Open Access
Add to BibSonomy
Abstract
Single-crystalline layers of 12 at.% Tb3+, 5 at.% Gd3+:LiYF4 were grown by the liquid phase epitaxy method on (001) oriented bulk undoped LiYF4 substrates using LiF as a solvent. The growth temperature was 737–740 °C, the growth duration was 15 - 25 min, and the layer thickness was 40–90 µm. The structural, morphological, vibronic and spectroscopic properties of the layers were studied. Tb3+ ions were uniformly distributed in the layers with no diffusion into the substrate. Polarized Raman spectroscopy confirmed the orientation of the layers (growth along the [001] direction). Under excitation in the blue, the layers exhibited intense green emission. For the 5D4 → 7F5 Tb3+ transition, the peak stimulated-emission cross-section is 1.28 × 10−21 cm2 at 542.0 nm for π-polarization. The luminescence lifetime of the 5D4 Tb3+ state is 5.05 ms at room temperature. The crystal-field splitting of Tb3+ multiplets was determined at low temperature. The developed epitaxies are promising for green and yellow waveguide lasers.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Trivalent terbium ions (Tb3+) possess an electronic configuration of [Xe]4f8, with a group of lower-lying 7FJ multiplets (J = 0 - 6, where 7F6 is the ground state) and a metastable state 5D4. The Tb3+ ion presents multi-color emissions in the visible due to the 5D4 → 7FJ transitions, with the most intense one at ∼544 nm falling into the green spectral range (the 5D4 → 7F5 transition) [1]. The 5D4 Tb3+ state does not suffer from cross-relaxation and presents a relatively long lifetime (about 5 ms for fluoride materials) even at high Tb3+ doping concentrations [2]. One of the difficulties for pumping terbium ions is that the transition in absorption 7F6 → 5D4 is spin-forbidden, so it corresponds to very low absorption cross-section, in the order of 10−22 cm2. Still, terbium ions can be efficiently excited in the UV spectral range [3].
Tb3+-doped materials are widely used as green phosphors [4–6], e.g., in fluorescent lamps. Eu2+-and Eu3+-based blue / red phosphors combined with Tb3+-based ones can provide trichromatic lighting technology. Terbium is also used as a probe in biochemistry [7,8]. Note that terbium-containing crystals are also often used in magneto-optical devices, because of their large effective magnetic moment and high paramagnetic susceptibilities. Recent studies show that Faraday rotator properties can be improved by incorporating Tb [9,10].
Tetravalent Tb4+ ions also exist, but previous studies have shown that they do not contribute to any visible emission and just act as quenching centers, with absorption bands in the visible [11,12].
Tb3+ ions are also suitable for generating laser emission in two distinct spectral ranges: in the mid-IR for low-phonon-energy hosts like chalcogenide glasses (hνph ∼ 250 - 350 cm-1), with continuous-wave laser emission observed at 4.9 - 5.5 µm (the 7F5 → 7F6 transition) [13,14], and in the visible with fluoride crystals, mainly LiYF4, at 544 nm (green lasers) and 587 nm (yellow ones) [15–17]. Efficient lasing in the visible was difficult to obtain at first because of the low absorption cross-sections and the risk of exited-state absorption. The first significant results about a Tb3+ visible laser were published in 2007 [18]. Currently, the main trends for the development of visible Tb3+ lasers are: (i) the use of blue GaN laser diodes as pump sources, and (ii) the use of heavily doped materials for boosting the pump absorption [15]. Lasing in Tb3+-based stoichiometric crystals (LiTbF4, TbF3) was also achieved [1,2].
Currently, Tb3+ visible lasers are based only on fluoride crystals, mainly LiYF4, which is a well-known laser host matrix. It has a scheelite (CaWO4) like tetragonal structure and belongs to the I41/a space group. It has a broad transparency range (0.12 - 7.5 µm) and exhibits low phonon energy (hνph = 444 cm-1). LiYF4 features good thermo-mechanical properties, i.e., high thermal conductivity and weak anisotropy of thermal expansion. It has a single substitution rare-earth site (Y3+ site, S4 symmetry), and in the case of Tb3+ doping, Li1-xTbxF4 solid-solutions exist up to 100 at.% substitution, i.e., reaching the LiTbF4 stoichiometric composition [19].
Liquid Phase Epitaxy (LPE) is a well-known method for growing oriented single-crystalline layers of high optical quality which are particularly well suited for applications as planar laser waveguides [20]. LiYF4 is very suitable for the LPE growth [21] and crystalline LiYF4 layers doped with various rare-earth ions such as Tm3+ [22], Er3+ [23], Yb3+ [24], Ho3+ [25] or Pr3+ [26] have already been elaborated by LPE, including heavily doped (20 at.%) ones [23,27,28]. In particular, visible waveguide lasers have been demonstrated employing Pr:LiYF4 epitaxial layers codoped with Gd3+ and Lu3+ buffer ions to address the green (522 nm), red (639 nm) and orange (605 nm) spectral ranges [26]. LPE is capable of producing low-loss (down to 0.14 dB/cm in the red) LiYF4-based crystalline layers, with a thickness between a few µm to several hundreds of µm, on relatively large substrates (a few cm2) and with high growth rates (up to a few µm/min). The stress induced at the layer / substrate interface is reduced for a homo-epitaxial growth, i.e., when the layer and the substrate are of the same nature, as in the case of rare-earth-doped LiYF4 on undoped LiYF4. So far, the growth of Tb3+-doped LiYF4 epitaxial layers by LPE has never been reported.
In the present work, we report on the LPE growth and morphological, vibronic, and polarized spectroscopic properties of single-crystalline Tb3+,Gd3+-codoped LiYF4 epitaxial layers with the goal of developing novel active media for visible waveguide lasers.
2. Liquid phase epitaxy growth
Single-crystalline layers of LiYF4 codoped with Tb3+ and Gd3+ ions were grown using the LPE method on undoped bulk LiYF4 substrates. Here, Tb3+ ions were introduced as active optical centers responsible of the visible emission and Gd3+ ones (being optically passive) – to increase the refractive index contrast between the layer and the substrate for waveguide applications. The starting batch composition was 73 mol% LiF - 27 mol% REF3 (RE = Y, Tb, Gd, rare-earth) as represented by the dashed red line on the phase diagram of the LiF – YF3 binary system [29], see Fig. 1. Lithium fluoride (LiF) served as both the solvent and the constituent material of the LiREF4 phase. Here, the temperatures corresponding to the eutectic (E) and peritectic (P) points are 695 °C and 815 °C, respectively. To analyze the expected effect of heavy Tb3+ doping on the growth conditions, we also plotted the phase diagram of the LiF - TbF3 system [30] corresponding to the crystallization of the LiTbF4 stoichiometric phase. The replacement of Y3+ by Tb3+ is expected to shift both the E and P points and the liquidus curve. For the studied 73 mol% LiF - 27 mol% REF3 composition, a slightly lower (by 1-1.5 °C) onset of apparent crystallization is indeed observed in line with the phase diagrams. Note the non-congruent melting of both LiYF4 and LiTbF4 phases.
Fig. 1. Phase diagram of the LiF – YF3 (black) and LiF – TbF3 (blue) binary systems [29,30]. The dashed red line corresponds to the composition selected for the LPE growth, and red circle - to the apparent onset of crystallization.
The layers were doped with 12 at.% Tb3+ and 5 at.% Gd3+ (starting composition), with both ions substituting yttrium ions Y3+. Their ionic radii for VIII-fold F- coordination are RTb =1.18 Å, RGd =1.19 Å and RY =1.16 Å [31], respectively, being relatively close, so that the stress induced by the doping is expected to be relatively weak.
The undoped LiYF4 substrates (lateral size: 33 × 11 mm2, thickness: 3 mm) were cut from a bulk crystal grown by the Czochralski method, oriented with their plane orthogonal to the [001] crystallographic axis, and then polished until very low surface rugosity was reached (<3 nm).
The reagents used for the LPE growth were lithium fluoride LiF (Alfa Aesar, 99.5% purity), and rare-earth oxides Y2O3, Gd2O3 and Tb2O3 (Alfa Aesar REaction, < 99.9% purity). The oxide precursors were fluorinated to obtain rare-earth fluorides (REF3) of high purity.
The growth bath containing thoroughly mixed precursors was first placed in the LPE chamber, which was degassed to a pressure of 2 × 10−4 mbar at 400 °C. Then, the bath was heated to a temperature higher than the liquidus point to melt every component. After homogenizing the bath with stirring, it was slowly cooled to a temperature slightly below the apparent onset of crystallization. Lowering the temperature and introducing the substrate in the supersaturated solution started crystallization of epitaxial layers. Several growth attempts were made with the growth temperature being in the range of 737–740 °C and the growth duration of 15 - 25 min. High-quality optical layers were achieved according to the following procedure: first setting the temperature to 738 °C for 15 min, then raising it to 740 °C for 10 min, with a rotation speed of 5 rpm. The layers were grown on both sides of the substrate. After the growth was completed, the epitaxies were slowly removed from the solution and pulled up from the LPE oven with a speed of a few mm per minute.
3. Characterization methods
The surface morphology of non-polished epitaxial layers was observed using an optical confocal microscope (Sensofar, S-neox), in reflection mode, using a blue GaN diode (405 nm) or an UV lamp (365 nm). The layer topography was studied in the interferometric mode. Energy Dispersive X-ray (EDX) spectra and element mapping were measured with a FEI Scios 2 Field Emission Scanning Electron Microscope with a focused Gallium ion beam (FESEM-FIB).
The phases crystallized from the molten bath of the LPE growth were identified by X-ray powder diffraction (XRD). The equipment used was a Bruker-AXS D8-Advance diffractometer using Cu Kα radiation, incident and diffracted beam Soller slits of 2.5° and a fixed receiving slit of 0.5°. The diffractograms were measured for 2θ angles from 5° to 80°, with a step size of 0.02° and a step time of 0.5 s. The detection of the diffracted X-rays was performed with a LynxEye-XE-T PSD detector with an opening angle of 2.94°.
The polarized µ-Raman and µ-luminescence spectra, as well as µ-luminescence maps were measured at room temperature (RT, 293 K) using a confocal Raman microscope (InVia Qontor, Renishaw), equipped with a × 50 Leica objective and an Ar+ ion laser (488 / 514 nm).
The excitation / luminescence spectra and the luminescence decay curves were also measured using a spectrofluorometer (QuantaMaster, Horiba). For low-temperature (LT, 12 K) measurements, the epitaxy was mounted on a APD DE-202 cryo-cooler equipped with a APD HC-2 Helium Vacuum Cryo Compressor. The spectral bandwidth (SBW) for LT studies was 0.1-0.25 nm, depending on the spectral range.
4. Results and discussion
4.1 Layer morphology
Figure 2(a) presents a photograph of an as-grown 12 at.% Tb, 5 at.% Gd:LiYF4/LiYF4 epitaxy (growth temperature / duration: 738 °C / 15 min, then 740 °C / 10 min). Approximately 2/3 of the substrate was vertically dipped into the solution and the epitaxial layers were grown over the whole dipped area on both sides of the substrate. A rounded-shaped growth meniscus is easily observed. The white color of the overgrown part of the epitaxy is due to crystallization of the residual solvent (LiF). The parts of the layer free of LiF crystallization were transparent. The same sample is shown in Fig. 2(b) under illumination with a UV lamp. An intense green emission due to the Tb3+ ions is observed.
Fig. 2. (a) A photograph of the as-grown Tb,Gd:LiYF4 / LiYF4 epitaxy (the growth temperature / duration is 738 °C for 15 min, then 740°C for 10 min), (b) the same sample illuminated by a UV lamp, λexc = 365 nm.
Confocal laser microscopy was employed to study the layer morphology, Fig. 3. Figures 3(a,b) represent the raw top surface, with “hills and valleys” topography, being typical of fluoride LPE growth. The epitaxial layer shows a smooth surface, while the crystallized residual solvent LiF presents a dendritic-like structure, Fig. 3(b). One of the lateral facets of the epitaxy was polished to observe the cross-sectional view, see Fig. 3(c). It indicates a clean and distinct interface between the substrate and the layer, with an average layer thickness of 74 ± 15 µm. For the performed LPE growth experiments, the layer thickness was ranging from 40 to 90 µm, depending on the growth temperature and duration, resulting in a range of growth rates of 1.6–3.6 µm/min. The residual solvent was easily removed from the top surface of the layer by polishing, as shown in Fig. 3(d), illustrating a final layer with a thickness of 30 µm. Both the substrate and the layer show no cracks nor inclusions of secondary phases.
Fig. 3. Confocal laser microscopy images of the Tb,Gd:LiYF4 epitaxial layers: (a,b) top view, raw surface; (c) side view, polished end-facet, (d) side view, top and side polished layer, reflection mode, λ = 405 nm.
The surface topography of the as-grown top layer surface was further revealed in the interferometric mode, Fig. 4(a). The root mean square (r.m.s.) surface roughness was about 1 µm as measured along the A-B line passing across several “hills” and “valleys”, Fig. 4(b). Note that for the substrate, the surface roughness measured in a similar way was about a few nm (after polishing).
Fig. 4. Surface topography of the as-grown Tb,Gd:LiYF4 epitaxial layer: (a) a 3D topography plot, (b) a surface roughness plot along the A – B line (r.m.s. – root mean square deviation).
4.2 XRD and EDX
After the epitaxial growth, the LPE molten bath was cooled down to room temperature and the crystallized materials were studied by powder XRD, see Fig. 5. Two phases were easily separated: transparent single-crystals and a translucent polycrystalline material. The single-crystals correspond to a tetragonal (sp. gr. I41/a) scheelite-type LiYF4-based phase with lattice parameters a = 5.1754 Å and c = 10.7650 Å. The translucent polycrystalline material is composed of the same LiYF4-based phase, as well as a small amount of LiF with a face centred cubic structure (sp. gr. Fm-3 m).
Fig. 5. X-ray powder diffraction (XRD) patterns of the crystallized molten bath of the LPE experiment: a translucent polycrystalline material and transparent single crystals, the assignment of the diffraction peaks is after the PDF cards No. 81-1940 (LiYF4) and No. 04-0857 (LiF) from the ICDD database.
Assuming a very close composition of the single-crystals and the epitaxial layer, the lattice mismatch between the layer and the substrate could be estimated yielding Δa/asubstrate = 0.20% and Δc/csubstrate = 0.22%. Here, we used the lattice constants of undoped LiYF4: a = 5.164 Å and c = 10.741 Å [32]. The increase in the lattice constants of the layer with respect to the substrate are due to the larger ionic radii of both Tb3+ and Gd3+ as compared to that of Y3+ (see above). LiY1-x-yTbxGdyF4 (0 < x, y < 1) represents a substitutional solid-solution in the LiYF4-LiGdF4-LiTbF4 ternary system, so the variation of the lattice constants can be calculated from the weighted mean of the constituents’ lattice parameters (the Vegard's law). In this way, we achieved acalc = 5.172 Å and ccalc = 10.771 Å in a relatively good agreement with the experimental measurement. Here, the following lattice constants were used: a = 5.203 Å and c = 10.879 Å (LiTbF4) [19] and a = 5.235 Å and c = 11.019 Å (LiGdF4) [33].
The composition of the epitaxial layer was studied by EDX analysis and elemental mapping. The EDX spectrum of the epitaxial layer is shown in Fig. 6(a) and indicates the presence of Y, F and Tb (Li is too light to be detected by EDX analysis). Figure 6(b) shows a FESEM image of the lateral facet of the epitaxy, exhibiting the substrate, the layer and the residual solvent LiF on top of the latter. The elemental mapping was used to reveal the element distribution in both the layer and the substrate. Figure 6(c) shows a homogenous distribution of F in the layer and the substrate, with an increase in the area assigned to the residual solvent LiF. The Y content decreases in the layer compared to the substrate, Fig. 6(d), and Tb is uniformly distributed in the layer, Fig. 6(e), substituting for Y.
Fig. 6. (a) EDX spectrum of the Tb,Gd:LiYF4 epitaxial layer. (b-e) Element mapping across the polished end-facet of the epitaxy: (b) a FESEM image, (c-e) element maps: (c) F Kα1,2, (d) Y Lα1 and (e) Tb Mα1.
4.3 Raman spectra
The polarized µ-Raman spectra of the epitaxial layer are shown in Fig. 7(a). The measures were made in the a(ij)a configuration (where i, j = σ or π, using Porto’s notations for polarized Raman spectroscopy [34]) observing a polished side facet of the epitaxy with a spatial resolution better than 1 µm. At the center of the Brillouin zone Г (k = 0), vibrational modes are distributed over the irreducible representations of the C4h point group as follows: 3Ag + 5Bg + 5Eg + 5 Au + 3Bu + 5Eu. One Au and one Eu modes correspond to rigid translations of the whole crystal, and other Au and Eu modes are IR-active. The gerade (g) modes (5Bg + 5Eg) are Raman-active and the remaining modes (Bu) are silent. The a(ππ)a configuration shows the most intense Raman peak at 264 cm-1, corresponding to an Ag vibrational mode, with a full width at half maximum (FWHM) of 8.8 cm-1. The highest energy vibrations are observed at 424 cm-1 (Bg) and 444 cm-1 (Eg) [35].
Fig. 7. (a) Polarized µ-Raman spectra of the Tb,Gd:LiYF4 epitaxial layer in the a(ij)a geometry (i,j = π, σ), (b) a close look at the most intense Raman peak at ∼264 cm-1 for the substrate and the layer in the a(ππ)a geometry, λexc = 514 nm.
A slight blue-shift of 0.3 cm-1 is visible in the peak position of the band at ∼264 cm-1 between the Raman spectra of the LiYF4 substrate and the Tb,Gd:LiYF4 layer, Fig. 7(b), for the a(ππ)a configuration. This phenomenon is due to the variation of the crystalline structure generated by the Tb3+ and Gd3+ doping of the layer. The Raman peak for the layer is also slightly broadened and reduced in intensity as compared to that of the substrate indicating only a slight reduction in crystallinity as compared to the undoped single-crystal.
The well-preserved polarization anisotropy of the Raman spectra of the epitaxial layer indicates the conservation of its orientation with respect to the substrate (i.e., growth along the [001] crystallographic direction).
4.4 Optical spectroscopy
The excitation spectrum of Tb3+ ions in the epitaxial layer is shown in Fig. 8, when monitoring the green emission at 544 nm. The most intense band in the UV spectral range at ∼375.9 nm is due to the 7F6 → 5D3 transition. The transition to the metastable 5D4 Tb3+ level corresponds to a band falling in the blue spectral range with a maximum at 488 nm, which is commonly used for pumping Tb lasers. The spectrally overlapping bands in the UV are due to transitions to the higher-lying 5GJ, 5DJ, 5LJ, 5HJ multiplets of Tb3+ ions. Here, the assignment of the spectral bands was made according to Carnall et al. [36].
The tetragonal LiYF4 crystal is optically uniaxial and its optical axis is parallel to the c-axis. Two principal light polarizations are denoted π (E || c) and σ (E ⊥ c). The polarized luminescence spectra of Tb3+ ions in the epitaxial layer are shown in Fig. 9(a). The highest luminescence intensity is observed for π-polarization, for all the 5D4 → 7FJ transitions (J = 0-6). The most intense emission is related to the 5D4 → 7F5 transition between 539 and 555 nm, responsible for the green luminescence of Tb3+ ions. The CIE 1931 (Commision internationale de l’éclairage) colour coordinates for the luminescence of the Tb,Gd:LiYF4 epitaxial layer are x = 0.331 and y = 0.591 falling into the green spectral range. The dominant wavelength λd is 553.9 nm with colour purity p = 77.5%.
Fig. 9. (a) Polarized luminescence spectra of Tb3+ ions in the Tb,Gd:LiYF4 epitaxial layer for π and σ light polarizations, λexc = 488 nm; (b,c) stimulated-emission cross-sections for Tb3+ ions: (b) the 5D4 → 7F4 transition (yellow emission) and (c) the 5D4 → 7F5 transition (green emission).
The peak wavelengths of all the 5D4 → 7FJ Tb3+ emission bands in the epitaxial layer are listed in Table 1, together with the corresponding colours. Tb3+ ions provide multi-colour emission falling into the blue (J = 6), green (J = 5), yellow (J = 4), red (J = 2, 3) and deep-red (J = 0, 1) spectral ranges. The experimental luminescence branching ratios Bexp(JJ’) were calculated from the unpolarized luminescence spectra by integrating the luminescence intensity for each transition. The highest Bexp(JJ’) of 53.76% corresponds to the most intense 5D4 → 7F5 transition. This value agrees with those reported previously for a LiTbF4 stoichiometric crystal: 58.9% (experimental) and 58.18% (calculated using the Judd-Ofelt theory [27]). The 5D4 → 7F4 transition falling in the yellow spectral range corresponds to a smaller Bexp(JJ’) of 10.58%.
Table 1. Luminescence Branching Ratios for the 5D4 → 7FJ (J = 0 - 6) Transitions of Tb3+ Ions in the Tb,Gd:LiYF4 Epitaxial Layer
The micro-luminescence mapping was performed across the polished end-facet of the epitaxy, as shown in Fig. 10. The Tb3+ ions are only located in the layer, with a slight increase in concentration towards the surface of the layer, and no Tb3+ diffusion into the substrate is observed.
Fig. 10. Micro-luminescence mapping across a polished end-facet of a Tb,Gd:LiYF4 / LiYF4 epitaxy, λexc = 488 nm, λlum = 544 nm.
Polarized stimulated-emission (SE) cross-section, σSE, spectra for the 5D4 → 7F4,5 transitions were calculated from the corresponding polarized luminescence spectra using the Füchtbauer-Ladenburg (F-L) equation [37]:
For the 5D4 → 7F4 transition in the yellow, the maximum SE cross-section is 0.61 × 10−21 cm2 at 587.4 nm (for π-polarization) and 0.20 × 10−21 cm2 at 581.9 nm (for σ-polarization). For the 5D4 → 7F5 transition in the green, the corresponding values are higher, 1.28 × 10−21 cm2 at 542.0 nm (for π-polarization) and 1.25 × 10−21 cm2 at 544.0 nm (for σ-polarization).
The luminescence decay curves for Tb3+ ions in the epitaxial layer at both RT and LT (12 K) are shown in Fig. 11(a), observing the green emission (∼544 nm). The decay curves are single-exponential, in agreement with a single rare-earth site in LiYF4 (Y3+ ions, S4 symmetry, VIII-fold F- coordination). The luminescence lifetime τlum of the 5D4 Tb3+ state is 5.05 ms (at RT) and 5.41 ms (at 12 K).
Fig. 11. (a) Luminescence decay curves from the 5D4 state of Tb3+ ions in the Tb,Gd:LiYF4 epitaxial layer at 12 K and RT (black and green lines – single-exponential fits); (b) the luminescence lifetimes of the 5D4 state as a function of temperature, the data for a 11 at.% Tb:LiYF4 crystal are given for comparison.
The luminescence lifetime of the 5D4 Tb3+ state was also measured as a function of temperature, from 12 to 295 K, for both the 12 at.% Tb, 5 at.%Gd:LiYF4 epitaxial layer and a 11.2 at.% Tb:LiYF4 single-crystal studied for comparison, see Fig. 11(b). The τlum value slightly increases upon cooling for both samples and it is only slightly longer for the single-crystal, 5.14 ms (at RT) and 5.58 ms (at 12 K). The latter indicates a relatively low content of quenching centers (defects and impurities) in the epitaxial layer highlighting its good optical quality.
4.5 Low-temperature spectroscopy
The crystal-field splitting for Tb3 + multiplets in the Tb,Gd:LiYF4 epitaxial layer was determined from the LT (12 K) excitation and luminescence spectra, Fig. 12(a-e). The LT luminescence spectra corresponding to all the 5D4 → 7FJ (J = 0–6) transitions were measured under excitation at 375 nm (to the higher-lying 5G6 + 5D3 states). They were plotted vs. EZPL - Eph, where EZPL is the zero-phonon line (ZPL) energy for transitions between the lowest Stark sub-levels of the metastable state 5D4 and the ground-state 7F6, and Eph is the photon energy, thus directly giving the experimental energy of each Stark sub-levels of the 7FJ multiplets. The excitation spectrum corresponding to the 7F6 → 5D4 transition was measured when monitoring the green emission at 542 nm and it was plotted vs. Eph, directly giving the experimental energy of each Stark sub-level for the 5D4 metastable state.
Fig. 12. LT (12 K) spectroscopic studies of Tb3+ ions in the Tb,Gd:LiYF4 epitaxial layer: (a-d) luminescence spectra, λexc = 375 nm, (e) excitation spectrum, λlum = 542 nm. Vertical dashes – crystal-field splitting for Tb3+ ions in tetragonal LiTbF4 and CaWO4 crystals (after [33,34]).
For Tb3+ ions residing in S4 symmetry sites, any 2S + 1LJ multiplet with a total angular momentum J of 0, 1, 2, 3, 4, 5 and 6, will be split into a total of 1, 2, 4, 5, 7, 8 and 10 Stark sub-levels, respectively. The assignment of the electronic transitions in the LT excitation and luminescence spectra (indicated by the “+” symbols) was performed following the previous studies on the crystal-field splitting of Tb3+ ions in isostructural (scheelite-type) LiTbF4 [38] and CaWO4 [39] crystals. The experimental energies of Stark sub-levels are listed in Table 2, together with the expected number of sub-levels according to Christensen [38]. Only one experimental sub-level of the 7F6 multiplet is missing from the LT luminescence spectrum.
Table 2. Experimental Crystal-Field Splitting of the 7FJ (J = 0 - 6) and 5D4 Tb3+ Multiplets in the Tb,Gd:LiYF4 Epitaxial Layers
The experimental Stark sub-level energies were used to plot the Tb3+ energy-level diagram in Tb,Gd:LiYF4 epitaxial layers, Fig. 13, also showing the multi-colour 5D4 → 7FJ (J = 0 - 6) emissions falling into the blue, green, yellow, red and deep-red spectral ranges.
Fig. 13. Energy-level scheme of Tb3+ ions in the Tb,Gd:LiYF4 epitaxial layer, arrows indicate the transitions corresponding to visible absorption / emissions to / from the 5D4 metastable state.
5. Conclusion
We demonstrate the suitability of Liquid Phase Epitaxy for the growth of heavily Tb3+-doped single-crystalline oriented LiYF4 layers with a thickness of a few tens of µm on bulk undoped LiYF4 substrates. A starting layer composition of LiY0.83Tb0.12Gd0.05F4 was tested (with Tb3+ ions being the active centers and Gd3+ ones serving for increasing the refractive index contrast between the layer and the substrate). The layers exhibit (i) a single-crystalline structure without cracks and impurities and a clean and planar layer / substrate interface, (ii) a well-preserved orientation with respect to the substrate (growth along the [001] direction), (iii) a uniform distribution of Tb3+ ions with no diffusion into the substrate, (iv) strongly polarized emission properties and (v) a long lifetime of the metastable 5D4 Tb3+ level. We also report on key spectroscopic parameters of Tb3+ ions in Tb,Gd:LiYF4 epitaxial layers being relevant for laser operation, such as the polarized stimulated-emission cross-sections for green and yellow Tb3+ emissions, the luminescence branching ratios, the radiative Tb3+ lifetime and the crystal-field splitting. The developed epitaxies are promising for visible (green and yellow) waveguide lasers.
Future work will focus on the LPE growth of epitaxial layers with yet higher Tb3+ doping levels, up to the stoichiometric composition LiTbF4, with the goal of increasing the light absorption in the blue and UV spectral ranges limited by the low probabilities of the corresponding spin-forbidden Tb3+ transitions. The growth of stoichiometric epitaxial layers on top of a LiYF4 substrate should theoretically lead to a lattice mismatch of Δa/asubstrate = 0.69% and Δc/csubstrate = 1.28%.
Funding
Agence Nationale de la Recherche (ANR-22-CE08-0025-01, NOVELA); European Regional Development Fund; Contrat de plan État-Région (CPER) de Normandie; Agencia Estatal de Investigación (PID2019-108543RB-I00).
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.
References
1. C. Kränkel, D.-T. Marzahl, F. Moglia, G. Huber, and P. W. Metz, “Out of the blue: semiconductor laser pumped visible rare-earth doped lasers,” Laser Photonics Rev. 10(4), 548–568 (2016). [CrossRef]
2. P. W. Metz, D.-T. Marzahl, A. Majid, C. Kränkel, and G. Huber, “Efficient continuous wave laser operation of Tb3+-doped fluoride crystals in the green and yellow spectral regions,” Laser Photonics Rev. 10(2), 335–344 (2016). [CrossRef]
3. S. Kalusniak, H. Tanaka, E. Castellano-Hernández, and C. Kränkel, “UV-pumped visible Tb3+-lasers,” Opt. Lett. 45(22), 6170–6173 (2020). [CrossRef]
4. R. P. Rao, “Tb3+ activated green phosphors for plasma display panel applications,” J. Electrochem. Soc. 150(8), H165–H171 (2003). [CrossRef]
5. Z. Xia and R. S. Liu, “Tunable blue-green color emission and energy transfer of Ca2Al3O6F:Ce3+,Tb3+ phosphors for near-UV white LEDs,” J. Phys. Chem. C 116(29), 15604–15609 (2012). [CrossRef]
6. Y. Zhai, Q. Sun, S. Yang, Y. Liu, J. Wang, S. Ren, and S. Ding, “Morphology-controlled synthesis and luminescence properties of green-emitting NaGd(WO4)2:Tb3+ phosphors excited by n-UV excitation,” J. Alloys Compd. 781, 415–424 (2019). [CrossRef]
7. U. Yadav, Z. Abbas, R. J. Butcher, and A. K. Patra, “A luminescent terbium (III) probe as an efficient turn-on sensor for dipicolinic acid, a bacillus anthracis biomarker,” New J. Chem. 46(38), 18285–18294 (2022). [CrossRef]
8. A. Mohammadzadeh, A. Jouyban, M. Hasanzadeh, V. Shafiei-Irannejad, and J. Soleymani, “Ultrasensitive fluorescence detection of antitumor drug methotrexate based on a terbium-doped silica dendritic probe,” Anal. Methods 13(37), 4280–4289 (2021). [CrossRef]
9. V. Vasyliev, E. G. Villora, M. Nakamura, Y. Sugahara, and K. Shimamura, “UV-visible Faraday rotators based on rare-earth fluoride single crystals: LiREF4 (RE = Tb, Dy, Ho, Er and Yb), PrF3 and CeF3,” Opt. Express 20(13), 14460–14470 (2012). [CrossRef]
10. X. Xin, Y. Hao, L. Liu, J. Lv, J. Zhang, X. Fu, Z. Jia, and X. Tao, “Tb3Al3Ga2O12: A novel visible–infrared Faraday crystal exhibiting a superior magneto-optical performance,” Cryst. Growth Des. 22(9), 5535–5541 (2022). [CrossRef]
11. R. K. Verma, K. Kumar, and S. B. Rai, “Inter-conversion of Tb3+ and Tb4+ states and its fluorescence properties in MO–Al2O3:Tb (M = Mg, Ca, Sr, Ba) phosphor materials,” Solid State Sci. 12(7), 1146–1151 (2010). [CrossRef]
12. J. Kaszewski, B. S. Witkowski, Ł. Wachnicki, H. Przybylińska, B. Kozankiewicz, E. Mijowska, and M. Godlewski, “Reduction of Tb4+ ions in luminescent Y2O3:Tb nanorods prepared by microwave hydrothermal method,” J. Rare Earths 34(8), 774–781 (2016). [CrossRef]
13. M. F. Churbanov, B. I. Denker, B. I. Galagan, V. V. Koltashev, V. G. Plotnichenko, M. V. Sukhanov, S. E. Sverchkov, and A. P. Velmuzhov, “First demonstration of ∼ 5 µm laser action in terbium-doped selenide glass,” Appl. Phys. B 126(7), 117 (2020). [CrossRef]
14. V. S. Shiryaev, M. V. Sukhanov, A. P. Velmuzhov, E. V. Karaksina, T. V. Kotereva, G. E. Snopatin, B. I. Denker, B. I. Galagan, S. E. Sverchkov, V. V. Koltashev, and V. G. Plotnichenko, “Core-clad terbium doped chalcogenide glass fiber with laser action at 5.38 µm,” J. Non-Cryst. Solids 567, 120939 (2021). [CrossRef]
15. S. Kalusniak, E. Castellano-Hernández, H. Yalçinoğlu, H. Tanaka, and C. Kränkel, “Spectroscopic properties of Tb3+ as an ion for visible lasers,” Appl. Phys. B 128(2), 33 (2022). [CrossRef]
16. P. W. Metz, D.-T. Marzahl, G. Huber, and C. Kränkel, “Performance and wavelength tuning of green emitting terbium lasers,” Opt. Express 25(5), 5716–5724 (2017). [CrossRef]
17. E. Castellano-Hernández, P. W. Metz, M. Demesh, and C. Kränkel, “Efficient directly emitting high-power Tb3+:LiLuF4 laser operating at 587.5 nm in the yellow range,” Opt. Lett. 43(19), 4791–4794 (2018). [CrossRef]
18. T. Yamashita and Y. Ohishi, “Amplification and lasing characteristics of Tb3+-doped fluoride fiber in the 0.54 µm band,” Jpn. J. Appl. Phys. 46(No. 41), L991–L993 (2007). [CrossRef]
19. V. Vasyliev, E. G. Víllora, Y. Sugahara, and K. Shimamura, “Judd-Ofelt analysis and emission quantum efficiency of Tb-fluoride single crystals: LiTbF4 and Tb0.81Ca0.19F2.81,” J. Appl. Phys. 113(20), 203508 (2013). [CrossRef]
20. B. Ferrand, B. Chambaz, and M. Couchaud, “Liquid phase epitaxy: A versatile technique for the development of miniature optical components in single crystal dielectric media,” Opt. Mater. 11(2-3), 101–114 (1999). [CrossRef]
21. P. Rogin and J. Hulliger, “Liquid phase epitaxy of LiYF4,” J. Cryst. Growth 179(3-4), 551–558 (1997). [CrossRef]
22. P. Loiko, R. Soulard, G. Brasse, J.-L. Doualan, B. Guichardaz, A. Braud, A. Tyazhev, A. Hideur, and P. Camy, “Watt-level Tm:LiYF4 channel waveguide laser produced by diamond saw dicing,” Opt. Express 26(19), 24653–24662 (2018). [CrossRef]
23. L. Basyrova, G. Brasse, P. Loiko, C. Grygiel, R. M. Solé, M. Aguiló, F. Díaz, X. Mateos, A. Benayad, J.-L. Doualan, and P. Camy, “Liquid phase epitaxy growth and structural characterization of highly-doped Er3+:LiYF4 thin films,” Opt. Mater. 132, 112574 (2022). [CrossRef]
24. W. Bolaños, F. Starecki, A. Braud, J.-L. Doualan, R. Moncorgé, and P. Camy, “2.8 W end-pumped Yb3+:LiYF4 waveguide laser,” Opt. Lett. 38(24), 5377–5380 (2013). [CrossRef]
25. P. Loiko, R. Soulard, G. Brasse, J.-L. Doulan, A. Braud, A. Tyazhev, A. Hideur, and P. Camy, “Tm,Ho:LiYF4 planar waveguide laser at 2.05 µm,” Opt. Lett. 43(18), 4341–4344 (2018). [CrossRef]
26. W. Bolaños, G. Brasse, F. Starecki, A. Braud, J.-L. Doualan, R. Moncorgé, and P. Camy, “Green, orange, and red Pr3+∶YLiF4 epitaxial waveguide lasers,” Opt. Lett. 39(15), 4450–4453 (2014). [CrossRef]
27. G. Brasse, P. Loiko, C. Grygiel, A. Benayad, F. Lemarie, V. Zakharov, A. Veniaminov, J.-L. Doualan, A. Braud, and P. Camy, “Liquid Phase Epitaxy growth, structure and spectroscopy of highly-doped 20 at.% Yb3+:LiYF4 thin films,” J. Lumin. 236, 118071 (2021). [CrossRef]
28. R. Soulard, M. Salhi, G. Brasse, P. Loiko, J.L. Doualan, A. Braud, A. Tyazhev, A. Hideur, and P. Camy, “Laser operation of highly-doped Tm:LiYF4 epitaxies: Towards thin-disk lasers,” Opt. Express 27(6), 9287–9301 (2019). [CrossRef]
29. R. E. Thoma, C. F. Weaver, H. A. Freidman, H. Insley, L. A. Harris, and H. A. Yakel, “Phase equilibria in the system LiF–YF3,” J. Phys. Chem. 65(7), 1096–1099 (1961). [CrossRef]
30. P. P. Fedorov, B. P. Sobolev, L. V. Medvedeva, and B. M. Reiterov, “Revised phase diagrams of LiF-RF3 (R = La - Lu, Y) systems,” in Growth of Crystals, vol. 21 E. I. Givargizov and A. M. Mel’nikova, eds., (Springer US, 2002), pp. 141–154.
31. A. A. Kaminskii, Laser Crystals, vol. 14 Springer Series in Optical Sciences (Springer Berlin Heidelberg, 1990).
32. E. Garcia and R. R. Ryan, “Structure of the laser host material LiYF4,” Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 49(12), 2053–2054 (1993). [CrossRef]
33. A. Grzechnik, W. A. Crichton, P. Bouvier, V. Dmitriev, H.-P. Weber, and J.-Y. Gesland, “Decomposition of LiGdF4 scheelite at high pressures,” J. Phys.: Condens. Matter 16(43), 7779–7786 (2004). [CrossRef]
34. T. C. Damen, S. P. S. Porto, and B. Tell, “Raman effect in zinc oxide,” Phys. Rev. 142(2), 570–574 (1966). [CrossRef]
35. S. A. Miller, H. E. Rast, and H. H. Caspers, “Lattice vibrations of LiYF4,” J. Chem. Phys. 52(8), 4172–4175 (1970). [CrossRef]
36. W. T. Carnall, P. R. Fields, and K. Rajnak, “Electronic energy levels of the trivalent lanthanide aquo ions. III. Tb3+,” J. Chem. Phys. 49(10), 4447–4449 (1968). [CrossRef]
37. B. Aull and H. Jenssen, “Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections,” IEEE J. Quantum Electron. 18(5), 925–930 (1982). [CrossRef]
38. H. P. Christensen, “Spectroscopic analysis of lithium terbium tetrafluoride,” Phys. Rev. B 17(10), 4060–4068 (1978). [CrossRef]
39. R. P. Leavitt, C. A. Morrison, and D. E. Wortman, “Description of the crystal field for Tb3+ in CaWO4,” J. Chem. Phys. 61(3), 1250–1251 (1974). [CrossRef]
