Epitaxial Growth of Single Crystal YAG for Optical Devices

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Article Epitaxial Growth of Single Crystal YAG for Optical Devices

Syed N. Qadri 1,*, Woohong Kim 1, Shyam Bayya 1, L. Brandon Shaw 1, Syed B. Qadri 1, Joseph Kolis 2, Bradley Stadelman 2 and Jasbinder Sanghera 1

1 Naval Research Laboratory, Washington, DC 20375, USA; [email protected] (W.K.); [email protected] (S.B.); [email protected] (L.B.S.); [email protected] (S.B.Q.); [email protected] (J.S.) 2 Department of Chemistry, Clemson University, Clemson, SC 29634, USA; [email protected] (J.K.); [email protected] (B.S.) * Correspondence: [email protected]

Abstract: We report the latest progress on fabrication of rare earth doped single crystal yttrium aluminum garnet (YAG) core/undoped YAG cladded fibers. Rare-earth doped single crystal core fibers were grown with laser heated pedestal growth methods. In a second step, epitaxial methods were used to grow a single crystalline undoped YAG cladding onto the core fiber. Hydrothermal and liquid phase epitaxy methods utilize the core doped fiber as the seed. X-ray diffraction of cladding reveals an equilibrium (110) morphology. Energy-dispersive X-ray spectroscopy analysis shows there is minimal diffusion of rare-earth dopants into the cladding structure. The use of scandium doping is shown to substitute at the Al3+ site, thereby allowing an additional tunability of refractive index of core structure material besides conventional Y3+ site dopants. The use of these epitaxial growth methods enables material compatibility, tuning of refractive index, and conformal growth of cladding structures onto core fibers for optical devices.

Citation: Qadri, S.N.; Kim, W.; Keywords: liquid phase epitaxial growth; hydrothermal growth; fiber lasers; YAG; sesquioxides Bayya, S.; Shaw, L.B.; Qadri, S.B.; Kolis, J.; Stadelman, B.; Sanghera, J. Epitaxial Growth of Single Crystal YAG for Optical Devices. Coatings 1. Introduction 2021, 11, 644. https://doi.org/ Recently, there has been growing interest in developing high-power fiber lasers with 10.3390/coatings11060644 nearly diffraction limited beam quality. The fiber laser architecture has a number of advantages over bulk laser geometries. A key benefit of fiber-based geometry is reduced Academic Editor: Aivaras Kareiva thermal issues, including thermal beam distortions and fracture [1]. With planar waveguide geometries, the increased thermal load due to a surface area to volume ratio can result in Received: 30 April 2021 refractive-index changes based on thermal gradients. The long interaction lengths based Accepted: 20 May 2021 Published: 27 May 2021 on fiber lasers can result in high gain and impressive high power output. In a recent demonstration, a single mode output of 10 kW has been demonstrated from a single

Publisher’s Note: MDPI stays neutral aperture in a Yb-doped silica fiber [2]. However, operating at higher power outputs can with regard to jurisdictional claims in result in thermal effects including thermal mode instability (TMI) and thermal lensing. In published maps and institutional affil- addition, non-linear effects can result from having a long and thin gain medium profile. iations. Stimulated Brillouin scattering (SBS) is one effect seen at high power which can limit further power scaling [3,4]. Much work has been focused on silica-based fibers to date [5–7]. The ability to incorporate rare-earth dopants such as ytterbium as laser-active materials into the core of silica-fibers can mitigate photodarkening issues. However, the use of solution doping to incorporate the rare-earth ions can result in the presence of hydroxide impurities Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. in silica-based fibers. In addition, the laser-induced damage at higher powers with glass This article is an open access article fibers can damage the small cores. In an effort to mitigate SBS and TMI issues with fibers, distributed under the terms and one strategy would be to select a different host material, such as crystalline materials, with conditions of the Creative Commons lower SBS cross-sections and higher thermal conductivities. Attribution (CC BY) license (https:// One suggested crystalline host material for core fibers that has shown great promise is creativecommons.org/licenses/by/ yttrium aluminum garnet (YAG) [8]. YAG offers numerous advantages over silica including 4.0/). the potential for higher doping concentrations which would enable the use of shorter fiber

Coatings 2021, 11, 644. https://doi.org/10.3390/coatings11060644 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 644 2 of 13

lengths resulting in lower non-linear effects. YAG offers ∼10 higher thermal conductivity compared to silica which would reduce any stresses in radial direction of the core limiting melting or laser induced damage [9,10]. The lower SBS cross-section for YAG would lead to reduced non-linear effects and enable much greater potential for high peak and average laser power output [11]. YAG has the additional benefit of higher mechanical strength over silica-glass [12]. A recent study suggests that based on thermal and non-linear effects, a Ho doped silica fiber would have a limit of ∼3 kW single frequency power compared to an all crystalline Ho-doped YAG fiber which could generate 35 kW of single frequency eye-safe laser power [13]. In choosing cladding materials to apply to a crystalline YAG core, several characteris- tics must be considered. Minimizing any coefficient of thermal expansion (CTE) mismatch between materials would reduce any interfacial stress between the core and cladding. The ability to control numerical apertures (NA) through tailoring of the refractive indices can enable high power lasers. Finally, mechanical toughness similar to the core material would help ensure a rugged device. By choosing a proper crystalline cladding, thermal stresses are kept to a minimum between core and clad materials. Tuning of the NA is possible through changing refractive indices by tailoring the composition of the materials. While crystalline cladding selection offers better thermal and mechanical properties than glass, crystalline materials have very defined melting points and low viscosity in the molten state making it difficult to use a preform with a predefined core-clad structure to make a fiber as is commonly used for glass fibers. The challenge is to maintain the integrity of the crystalline core during the cladding process by maintaining a growth temperature below the melting point of the YAG core. Previously, methods including magnetron sputtering and in situ core-cladding pulling processes during laser-heated pedestal growth (LHPG) have been used to apply crystalline claddings to a crystalline core fiber [14–18]. These have had various successes in applying crystalline claddings, but there have only been limited reports on liquid phase epitaxy and hydrothermal growth being used for depositing crystalline cladding on crystalline core fibers. While the feasibility of using an epitaxial process to grow single crystal undoped YAG cladding on a rare-earth doped single crystal YAG core fiber has been shown previously in a report by Dubinskii et al., a detailed characterization of the crystal quality through single crystal X-ray diffraction has not been demonstrated [19]. Also, the optimization of these growth techniques to control etch-back on fibers followed by actual crystal growth on etched facets has not been reported. In fiber optics there has been a great deal of studies to explore doped-YAG where the dopant is Nd3+, Er3+, Yb3+, or Cr4+. However, due to their low thermal conductivity their applications as high power laser materials is limited [20]. Kim et al. reported on the synthesis and applications of doped sesquioxides such as Sc2O3,Y2O3, and Lu2O3 for high power laser applications because of their high thermal conductivity and high absorption and emission cross sections of 3+ rare-earth ions [21]. Thus based on the optical and thermal conductivity consideration a sesquioxide-doped YAG is a potential candidate for high-power applications. Here, we report on progress in the development of crystalline cladded doped YAG fiber structures as deposited successfully with the liquid phase epitaxy and hydrothermal methods. Physical and optical properties of these fibers are discussed. We also report on progress in the characterization of a scandium (Sc):YAG crystal cladding layer grown epitaxially on a YAG substrate as deposited successfully with the liquid phase epitaxy (LPE) method.

2. Experimental 2.1. Fabrication of Doped Yttrium Aluminum Garnet (YAG) Core Fibers In order to fabricate core and cladding structures for use as low loss waveguide fibers, a two step procedure is used in which the high quality single crystal core fiber must be first fabricated through LHPG. In the second step, the small core fiber is then overclad with other techniques. We have developed a high-precision LHPG system to draw single crystal fibers with lengths over 1 m and diameters as small as 17 µm (Figure1). The LHPG system Coatings 2021, 11,Coatings x FOR PEER 2021 ,REVIEW 11, x FOR PEER REVIEW 3 of 13 3 of 13

2. Experimental2. Experimental 2.1. Fabrication2.1. of DopedFabrication Yttrium of Doped Aluminum Yttrium Garnet Aluminum (YAG) GarnetCore Fibers (YAG) Core Fibers In order to fabricateIn order core to fabricateand cladding core andstructures cladding for structuresuse as low for loss use waveguide as low loss fi ‐waveguide fi‐ bers, a two step procedure is used in which the high quality single crystal core fiber must Coatings 2021, 11, 644bers, a two step procedure is used in which the high quality single crystal core fiber must 3 of 13 be first fabricatedbe first through fabricated LHPG. through In the LHPG.second Instep, the the second small step, core the fiber small is then core overclad fiber is then overclad with other techniques.with other We techniques. have developed We have a high developed‐precision a high LHPG‐precision system LHPG to draw system single to draw single crystal fibers withcrystal lengths fibers over with 1 lengths m and diametersover 1 m and as small diameters as 17 μasm small (Figure as 17 1). μ Them (Figure LHPG 1). The LHPG system is a modifiedsystemis a modified floatis a modified zone float crystal zone float growth crystal zone crystal growthtechnique growth technique that technique utilizes that a utilizes thathigh utilizes‐power a high-power astabi high‐ ‐power stabilized stabi‐ 2 lized CO2 laserlizedCO to2 melt laserCO a laser to seed melt to crystal melt a seed athat seed crystal is crystallowered that isthat loweredfrom is lowered above from the from above melt above theinto melt thethe molten intomelt the into molten the molten zone. zone. Once thezone.Once seed Once the comes seed the into comesseed contact comes into with contact into the contact withmolten with the zone, molten the themolten zone, feed zone, crystal the feed the is feed crystaladvanced crystal is advanced is advanced into into the moltenintothe zone moltenthe atmolten a zone constant zone at a constantrateat a constantwhile rate a feedback while rate while a feedback system a feedback maintains system system maintains ∼1% maintains core∼1% di‐ core∼1% diameter core di‐ ameter uniformityameteruniformity on uniformity small on diameter small on diameter small fibers. diameter fibers.The fibers fibers. The created fibers The fibers createdfrom LHPGcreated from demonstrate LHPGfrom LHPG demonstrate demonstrate good good mechanicalgoodmechanical strength mechanical strength and flexibility.strength and flexibility. and flexibility.

Figure 1. Schematic of laser‐heated pedestal growth (LHPG) technique at the Naval Research La‐ Figure 1. SchematicFigure 1. of Schematic laser-heated of laser pedestal‐heated growth pedestal (LHPG) growth technique (LHPG) at the technique Naval Research at the Naval Laboratory Research resulting La‐ in a fiber boratory resulting in a fiber with high flexibility. A feedback mechanism controls the pulling rate with high flexibility.boratory Aresulting feedback in mechanisma fiber with controlshigh flexibility. the pulling A feedback rate to achievemechanism as small controls as 17 theµm pulling fiber diameter rate from a to achieve as small as 17 μm fiber diameter from a 100 μm feedstock. 100 µm feedstock.to achieve as small as 17 μm fiber diameter from a 100 μm feedstock.

2.2 Undoped YAG2.22.2. Undoped Cladding Undoped YAG Processes YAG Cladding Cladding on Fibers Processes Processes on on Fibers Fibers 2.2.1. Hydrothermal2.2.1.2.2.1. Hydrothermal Hydrothermal Growth Growth Growth 3 5 12 HydrothermalHydrothermalHydrothermal synthesis of YIG synthesis synthesis (Y3Fe5O of of12 YIG) was YIG (Y first (YFe3Fe studiedO5O) 12was) wasby first Laudise first studied studied with by Laudisethe by goal Laudise with with the goal the of developingof goala developinglower of developing temperature a lower a lowerprocess temperature temperature compared process to process liquidcompared comparedphase to epitaxyliquid to liquidphase [22]. How epitaxy phase‐ epitaxy [22]. How [22].‐ ever, Czochralskiever,However, methods Czochralski Czochralski were methods used methods primarily were were used in used theprimarily field primarily to in grow the in the fieldlarge field to YAG togrow grow boules. large large YAG YAG boules. boules. Recently, McMillenRecently,Recently, et al. McMillen reported et growth al. reported reported of YAG growth growth through of YAG hydrothermal through hydrothermal method using method using YAG disks [23].YAGYAG Here, disks we [23]. [ 23report]. Here, on wehydrothermal report on hydrothermal growth using growtha Yb‐doped using YAG a Yb Yb-doped single‐doped YAG YAG single crystal core fibercrystalcrystal as seed core core material fiber fiber as seed(Figure seed material material 2). The (Figure (Figure seed substrate 22).). TheThe seedseed is suspended substrate iswithis suspendedsuspended a ladder withwith aa ladderladder in the upper portioninin the upper of the portionportion autoclave ofof the theand autoclave autoclave is kept slightly and and is is kept coolerkept slightly slightly than cooler lower cooler thanzone than lower where lower zone zone where where the the feedstock thematerialfeedstock feedstock is material provided material is providedthrough is provided a through baffle through located a baffle a bafflein located between located in betweentwo in zonebetween two through zone two throughout‐zone through the‐ out the growthoutgrowth process. the growth process. The feedstock process. The feedstock The of cladding feedstock of cladding component of cladding component oxides component in oxides the loweroxides in the portion lowerin the lower portion portion of the two zone furnace includes stoichiometric amounts of Y2O3 and2 Al32O3.K22CO3 3 mineralizer2 3 of the two zoneof thefurnace two zoneincludes furnace stoichiometric includes stoichiometric amounts of Y 2amountsO3 and Al of2O Y3.O K2 COand3 AlminO‐ . K CO min‐ and water are added to reach a fill level of approximately 50 to 70%. The sealed autoclave eralizer and watereralizer are and added water to reachare added a fill tolevel reach of approximatelya fill level of approximately 50 to 70%. The 50 sealed to 70%. The sealed is run between 600 and 640 ◦C to achieve a thermal gradient between dissolution and autoclave is runautoclave between is 600run andbetween 640 °C 600 to and achieve 640 °Ca thermal to achieve gradient a thermal between gradient dissolu between‐ dissolu‐ growth zones, which are separated by a baffle. The thermal gradient and pressures of tion and growthtion zones, and growth which zones,are separated which areby separateda baffle. The by thermala baffle. gradientThe thermal and gradientpres‐ and pres‐ 15−25 kpsi result in a super-saturation of the cooling region and encourages crystal growth sures of 15−25sures kpsi resultof 15− 25in akpsi super result‐saturation in a super of ‐thesaturation cooling ofregion the cooling and encourages region and crys encourages‐ crys‐ of undoped YAG onto the seed substrate. The core fiber drawn from the LHPG process tal growth of undopedtal growth YAG of undoped onto the YAG seed ontosubstrate. the seed The substrate. core fiber The drawn core from fiber the drawn LHPG from the LHPG serves as the seed single crystal fiber in this cladding process. process servesprocess as the seedserves single as the crystal seed singlefiber in crystal this cladding fiber in process.this cladding process.

Figure 2. Hydrothermal chamber for cladding growth showing the bottom dissolving zone and an upper growth zone for growing single crystal cladding on single crystal ﬁber.

Various dopants and concentrations can be used with this method to deposit cladding onto the rare-earth doped YAG core fiber. In Figure3, a 100 µm square shaped Ho3+ doped single crystal YAG fiber was cladded with YAG containing a small dopant concentration (0.25%) of Co2+/Si4+. The Co2+ doping allowed for a simplified blue color identification Coatings 2021, 11, x FOR PEER REVIEW 4 of 13 Coatings 2021, 11, x FOR PEER REVIEW 4 of 13

Figure 2. Hydrothermal chamber for cladding growth showing the bottom dissolving zone and an upperFigure growth 2. Hydrothermalzone for growing chamber single forcrystal cladding cladding growth on singleshowing crystal the bottomfiber. dissolving zone and an upper growth zone for growing single crystal cladding on single crystal fiber. Various dopants and concentrations can be used with this method to deposit clad‐ Coatings 2021, 11, 644 Various dopants and concentrations can be used with this method to deposit4 of 13 clad‐ ding onto the rare‐earth doped YAG core fiber. In Figure 3, a 100 μm square shaped Ho3+ ding onto the rare‐earth doped YAG core fiber. In Figure 3, a 100 μm square shaped Ho3+ doped single crystal YAG fiber was cladded with YAG containing a small dopant concen‐ doped single crystal YAG fiber was cladded with YAG containing a small dopant concen‐ tration (0.25%) of Co2+/Si4+. The Co2+ doping allowed for a simplified blue color identifica‐ tration (0.25%) of Co2+/Si4+. The4+ Co2+ doping allowed for a simplified blue color identifica2+ ‐ tionof the of cladding the cladding structure. structure. The SiThe co-dopantSi4+ co‐dopant was usedwas used for charge for charge compensation compensation as Co as tion4+ of the cladding structure.3+ The Si4+ co‐dopant was used for charge compensation as Coand2+ Siand substituteSi4+ substitute at the at Al the siteAl3+ in site YAG in YAG crystal. crystal. Crystal Crystal core/clade core/clade fiber withfiber awith diameter a di‐ Co2+ and Si4+ substitute at the Al3+ site in YAG crystal. Crystal core/clade fiber with a di‐ ameterof 140 µofm 140 was μm successfully was successfully fabricated fabricated as shown as shown in Figure in Figure3. Tuning 3. Tuning of the of feedstock the feed‐ ameter of 140 μm was successfully fabricated as shown in Figure 3. Tuning of the feed‐ stockconcentrations concentrations were were investigated investigated in order in order to understand to understand boundary boundary conditions conditions for this for stock concentrations were investigated in order to understand boundary conditions for thismethod. method. Specifically, Specifically, the quantitythe quantity of feedstock of feedstock and theand presence the presence of other of other reactants reactants were this method. Specifically, the quantity of feedstock and the presence of other reactants werefound found to determine to determine the etchback the etchback conditions. conditions. Etchback Etchback occurs whenoccurs the when feedstock the feedstock is nutrient is were found to determine the etchback conditions. Etchback occurs when the feedstock is nutrientdeficient deficient and the fiber and wouldthe fiber dissolve would in dissolve a preferential in a preferential direction. This direction. may be This used may in the be nutrient deficient and the fiber would dissolve in a preferential direction. This may be usedfuture in to the achieve future smaller to achieve core smaller diameter core fibers diameter as well fibers as designing as well as specific designing geometries. specific used in the future to achieve smaller core diameter fibers as well as designing specific geometries.Faceted growth Faceted may growth be seen may on thebe seen end on face the of end an epitaxially face of an epitaxially cladded fiber cladded as shown fiber inas geometries. Faceted growth may be seen on the end face of an epitaxially cladded fiber as shownFigure4 in. A Figure cleaving 4. A and cleaving polishing and polishing step is used step to is reveal used theto reveal core-clad the core structure‐clad structure prior to shown in Figure 4. A cleaving and polishing step is used to reveal the core‐clad structure priorsplicing to splicing between between dissimilar dissimilar materials materials such as silicasuch as [24 silica]. [24]. prior to splicing between dissimilar materials such as silica [24].

(a) (b) (a) (b) Figure 3. Photos of a 19 mm long, 90 μµm core core-cladded‐cladded fiber fiber using the hydrothermal method. ( a) Optical microscope image Figure 3. Photos of a 19 mm long, 90 μm core‐cladded fiber using the hydrothermal method. (a) Optical microscope image of thethe side side profile profile showing showing core core inside inside the the cladding cladding structure. structure. (b) Scanning(b) Scanning electron electron microscope microscope (SEM) (SEM) image image of cross-section, of cross‐ of the side profile showing core inside the cladding structure. (b) Scanning electron microscope (SEM) image of cross‐ section, showingµ the 90 μm yttrium aluminum garnet (YAG) core fiber cladded2+ with4+ Co2+:Si4+ doped YAG cladding. showingsection, the showing 90 m yttrium the 90 aluminum μm yttrium garnet aluminum (YAG) garnet core fiber (YAG) cladded core fiber with cladded Co :Si withdoped Co2+ YAG:Si4+ doped cladding. YAG cladding.

(a) (b) (a) (b) Figure 4. As grown end face of 208 μm × 208 μm fiber, followed polishing. (a) SEM image of as grown end face after µ × µ hydrothermalFigure 4. growth.As grown (b )end ConfocalFigure face of 4. microscope 208As μ grownm × 208image end μm face of fiber, polished of 208 followed fiberm crosspolishing.208 ‐section.m fiber, (a) SEM followed image polishing. of as grown (a) SEMend face image after of hydrothermal growth. (asb) grown Confocal end microscope face after image hydrothermal of polished growth. fiber cross (b) Confocal‐section. microscope image of polished fiber 2.2.2.cross-section. Liquid Phase Epitaxial Growth 2.2.2. Liquid Phase Epitaxial Growth 2.2.2.In Liquid a different Phase approach Epitaxial to Growth achieve an all crystalline core/clad laser fiber, an approach In a different approach to achieve an all crystalline core/clad laser fiber, an approach using liquid phase epitaxial growth has shown considerable promise. The approach is usingIn a different liquid phase approach epitaxial to achieve growth an has all crystalline shown considerable core/clad laser promise. fiber, anThe approach approach is similar to the hydrothermal growth process where the solution phase is initially fully sat‐ usingsimilar liquid to phase the hydrothermal epitaxial growth growth has process shown where considerable the solution promise. phase The is initially approach fully is sat‐ uratedsimilar with to the dissolved hydrothermal YAG. The growth YAG process powder where or mixture the solution of Y2O3 and phase Al2 isO3 initially reactants fully are urated with dissolved YAG. The YAG powder or mixture of Y2O3 and Al2O3 reactants are added to molten flux at high temperature (>1100 °C) until it exceeds saturation. The mol‐ saturatedadded with to molten dissolved flux YAG.at high The temperature YAG powder (>1100 or mixture °C) until of it Y exceeds2O3 and saturation. Al2O3 reactants The mol‐ tenare flux added containing to molten PbO flux and at highB2O3 temperatureis stirred in the (>1100 platinum◦C) until crucible it exceeds with the saturation. YAG material The ten flux containing PbO and B2O3 is stirred in the platinum crucible with the YAG material while the single crystal core fiber is immersed in the flux as a seed material and crystal is moltenwhile flux the containing single crystal PbO core and fiber B2O is3 isimmersed stirred in in the the platinum flux as a seed crucible material with and the YAGcrystal is grown around the fiber (Figure 5). With a reaction temperature kept between 1000−1050 materialgrown while around the singlethe fiber crystal (Figure core 5). fiber With is immerseda reaction temperature in the flux as kept a seed between material 1000 and−1050 °C,crystal well is below grown the around melting the temperature fiber (Figure of5 ).YAG, With the a reactionseed‐doped temperature YAG fiber kept serves between as the °C, well below the melting temperature of YAG, the seed‐doped YAG fiber serves as the 1000−1050 ◦C, well below the melting temperature of YAG, the seed-doped YAG fiber serves as the nucleation site for crystal growth for undoped YAG. As the temperature of the molten flux is slowly lowered, the saturation limit for dissolved YAG drops and the excess comes out of solution and deposits on the fiber. Isothermal growth conditions within the furnace ensure a uniform and homogenous composition throughout the grown cladding. Coatings 2021, 11, x FOR PEER REVIEW 5 of 13

nucleation site for crystal growth for undoped YAG. As the temperature of the molten flux is slowly lowered, the saturation limit for dissolved YAG drops and the excess comes Coatings 2021, 11, 644 5 of 13 out of solution and deposits on the fiber. Isothermal growth conditions within the furnace ensure a uniform and homogenous composition throughout the grown cladding.

Figure 5. Schematic showing liquid phase epitaxy growth setup of YAG crystal cladding. Figure 5. Schematic showing liquid phase epitaxy growth setup of YAG crystal cladding.

Sc:YAG crystal layers were grown from PbO−B2O3 fluxed melts using an isothermal LPESc:YAG dipping crystal method layers similar were to grown that described from PbO in−B the2O3 literature fluxed melts [25 ,using26]. The an isothermal substrates wereLPE dipping polished method (100) wafers similar of to Czochralski-grown that described in the yttrium literature aluminum [25,26]. garnet The substrates of dimensions were 10polished× 10 mm (100)2 and wafers approximately of Czochralski 0.5 mm‐grown thick. yttrium The melts aluminum were prepared garnet of from dimensions an appropri- 10 × 10ate mm amount2 and of approximately oxides in the same0.5 mm stoichiometric thick. The melts ratios were as desired prepared film from compositions. an appropriate Upon amountremoval of from oxides the melt,in the sample same stoichiometric was rinsed with ratios HCl acidas desired to remove film anycompositions. residual flux. Upon removal from the melt, sample was rinsed with HCl acid to remove any residual flux. 3. Results 3.3.1. Results Characterization 3.1. CharacterizationEnergy-dispersive X-ray spectroscopy (EDX) was used to characterize the composition of theEnergy cladded‐dispersive YAG fiber. X‐ Elementalray spectroscopy analysis (EDX) of the was cladding used and to characterize core regions the reveals composi a sto-‐ tionichiometry of the cladded close to YAG the desired fiber. Elemental Y3Al5O12 analysisstoichiometry of the ofcladding undoped and YAG core phases regions (Table reveals1 ). The core fiber shows a dopant concentration of roughly 14% Yb, as expected from the a stoichiometry close to the desired Y3Al5O12 stoichiometry of undoped YAG phases (Ta‐ bledoped 1). The core core fiber fiber grown shows from a LHPG.dopant In concentration addition, there of isroughly a sharp 14% gradient Yb, as in expected the detection from theof the doped concentration core fiber ofgrown Yb at from the core–clad LHPG. In interface. addition, Thisthere suggests is a sharp minimal gradient diffusion in the de of‐ tectionrare-earth of the ions concentration occurred during of Yb the at the hydrothermal core–clad interface. growth ofThis cladding. suggests Diffusion minimal ofdiffu the‐ siondopant of rare would‐earth cause ions scattering occurred losses during and the less hydrothermal confinement growth of light of in cladding. the core. Diffusion of the dopant would cause scattering losses and less confinement of light in the core. Table 1. Atomic % composition of fiber core and cladding regions determined by energy-dispersive TableX-ray spectroscopy1. Atomic % composition (EDX, uncalibrated of fiber measurement). core and cladding regions determined by energy‐disper‐ sive X‐ray spectroscopy (EDX, uncalibrated measurement). Composition O Al Y Yb Core (10%Composition Yb:YAG) 65O 21Al 12Y 2Yb CladdingCore (10% (YAG) Yb:YAG) 6765 1921 1412 02 Cladding (YAG) 67 19 14 0 Single crystal X-ray diffraction (XRD) was used to identify the crystal structure of Single crystal X‐ray diffraction (XRD) was used to identify the crystal structure of the the cladding and identify the crystallographic directions of the different facets (Figure6). cladding and identify the crystallographic directions of the different facets (Figure 6). In‐ Initially the Yb:YAG core fiber shows multiple facets and possible growth planes prior to itially the Yb:YAG core fiber shows multiple facets and possible growth planes prior to hydrothermal cladding. However, when there was sufficient growth time in the autoclave, hydrothermal cladding. However, when there was sufficient growth time in the auto‐ the equilibrium morphology that was favored was clearly the (110) form as demonstrated clave,by undoped the equilibrium YAG cladding morphology growth planes. that was The favored angles was between clearly (110) the and (110) (11 form0) are as predicted demon‐ 1 stratedto be 90 by degrees undoped and YAG this is cladding visually growth depicted planes. as a square The angles cladding. between The (110) doped and core (1 region0) are predictedhas slightly to lowerbe 90 degrees unit cell and parameters this is visually due to depicted the smaller as a ionic square radius cladding. of Yb The3+ (0.86 doped Å) 3+ corecompared region to has the slightly Y3+ (0.89 lower Å) in unit YAG cell garnet parameters (Table2 due). to the smaller ionic radius of Yb (0.86 Å) compared to the Y3+ (0.89 Å) in YAG garnet (Table 2).

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(a) (b)

Figure 6. X‐ray diffractionFigure 6. X-ray (XRD) diffraction of a single (XRD) crystal of adoped single YAG crystal core doped before YAG and core after before cladding and afterwith claddingdimensions with 208 μm. (a) Miller indicesdimensions of Yb:YAG 208 coreµm. fiber (a) crystal Miller indicesplanes ofbefore Yb:YAG cladding; core ﬁber (b) Miller crystal indices planes beforeof undoped cladding; YAG (b cladding) Miller after hydrothermal growth.indices of undoped YAG cladding after hydrothermal growth.

Table 2. XRD lattice parametersTable 2. XRD of the lattice doped parameters YAG core of and the the doped undoped YAG YAG core cladding.and the undoped YAG cladding. Composition Phase ID Space Group Cell Parameters (Å) Volume (Å3) Composition Phase ID Space Group Cell Parameters (Å) Volume (Å3) 10.0% Yb:YAG core before Hy‐ 10.0% Yb:YAG core before YAG F‐43m (216) a = b = c = 11.9787 1718.796 YAGdrothermal F-43m growth (216) a = b = c = 11.9787 1718.796 Hydrothermal growth Undoped YAG cladding after Undoped YAG cladding after YAG F‐43m (216) a = b = c = 12.0113 1732.871 YAGHydrothermal F-43m Growth (216) a = b = c = 12.0113 1732.871 Hydrothermal Growth The original core fiber as drawn from LHPG used for the hydrothermal cladding The originalprocess core shown fiber in as Figure drawn 7a from can be LHPG seen usedwith corners. for the hydrothermal With insufficient cladding feedstock pro- reactants, cess shownthe in dissolution Figure7a can of be a core seen fiber with can corners. result With in slight insufficient etchback feedstock of the original reactants, substrate the (Figure dissolution7b of− ac). core The fiber radius can of result curvature in slight remains etchback similar, of the originalyet the substratefaces of the (Figure dissolution7b–c ). form (D‐ The radiusform) of curvature can flatten remains to form similar, additional yet the faces. faces It of has the been dissolution suggested form that (D-form) the growth can form (G‐ flatten to formform) additional of a single faces. crystal It hassubstrate been suggested will eventually that the switch growth from form the (G-form)fastest growing of a faces to single crystalthe substrate slowest and will reach eventually an equilibrium switch from state the [27]. fastest The growing D‐form for faces a substrate to the slowest will exhibit the and reach anfaces equilibrium with the greatest state [27 shift]. The velocities. D-form This for a has substrate been shown will exhibit for acid the etching faces with of garnet crys‐ the greatesttals, shift where velocities. the (111) This form has been was shown the most for acidfrequent etching D‐form of garnet for single crystals, crystal where sphere sub‐ the (111) formstrates was [28]. the mostFor the frequent growth D-form of bulk for YAG, single a crystal chemical sphere bonding substrates theory [28 of]. Forsingle crystal the growthgrowth of bulk was YAG, used a chemicalto determine bonding the (100) theory growth of single direction crystal to growthbe the slowest was used [29]. In initial to determineexperiments, the (100) growth the growth direction in (111) to bedirections the slowest was [the29]. fastest In initial‐resulting experiments, cladding. the For the per‐ growth infect (111) crystal, directions the lowest was the energy fastest-resulting plane would cladding. have the Forslowest the perfectgrowth crystal, rate, which the would be lowest energythe (110) plane plane. would In have a terminated the slowest growth growth reaction, rate, which the observed would be end the‐face (110) of plane. a fiber with the In a terminated growth reaction, the observed end-face of a fiber with the hexagonal (111) hexagonal (111) planes growing out and eventually would be bound by the lowest energy planes growing out and eventually would be bound by the lowest energy plane (110), plane (110), before coarsening completely to the equilibrium morphology. These etch‐back before coarsening completely to the equilibrium morphology. These etch-back and growth and growth processes require a considerable amount of effort to understand and optimize. processes require a considerable amount of effort to understand and optimize. However, However, the tunability of experimental conditions such as nutrient concentration and the tunability of experimental conditions such as nutrient concentration and processing processing temperature can result in a well‐defined faceted core fiber followed by epitax‐ temperature can result in a well-defined faceted core fiber followed by epitaxial cladding ial cladding growth [30]. growth [30]. A high-resolution X-ray diffractometer using CuKα1 radiation was used to measure the rocking curve for the hydrothermally grown cladding using (022) reflection and a FWHM of 0.047◦ was determined (Figure8). The intensity for a polycrystalline sample varies as a function of square of the amplitude of the structure factor:

I ∝ |F|2 ⇒ I = C |F|2 (1)

Figure 7. (a) Original core fiber as drawn from LHPG to be used as seed and core fiber for clad‐ ding by hydrothermal method. (b) Doped core fiber cladded with undoped YAG cladding. (c) A single crystalline core reveals some etchback had occurred during hydrothermal process.

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(a) (b) Figure 6. X‐ray diffraction (XRD) of a single crystal doped YAG core before and after cladding with dimensions 208 μm. (a) Miller indices of Yb:YAG core fiber crystal planes before cladding; (b) Miller indices of undoped YAG cladding after hydrothermal growth.

Table 2. XRD lattice parameters of the doped YAG core and the undoped YAG cladding.

Composition Phase ID Space Group Cell Parameters (Å) Volume (Å3) 10.0% Yb:YAG core before Hy‐ YAG F‐43m (216) a = b = c = 11.9787 1718.796 drothermal growth Undoped YAG cladding after YAG F‐43m (216) a = b = c = 12.0113 1732.871 Hydrothermal Growth

The original core fiber as drawn from LHPG used for the hydrothermal cladding process shown in Figure 7a can be seen with corners. With insufficient feedstock reactants, the dissolution of a core fiber can result in slight etchback of the original substrate (Figure 7b−c). The radius of curvature remains similar, yet the faces of the dissolution form (D‐ form) can flatten to form additional faces. It has been suggested that the growth form (G‐ form) of a single crystal substrate will eventually switch from the fastest growing faces to Coatings 2021, 11, x FOR PEER REVIEW 7 of 13 the slowest and reach an equilibrium state [27]. The D‐form for a substrate will exhibit the faces with the greatest shift velocities. This has been shown for acid etching of garnet crys‐ tals, where the (111) form was the most frequent D‐form for single crystal sphere sub‐ stratesA high[28].‐ resolutionFor the growth X‐ray diffractometer of bulk YAG, using a chemical CuKα1 radiation bonding was theory used to of measure single crystal growththe rocking was curveused tofor determine the hydrothermally the (100) growngrowth cladding direction using to be (022) the reflectionslowest [29].and Ina initial experiments,FWHM of 0.047° the growthwas determined in (111) directions(Figure 8). Thewas intensitythe fastest for‐resulting a polycrystalline cladding. sample For the per‐ fectvaries crystal, as a function the lowest of square energy of planethe amplitude would haveof the the structure slowest factor: growth rate, which would be the (110) plane. In a terminated growthI  F2  reaction, I  C F2 the observed end‐face of a fiber(1) with the hexagonalIf we take (111) the planes intensity growing ratio of out the and 400 eventuallyand 800 peaks would as previously be bound reported by the [31],lowest energy plane (110), before coarsening completely to the equilibrium morphology. These etch‐back 2/ 2 and growth processes require a considerableI400/I800 = F400 amount F800 ∙ =of 29/9 effort = 3.22 to understand and (2)optimize. However, the tunability of experimental conditions such as nutrient concentration and  F400/F800  1.8 (3) Coatings 2021, 11, 644 processing temperature can result in a well‐defined faceted core fiber followed7 of by 13 epitax‐ For a nearly perfect crystal the intensity of the different ratios of peaks varies as: ial cladding growth [30]. I  F (4) Thus we expect the ratio of intensities for a nearly perfect crystal to be:

I400/I800 = F400/F800  1.8 (5) For a single crystal YAG cladding grown epitaxially, our measurements show the ratio of the measured intensities of 400 to 800 reflections are ∼1.8 in agreement with the expected value. In addition, the full width half maximum (FWHM) of (400) is 169 arc secs, consistent with very high crystalline quality [32,33]. The values for FWHM demonstrates lower con‐ tributions from dislocation density, strain, and any other defects present in the crystal. In

comparison, previous reports of YAG garnets grown with the Czochralski method and Figure 7. (a) Original core fiber as drawn from LHPG to be used as seed and core fiber for cladding Figurehorizontal 7. (a )directional Original core solidification fiber as drawn method from were LHPG determined to be used to as have seed FWHM and core of fiber1372 forand clad ‐ by hydrothermal method. (b) Doped core fiber cladded with undoped YAG cladding. (c) A single ding947 arcby hydrothermalsecs, respectively method. [34,35]. (b ) Doped core fiber cladded with undoped YAG cladding. (c) A singlecrystalline crystalline core reveals core somereveals etchback some etchback had occurred had during occurred hydrothermal during hydrothermal process. process.

FigureFigure 8. RockingRocking curve curve of of hydrothermally hydrothermally grown grown undoped undoped YAG YAG cladding cladding using using a a (022) (022) reflection reﬂection

andand CuKKαα11 radiation.radiation.

3.2. DemonstratedIf we take the Power intensity Gain ratioin Hydrothermal of the 400 and Cladded 800 peaks Fiber as previously reported [31], A 100 μm Yb:YAG core, undoped YAG fiber structure whose cladding was fabricated I /I = |F |2/|F |2 = 29/9 = 3.22 (2) by the hydrothermal crystal400 growth800 400method was800 used for the gain measurements. The cladding thickness varied radially and ranged from∼ ∼15−40 μm. The refractive index of ∴ |F400|/|F800| = 1.8 (3) undoped YAG is lower than that of doped YAG. Based on the index of 10% Yb:YAG and undopedFor a YAG, nearly the perfect numerical crystal aperture the intensity (NA) ofof the the different fiber core ratios was of∼0.1. peaks Gain varies was as: meas‐ ured using a 1025 nm DFB laser as the seed and a 970 nm diode pump operating at 10% I ∝ |F| (4) duty cycle to minimize heating of the sample (Figure 9). Both pump and seed were launchedThus into we expectthe core the of ratio the of10% intensities Yb; YAG for fiber. a nearly The perfectlaunched crystal seed to signal be: temporally overlapping the pump was 0.268 mW. A net peak gain of 14 dB was measured in a 7 mm ∼ long sample. For single crystalI400 fiber/I800 structures= |F400|/|F to be800 a| viable= 1.8 technology, mating the fiber (5)

For a single crystal YAG cladding grown epitaxially, our measurements show the ratio of the measured intensities of 400 to 800 reﬂections are ∼1.8 in agreement with the expected value. In addition, the full width half maximum (FWHM) of (400) is 169 arc secs, consistent with very high crystalline quality [32,33]. The values for FWHM demonstrates lower contributions from dislocation density, strain, and any other defects present in the crystal. In comparison, previous reports of YAG garnets grown with the Czochralski method and horizontal directional solidiﬁcation method were determined to have FWHM of 1372 and 947 arc secs, respectively [34,35].

3.2. Demonstrated Power Gain in Hydrothermal Cladded Fiber A 100 µm Yb:YAG core, undoped YAG ﬁber structure whose cladding was fabricated by the hydrothermal crystal growth method was used for the gain measurements. The cladding thickness varied radially and ranged from ∼15−40 µm. The refractive index of undoped YAG is lower than that of doped YAG. Based on the index of 10% Yb:YAG and

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undoped YAG, the numerical aperture (NA) of the fiber core was ∼0.1. Gain was measured using a 1025 nm DFB laser as the seed and a 970 nm diode pump operating at 10% duty cycle to minimize heating of the sample (Figure9). Both pump and seed were launched Coatings 2021, 11, x FOR PEER REVIEW 8 of 13 into the core of the 10% Yb; YAG fiber. The launched seed signal temporally overlapping the pump was 0.268 mW. A net peak gain of 14 dB was measured in a 7 mm long sample. For single crystal fiber structures to be a viable technology, mating the fiber to silica fiber to technologysilica fiber technology is critical. We is critical. have demonstrated We have demonstrated splicing of unclad splicing 100 ofµ uncladm single 100 crystal μm sin YAG‐ glefiber crystal to 65 YAGµm fiber core/125 to 65µ μmm clad core/125 silica μ fiber.m clad A splice silica lossfiber. of A 0.33 splice dB wasloss measuredof 0.33 dB withwas a measuredsplice tensile with strengtha splice tensile of ∼50 strength kpsi. of ∼50 kpsi.

Output Power (mW) 24 20 Net Gain (dB) 22 18

20 16

18 14

16 12 14 10 (dB) Gain Net 12 8 10 6 8 Output Power@ 1030 nm (mW) 0.40.60.81.0 Input power @ 1030 nm (mW)

FigureFigure 9. Gain 9. Gain measurements measurements in incladded cladded fiber. ﬁber. The The pump pump was was coupled coupled into into cladding cladding and and signal signal into intothe the core. core. A A core core loss loss of of 0.15 0.15 dB/cm dB/cm (measured (measured at at 1300 1300 nm) nm) is is reported. reported. 3.3. Liquid Phase Epitaxial Growth of Scandium (Sc):YAG 3.3. Liquid Phase Epitaxial Growth of Scandium (Sc):YAG For an all crystalline core/clad ﬁber, the epitaxial growth techniques allow for match- For an all crystalline core/clad fiber, the epitaxial growth techniques allow for match‐ ing of cladding crystal structures to the core. Matching of the lattice constants and thermal ing of cladding crystal structures to the core. Matching of the lattice constants and thermal expansion minimizes stresses and interface losses. The undoped YAG material has the low- expansion minimizes stresses and interface losses. The undoped YAG material has the est refractive index (1.814) compared to other garnets. While doping allows for tunability lowest refractive index (1.814) compared to other garnets. While doping allows for tuna‐ of the refractive index within the garnet family, most reported doping is restricted at the bility of the refractive index within the garnet family, most reported doping is restricted Y-site of standard Y3Al5O12. Here, we expand the range of tuning of the refractive index at the Y‐site of standard Y3Al5O12. Here, we expand the range of tuning of the refractive by doping with Sc where the Al ions can be substituted and report on investigating some index by doping with Sc where the Al ions can be substituted and report on investigating of the properties within the Sc-substituted YAG composition, Y3(Sc,Al)5O12. some of the properties within the Sc‐substituted YAG composition, Y3(Sc,Al)5O12. In order to better predict the impact of Sc3+ doping, we investigated possible effects 3+ onIn the order overall to better structure. predict The the crystal impact structure of Sc doping, of YAG andwe investigated lattice substitutions possible haseffects been onreported the overall in thestructure. literature The [ 36crystal]. The structure crystal structure of YAG and of YAG lattice and substitutions the local environment has been reportedaround in Al the3+ andliterature Y3+ ions [36]. is shownThe crystal in Figure structure 10. Theof YAG larger and Y3+ theion local (1.019 environment Å) goes in a 3+ 3+ 3+ arounddodecahedral Al and siteY ions where is shown it is surrounded in Figure 10. by The 8 O 2larger− ions. Y The ion smaller (1.019 Å) Al goes3+ ion in goes a do‐ on 2− 3+ decahedraloctahedral site and where tetrahedral it is surrounded sites. The by ionic 8 O radiiions. ofThe Al smaller3+ in octahedral Al ion goes and on tetrahedral octahe‐ 3+ dralcoordination and tetrahedral are 0.535sites. The Å and ionic 0.39 radii Å of respectively Al in octahedral [36]. Theand tetrahedral occupancy coordination of Al3+ ion in arethe 0.535 octahedral:tetrahedral Å and 0.39 Å respectively sites is [36]. in a The 2:3 occupancy ratio. If A representsof Al3+ ion in Y3+ thein octahedral:tetra the dodecahedral‐ hedralsite while sites is B andin a C2:3 represent ratio. If A Al represents3+ in octahedral Y3+ in andthe dodecahedral tetrahedral sites, site respectively, while B and then C represent Al3+ in octahedral and tetrahedral sites, respectively, then the YAG structure3+ can the YAG structure can be represented as A3(B2C3)O12. The rare-earth ions (Yb and beHo represented3+ in our case)as A3(B substitute2C3)O12. The for Yrare3+ ‐ionearth on ions the A(Yb site.3+ and The Ho ionic3+ in our radii case) of Sc substitute3+ in eight- forfold Y3+ ion coordination on the A site. (A-site) The ionic is 0.87 radii Å whileof Sc3+ in in six-foldeight‐fold coordination coordination (B-site) (A‐site) it is is 0.87 0.745 Å Å. whileScandium in six‐fold substitution coordination on both (B‐site) A-site it is (for 0.745 Y3+ Å.) and Scandium B-site (for substitution Al3+) has beenon both reported A‐site in (forthe Y literature3+) and B‐ [site37]. (for When Al3+ the) has Sc3+ beenion substitutesreported in for the larger literature Y3+ ion [37]. (on When A-site) the the Sc unit3+ ion cell substitutessize decreases for larger and whenY3+ ion it (on substitutes A‐site) the for unit the cell smaller size Aldecreases3+ ion (on and B-site) when the it substitutes unit cell size forincreases the smaller [37 ].Al When3+ ion (on Sc substitutesB‐site) the unit on all cell the size B-sites increases the formula[37]. When can Sc be substitutes represented on as allY the3Sc 2BAl‐sites3O12 the. We formula used VESTA can be analysis represented software as Y to3Sc calculate2Al3O12. theWe crystalused VESTA structure analysis of YAG softwareas a function to calculate of Sc substitutionthe crystal structure on the B-site. of YAG The as shift a function in d400 peakof Sc as substitution would be observed on the B‐site.using The X-ray shift diffraction in d400 peak is shown as would in Figure be observed 11a. This using is shown X‐ray as andiffraction increase inis d-spacingshown in in FigureFigure 11a. 11 bThisor as is ashown shift to as lower an increase angles for in d the‐spacing (400) peak in Figure in 2θ as 11b shown or as in a Figureshift to 11 lowera. Our anglesinitial for results the (400) based peak on sixin 2 variousθ as shown compositions in Figure indicate 11a. Our greater initial Sc results substitution based on leads six to various compositions indicate greater Sc substitution leads to larger unit cell parameters (Figure 11b). However, it is possible the Sc3+ (0.745 Å) ions can substitute the larger Y3+ (1.019 Å) ions. The Y−O polyhedral sites contain a dodecahedral site for the Y3+ ions, which is typically the site for rare‐earth dopants such as Nd3+. In this approximation, substitution at the A‐site would lead to decreased unit cell parameters as Sc3+ dopant concentration

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Coatings 2021, 11, x FOR PEER REVIEW 9 of 13 3+ Coatings 2021, 11, x FOR PEER REVIEW larger unit cell parameters (Figure 11b). However, it is possible the Sc (0.745 Å) ions can9 of 13 substitute the larger Y3+ (1.019 Å) ions. The Y−O polyhedral sites contain a dodecahedral site for the Y3+ ions, which is typically the site for rare-earth dopants such as Nd3+. In thisincreased, approximation, as previously substitution shown at the A-site in one would report lead to[38]. decreased The compositions unit cell parameters of the planar sub‐ 3+ increased,asstrates Sc dopant aswere previously analyzed concentration shown with increased, in energy one report as‐dispersive previously [38]. The shownspectroscopy compositions in one report (EDS)of the [38 andplanar]. The confirmed sub‐ to be stratescompositionssimilar were to analyzed the of thecomposition planar with substrates energy of ‐thedispersive were prepared analyzed spectroscopy melt with energy-dispersive(Figure (EDS) 12). and The spectroscopyconfirmed measured to concentration be (EDS) and conﬁrmed to be similar to the composition of the prepared melt (Figure 12). The similarlevel to of the Y compositionis consistent of with the prepared that of undoped melt (Figure YAG, 12). suggesting The measured the concentration Sc doping is confined levelmeasured of Y is concentrationconsistent with level that of Y of is undoped consistent withYAG, that suggesting of undoped the YAG, Sc doping suggesting is confined the Scto dopingthe Al is site. conﬁned to the Al site. to the Al site.

Figure 10. Unit cell showing the positions of Y3+, Al3+ and O2− in YAG cubic crystal structure. Y3+ in Figure 10. Unit cell showing the positions of Y3+, Al3+ and O2− in YAG cubic crystal structure. Y3+ Figure 10. Unit cell showing3+ the positions of Y3+3+, Al3+ and O2− in YAG cubic crystal structure. Y3+ in A‐site (dodecahedral), Al in3+ B‐site (octahedral) and Al 3+ in C‐site (tetrahedral) are also shown. inA‐ A-sitesite (dodecahedral), (dodecahedral), Al Alin3+ B-sitein B‐site (octahedral) (octahedral) and Al andin C-siteAl3+ in (tetrahedral) C‐site (tetrahedral) are also shown. are also shown.

(a) (b)

FigureFigure 11. (a 11.) A (shifta) A shiftto lower to lower 2θ angles 2θ angles is predicted is predicted in in X X-ray‐ray diffraction diffraction (XRD) (XRD) diffraction diffraction peak peak for for (400) (400) based based on increased on increased Sc3+ substitutionSc3+ substitution (x) on (x )B on‐site. B-site. (b) (Lineb) Line shows(a) shows theoretical theoretical d d-spacing‐spacing calculationscalculations based based on on Sc3+ Scsubstitution3+( bsubstitution) at the at B-sites the B‐ insites in Y3(Al2Y‐xSc(Alx)AlSc3Ox)Al12. ExperimentalO . Experimental values values from from Sc Scsubstitution substitution shown shown as as points, showing showing an an increased increased d-spacing d‐spacing for greater for greater Figure3 11.2- x(a) A shift3 12 to lower 2θ angles is predicted in X‐ray diffraction (XRD) diffraction peak for (400) based on increased ScandiumScandium content content in the in thecomposition composition series. series. Sc3+ substitution (x) on B‐site. (b) Line shows theoretical d‐spacing calculations based on Sc3+ substitution at the B‐sites in Y3(Al2‐xScx)Al3O12. ExperimentalSc-YAG values filmsfrom wereSc substitution measured using shown ellipsometry as points, forshowing changes an in increased polarization d‐spacing of light for greater Scandium content in the compositionreflected from series. samples using a knife-edge illumination technique. This technique avoids the disturbing backside-reflections that can occur from thin transparentElement substratesAtomic by using% a beam cutter. Refractive index and a non-zero extinction coefficient were obtained by O 61 fitting a Cauchy optical model to measurement data, as shown in Figure 13 for an undoped YAG substrate. For the uncoated YAG substrate measurement,Al a refractive index,22 n, of Sc Element 3 Atomic % Y O 14 61 Al 22 Sc 3 Y 14

Figure 12. Energy‐dispersive spectroscopy (EDS) analysis of planar substrate reveals deposited Sc‐ doped composition is similar to the prepared melt composition Y3(Sc0.67Al1.33)Al3O12.

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increased, as previously shown in one report [38]. The compositions of the planar sub‐ strates were analyzed with energy‐dispersive spectroscopy (EDS) and confirmed to be similar to the composition of the prepared melt (Figure 12). The measured concentration level of Y is consistent with that of undoped YAG, suggesting the Sc doping is confined to the Al site.

Figure 10. Unit cell showing the positions of Y3+, Al3+ and O2− in YAG cubic crystal structure. Y3+ in A‐site (dodecahedral), Al3+ in B‐site (octahedral) and Al3+ in C‐site (tetrahedral) are also shown.

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(a) 1.820 is measured at 1 µm that is slightly(b) higher than the theoretical values of 1.814 for an undoped Y3Al5O12 (YAG) at 1 µm. After the epitaxial growth of Sc-YAG films, refrac- Figure 11. (a) A shift to lower 2θ angles is predictedtive indices in X‐ray at 1diffractionµm were (XRD) measured diffraction to be 1.837 peak andfor (400) 1.841 based for Y on3(Sc increased0.33Al1.67 )Al3O12 and 3+ 3+ 3+ Sc substitution (x) on B‐site. (b) Line showsY theoretical3(Sc0.67Al1.33 d‐)Alspacing3O12, calculations respectively. based The addition on Sc substitution of Sc leads at to the a higher B‐sites refractive in index, Y3(Al2‐xScx)Al3O12. Experimental values from whileSc substitution YAG is still shown ideal as for points, the outer showing cladding an increased of a fiber. d Any‐spacing need for for greater additional dopants Scandium content in the composition series. allows for precise tailoring of the inner cladding refractive index for an all-crystalline double-clad fiber laser.

Element Atomic % O 61 Al 22 Sc 3 Y 14

Figure 12.FigureEnergy-dispersive 12. Energy‐dispersive spectroscopy spectroscopy (EDS) analysis (EDS) of planar analysis substrate of planar reveals substrate deposited reveals Sc-doped deposited composition Sc‐ is similar todoped the prepared composition melt composition is similar Yto3 (Scthe0.67 preparedAl1.33)Al 3meltO12. composition Y3(Sc0.67Al1.33)Al3O12.

Figure 13. Optical model and ﬁt for a bare undoped YAG substrate from measurements using ellipsometry, showing n, refractive index, and k, a non-zero extinction coefﬁcient, plotted against wavelength. A refractive index of 1.820 is measured at 1 µm for this sample.

In order to further examine the possible sites for scandium substitution, we used Raman spectroscopy to look at the stretching vibrations for the (AlO4) unit. The lattice strains that are introduced by doping YAG at the Y3+ site by Nd3+ typically result in a blue shifting of modes [39]. In the Sc-doped YAG ﬁlms, we observe that the longer Sc–O bonds results in distinct peaks in the Raman spectra based on a regular substituting of Al3+ by heavier Sc3+ ions. In Figure 14, the peaks seen between 680 and 1350 cm−1 can be assigned to the internal stretching vibrations between scandium ions and oxygen in the tetrahedral (Sc,Al−O4) unit [40,41]. A detailed analysis of the Raman spectra of compositions will described in a subsequent communication.

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Figure 14. Raman spectra for undoped YAG (blue) and scandium (Sc):YAG compositions at room temperature.

4. Summary The hydrothermal and liquid phase epitaxial growth of YAG shows great promise for single crystal cladding onto single crystal core fibers. These epitaxial techniques are versatile epitaxial methods to growth undoped YAG cladding of various thicknesses onto doped YAG core fibers, potentially for tailoring of optical laser devices. These techniques also potentially allow for double cladding of doped core fibers. We have demonstrated that careful consideration of feedstock concentrations is important for controlled etchback of original core fiber. In addition, growth experiments of single crystal YAG cladding demonstrated (111) morphology if terminated early before reaching the equilibrium (110) form. Our present work shows a demonstration of the versatility of these epitaxial growth methods to grow single crystal cladding onto a single crystal core fiber. A Yb-doped YAG core fiber was cladded with undoped YAG and demonstrated a 14 dB gain. A liquid phase epitaxial method was used to grow varied compositions of single crystal Sc:YAG films onto YAG substrates, showing a higher refractive index for doped compositions. This work may serve as a basis for the continued development of epitaxially grown films for other doped YAG compositions in optical devices.

Author Contributions: Conceptualization, W.K., L.B.S., S.B., and J.S.; methodology, S.N.Q., J.K., and B.S.; formal analysis, S.N.Q. and S.B.Q.; writing—original draft preparation, S.N.Q.; writing—review and editing, W.K.; funding acquisition, J.S. and J.K. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Office of Naval Research, grant No. N0001418WX00846. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article. Acknowledgments: The authors would like to thank Alyssa Mock for valuable discussions on ellipsometry. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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