Deep-red planar waveguide laser operation of
Eu3+:KY(WO4)2 layers

Amandine Baillard, Ji Eun Bae, Mailyn Ceballos, Pavel Loiko, Rosa Maria

Solé, Magdalena Aguiló, Francesc Díaz, Gurvan Brasse, Xavier Mateos,
Patrice Camy

► To cite this version:

Amandine Baillard, Ji Eun Bae, Mailyn Ceballos, Pavel Loiko, Rosa Maria Solé, et al. Deep-red planar waveguide laser operation of Eu3+:KY(WO4)2 layers. Optics Express, 2025, 33 (3), pp.4926-4939. 10.1364/oe.537598. hal-05371828

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**Title:** Deep-red planar waveguide laser operation of Eu3+:KY(WO4)2 layers

**Authors:** Amandine Baillard, Ji Eun Bae, mailyn ceballos, Pavel Loiko, Rosa Maria Solé, Magdalena Aguilo, Francesc Diaz, Gurvan BRASSE, Xavier Mateos, Patrice Camy

**Accepted:** 10 November 24

**Posted** 11 November 24

**DOI:** https://doi.org/10.1364/OE.537598

Published by Optica Publishing Group under the terms of the [Creative Commons Attribution 4.0 License](https://creativecommons.org/licenses/by/4.0/). Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

OPTICA
PUBLISHING GROUP
# Deep-red planar waveguide laser operation of Eu³⁺:KY(WO₄)₂ layers

AMANDINE BAILLARD,¹ JI EUN BAE,¹ MAILYN CEBALLOS,² PAVEL LOIKO,¹ ROSA MARIA SOLÉ,² MAGDALENA AGUILÓ,² FRANCESC DÍAZ,² GURVAN BRASSE,¹ XAVIER MATEOS,²,† AND PATRICE CAMY¹,*

¹Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), UMR 6252 CEA-CNRS-ENSICAEN, Université de Caen Normandie, 6 Boulevard Maréchal Juin, 14050 Caen Cedex 4, France

²Física i Cristal·lografiá de Materials (FiCMA), Universitat Rovira i Virgili (URV), 43007 Tarragona, Spain. †Serra Húnter Fellow, Spain

*patrice.camy@ensicaen.fr

**Abstract:** We report on the first deep-red Europium planar waveguide laser. It employs 30-µm thick heavily-doped 11.5 at.% Eu:KY(WO₄)₂ layers grown by Liquid Phase Epitaxy on undoped substrates using K₂W₂O₇ as a solvent. When pumped with a green laser at 532 nm, the Eu-waveguide laser delivers a maximum output power of 7 mW at 704.7 nm with a slope efficiency of 9.5%, a laser threshold of 21 mW and linear polarization. The waveguide propagation losses are as low as 0.05 dB/cm. The morphology, vibronic and spectroscopic properties of the epitaxial layers are also studied. In the deep red, for the ⁵D₀ → ⁷F₄ transition, the stimulated-emission cross-section reaches 1.76×10⁻²⁰ cm² at 704.7 nm for light polarization *E* || *N*ₚ, and the luminescence lifetime of the ⁵D₀ manifold is 472 µs.

© 2024 Optica Publishing Group under the terms of the Optica Publishing Group Open Access Publishing Agreement

## 1. Introduction

Trivalent europium ions (Eu³⁺, electronic configuration: [Xe]4f⁶) are known for their multicolor emissions ranging from yellow to deep-red, due to radiative transitions originating from the metastable state ⁵D₀ to a group of lower-lying manifolds ⁷Fⱼ (j = 0 – 6, ⁷F₀ being the ground state), Fig. 1. Owing to a large energy-gap between the ⁵D₀ state and the next lower-lying level ⁷F₆ (about 12000 cm⁻¹), no depopulation of the emitting state occurs by multiphonon non-radiative relaxation, even in high-phonon energy matrices. Eu³⁺-doped materials do not suffer from self-quenching via cross-relaxation enabling high available doping concentrations [1]. Usually, the red emission at 613 nm (the electric dipole ⁵D₀ → ⁷F₂ transition) dominates in the spectra of Eu³⁺-doped materials (i.e., it corresponds to the highest luminescence branching ratio) paving the way towards high color purity luminophores (phosphors) [2]. In addition, the presence of a purely magnetic dipole ⁵D₀ → ⁷F₁ transition at 591 nm, whose probability is almost independent of the host matrix, enables the use of Eu³⁺ ions as structural probes: the intensity ratio between the “hypersensitive” ⁵D₀ → ⁷F₂ transition and the ⁵D₀ → ⁷F₁ one is often calculated to evaluate the site symmetry where Eu³⁺ is residing [3]. Recently, Eu³⁺ ions attracted a lot of attention for the development of quantum technologies, in particular for their unusually long coherence lifetimes and ultra-narrow optical homogeneous linewidths [4,5].

Moreover, Eu³⁺ ions are suitable for the development of visible lasers emitting in the red and deep red. Despite a recent breakthrough in rare-earth ion-based lasers directly emitting in the visible under pumping in the blue [6], there exist very few studies on Eu visible lasers. This is probably due to the extremely narrow and weak absorption lines of Eu³⁺ (due to the spin-forbidden nature of the corresponding transitions, ΔS ≠ 0), imposing serious requirements on the pump sources. The first room-temperature solid-state Eu laser was demonstrated in 2004 by Park and Steckl, using 1-3 at.% Eu³⁺:GaN semiconductor thin films grown by molecular beam epitaxy on sapphire substrates. The 0.6-µm-thick films were optically pumped by a UV nitrogen gas laser emitting at 337 nm, achieving stimulated emission in the red, at 620 nm [7].
This result also represents the first Eu waveguide laser action. Further progress in bulk Eu lasers was achieved implementing double tungstate crystals heavily doped with Eu³⁺ (25 at.%) [8,9]. Finally, a breakthrough was achieved in 2021 with a stoichiometric KEu(WO₄)₂ compound, delivering Watt-level output power at 703 nm (in the deep-red) with a slope efficiency of 43.2%, a low laser threshold of 64 mW and a linearly polarized emission [10].

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Partial energy-level scheme of Eu³⁺ ions (after Carnall *et al.* [11]). Arrows – transitions in absorption and emission, NR – multiphonon non-radiative relaxation, ED and MD – electric and magnetic dipole transitions, respectively (crosses indicate transitions forbidden by selection rules), numbers in parentheses – luminescence branching ratios, *B*(JJ').</figcaption></figure>

Red and deep-red laser sources are extensively employed in our daily life, for example in display technologies [12] and optical data storage [13], but also in cutting-edge applications such as medicine, fluorescence microscopy or contactless sensing. In medicine, red and deep-red lasers are of great interest for the photochemical activation of specific drugs and substances [14]. As red light offers deep penetration into bio-tissues, red lasers have found their niche in dermatology [15]. Red lasers are also relevant for stimulated-emission-depletion microscopy, allowing to overcome the limit of resolution imposed by diffraction of light [16]. The laser-based interface measurement technique is using red lasers to measure concentration and temperature variations in liquids, at transparent interfaces between liquids and solids. These measurements are essential for microfluidic, studies of heat transfer and mass diffusion, and micro-electro-mechanical systems [17]. Red light irradiation is also found in agriculture for biostimulation purposes [18].

The crystal family of monoclinic potassium rare-earth double tungstates KR(WO₄)₂ (R = Y, Lu, Gd) has been extensively studied for doping with trivalent rare-earth ions for near-infrared lasers [19-21]. These materials combine several advantages, i.e., i) very high transition cross-sections for polarized light, ii) high available rare-earth doping levels up to stoichiometric compositions, iii) weak concentration quenching of luminescence due to the large interatomic distances, iv) Raman activity, and v) reasonable thermal and thermo-optical properties. Recently, there is raising interest in visible lasers employing monoclinic double tungstates doped with Dy³⁺ [22], Eu³⁺ [8,9] and Sm³⁺ [23,24] ions.

Waveguide lasers represent an important component of integrated photonic circuits. They are compatible with optical fibers and on-chip devices leading to applications in optical sensing, on-chip spectroscopy, and microfluidics. This laser architecture benefits from small mode areas leading to high intracavity light intensities, low laser thresholds, good spatial beam quality and high efficiency [25,26].

This paper reports on a deep-red planar waveguide laser based on Eu³⁺-doped monoclinic double tungstate epitaxial layers grown by Liquid Phase Epitaxy (LPE). The LPE technique is well suited for elaboration of single-crystalline oriented layers of high optical quality (with propagation losses down to 0.1 dB/cm [27]), even for strongly anisotropic materials like double tungstates. Growth, spectroscopy and laser action in the near-infrared spectral range (1 – 2 µm) of KR(WO₄)₂ epitaxial layers doped with Yb³⁺, Tm³⁺ and Ho³⁺ ions were reported [28-30].
There exist studies on fabrication of Eu³⁺-doped luminescent crystalline layers of various compositions. In general, Eu³⁺-doped garnets have been fabricated by LPE for scintillators [31,32], while epitaxial layers of Eu₃Ga₅O₁₂ were also studied for optical waveguides [33]. Other physical and chemical methods are also available for the growth of films doped with Eu³⁺ ions, such as sol-gel [34], atomic layer deposition [35], chemical vapour deposition [36], sputtering [37], metal organic vapour phase epitaxy with ion implantation [38], matrix-assisted pulsed laser evaporation technique [39], spray pyrolysis [40], pulsed laser deposition [41], electrodeposition [42], or chemical-bath deposition [43]. Nevertheless, none of these Eu³⁺-doped layers were involved in planar waveguide laser experiments.

## 2. Liquid Phase Epitaxy growth

Eu³⁺-doped KY(WO₄)₂ layers were grown on undoped bulk KY(WO₄)₂ substrates by the Liquid Phase Epitaxy method using potassium ditungstate K₂W₂O₇ as a solvent in air atmosphere. The solute to solvent ratio was 7 mol% KY₀.₉₀Eu₀.₁₀(WO₄)₂ – 93 mol% K₂W₂O₇. The bulk crystals used for the substrate preparation were grown by the Top-Seeded Solution Growth (TSSG) method. The substrates with dimensions of 8×22 mm² (width × length (||c crystallographic direction)) and a thickness of 2 mm were cut with their plane orthogonal to the [010] crystallographic axis. They were polished to laser-grade quality from both sides (surface roughness: < 1 nm) and thoroughly cleaned before the epitaxial growth. After carefully determining the saturation temperature of the solution, the substrates were slowly introduced in the furnace to avoid thermal shocks. The substrates were first heated for 1 h above the solution surface and then dipped vertically several millimeters in the solution at a temperature 1 °C higher than the saturation temperature for 5 min, to dissolve slightly the outer part of the substrate. The growth was performed by slow cooling of 4 °C from the saturation temperature (845 °C) at a rate of 0.2 °C/h and rotating the substrate at 10 rpm resulting in a layer thickness of ~100 µm (on both sides of the substrate). The growth duration was 20 h, and the epitaxial growth rate was around 9.5 mg/h. After the growth, the epitaxy was removed from the flux and slowly cooled down to room temperature to prevent any thermal stress that could produce cracks. No additional cladding layer was deposited. Figure 2(a,b) depicts the photographs of the as-grown epitaxy, both under natural light and UV illumination. The layers are transparent and nearly colorless. When exposed to a UV lamp, they emit bright orange-red luminescence due to Eu³⁺ ions.

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** (a,b) Photographs of the as-grown 11.5 at.% Eu:KY(WO₄)₂ epitaxy: (a) in natural light, (b) under UV illumination; (c) orientation of the optical indicatrix frame {Nₚ, Nₘ, N₉} with respect to the crystallographic axes {a, b, c} using the standard C2/c setting reported in [44].</figcaption></figure>

For laser experiments, the top surface of the epitaxy was polished to laser-grade quality reducing the layer thickness down to 30±1 µm, as well as the two side facets, resulting in a length *l* of 8.37 mm.

The actual Eu³⁺ doping level in the layers was determined from the polarized absorption spectra and absorption cross-sections measured independently from a reference stoichiometric KEu(WO₄)₂ crystal. This analysis yielded 11.5 at.% Eu (ion density: Nₐu = 7.3×10²⁰ at/cm³) and a segregation coefficient Kₐu = Cₐlayer/Cₐsolution of 1.15. The Eu³⁺ ions are substituting Y³⁺ ones in a single type of sites with C₂ symmetry. Their ionic radii are Rₐu = 1.066 Å and Rₚ = 1.019 Å for VIII-fold oxygen coordination [45].
Undoped KY(WO₄)₂ crystals are monoclinic (space group C62h - C2/c). They are optically biaxial, and their optical properties are described within the optical indicatrix frame {Nₚ, Nₘ, Nₗ}, corresponding to the principal axes of the index ellipsoid, which are labelled following the convention nₚ < nₘ < nₗ. The orientation of the optical indicatrix frame with respect to the crystallographic axes is depicted in Fig. 2(c) at the wavelength of 633 nm (in the red, note the apparent rotation of the dielectric frame with the wavelength) [44]. The Nₚ-axis is parallel to the *b* crystallographic axis which is the 2-fold symmetry axis (C₂), and Nₘ and Nₗ ones lie in the *a*-c mirror plane (m) [19]. The angle between the Nₘ and *a* axes is 59.6° (within the monoclinic angle β = *a*^*c*), and the one between the Nₗ and *c* axis is 18.9° (out of the monoclinic angle). Therefore, the absorption and emission properties of Eu³⁺ ions were studied for the principal light polarizations *E* || Nₚ, *E* || Nₘ, *E* || Nₗ (note that it was predicted that for monoclinic crystals, the eigen frames of absorption / emission may not necessarily coincide with that of the optical indicatrix [46]).

<table><caption>**Table 1.** Measured refractive indices of the KY(WO₄)₂ substrate and the Eu³⁺-doped KY(WO₄)₂ epitaxial layer at 632.8 nm.</caption><thead><tr><th>Material</th><th>n<sub>p</sub></th><th>n<sub>m</sub></th><th>n<sub>g</sub></th></tr></thead><tbody><tr><td>KY(WO<sub>4</sub>)<sub>2</sub> substrate</td><td>1.9977</td><td>2.0405</td><td>2.0851</td></tr><tr><td>KY<sub>0.885</sub>Eu<sub>0.115</sub>(WO<sub>4</sub>)<sub>2</sub> layer</td><td>2.0002</td><td>2.0419</td><td>2.0853</td></tr></tbody></table>

The principal refractive indices of both the undoped substrate and the Eu³⁺-doped epitaxial layer were measured using a prism-film coupler (Metricon 2010) equipped with a He-Ne laser at 632.8 nm. The results are listed in Table 1. The refractive indices of the substrate are in line with the previously reported values for KY(WO₄)₂ [22]. All three principal refractive indices of the epitaxial layer increase upon substituting Y³⁺ by Eu³⁺. For the light polarization *E* || Nₚ (the laser polarization for the developed waveguide), the refractive index contrast Δn = nₗlayer - nₗsubstrate = 2.5×10⁻³.

## 3. Results and discussion

### 3.1 Layer morphology, composition and structure

The layer morphology was observed by confocal laser microscopy, Fig. 3.

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Observation of the Eu:KY(WO₄)₂ epitaxial layers by confocal laser microscopy: (a) side view on the polished end-facet; (b) top view of the as-grown surface; (c) topography of the as-grown surface highlighting the growth steps typical of the LPE growth of double tungstate layers; (d) surface roughness plot along the A-B red line. Reflection mode, λ = 405 nm.</figcaption></figure>
The as-grown surface shown in Fig. 3(b) exhibits growth steps typical of double tungstates elaborated by LPE, with no residual solvent. This behavior can be explained by the growth mechanism (layer by layer), good wettability of the solvent on the substrate surface, and orientation of the substrate. The top surface was further examined in the interferometric mode, and a surface roughness plot revealed very flat terraces with growth steps of about 0.5 µm in height, Fig. 3(c,d). The top and side surfaces of the epitaxy were then polished to laser-grade quality, reducing the layer thickness down to about 30 µm. The epitaxial layer is single crystalline, uniform and free of defects, cracks and inclusions, and the interface between the layer and the substrate is clean and distinct, Fig. 3(a). After polishing to laser-grade quality, the parallelism of the faces was better than 15', and the right angle between the top and the output facet was 90° with an accuracy of 30 arcsec.

The composition of the epitaxial layer, as well as the homogeneous distribution of the constituting elements were studied by Energy Dispersive X-ray (EDX) spectroscopy associated with the field emission scanning electron microscopy (FESEM) technique, Fig. 4. This analysis indicated a partial substitution of Y³⁺ by Eu³⁺ within the layer, as well as a homogeneous distribution of the Eu³⁺ dopant (the weak signal of Eu³⁺ from the substrate is due to the noise from the detector).

<figure><img src="image_5.png" /><figcaption>**Fig. 4.** Energy dispersive X-ray (EDX) based elemental mapping over an end-facet of the Eu:KY(WO₄)₂ epitaxial layer: (a-d) element maps: (a) a FESEM image, (b) O Kα₁, (c) Y Lα₁, (d) Eu Lα₁; (e) EDX spectrum of the Eu-doped layer.</figcaption></figure>

The polarized Raman spectra of the 11.5 at.% Eu:KY(WO₄)₂ epitaxial layer are displayed in Fig. 5(a). The measurements were performed in the *p*(ij)π̄ geometry (where *i*, *j* = *m*, *g*) using Porto's notation for polarized Raman spectroscopy [47]. The factor group analysis predicts the following set of irreducible representations at the center of the Brillouin zone Γ(*k* = 0) for monoclinic double tungstates: 17A_g + 19B_g + 17A_u + 19B_u, with 36 even (*gerade*) modes, A_g + B_g, being Raman active. The assignment of the Raman peaks was done according to the results reported in [48,49]. The phonons having energies below 250 cm⁻¹ are associated with translation motions (*T*) of the cations inside the crystalline structure (namely, K⁺, W⁶⁺, and Y³⁺|Eu³⁺). The in- (*δ*) and out-of-plane (*γ*) bending modes are found below 380 cm⁻¹. Finally, stretching modes (*ν*) are located between 400 cm⁻¹ and 915 cm⁻¹, with the peaks in the range from 430 cm⁻¹ to 750 cm⁻¹ corresponding to vibrations of oxygen bridges in compounds where tungsten appears in a V- or VI-fold oxygen coordination (for the studied crystal, these are [WO₆] polyhedra). The highest-energy modes are observed at 903 cm⁻¹ and 917 cm⁻¹, with the first one attributed to stretching vibrations within the [WO₆] polyhedra [49].

A comparison was made between the intense Raman peak located at 903 cm⁻¹ for both the epitaxial layer and the undoped substrate, Fig. 5(b). A slight red shift of 0.3 cm⁻¹ manifests the incorporation of Eu³⁺ ions substituting Y³⁺ ones, thus inducing a small variation of the crystalline structure.
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** (a) Polarized µ-Raman spectra of Eu:KY(WO₄)₂ epitaxial layer in the *p*(ij)π*p* geometry (i, j = *m*, *g*); (b) a close look at the Raman peak at 903 cm⁻¹ for the epitaxial layer and the undoped substrate in the *p*(gg)π*p* geometry, λ<sub>exc</sub> = 514 nm. *Numbers* – Raman frequencies in cm⁻¹, *ν* – stretching modes, *δ* and *γ* – in-plane and out-of-plane bending modes, *T* – translational modes, *\*-* coupling of different modes, # – modes not assigned.</figcaption></figure>

## 3.2 Optical spectroscopy

Prior to laser experiments, we studied polarized absorption and emission properties of Eu³⁺ ions in the grown epitaxial layers. The absorption cross-sections, σₐbs = αₐbs/Nₐu (αₐbs – absorption coefficient), of Eu³⁺ in the blue-green spectral range are plotted in Fig. 6 for the two available light polarizations, *E* || *N*<sub>m</sub> and *N*<sub>g</sub>. Eu³⁺ presents a strong anisotropy of absorption properties, with the highest cross-sections for light polarized along the *N*<sub>m</sub>-axis. Within the studied spectral range, the absorption transitions originate from the ground-state ⁷F₀ and the thermally populated lower-lying excited-state ⁷F₁ and terminate at the ⁵D₁, ⁵D₂, ⁵D₃ and ⁵L₆ higher-lying manifolds. Due to the spin-forbidden nature of these transitions (Δ*S* ≠ 0), they are relatively weak. The weak electron-phonon coupling inherent for Eu³⁺ results in rather narrow absorption linewidths (typically below 1 nm), setting a strong constraint for the choice of the pump wavelength and laser linewidth. Fortunately, due to the lack of cross-relaxation processes among Eu³⁺ ions, it can be leveraged by using heavily doped materials. The pump sources envisioned for Eu lasers are blue (465 nm) GaN-based laser diodes and green (532 nm) frequency-doubled optically pumped semiconductor lasers (2ω-OPSL).

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** (a-c) Polarized absorption cross-section, σ<sub>abs</sub>, spectra of Eu<sup>3+</sup> ions in the 11.5 at.% Eu:KY(WO<sub>4</sub>)<sub>2</sub> epitaxial layer for light polarizations *E* || *N*<sub>m</sub> and *N*<sub>g</sub>. In (c), the spectra for a stoichiometric Eu(WO<sub>4</sub>)<sub>2</sub> crystal are added for comparison (dashed lines). The arrow in (c) indicates the pump wavelength.</figcaption></figure>
The $ ^{7}\text{F}_{0,1} \rightarrow {}^{5}\text{L}_{6} $ transition corresponds to the highest absorption cross-section of 3.51×10<sup>-20</sup> cm<sup>2</sup> at 394.8 nm for light polarization *E* || *N*<sub>m</sub>, Fig. 6(a). For the $ ^{7}\text{F}_{0} \rightarrow {}^{5}\text{D}_{2} $ absorption band which can be addressed by 465-nm GaN laser diodes, two intense absorption peaks are found at 465.2 nm and 466.5 nm, with $ \sigma_{\text{abs}} $ of 1.61×10<sup>-20</sup> cm<sup>2</sup> and 1.45×10<sup>-20</sup> cm<sup>2</sup>, respectively, also for *E* || *N*<sub>m</sub>, Fig. 6(b). The $ ^{7}\text{F}_{1} \rightarrow {}^{5}\text{D}_{1} $ transition falling in the green exhibits lower cross-sections, $ \sigma_{\text{abs}} = 0.53 \times 10^{-20} $ cm<sup>2</sup> at 534.3 nm (absorption linewidth: 0.9 nm) again for *E* || *N*<sub>m</sub>, Fig. 6(c). In the latter figure, the spectra measured for the KEu(WO<sub>4</sub>)<sub>2</sub> crystal are shown for comparison, adding the light polarization *E* || *N*<sub>p</sub> inaccessible for epitaxial layers.

The pump source employed in the present study was a continuous-wave (CW) solid-state laser emitting in the green (at 532.2 nm), thus falling on the side (phonon sideband) of the $ ^{7}\text{F}_{1} \rightarrow {}^{5}\text{D}_{1} $ absorption band (so called “wing-pumping”). The pump polarization corresponded to $ E_{p} $ || $ N_{p} $ (in the layer), and the corresponding absorption cross-section $ \sigma_{\text{abs}} $ was only 0.7×10<sup>-22</sup> cm<sup>2</sup> (measured for the KEu(WO<sub>4</sub>)<sub>2</sub> crystal).

<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Polarized emission properties of Eu<sup>3+</sup> ions in the 11.5 at.% Eu:KY(WO<sub>4</sub>)<sub>2</sub> epitaxial layer: (a) luminescence spectra, $ \lambda_{\text{exc}} = 457 $ nm; (b,c) stimulated-emission cross-sections, $ \sigma_{\text{SE}} $: (b) the $ {}^{5}\text{D}_{0} \rightarrow {}^{7}\text{F}_{2} $ transition in the red, (c) the $ {}^{5}\text{D}_{0} \rightarrow {}^{7}\text{F}_{4} $ transition in the deep-red. The light polarizations are $ E \parallel N_{p}, N_{m}, $ and $ N_{g} $. The arrow in (c) indicates the laser wavelength.</figcaption></figure>

The polarized luminescence spectra of Eu:KY(WO₄)₂ epitaxy in the visible spectral range are shown in Fig. 7(a). Eu³⁺ ions exhibit strong anisotropy of emission properties which is a prerequisite for linearly polarized laser emission. The most intense emission corresponds to the $ {}^{5}\text{D}_{0} \rightarrow {}^{7}\text{F}_{2} $ transition in the red. The stimulated-emission (SE) cross-sections, $ \sigma_{\text{SE}} $, for the $ {}^{5}\text{D}_{0} \rightarrow {}^{7}\text{F}_{2} $ and $ {}^{7}\text{F}_{4} $ transitions were calculated via the Füchtbauer-Ladenburg formula [50]:

$$ \sigma_{SE}^{i}(\lambda) = \frac{\lambda^{5}}{8\pi c \langle n \rangle^{2} \tau_{\text{rad}}} \times \frac{B(JJ')W_{i}(\lambda)}{\frac{1}{3} \sum_{j=p,m,g} \int \lambda W_{j}(\lambda) d\lambda} \quad (1) $$

where $ \lambda $ is the light wavelength, $ c $ is the speed of light, $ \langle n \rangle $ is the mean refractive index of the host matrix at the emission wavelength, $ \tau_{\text{rad}} $ is the radiative lifetime of the emitting state (⁵D₀), $ W_i(\lambda) $ is the measured luminescence spectrum for the i-th polarization (i = p, m, g), and $ B(JJ') $ is the luminescence branching ratio. The results are displayed in Fig. 7(b,c).

In the red, for the $ {}^{5}\text{D}_{0} \rightarrow {}^{7}\text{F}_{2} $ transition, the maximum $ \sigma_{\text{SE}} $ is 3.89×10<sup>-20</sup> cm<sup>2</sup> at 613.7 nm for light polarization $ E \parallel N_{m} $ (experimental luminescence branching ratio: $ B(0 \rightarrow 2) = 76.6\% $). In the deep-red, for the $ {}^{5}\text{D}_{0} \rightarrow {}^{7}\text{F}_{4} $ transition, the peak $ \sigma_{\text{SE}} $ is smaller, measuring 1.76×10<sup>-20</sup> cm<sup>2</sup> at 704.7 nm for $ E \parallel N_{p} $ ($ B(0 \rightarrow 4) = 11.7\% $), corresponding to an emission linewidth of 0.6 nm. The luminescence branching ratios of each Eu³⁺ radiative transition in the KY(WO₄)₂ epitaxial layer
are given in Fig. 1. The near-infrared emissions due to transitions $^5D_0 \rightarrow {}^7F_5$ and $^5D_0 \rightarrow {}^7F_6$ are not included in Fig. 7(a) due to their low intensity (the corresponding luminescence branching ratios are $B(0 \rightarrow 5) = 0.5\%$ and $B(0 \rightarrow 6) = 1.3\%$).

<figure><img src="image_9.png" /><figcaption>**Fig. 8.** Luminescence decay curve from the $^5D_0$ emitting state of Eu$^{3+}$ in the 11.5 at.% Eu:KY(WO$_4$)$_2$ epitaxial layer, circles – experimental data, line – their exponential fit, $\lambda_{exc} = 395$ nm, $\lambda_{lum} = 610$ nm. The decay curve for a KEu(WO$_4$)$_2$ bulk crystal is added for comparison.</figcaption></figure>

The luminescence decay from the $^5D_0$ emitting state of Eu$^{3+}$ ions in the epitaxial layer was studied by monitoring the red luminescence at 610 nm and exciting the active ions at 395 nm, Fig. 8. The decay curve for the stoichiometric KEu(WO$_4$)$_2$ crystal was added for comparison. The measured luminescence lifetime $\tau_{lum}$ amounted to 472 µs for the epitaxial layer, being only slightly shorter than that for the single crystal (479 µs). It is also close to the radiative lifetime ($\tau_{rad} = 464$ µs) calculated using the Judd-Ofelt theory in a previous work [2] thus indicating a luminescence quantum efficiency close to unity. This highlights the high optical quality and crystallinity of the layer being free of quenching centers such as defects and impurities. The decay curves are single exponential, in agreement with the single type of sites ($C_2$ symmetry) for Eu$^{3+}$ ions in KY(WO$_4$)$_2$ crystals.

The homogeneity of the layer, including the homogeneity of distribution of active ions, was confirmed by $\mu$-luminescence and $\mu$-Raman mapping performed across the polished end-facet of the Eu:KY(WO$_4$)$_2$ / KY(WO$_4$)$_2$ epitaxy, Fig. 9. The Eu$^{3+}$ ions are uniformly distributed in the layer and no diffusion into the substrate can be observed. The $\mu$-Raman mapping confirms the slight modification of the material structure due to the heavy Eu$^{3+}$ doping.

<figure><img src="image_10.png" /><figcaption>**Fig. 9.** Mapping across the polished end-facet of the Eu:KY(WO$_4$)$_2$ / KY(WO$_4$)$_2$ epitaxy: (a) $\mu$-luminescence mapping monitoring the peak intensity at 614 nm (the $^5D_0 \rightarrow {}^7F_2$ transition); (b) $\mu$-Raman mapping monitoring the peak position at 764 cm$^{-1}$, in the $\mathbf{p(gm)\bar{p}}$ geometry.</figcaption></figure>

### 3.3 Planar waveguide laser

The layout of the laser set-up, as well as a photograph of the laser cavity are depicted in Fig. 10. The pump source was a commercial CW green solid-state laser delivering multi-Watt output at 532.2 nm (laser linewidth: 0.15 nm) with linear polarization and almost diffraction-limited beam (measured beam quality factor $M^2 = 1.02$), addressing the $^7F_1 \rightarrow {}^5D_1$ absorption band of Eu$^{3+}$ ions. The pump polarization was adjusted using an anti-reflection (AR) coated half-wave plate corresponding to $E_p \parallel N_p$ (vertical) in the waveguide. The pump radiation was coupled into the layer using an AR coated achromatic lens (focal length: $f = 60$ mm). The linear laser cavity was formed by a plane pump mirror (PM) coated for high transmission (HT, $T > 97.0\%$) at 532 nm and high reflection (HR, $R > 99.8\%$) at 705 nm, and a set of plane output couplers
(OCs) with a transmission $T_{OC}$ ranging from 0.3% to 3.0% at the laser wavelength. Both bulk cavity mirrors were slightly pressed towards the waveguide end-facets, consequently forming a compact quasi-monolithic cavity. No index-matching fluid was used. The epitaxy was mounted on a Cu-holder using a silver paint for better heat removal. The waveguide length $\ell$ was 8.37 mm, and it was oriented for light propagation along the $N_m$-axis ($N_m$-cut). A mechanical chopper was implemented to modulate the pump beam (duty cycle: 1:12, pump pulse duration: 40 ms) to avoid detrimental thermal effects. Below, we will refer to this as quasi-CW excitation. The residual pump after the OC was filtered out using a long-pass filter (FELH0650, Thorlabs). The single-pass pump absorption was relatively low (3.1%) as the pump source enabled wing-pumping.

<figure><img src="image_11.png" /><figcaption>**Fig. 10.** Planar Eu waveguide laser: (a) scheme of the laser set-up, (b) a photograph of the laser cavity.</figcaption></figure>

The laser emission spectra were measured using an optical spectrum analyzer (Yokogawa, AQ6373). The oscilloscope traces of laser emission were captured using a fast photodiode and an 8 GHz digital oscilloscope (DSA70804B, Tektronix), under quasi-CW excitation. The spatial profiles of the pump and laser modes, as well as the guided luminescence in the near field were taken using a short focal length ($f=15$ mm) spherical uncoated CaF₂ lens (Thorlabs) and a CCD camera (BladeCam2-XHR, DataRay). For scale calibration, the waveguide with a known thickness was illuminated using a white light source. The input-output dependencies were also measured under quasi-CW excitation.

<figure><img src="image_12.png" /><figcaption>**Fig. 11.** Eu:KY(WO₄)₂ deep-red planar waveguide laser: (a) input-output dependences under quasi-CW excitation, η – slope efficiency, $P_{th}$ – laser threshold; (b) a typical laser spectrum, $T_{OC}$ = 1.0%, red curve – luminescence spectrum (for both, the polarization state is $E \parallel N_p$); (c) a typical oscilloscope trace of laser emission under quasi-CW excitation; (d) estimation of the roundtrip passive loss L via the Findlay-Clay analysis.</figcaption></figure>
The power characterization of the developed laser is shown in Fig. 11(a). The deep-red planar waveguide laser generated a maximum output power of 7 mW at 704.7 nm with a slope efficiency η of 9.5% (vs. the absorbed pump power) and a very low laser threshold $P_{th}$ of 21 mW, when using 1.3% output coupling. On decreasing the transmission of the output coupler, the threshold gradually decreased from 23 mW ($T_{OC}$ = 3.0%) to 14 mW ($T_{OC}$ = 0.3%). Such a low-threshold behavior is related to the four-level scheme of the Eu laser without reabsorption, a relatively long luminescence lifetime of the upper laser level ($^5D_0$), and a small laser mode area. The laser emission was linear polarized ($E_L \parallel N_p$), and the polarization state was naturally selected by the anisotropy of the emission, Fig. 7(c). A typical spectrum of laser emission for $T_{OC}$ = 1.0% is given in Fig. 11(b) and compared with the fluorescence spectrum for the respective light polarization. The laser emission spectrum was relatively broad, and its multi-line structure was determined by the etalon (Fabry-Perot) effects at the crystal / mirror interfaces (laser linewidth: ~0.6 nm). A typical oscilloscope trace of laser emission measured under quasi-CW excitation reveals a spiking behavior, see Fig. 11(c).

The Findlay-Clay analysis was applied to evaluate the waveguide propagation losses [51], as shown in Fig. 11(d). The following formula links the laser threshold $P_{th}$ and the output coupler reflectivity, $R_{OC} = 1-T_{OC}$:

$$-\ln(R_{OC}) = 2KP_{th}-L, \quad (2)$$

where K is a constant, and L is the roundtrip passive loss corresponding to the y-intercept of the linear fit. This analysis yields $L = 1.8 \pm 0.5\%$, giving an upper estimate for the waveguide propagation loss $\delta_{loss} = 4.34L/(2\ell)$ of 0.05 dB/cm.

The near-field modal profiles of the guided green pump radiation, red luminescence and deep-red laser emission are shown in Fig. 12. Both the pump and laser modes are well confined within the epitaxial layer, see Fig. 12(a,c). The photographs of the red luminescence and deep-red laser beam are also given as insets in Fig. 12(b) and (c), respectively. They exhibit a stripe-like shape typical for planar waveguides with light confinement in only one direction (vertical).

<figure><img src="image_13.png" /><figcaption>Fig. 12. Modal behavior of the Eu:KY(WO$_4$)$_2$ planar waveguide: (a-c) near field mode profiles: (a) guided pump at 532 nm ($E_P \parallel N_p$), (b) guided luminescence at 613 nm (unpolarized), (c) laser mode at 705 nm ($E_L \parallel N_p$). White lines – the air / layer and layer / substrate interfaces. The insets show photographs of (b) the red luminescence in the far-field and (c) the deep-red laser beam; (d) 1D intensity profiles of the laser mode in the horizontal and vertical directions, symbols – experimental data, curve – Gaussian fit.</figcaption></figure>

The 1D intensity profiles of the laser mode are displayed in Fig. 12(d). The intensity profile is reasonably well fitted with a Gaussian distribution in the horizontal direction (i.e., in the waveguide plane) yielding a mode diameter (at the $1/e^2$ level) of 179 µm, and, as expected due to the lack of cladding, the intensity profile is strongly asymmetric in the vertical direction
(beam diameter: 16 µm). According to the measured refractive index contrast, the waveguide is strongly multimode in the latter direction.

Eu³⁺ ions exhibit their most intense emission in the red, at 613 nm. However, it was proven difficult to achieve room temperature lasing in this spectral range [1,52]. This could be explained by resonant excited-state absorption processes originating from the upper laser level (⁵D₀) to higher-lying 4f manifolds, and / or reabsorption from the thermally populated terminal laser level (⁷F₂). Stimulated emission of Eu³⁺ ions in the red was only observed at cryogenic temperatures owing to the decreased thermal population of the ⁷F₂ manifold [53,54].

**4. Conclusions**

To conclude, we report on the first Liquid Phase Epitaxy growth of KY(WO₄)₂ layers heavily doped (11.5 at.%) with Eu³⁺ ions, as well as the first deep-red Eu waveguide laser employing a dielectric material as gain medium. The grown epitaxial layers feature i) a well-preserved orientation, ii) a uniform distribution of the dopant ions with no migration into the substrate, iii) a strong polarization of emission properties of Eu³⁺ ions in the red and deep-red giving rise to linear laser polarization, iv) a relatively long upper laser level luminescence lifetime and almost no quenching via energy migration to defects and impurities, and v) low waveguide propagation losses (0.05 dB/cm) in the deep-red. Altogether, this makes Eu:KY(WO₄)₂ layers a promising platform for compact, low threshold visible laser sources.

Further power scaling and improvement of the laser efficiency is expected by boosting the pump absorption in the layers, namely via their proper orientation to access the desired light polarization *E* || *N*<sub>m</sub> with the highest absorption cross-sections, as well as increasing the Eu³⁺ content in the layers potentially up to the stoichiometric composition. Diode-pumping of such waveguides could be another solution, e.g., when using 465 nm GaN-based blue laser diodes addressing the ⁷F₀ → ⁵D₂ absorption band. The microstructuring of grown layers by means of precision diamond saw dicing could lead to fabrication of surface channel (ridge) waveguides with yet lower laser thresholds and improved slope efficiencies owing to the smaller mode areas and better overlap between the pump and laser modes.

**Funding.** Région Normandie, France (Contrat de plan État-Région (CPER), Programme WINNINGNormandy); Agence Nationale de la Recherche, France (ANR-22-CE08-0025-01, NOVELA, ANR-23-LCV2-0001-01, LOVINOV); Project PID2022-1414990B0-100, funded by MICIU/AEI/10.13039/501100011033/ and by FEDER/UE; H2020 Marie Skłodowska-Curie Actions (101034329).

**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. P. Loiko, V. Dashkevich, S. Bagaev, V. Orlovich, A. Yasukevich, K. Yumashev, N. Kuleshov, E. Dunina, A. Kornienko, and S. Vatnik, "Spectroscopic characterization and pulsed laser operation of Eu³⁺:KGd(WO₄)₂ crystal," Laser Phys. 23(10), 105811 (2013).

2. P. Loiko, V. Dashkevich, S. Bagaev, V. Orlovich, A. Yasukevich, K. Yumashev, N. Kuleshov, E. Dunina, A. Kornienko, S. Vatnik, and A. Pavlyuk, "Spectroscopic and photoluminescence characterization of Eu³⁺-doped monoclinic KY(WO₄)₂ crystal," J. Lumin. 153, 221-226 (2014).

3. R. Jagannathan and M Kottaisamy, "Eu³⁺ luminescence: A spectral probe in M₅(PO₄)₃X apatites (M=Ca or Sr; X=F, Cl, Br or OH⁻)," J. Phys.: Condens. Matter 7(44), 8453-8466 (1995).

4. M. Scarafagio, A. Tallaire, K.-J. Tielrooij, D. Cano, A. Grishin, M.-H. Chavanne, F. Koppens, A. Ringuedé, M. Cassir, D. Serrano, P. Goldner, and A. Ferrier, "Ultrathin Eu- and Er-doped Y₂O₃ films with optimized optical properties for quantum technologies," J. Phys. Chem. C 123(21), 13354-13364 (2019).

5. A. Arcangeli, M. Lovrić, B. Tumino, A. Ferrier, and P. Goldner, "Spectroscopy and coherence lifetime extension of hyperfine transitions in ¹⁵¹Eu³⁺:Y₂SiO₃," Phys. Rev. B 89(18), 184305 (2014).

6. C. Kränkel, D. Marzal, F. Moglia, G. Huber, and P. Metz, "Out of the blue: semiconductor laser pumped visible rare-earth doped lasers," Laser Photonics Rev. 10(4) 548-568 (2016).

7. J. Park and A. Steckl, "Laser action in Eu-doped GaN thin-film cavity at room temperature," Appl. Phys. Lett. 85(20), 4588-4590 (2004).

8. S. Bagaev, V. Dashkevich, V. Orlovich, S. Vatnik, A. Pavlyuk, and A. Yurkin, "25%Eu:KGd(WO₄)₂ laser crystal: spectroscopy and lasing on the ⁵D₀→⁷F₄ transition," Quantum Electron. 41(3), 189-192 (2011).
9. V. Dashkevich, S. Bagayev, V. Orlovich, A. Bui, P. Loiko, K. Yumashev, N. Kuleshov, S. Vatnik, and A. Pavlyuk, "Quasi-continuous wave and continuous wave laser operation of Eu:KGd(WO₄)₂ crystal on a ²D₀ → ⁷F₄ transition," Laser Phys. Lett. 12(1), 015006 (2015).

10. P. Loiko, D. Rytz, S. Schwung, P. Pues, T. Jüstel, J.-L. Doualan, and P. Camy, "Watt-level europium laser at 703 nm," Opt. Lett. 46(11), 2702-2705 (2021).

11. W. Carnall, P. Fields, and K. Rajnak, "Electronic energy levels of the trivalent lanthanide aquo ions. IV. Eu³⁺," J. Chem. Phys. 49(10), 4450-4455 (1968).

12. T. Nishida, N. Shimada, K. Ono, T. Yagi, and A. Shima "Highly reliable 637-639 nm red high-power LDs for displays," Proc. SPIE 7583, 758303 (2010).

13. X. Lin, J. Liu, J. Hao, K. Wang, Y. Zhang, H. Li, H. Horimai, and X. Tan, "Collinear holographic data storage technologies," Opto-Electron. Adv. 3(3), 190004 (2020).

14. L. Walsh, "The current status of laser applications in dentistry," Aust. Dent. J. 48(3), 146-155 (2003).

15. C. Dupuy, C. Hwang, D. Benenati, H. Simmonds, R. Fu, and D. Bull, "High-power red diode laser system for photodynamic therapy," Proc. SPIE 231, 333-341 (1994).

16. D.-R. Lee, "Progresses in implementation of STED microscopy," Meas. Sci. Technol. 34(10), 102002 (2023).

17. C. Fan and J. Longtin, "Laser-based measurement of liquid temperature or concentration at a solid-liquid interface," Exp. Therm. Fluid Sci. 23(1-2), 1-9 (2000).

18. A. Hernandez, P. Dominguez, O. Cruz, R. Ivanov, C. Carballo, and B. Zepeda, "Laser in agriculture," Int. Agrophys. 24(4), 407-422 (2010).

19. V. Petrov, M. Pujol, X. Mateos, Ó. Silvestre, S. Rivier, M. Aguiló, R. Solé, J. Liu, U. Griebner, and F. Díaz, "Growth and properties of KLu(WO₄)₂, and novel ytterbium and thulium lasers based on this monoclinic crystalline host," Laser Photonics Rev. 1(2), 179-212 (2007).

20. V. Jambunathan, X. Mateos, P. Loiko, J. Serres, U. Griebner, V. Petrov, K. Yumashev, M. Aguiló, and F. Díaz, "Growth, spectroscopy and laser operation of Ho:KY(WO₄)₂," J. Lumin. 179, 50-58 (2016).

21. J. Serres, P. Loiko, V. Jambunathan, X. Mateos, V. Vitkin, A. Lucianetti, T. Mocek, M. Aguiló, F. Díaz, U. Griebner, and V. Petrov, "Efficient diode-pumped Er:KLu(WO₄)₂ laser at ~1.61 µm," Opt. Lett. 43(2), 218-221 (2018).

22. A. Kaminskii, J. Gruber, S. Bagaev, K.-I. Ueda, U. Hömmerich, J. Seo, D. Temple, B. Zandi, A. Kornienko, E. Dunina, A. Pavlyuk, R. Klevtsova, and F. Kuznetsov, "Optical spectroscopy and visible stimulated emission of Dy³⁺ ions in monoclinic α-KY(WO₄)₂ and α-KGd(WO₄)₂ crystals," Phys. Rev. B 65, 125108 (2002).

23. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO₄)₂ laser at 649 nm," Opt. Lett. 48(18), 4721-4724 (2023).

24. A. Baillard, P. Loiko, D. Rytz, S. Schwung, A. Pavlyuk, A. Kornienko, E. Pimor, M. Badtke, C. Kränkel, E. Dunina, L. Fomicheva, L. Guillemot, A. Braud, and P. Camy, "Polarized spectroscopy of Sm³⁺ ions in monoclinic KGd(WO₄)₂ crystals for lasers emitting in the red," J. Lumin. 273, 120641 (2024).

25. J. Mackenzie, "Dielectric solid-state planar waveguide lasers: a review," IEEE J. Sel. Top. Quantum Electron. 13(3), 626-637 (2007).

26. E. Kifle, P. Loiko, C. Romero, J. de Aldana, M. Aguiló, F. Díaz, P. Camy, U. Griebner, V. Petrov, and X. Mateos, "Watt-level ultrafast laser inscribed thulium waveguide lasers," Prog. Quantum Electron. 72, 100266 (2020).

27. K. Van Dalfsen, S. Aravazhi, C. Grivas, S. García-Blanco, and M. Pollnau, "Thulium channel waveguide laser with 1.6 W of output power and ~80% slope efficiency," Opt. Lett. 39(15), 4380-4383 (2014).

28. D. Geskus, S. Aravazhi, E. Bernhardi, C. Grivas, S. Harkema, K. Hametner, D. Günther, K. Wörhoff, and M. Pollnau, "Low-threshold, highly efficient Gd³⁺, Lu³⁺ co-doped KY(WO₄)₂:Yb³⁺ planar waveguide lasers," Laser Phys. Lett. 6(11), 800-805 (2009).

29. J. Carvajal, W. Bolaños, X. Mateos, F. Díaz, and M. Aguiló, "Tm³⁺-based waveguide lasers in monoclinic double tungstates," J. Lumin. 133, 262-267 (2013).

30. X. Mateos, S. Lamrini, K. Scholle, P. Fuhrberg, S. Vatnik, P. Loiko, I. Vedin, M. Aguiló, F. Díaz, U. Griebner, and V. Petrov, "Holmium thin-disk laser based on Ho:KY(WO₄)₂/KY(WO₄)₂ epitaxy with 60% slope efficiency and simplified pump geometry," Opt. Lett. 42(17), 3490-3493 (2017).

31. V. Gorbenko, T. Zorenko, A. Kaczmarek, R. Van Deun, A. Fedorov, and Y. Zorenko, "Eu³⁺ multicenter formation and luminescent properties of Ca₃Sc₂Si₃O₁₂:Eu and Ca₂YScMgSiO₁₂:Eu single crystalline films," Opt. Mater. 90, 70-50 (2019).

32. N. Vasil'eva, D. Spassky, S. Omelkov, V. Plotnichenko, and D. Vasil'ev, "Growth, optical absorption, and photoluminescence of epitaxial (Pb,Gd)₃(Al,Ga)₃O₁₂:Ce³⁺,Eu³⁺ films," Inorg. Mater. 58(2), 165-171 (2022).

33. P. Tien, R. Martin, S. Blank, S. Wemple, and L. Varnerin, "Optical waveguides of single-crystal garnet films," Appl. Phys. Lett. 21(5), 207-209 (1972).

34. Á. de Jesús Morales-Ramírez, M. García-Hernandez, M. Carrera-Jota, P. Garnica-Chávez, B. González-Penguelly, and A. Garrido-Hernández, "Photoluminescence properties of Lu₂Si₂O₇:Eu³⁺ sol-gel thin films obtained using GLYMO as a silicon source," Ceram. Int. 49(23), Part A, 38165-38173 (2023).

35. M. Scarafagio, A. Tallaire, M.-H. Chavanne, M. Cassir, A. Ringuedé, D. Serrano, P. Goldner, and A. Ferrier, "Improving the luminescent properties of atomic layer deposition Eu:Y₂O₃ thin films through optimized thermal annealing," Phys. Status Solidi A 217(8), 1900909 (2020).
36. N. Harada, A. Ferrier, D. Serrano, M. Persechino, E. Briand, R. Bachelet, I. Vickridge, J.-J. Ganem, P. Goldner, and A. Tallaire, "Chemically vapor deposited Eu<sup>3+</sup>:Y<sub>2</sub>O<sub>3</sub> thin films as a material platform for quantum technologies," J. Appl. Phys. **128**, 055304 (2020).

37. K. Ezirmik, "Concentration quenching of photoluminescence in optically transparent Lu<sub>2</sub>O<sub>3</sub>:Eu thin films deposited to be used as scintillators," Mater. Res. Express **8**(1), 016407 (2021).

38. S. Miranda, M. Peres, T. Monteiro, E. Alves, H. Sun, T. Geruschke, R. Vianden, and K. Lorenz, "Rapid thermal annealing of rare earth implanted ZnO epitaxial layers," Opt. Mater. **33**(7), 1139-1142 (2011).

39. I. Camps, M. Borlaf, M. Colomer, R. Moreno, L. Duta, C. Nita, A. Perez del Pino, C. Logofatu, R. Serna, and E. György, "Structure-property relationships for Eu doped TiO<sub>2</sub> thin films grown by a laser assisted technique from colloidal sols," RSC Adv. **7**(60), 37643-37653 (2017).

40. S. Al-Shomar, "Synthesis and characterization of Eu<sup>3+</sup> doped TiO<sub>2</sub> thin films deposited by spray pyrolysis technique for photocatalytic application," Mater. Res. Express **8**(2), 026402 (2021).

41. M. Novotný, M. Vondráček, E. Marešová, P. Fitl, J. Bulíř, P. Pokorný, Š. Havlová, N. Abdellaoui, A. Pereira, P. Hubík, J. More-Chevalier, and J. Lančok, "Optical and structural properties of ZnO:Eu thin films grown by pulsed laser deposition," Appl. Surf. Sci. **476**, 271-275 (2019).

42. G.-R. Li, C.-R. Dawa, X.-H. Lu, X.-L. Yu, and Y.-X. Tong, "Use of additives in the electrodeposition of nanostructured Eu<sup>3+</sup>/ZnO films for photoluminescent devices," Langmuir **25**(4), 2378-2384 (2009).

43. P. Chen, Z. Wang, S. Wang, M. Lyu, M. Hao, M. Ghasemi, M. Xiao, J.-H. Yun, Y. Bai, L. Wang, "Luminescent europium-doped titania for efficiency and UV-stability enhancement of planar perovskite solar cells," Nano Energy **69**, 104392 (2020).

44. P. Loiko, P. Segonds, P. Inácio, A. Peña, J. Debray, D. Rytz, V. Filippov, K. Yumashev, M. Pujol, X. Mateos, M. Aguiló, F. Díaz, M. Eichhorn, and B. Boulanger, "Refined orientation of the optical axes as a function of wavelength in three monoclinic double tungstate crystals KRE(WO<sub>4</sub>)<sub>2</sub> (RE = Gd, Y or Lu)," Opt. Mater. Express **6**(9), 2984-2990 (2016).

45. R. Shannon, "Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides," Acta Crystallogr. A **32**, 751-767 (1976).

46. Y. Petit, S. Joly, P. Segonds, and B. Boulanger, "Recent advances in monoclinic crystal optics," Laser Photonics Rev. **7**(6), 920-937 (2013).

47. T. Damen, S. Porto, and B. Tell, "Raman effect in zinc oxide," Phys. Rev. **142**(2), 570-574 (1966).

48. L. Macalik, P. Deren, J. Hanuza, W. Stręk, A. Demidovich, and A. Kuzmin, "Effect of random distribution and molecular interactions on optical properties of Er<sup>3+</sup> dopant in KY(WO<sub>4</sub>)<sub>2</sub> and Ho<sup>3+</sup> in KYb(WO<sub>4</sub>)<sub>2</sub>," J. Mol. Struct. **450**(1-3), 179-192 (1998).

49. L. Macalik, J. Hanuza, A. Kaminskii, "Polarized infrared and Raman spectra of KGd(WO<sub>4</sub>)<sub>2</sub> and their interpretation based on normal coordinate analysis," J. Raman Spectrosc. **33**(2), 92-103 (2002).

50. 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).

51. D. Findlay and R. Clay, "The measurement of internal losses in 4-level lasers," Phys. Lett. **20**(3), 277-278 (1966).

52. A. Baillard, P. Loiko, A. Pavlyuk, G. Brasse, A. Braud, and P. Camy, "Deep-red laser operation of cleaved single-crystal plates of Eu:CsGd(MoO<sub>4</sub>)<sub>2</sub> molybdate," Opt. Lett. **48**(11), 2977-2980 (2013).

53. N. Chang, "Fluorescence and stimulated emission from trivalent europium in yttrium oxide," J. Appl. Phys. **34**(12), 3500-3504 (1963).

54. E. Schimitschek, "Stimulated emission in rare-earth chelate (europium benzoylacetonate) in a capillary tube," Appl. Phys. Lett. **3**(7), 117-118 (1963).