig. 4(c)) initially increase with increasing erbium concentration due to reabsorption effects, then fall due to quenching.
Figure 5(a) plots the Er$^{3+}$ $^4I_{11/2} \rightarrow ^4I_{13/2}$ fluorescence decay curves measured using mechanically chopped 975 nm pump light (black dotted line). The decays are non-monoexponential and were calculated using the mean lifetime method above (Fig. 5(b)). The lifetime of the $^4I_{11/2} \rightarrow ^4I_{13/2}$ decay of 0.1ErLT was measured to be 279$\pm$22 $\mu \rm {s}$. This is in excellent agreement with what is reported in an Er$^{3+}$ doped lead lanthanum zirconate titanate (PLZT) transparent ceramic by de Camargo et al., who measured ${\tau }\approx$ 280 $\mu$s for the 2.7 $\mu \rm {m}$ emission decay [31]. In the classical laser crystal Er:YAG, this transition has a lifetime of about 120 $\mu$s [32,33]. The lifetime steadily decreases as $x$ increases from 0.1 to 9. At 9% Er$_2$O$_3$, the lifetime of the $^4I_{11/2}$ level is 163 $\mu$s. Overall, the lifetime of $^4I_{13/2}$ decreases at a faster rate than that of $^4I_{11/2}$, which is viewed as beneficial to laser operation of Er$^{3+}$ at $\approx$2.8 $\mu$m [34].
Fig. 5. (a) Er$^{3+}$ $^4I_{11/2} \rightarrow ^4I_{13/2}$ fluorescence decays of select ErLT glasses and (b) mean lifetime values as a function of erbium content, with linear fit to guide the eye. Measurements were made with a HgCdTe photodetector and a 2700 nm bandpass filter.
Visible and near-infrared upconversion at 525, 547, 660, and 820 nm took place in all samples under 975 nm excitation, as is typically seen for erbium-doped materials [35] and reported by Pan et al. [27] previously for erbium-doped lanthanum titanate glasses.
The mid-infrared fluorescence lineshapes of 1ErLT and 1DyLT are shown in Fig. 6. Spectra were acquired using the free space input port of a ThorLabs Redstone OSA305 scanning Michelson interferometer purged with flowing N$_2$. The spectrum for $^4I_{11/2} \rightarrow ^4I_{13/2}$ in 1ErLT was measured under 975 nm excitation ($^4I_{15/2} \rightarrow ^4I_{11/2}$) and required no correction. The lineshape is characterized by a peak wavelength at 2716 nm, a mean wavelength of 2746 nm and an effective linewidth of 125 nm.
Fig. 6. Area normalized mid-infrared fluorescence of 1RE$_2$O$_3$-16La$_2$O$_3$-83 TiO$_2$ with RE=Dy in blue (dashed) and RE=Er in red (solid).
The spectrum for 1DyLT was measured under 910 nm excitation ($^6H_{15/2} \rightarrow ^6F_{7/2}$). At the pump power used for acquisition, sample heating resulted in a thermal background, which was pronounced beyond 3500 nm. The background was fit with the functional form of a blackbody and subtracted (see Supplement 1). The corrected lineshape of the 1DyLT sample is characterized by a peak wavelength at 2957 nm, a mean wavelength of 2990 nm and an effective linewidth of 474 nm - although there is moderate uncertainty in these values given the necessity of the background correction. Overall though, the lineshape is very similar to that measured for sodium zinc tellurite (NZT) glass [6] as well as for yttrium aluminum perovskite (YAP) [36].
To measure the lifetime of the $^6H_{13/2} \rightarrow ^6H_{15/2}$ in dysprosium doped lanthanum titanate glasses, a Q-switched Nd:YAG laser at 1064 nm was used to excite $^6H_{15/2} \rightarrow ^6H_{7/2},^6F_{9/2}$ (see Fig. 3). The measured curves (Fig. 7) are described well by a bi-exponential to capture the filling of the $^6H_{13/2}$ level and the subsequent decay to $^6H_{15/2}$. The fitted rise times are 1.41 and 1.35 $\mu$s for 0.1DyLT and 1DyLT, respectively, indicating the lifetimes of $^6H_{7/2},^6F_{9/2},^6H_{9/2},^6F_{11/2}\,\rm {and}\,^6H_{11/2}$ levels are sufficiently short. The corresponding fitted decay times are 9.91 and 9.09 $\mu$s. It is worth mentioning here that Gomes et al. pumped an NZT glass with 3 wt% Dy$^{3+}$ at 805 nm and measured $\tau _{\rm {rise}}$=1.25 $\mu$s and $\tau _{\rm {decay}}$=9.7 $\mu$s [6].
Fig. 7. Dy$^{3+}$ $^6H_{13/2} \rightarrow ^6H_{15/2}$ fluorescence decays following 1064 nm pumping recorded with HgCdTe photodetector and 2950 nm bandpass filter. Discrete data points are experimentally measured and solid lines are bi-exponential fits.
In carrying out this work, it was observed that intrinsic losses of optical elements used for measurements combined with blackbody radiation emitted from the sample due to heating under laser irradiation can give the appearance of mid-infrared fluorescence bands between 3.0-5.0 $\mu$m. This can be especially pronounced in heavily doped glasses, in particular those containing erbium, where excited state absorption processes lead to significant internal heat generation [37].
The spectra presented in Fig. 8 illustrate the artifact bands due to heating of the sample under laser excitation, though the possibility of contributions from heating of the surrounding area by stray pump light cannot be excluded. The individual panels were each measured using comparable samples and power levels, but with different light collection arrangements as indicated on the right side of each panel. In Fig. 8(a), when the sample is excited immediately in front of the free space optical input and the fluorescence is collimated with a silicon lens before entering the spectrometer, the $^4I_{11/2} \rightarrow ^4I_{13/2}$ erbium emission is present and a long wavelength background steadily increases with increasing pump power. The increasing energy of this tail with increasing pump power ultimately leads it to overlapping with shorter wavelength fluorescence bands (see Supplement 1). The problems that can be encountered due to this overlap are illustrated in Fig. 8(b) and Fig. 8(c). In both cases, bands develop between 3-5 microns with increasing pump power. One might be led to assign such features to excited state processes in Er$^{3+}$, an ion whose complex energy level structure unwittingly welcomes erroneous interpretation. However, we find that these bands, which resemble reports in the literature [38], are not due to emission from any rare-earth ion.
Fig. 8. Dependence of measured optical spectra with increasing pump power for different light collection methodologies (a) free space, (b) Si-Ge lens array and ZrF$_4$ multimode fiber, and (c) SiO$_2$ and CaF$_2$ lens array and ZrF$_4$ multimode fiber. The broadband emission above 3 $\mu$m in (b) and (c) is not real fluorescence from erbium ions - see text for details.
Qualitatively, the lineshapes in Fig. 8(b) and Fig. 8(c) follow the absorption spectrum of the collection optics. In Fig. 8(c), the lens array includes a silica (Infrasil 302) and CaF$_2$ doublet as well as a 1 meter long ZrF$_4$ multimode fiber to deliver light to the spectrometer. The transmission of Infrasil falls off rapidly above 3.5 $\mu$m, similar to the measured envelope in Fig. 8(c). Replacing this doublet, which is sold for use below 3 $\mu$m, with a Si-Ge lens array optimized for 3-5 micron applications while using the same ZrF$_4$ multimode fiber produced the results in Fig. 8(b). A one meter long section of ZrF$_4$ is only expected to transmit, approximately, 50% of incident light around 4.4-4.6 microns, as it approaches 0% transmission above 5 $\mu$m for a 1 meter long propagation length. This would be consistent with the measured lineshape in Fig. 8(b), where the measured spectra also goes to zero in the same vicinity.
From a more quantitative vantage point, we could measure no fluorescence lifetimes between 3 and 4 microns employing various bandpass filters in this spectral range. The authors of Ref. [38] measured the lifetimes of the visible upconversion from the $^4S_{3/2}$ and $^4F_{9/2}$ levels, which yield green and red light emissions upon radiative relaxation to the ground state. However, each of these levels can generate 3.4-3.6 micron emission when relaxing to the level immediately below [39]. The authors of Ref. [38] reported 3.4-3.6 micron spectral activity, similar to that seen in Fig. 8(c) but measured the visible emission lifetimes of the $^4S_{3/2}$ and $^4F_{9/2}$ levels, which they found to be between 10-100 $\mu$s. On the other hand, Pan et al. reported lifetimes of the $^4S_{3/2}$ level in erbium doped lanthanum titanate glass to be about 150 $\mu$s. Like Pan et al., we observed strong green upconversion under near infrared excitation, as is typical of erbium doped materials [35] and is indicative that the $^4S_{3/2}$ level populates from excited state absorption in our glasses. Moreover, a lifetime on the order of 150 $\mu$s for the $^4S_{3/2}$ level suggests that if MIR radiative decay did indeed occur from the $^4S_{3/2}$ level, our experimental setup would have been entirely capable of recording this decay (20 $\mu$s pulses, 1 MHz bandwidth HgCdTe photodetector, 350 MHz oscilloscope). However, no signal could be detected beyond 3 microns with various bandpass filters, even when employing peak powers between 3 and 5 times larger than used when producing measurements in Fig. 8(b) and Fig. 8(c). Considering these experimental findings, we advise a healthy skepticism of reports claiming long wavelength mid-infrared emission in oxide glasses, or other materials where the employed excitation power is moderate to high.
As described above, the 2.7-3$\mu$m emission properties in titanates closely resembles that measured in tellurite systems. Tellurites have received attention for over two decades as a material family for MIR applications due to the promise of good optical performance coupled with superior physical properties relative to fluorides and chalcogenides (sulphides, selenides, tellurides). It has been shown here through the time-dependent fluorescence measurements that mixed rare-earth titanate glasses have nearly identical near- and mid-infrared behavior to that reported for tellurites [5,6,22,40], with much higher intrinsic thermal stability. The ’soft’ glass-like photonic properties of these glasses is likely rooted in their unique structural arrangements. Most glass-forming oxides produce three-dimensional networks of corner-sharing polyhedra where the average coordination number of the primary cation is 3 or 4 [41]. In contrast to this, rare-earth titanate glasses form an octahedral network with a significant fraction ($\approx 20-30{\%}$) of edge-sharing polyhedra [24] - strongly violating Zachariasen’s rules [42]. The phonon energy of corner and edge-sharing titanate polyhedra is approximately 740 and 630 cm$^{-1}$, respectively [43]. The latter of these is close to the 600 cm$^{-1}$ stretching frequency of corner-sharing AlF$_6$ octahedra in fluoroaluminate glasses [44]. These considerations suggest that the fairly low phonon energy sites (for a durable oxide glass) made possible by the unique rare-earth titanate glass network promote the desirable performance of active rare-earth ions.
Noting how Weber and co-workers pulled fibers from levitated rare-earth aluminate melts [45], it seems necessary to explore the possibility of drawing titanate fibers. Given the sensitivity of this process to melt viscosity, additional components may be necessary, though they should be chosen as to not increase the phonon energy of the system [46]. Their high glass transition temperature implies that fibers based on mixed rare-earth titanate glasses may be viable materials for high power, high energy, and/or high intensity applications in the mid-infrared - assuming background losses are not prohibitively high. For example (assuming a step-index architecture could be achieved by means of a pseudo-double crucible procedure or a post-processing method) Raman fiber lasers with high gain bandwidths look very promising. In terms of active discs or fiber, co-doping schemes may reasonably be expected to increase the efficiency, such as Er/Pr [11,47], Er/Nd [48], or Dy/Tm [49]. Of course, while the current synthesis route of levitation melting allows up to around $\approx$7 mm spheres to be produced (depending on the fabrication instrument and composition [50]), this is nowhere near the scale of large billets and preforms that can be produced with current technology using tellurites, germanates, gallates, and heavy metal oxide glasses [22,51–54]. Nevertheless, the similarity of the spectroscopic properties of titanate glasses to these more established candidate oxide systems as seen here motivates future study of optically active titanates simply for deeper insight into glassy state of matter. Moreover, the high doping concentrations allowed in the types of reluctant glass formers that can prepared by the levitation method allow short material lengths with high effective gain, or other strong optical effects, when in a fiber, disc, or sphere geometry. This allows, in addition to the prospect of short gain fiber, the fabrication of in-line fiber or on-chip devices [10,55–58].
4. Conclusion
We prepared lanthanum titanate glasses doped with erbium and dysprosium. Disc samples were transparent throughout the entire 3-5 $\mu$m atmospheric window and showed very low hydroxyl content. The diverse structural arrangement of the titanate polyhedra yields a broad Raman gain spectrum with a 315 cm$^{-1}$ 3 dB bandwidth. The near infrared fluorescence from erbium was similarly broad, comparable to mixed anion glasses, and displayed an intrinsic lifetime of about 3 ms. Mid-infrared fluorescence from Er$^{3+}$ and Dy$^{3+}$ were comparable to what is found in tellurites. Although the Dy$^{3+}$ lifetime is likely too short to allow efficient lasing, the Er$^{3+}$ MIR lifetime is very promising. Additional results from this study suggest that claims of efficient luminescence of Er$^{3+}$ or other rare-earth dopants beyond 3 $\mu$m may actually be misinterpreted data.
Funding
Air Force Research Laboratory (FA9451-22-2-0016); U.S. Department of Energy (DE-SC0018601); National Aeronautics and Space Administration (80NSSC19K1288).
Acknowledgments
This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is managed by Triad National Security, LLC for the U.S. Department of Energy’s NNSA, under contract 89233218CNA000001. M.T.P. acknowledges support from Laboratory Directed Research and Development (LDRD) award 20230014DR. EDS measurements were made at the EPIC Facility of Northwestern University’s NUANCE Center, which has received support from the SHyNE Resource (NSF-ECCS-2025633), the IIN, and Northwestern’s MRSEC program (NSF DMR-1720139). BT thanks Doris Ehrt for the Er doped NCS and FP20 glasses. BT gratefully acknowledges support in the form of a Postdoctoral Appointment under the supervision of Ganesh Balakrishnan.
Disclosures
The authors declare no conflicts 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.
Supplemental document
See Supplement 1 for supporting content.
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