Laser floating zone growth: Overview, singular materials, broad applications, and future perspectives

crystals Review Laser Floating Zone Growth: Overview, Singular Materials, Broad Applications, and Future Perspectives Francisco Rey-García1,* , Rafael Ibáñez 2, Luis Alberto Angurel 1, Florinda M. Costa 3and Germán F. de la Fuente 1,*   Citation: Rey-García, F.; Ibáñez, R.; Angurel, L.A.; Costa, F.M.; de la Fuente, G.F. Laser Floating Zone Growth: Overview, Singular Materials, Broad Applications, and Future Perspectives. Crystals 2021,11, 38. https://doi.org/ 10.3390/cryst11010038 Received: 28 November 2020 Accepted: 29 December 2020 Published: 31 December 2020 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Instituto de Nanociencia y Materiales de Aragón (CSIC-Universidad de Zaragoza), María de Luna 3, E-50018 Zaragoza, Spain; [email protected] 2Institut de Ciència dels Materials de la Universitat de Valéncia, C/ Catedrático JoséBeltrán, 2, E-46980 Paterna, Spain; [email protected] 3Departamento de Física & i3N, Campus de Santiago s/n, Universidade de Aveiro, PT-3810-193 Aveiro, Portugal; [email protected] *Correspondence: francisco.rey[email protected] (F.R.-G.); [email protected] (G.F.d.l.F.) Abstract: The Laser Floating Zone (LFZ) technique, also known as Laser-Heated Pedestal Growth (LHPG), has been developed throughout the last several decades as a simple, fast, and crucible-free method for growing high-crystalline-quality materials, particularly when compared to the more conventional Verneuil, Bridgman–Stockbarger, and Czochralski methods. Multiple worldwide efforts have, over the years, enabled the growth of highly oriented polycrystalline and single-crystal highmelting materials. This work attempted to critically review the most representative advancements in LFZ apparatus and experimental parameters that enable the growth of high-quality polycrystalline materials and single crystals, along with the most commonly produced materials and their relevant physical properties. Emphasis will be given to materials for photonics and optics, as well as for electrical applications, particularly superconducting and thermoelectric materials, and to the growth of metastable phases. Concomitantly, an analysis was carried out on how LFZ may contribute to further understanding equilibrium vs. non-equilibrium phase selectivity, as well as its potential to achieve or contribute to future developments in the growth of crystals for emerging applications. Keywords: laser floating zone; laser-heated pedestal growth; single crystals; textured materials; congruent/incongruent melting 1. Introduction Growth of crystalline solids can be achieved by a large variety of methods driven by thermal or chemical potential gradients. Crystal growth in nature is achieved under many different geological conditions, in some cases under extremely high temperatures and pressure and in others by very slow solution processes [ 1 ]. Many current laboratory methods are nothing but transcripts of those that we can find in nature, as in the case of hydrothermal processes involved in the preparation of many functional materials [ 2 ]. A comprehensive classification of crystal growth techniques can be found in the classical work of Pamplin [ 3 ]. In this work, four main categories of growth methods are established: Growth from solid (S → S), melt (L → S), vapor (V → S), and solution (sol → S). Melt growth techniques are the choice methods to obtain bulk single crystals of inorganic materials and they could be divided into four categories: crystal pulling, directional solidification, floating zone, and Verneuil techniques [ 4 ]. Crystallization techniques such as Verneuil, Bridgman–Stockbarger, and Czochralski methods are usually employed for large and conventional production of gemstones (Al 2 O 3 allomorphs and doped crystals, TiO 2 , SrTiO 3 ), semiconductor single crystals (Si, Ge, GaAs), or metals (Pd, Pt, Ag, Au). In these methods, a material of approximately the correct composition is melted congruently, i.e., the crystalline phase is maintained before and after melting, being solidified in a carefully controlled fashion causing the formation of a single crystal from a well-oriented Crystals 2021,11, 38. https://doi.org/10.3390/cryst11010038 https://www.mdpi.com/journal/crystals Crystals 2021,11, 38 2 of 29 seed material [ 5 ]. However, they present some disadvantages when the materials are highly refractory or exhibit incongruent melting. Likewise, they usually imply the use of a large amount of starting precursor materials, long processing times, and the use of crucibles that could introduce impurities on crystals with consequences in their physical properties. In contrast, the study of new materials implies limited quantities of precursors, the required highest purity possible to present the best performance, and, considering these competitive times, their production in the shortest time possible. Thus, the Laser Floating Zone (LFZ) technique, namely, Laser-Heated Pedestal Growth (LHPG), has been revealed as a suitable prototyping technique that enables the production of high-quality crystalline materials in a simple, fast, and crucible-free method with low consumption of precursor materials. In addition, several studies have allowed us to easily process materials presenting incongruent melting, as well as obtaining non-equilibrium phases. The origin of this technique may be assigned to Haggerty in 1972 [ 6 ], who defined a Laser-Heated floating zone growth process of Al 2 O 3 (sapphire) fibers for NASA (National Aeronautics and Space Administration; Cleveland, OH, USA). Feigelson later applied the method to the growth of single-crystal fibers with potential use in solid-state lasers [ 7 ]. A good number of materials have been obtained by LFZ since then, for a wide range of applications taking into account the continuous improvement of the technique along these years, considering both laser source and optical setup [ 7 ]. Thus, materials for photonics and electrical applications can be highlighted as the most produced by LFZ together with the industry interest on environmentally friendly processes and materials, reducing energy consumption and enhanced properties of materials. It must be noted that, since the reviews published by Feigelson in 1989 [ 7 ], Rudolph and Fukuda in 1999 [ 8 ], and Andreeta et al. in 2010 [ 9 ], there is no other review about materials processed by the LFZ (or LHPG) technique. Taking into account the materials produced during the past number of years, as well as the latest apparatus advances for enhancing the quality of the molten zone and the consequent solidification, this call for a review focused on the technique and its materials’ development. Concomitantly, most promising fields are noted, highlighting the corresponding materials produced, aiming to define the future perspectives of the LFZ technique. Before describing in detail the LFZ technique, focusing on materials developed for photonic or electrical applications, however, it is convenient to mention the analogous Optical Floating Zone (OFZ) technique [ 10 – 15 ]. In fact, a number of interesting papers have been published regarding the growth of different kinds of single-crystal materials. These include, for example, high-quality rubies [ 16 ], europium-doped, yttria-stabilized hafnia (YSH) [ 17 ], multifunctional BaZrO 3 [ 18 ], the Ba 2 PrFeNb 4 O 15 ferroelectric relaxor [ 19 ], or rare-earth disilicates of Er, Ho, and Tm with a rich range of interesting magnetic properties [ 20 ]. Technically, the difference between OFZ and LFZ is in the optical radiation sources used to attain heating and melting. Usually OFZ makes use of ellipsoidal mirrors with a secondary focal point at the center of the growing rod or crystal, while the primary focus contains a powerful halogen or xenon lamp. This setup allows achieving high melting temperatures with a significantly lower energy consumption, as compared to conventional crystallization methods described above [ 13 , 14 ]. However, the LFZ technique allows better control of the temperature gradient compared to OFZ, as well as higher temperatures [ 21 ] since the light focus in OFZ is broad and the temperature gradient at the interface between the solid and the liquid is less abrupt. This makes the melt seriously attack the feed rod and spill over to the crystal, eventually making the growth unstable [ 22 ]. Likewise, the use of lasers allows implementation of a high-strength metal growth chamber, permitting high pressures, up to 1000 bar [ 23 ]. Meanwhile, in conventional mirror-based designs, namely, OFZ chambers, apparently the maximum pressure achieved is in the proximity of 300 bar [ 15 ]. In both cases, high pressures are desired to enable the growth of highly volatile and metastable materials. Crystals 2021,11, 38 3 of 29 2. LFZ Technical Developments The first equipment developed by Haggerty in 1972 [ 6 ] was proposed for the production at ambient atmospheres of Al 2 O 3 :Cr, TiC, and Y 2 O 3 fibers, noting that there were no available crucibles for the melting of the first two materials. Thus, these materials were produced using an apparatus provided with a small 10-W CO 2 laser, coupled to a basic optical system composed of (1) the beam expanding and pointing optics, (2) the beam splitter, and (3) the beam splitting and focusing optical bench. The beam splitter consisted of a water-cooled, coated GaAs window and a front surface mirror. Meanwhile, the two furnace windows—after beam splitter—were made of NaCl. After that, beams were intercepted by semicircular and spherical mirrors and focused at the position of the molten zone. Finally, fiber withdrawal and feed-rod insertion mechanisms were installed inside the furnace as pulling heads. Andreeta et al. [ 9 ] summarized the technical advances of existing prototypes through 2010. Among all advances, the introduction of the reflaxicon by Fejer et al. [ 24 , 25 ] may be highlighted. It enabled a circular, crown-shaped laser beam focus and, therefore, uniform radial heating. This term describes a setup of a two-stage pair of reflective linear axicon surfaces [ 21 ], which was invented by Martin in 1948 [ 26 ] and later improved by Nubling and Harrington in 1997 [ 27 ] (Figure 1). Likewise, processing under high-vacuum conditions, achieved for the first time by Brueck et al. in 1996 [ 28 ], must also be highlighted. Another remarkable advancement was achieved by Carrasco et al., in 2004 [ 29 ], through the application of an electrical current during processing. By establishing the Electrically Assisted Laser Floating Zone (EALFZ) (Figure 2) they demonstrated impressive preferential texture and the consequent enhancement in electrical properties of high Tc superconductors. In the same year and after previous experiments employing floating zone combined with a YAG (Y 3 Al 5 O 12 ) laser, Geho et al., in 2004 [ 30 ], reported the design of a hybrid laser floating zone machine for the successful growth of incongruently melting Tb 3 Al 5 O 12 (TAG) single crystals (Figure 3). This new device combined four CO 2 lasers and four halogen lamps, aiming to reduce temperature gradients during solidification. Likewise, Sekijima and Geho, in 2004 [ 31 ], patented an apparatus composed of two CO 2 lasers for growing TAG single crystals. Crystals 2021, 11, x FOR PEER REVIEW 3 of 30 2. LFZ Technical Developments The first equipment developed by Haggerty in 1972 [6] was proposed for the production at ambient atmospheres of Al2O3:Cr, TiC, and Y2O3 fibers, noting that there were no available crucibles for the melting of the first two materials. Thus, these materials were produced using an apparatus provided with a small 10-W CO2 laser, coupled to a basic optical system composed of (1) the beam expanding and pointing optics, (2) the beam splitter, and (3) the beam splitting and focusing optical bench. The beam splitter consisted of a water-cooled, coated GaAs window and a front surface mirror. Meanwhile, the two furnace windows—after beam splitter—were made of NaCl. After that, beams were intercepted by semicircular and spherical mirrors and focused at the position of the molten zone. Finally, fiber withdrawal and feed-rod insertion mechanisms were installed inside the furnace as pulling heads. Andreeta et al. [9] summarized the technical advances of existing prototypes through 2010. Among all advances, the introduction of the reflaxicon by Fejer et al. [24,25] may be highlighted. It enabled a circular, crown-shaped laser beam focus and, therefore, uniform radial heating. This term describes a setup of a two-stage pair of reflective linear axicon surfaces [21], which was invented by Martin in 1948 [26] and later improved by Nubling and Harrington in 1997 [27] (Figure 1). Likewise, processing under high-vacuum conditions, achieved for the first time by Brueck et al. in 1996 [28], must also be highlighted. Another remarkable advancement was achieved by Carrasco et al., in 2004 [29], through the application of an electrical current during processing. By establishing the Electrically Assisted Laser Floating Zone (EALFZ) (Figure 2) they demonstrated impressive preferential texture and the consequent enhancement in electrical properties of high Tc superconductors. In the same year and after previous experiments employing floating zone combined with a YAG (Y3Al5O12) laser, Geho et al., in 2004 [30], reported the design of a hybrid laser floating zone machine for the successful growth of incongruently melting Tb3Al5O12 (TAG) single crystals (Figure 3). This new device combined four CO2 lasers and four halogen lamps, aiming to reduce temperature gradients during solidification. Likewise, Sekijima and Geho, in 2004 [31], patented an apparatus composed of two CO2 lasers for growing TAG single crystals. Figure 1. Schematic diagram of the typical Laser Floating Zone (or LHPG) setup, detailing both reflaxicon as molten zone area (see further details in [9,21]). Plane mirror Parabolic mirror Reflaxicon Motorized holder Motorized holder CHAMBER ZnSe window Laser source Inner cone Outer cone Outer cone Laser beam Molten Zone Laser beam Fiber Feed rod crown-shaped laser beam Figure 1. Schematic diagram of the typical Laser Floating Zone (or LHPG) setup, detailing both reflaxicon as molten zone area (see further details in [9,21]). Crystals 2021,11, 38 4 of 29 Crystals 2021, 11, x FOR PEER REVIEW 4 of 30 Figure 2. Schematic drawing of Electrically Assisted Laser Floating Zone (EALFZ) at the processing area, highlighting the D.C. power supply unit, Carrasco et al., 2004 [29]. Outstanding advances were achieved during the last 10 years, also focused on improving radial heating uniformity. Most of these implied the use of laser diodes both to improve radial heating, envisaging the growth of incongruently melting materials, as well as to substantially increase total laser power. The first study was reported by Ito et al., in 2013 [22], who successfully grew incongruent materials such as BiFeO 3 and (La,Ba) 2 CuO 4 by developing a laser–diode-heated floating zone (LDFZ) apparatus. The latter made use of five laser diodes emitting at a wavelength of 975 nm, with a total laser power output of 350 W and without use of a reflaxicon (Figure 4). In addition, these authors studied the effects of the number of laser diodes (3 to 8) on the quality of radial heating. A similar apparatus was recently employed by Kaneko NS Tokura, in 2020 [32], to grow refractory (Al 2 O 3 :Cr, SmB 6 ), incongruent melting (Ba 2 Co 2 Fe 12 O 22 ) and volatile (Nd 2 Mo 2 O 7 , SrRuO 3 ) materials by LDFZ, employing five laser diodes emitting at 940-nm wavelength with a total power of 1 kW. Thus, the use of various lasers enables uniform irradiation intensity distribution on the periphery of the raw material. Moreover, a vertical irradiation intensity can be designed to exhibit a flat or bell-shape distribution to improve relaxation of residual thermal strain in the grown crystal [22,32]. Nowadays, the Crystal Systems Corporation (Hokuto, Yamanashi, Japan) commercializes a laser floating zone furnace that provides a total laser power output of 5 kW, employing five laser diodes emitting at a wavelength of 808 nm [33]. In addition, similar equipment from Quantum Design International (San Diego, CA, USA) allows monitoring the temperature in the range 1173–3273 K while achieving a maximum laser output power of 2 kW from five laser diodes [34]. Figure 2. Schematic drawing of Electrically Assisted Laser Floating Zone (EALFZ) at the processing area, highlighting the D.C. power supply unit, Carrasco et al., 2004 [29]. 1 Figure 3. Top-view schematic drawing based on the hybrid Laser Floating Zone designed by Geho et al., in 2004 [30], and provided with four CO2lasers and four halogen lamps. Outstanding advances were achieved during the last 10 years, also focused on improving radial heating uniformity. Most of these implied the use of laser diodes both to improve radial heating, envisaging the growth of incongruently melting materials, as well as to substantially increase total laser power. The first study was reported by Ito et al., in 2013 [ 22 ], who successfully grew incongruent materials such as BiFeO 3 and (La,Ba) 2 CuO 4 by developing a laser–diode-heated floating zone (LDFZ) apparatus. The latter made use of five laser diodes emitting at a wavelength of 975 nm, with a total laser power output of 350 W and without use of a reflaxicon (Figure 4). In addition, these authors studied the effects of the number of laser diodes (3 to 8) on the quality of radial heating. A similar apparatus was recently employed by Kaneko NS Tokura, in 2020 [ 32 ], to grow refractory (Al 2 O 3 :Cr, SmB 6 ), incongruent melting (Ba 2 Co 2 Fe 12 O 22 ) and volatile (Nd 2 Mo 2 O 7 , SrRuO 3 ) materials by LDFZ, employing five laser diodes emitting at 940-nm wavelength with a total power Crystals 2021,11, 38 5 of 29 of 1 kW. Thus, the use of various lasers enables uniform irradiation intensity distribution on the periphery of the raw material. Moreover, a vertical irradiation intensity can be designed to exhibit a flat or bell-shape distribution to improve relaxation of residual thermal strain in the grown crystal [ 22 , 32 ]. Nowadays, the Crystal Systems Corporation (Hokuto, Yamanashi, Japan) commercializes a laser floating zone furnace that provides a total laser power output of 5 kW, employing five laser diodes emitting at a wavelength of 808 nm [ 33 ]. In addition, similar equipment from Quantum Design International (San Diego, CA, USA) allows monitoring the temperature in the range 1173–3273 K while achieving a maximum laser output power of 2 kW from five laser diodes [34]. 1 Figure 4. Top-view schematic drawing based on the Laser Diode Floating Zone (LDFZ) equipment designed by Ito et al., in 2013 [ 22 ], provided with five laser diodes, allowing a uniform irradiation intensity distribution on the periphery of the raw material [22,32]. Finally, Schmehr et al., in 2019 [ 23 ], developed a high-pressure LFZ (HP-LFZ) apparatus provided with seven laser diodes, emitting at a wavelength of 810 nm with a total laser output power of 700 W. They particularly aimed at avoiding outgassing/volatily effects inherent to materials that exhibit high vapor pressures at high temperatures. In addition, they also addressed the melting of highly refractory materials with volatile component loss reduction, by applying high processing gas pressures. Thus, Cu 2 O, Nd 2 Zr 2 O 7, and LiCuO2crystals have been successfully grown using this last apparatus. 3. Experimental Procedure (Standard) LFZ growth requires precursor powders in the form of cylindrical rods to be used as feed and seed and there are three typical processes to produce these precursor rods: (1) The extrusion process is the most common way to prepare the precursor rod cylinders for the LFZ process, since it is a simple method, not requiring special equipment or additional hands [ 21 , 29 ]. Thus, the commercial raw oxide powders are mixed, according to the desired stoichiometry, and reduced in grain size with an agate ball mill or similar equipment. The purity of the precursors depends on the desired application. For example, the use of powders of 5–6 N of purity should be envisaged for Crystals 2021,11, 38 6 of 29 photonic applications. Aiming to bind the powder mixture for the extrusion process, polyvinyl alcohol (PVA, 0.1 g/mL) is added, mashing the powders until a compact and plastic paste is achieved. The obtained clay is then extruded into cylindrical rods, with diameters that can reach up to 5 mm, depending on the material’s nature and its application. After extrusion, the cylindrical rods are dried in air and ready to be used as feed and seed materials (also known as green rods). However, it is important to emphasize that for support the extruded bars should have slots, aiming to guarantee their alignment during the drying process. (2) Alternatively, single crystals or dense ceramics appropriately cut can also be used as seed or feed rods instead of green rods [ 26 ]. The use of bulk-grown crystal seeds favors the formation of single-crystalline fibers. This approach helps laser processing and allows enhancing the structural characteristics of the single-crystal fiber produced. Similarly, in the last 10 years, cladded, single-crystalline fibers, mainly used as amplifiers, have been produced from bulk crystal seeds covered by Sol-Gel or embedded into silica or borosilicate hollow tubes, among other coating approaches [ 35 – 39 ]. Furthermore, some works from Rutgers University (Piscataway, NJ, USA) reported the growth from Pt wires together with the use of seed crystals or presintered ceramics as feed rods [40–42]. (3) Likewise, precursor rods can be also prepared by cold isostatic pressing [ 43 ]. Through this compaction method, both mixed raw and presintered powders with the desired composition are enclosed in a flexible mold. This flexible bag is introduced into a perforated support inside a pressure container. Once this setup is sealed, fluid pressure is exerted over the outside surface of the container, allowing the container to press all around the bag and inducing uniform compaction of the powder and, consequently, a uniform density within the compacted rod [44]. The LFZ equipment usually comprises a CO 2 laser coupled to a reflective optical setup which includes a reflaxicon, described in the previous section. In the case of LDFZ, this optical setup is not present. Once seed and feed fibers are placed on the respective holders, a molten zone is formed by irradiating the densified rods with the CO 2 laser and a different optical configuration. Fibers can be grown in the ascending or descending direction from this molten region at a defined growth or pulling rate, in air or special atmospheres, depending on the materials’ physico-chemical properties. Likewise, feed and seed rods should be rotated, favoring the mixing of precursors in the melt, homogenizing the temperature of the molten material, and contributing to reduce unsymmetrical thermal stresses. Concomitantly, occurrence of constitutional supercooling is reduced. Thus, the melt rotation stabilizes the heat and mass flow against undesirable temperature perturbations [ 45 ]. The growth process can end abruptly or by reducing the laser power gradually. This procedure is very important to reduce the thermal stresses and, therefore, to avoid crack formation [9,21]. 4. Materials for Photonic and Optical Applications Several high-quality crystals for photonic and optical applications have been obtained via the LFZ technique. These include laser garnets, luminescent, fluorescent, optoelectronic, and photorefractive materials. Indeed, Haggerty, in 1972 [ 6 ], produced ruby and yttria when the technique was reported. Likewise, Nd:YAG fibers were obtained by Stone et al., in 1976 [ 46 ], from a Nd:YAG preformed rod with a CO 2 laser. This was not the conventional LFZ method described here since a platinum wire was dipped into the melt and raised slowly to pull the desired crystal. The first notice about the production by LFZ (or LHPG) of crystals for laser applications was reported by Fejer et al., in 1984 [ 25 ], when the reflaxicon was introduced. The suitability of the LFZ apparatus developed was thus tested, producing sapphire, ruby, LiNbO3, and Nd:YAG laser media. Due to the crystal requirements for optics and photonics [ 47 ], defect-free, transparent, and highly pure materials with lower thermal conductivity [ 48 ] have been produced since the technique’s origin [ 6 ]. An excellent compilation of active and passive photonic materials Crystals 2021,11, 38 7 of 29 was conveniently reported by Rudolph and Fukuda in 1999 [ 8 ]. Likewise, Maxwell et al., in 2017 [ 49 ], reported the LFZ production of neodymiumand ytterbium-doped singlecrystalline YAG cylinders (SCF) with diameters under 1 mm and, in addition, processing in combination with the Sol-Gel method for obtaining cladded fibers. Moreover, the increasing interest of industry on efficient optical processes required an updated revision from that reported by Rudolph and Fukuda in 1999 [ 8 ]. Indeed, it is remarkable the great interest that has arisen recently on crystals for ultrashort pulsed laser systems [ 50 , 51 ] and for LED devices applied for indoor plants’ growth [ 52 ], as a consequence of the increasing food requirements derived from the human population increase. One of the most LFZ-developed materials is single crystal Y 3 Al 5 O 12 (YAG) as a laser medium. Despite the fact that it was produced by Fejer et al. in 1984 [ 25 ], a considerable amount of work has appeared during the last 20 years (Table 1) reporting its growth [ 35 – 38 , 40 – 42 , 49 – 51 , 53 – 72 ], improving the production efficiency [ 59 , 71 ], and also enhancing its mechanical and optical characteristics [ 38 ]. Among those studies, the work of Nie et al., in 2015 [ 37 ], who produced YAG single crystals doped with Er 3+ , Ho 3+ , Tm 3+ , Nd 3+ , and Yb 3+ may be highlighted here. They studied the dopant distribution following the ideal concentration for optimal photonic properties, together with the enhanced mechanical and microstructural characteristics. Likewise, diode-pumped garnets based on YAG crystals doped with metallic ions such as Cr 4+ for Q-switched laser operation were also developed [ 53 , 54 ]. Focusing on this composition, both Lai et al., in 2009 [ 60 ], and Yi et al., in 2010 [ 61 ], developed laser gain media and multi-pass ring laser devices, respectively, from single crystal fibers (SCF) with the double-clad structure. The doubleclad structure fabrication has been generally achieved by covering the single-crystal fiber through Sol-Gel and later sintering [ 49 ] or basically inserting a diameter-reduced grown rod into silicaor glass-based capillary tubes [ 35 , 50 , 63 , 66 , 68 ]. These kinds of materials, namely, the cladded, single-crystal fibers, have been largely applied as fiber amplifiers of the laser radiation [ 40 , 67 , 70 ]. Recently, Kim et al., in 2019, produced Yb:YAG core/YAG-clad fibers by a three-step process, combining an initial growth of doped YAG fibers, followed by acid etching and, finally, an epitaxial hydrothermal growth of the undoped YAG cladding [ 72 ]. On the other hand, Ye et al., in 2005 [ 57 ], grew an excellent YAG:Cr 3+ crystal to be used as a fiber thermometer. This was based on fluorescence lifetime and found applications in temperature monitors for microwave treatments and medium-voltage substations. This application for temperature sensing has been also successfully explored in work from Zhejiang University (Hangzhou, Zhejiang, China), reporting the growth of Y 2 O 3 :Er 3+ /Yb 3+ [ 73 ] and Y 2 O 3 :Ho 3+ /Yb 3+ [ 74 ] single-crystal fibers with up-conversion luminescent characteristics. Other complex oxides have also been developed by LFZ. Harrington, in 2014 [ 42 ], reported, for example, the growth of single-crystal fiber garnets of MgAl 2 O 4 (spinel) from commercial YAG seeds and Pt wires. In 2004, Romero et al. [ 75 ] produced multiwavelengthlaser, single-crystal aluminates of YAlO 3 doped with Nd 3+ (0.5–1.5 mol%). Recently, single crystals of orthosilicates (RE 2 SiO 5 ) also doped with rare earths (RE) have been produced by LFZ two to three times faster than conventional methods like Czochralski [ 76 – 78 ], maintaining or even improving laser performance by doping with Nd 3+ or Yb 3+ [ 48 ]. Other typical garnets usually employed in commercial laser resonators are based on lanthanum vanadates. Such is the case for LaVO 4 , Gd 1−x La x VO 4, and Y 1−x La x VO 4 , produced by Andreeta et al. in 2006 [ 79 ] from mixtures of raw oxide powders. These were processed in air at relatively high growth rates (9–18 mm/h), as compared to those applied on Czochraslki. From the research group of Andreeta et al. (Universidade Federal de São Carlos; São Carlos, Brasil), erbium-doped and undoped CaNb 2 O 6 and CaTa 2 O 6 single crystals, suitable as laser active media, were also developed and produced [ 80 ]. The optically transparent CaTa 2 O 6 single crystals were also grown by Almeida et al., in 2013 [ 81 ], exploring the three polymorphic modifications that this material exhibits at room temperature. Focusing on niobates, the production of promising optical active elements for lasers such as lithium niobates (LiNbO 3 ) [ 55 , 82 – 85 ] or incongruent melting lithium-potassium niobate (K 3 Li 2−x Nb 5+x O 15+2x , KLN) [ 86 – 88 ] (Table 2) must be also high- Crystals 2021,11, 38 8 of 29 lighted. In addition, more niobates were also grown by LFZ for holographic devices, as photorefractive materials by doping with Fe [ 89 , 90 ], for nonlinear optics and acoustic wave devices [ 91 ], for wavelength and frequency modulators [ 92 , 93 ], and, in the case of the EuNbO4, for optoelectronics as light emitters [94]. Table 1. Y3Al5O12 (YAG) garnets grown by LFZ since the review by Rudolph and Fukuda in 1999 [8]. Year Authors Dopant Application Ref. 2000 Ishibashi & Naganuma Cr4+ Diode-pumped garnet [53] 2002 Shen et al. Cr4+ Garnets for Q-switch lasers [54] Nd3+ 2003 Boulon et al. - Laser media [55] 2003 Yoshikawa et al. Yb3+ Laser media [56] 2005 Ye et al. Cr3+ Temperature sensor [57] 2006 Bufetova et al. Nd3+ Laser media [58] 2009 Chen et al. - Laser media [59] 2009 Lai et al. Cr4+ Cladded Laser media [60] 2010 Yi et al. Cr4+ Cladded Multi-pass ring laser [61] 2011 Chang et al. Cr3+ Laser media [62] 2011 Zhu et al. - Laser amplifier [40] 2012 Lautsen & Harrington - Laser media [41] 2012 Wang et al. Cr4+ Cladded fiber amplifier [35] 2012 Kim et al. Ho3+ Cladded High power laser media [63] Yb3+ Cladded High power laser media 2013 Hsu et al. - Cladded Laser gain media [64] 2013 Hsu et al. Multi-mode media [65] 2013 Kim et al. Cladded laser media [36] 2013 Lai et al. Cr4+ Double-cladded fiber amplifier [66] 2013 Maxwell et al. Er3+ Cladded laser media [67] Nd3+ Yb3+ 2014 Harrington - Laser media [42] 2014 Wang et al. Nd3+ Waveguide [68] 2015 Nie et al. Er3+ Garnets [37] Ho3+ Nd3+ Tm3+ Yb3+ 2016 Kim et al. - High power fiber lasers [50] Ho3+ Yb3+ 2016 Oliete et al. - YbAG comparative study [69] 2017 Liu et al. Cr4+ Fiber transmission systems [70] 2017 Maxwell et al. Er3+ Laser performance of cladded-core [49] Nd3+ Yb3+ 2019 Bufetova et al. Er3+ Laser media [71] 2019 Kim et al. Ho3+ High power fiber lasers [72] Yb3+ High power cladded fiber lasers 2019 Wang et al. Yb3+ High power laser [51] 2020 Bera et al. Nd3+ Garnets [38] Nd3+/Ho3+ Following the production of laser garnets, gadoliniumand gallium-based compositions have also been produced during the last several years. Taking into account the ease of evaporation of gallium oxide (Ga 2 O 3 ), an excess of the same must be introduced, aiming to obtain the desired compositions [ 95 ]. Thus, Harrington reported, for the first time in 2014 [ 42 ], the single-crystal production of Gd 3 Ga 5 O 12 (GGG) laser garnet. Recently, Crystals 2021,11, 38 9 of 29 Rey-García et al., in 2020 [ 21 ], produced, in air atmosphere, gadolinium oxyorthosilicate single crystals, suitable to be employed as laser host matrices. Table 2. Niobates grown by LFZ since the review by Rudolph and Fukuda in 1999 [8]. Year Authors Material Application/Study Aim Ref. 2001 Reyes-Ardila et al. LiNbO3LHPG technique development [82] 2001 Matsukura et al. KLN 1Optical applications [86] 2002 Andreeta et al. LiNbO3LHPG technique development [83] 2003 Boulon et al. LiNbO3Laser media [55] 2003 Bourson et al. LiNbO3:Fe Photorefractive material [89] 2003 Cochez et al. LiNbO3:Fe Photorefractive material [90] 2004 Guo et al. KLN 1:Zn2+ SHG 2blue laser [87] 2004 Nagashio et al. LiNbO3 Nonlinear optics and acoustic wave [91] 2005 Chen et al. LiNbO3Optical material growth & study [84] 2005 Lee et al. LiNbO3:MgO Wavelength modulator [92] 2007 Chen et al. LiNbO3Optical material growth & study [85] 2008 De Camargo et al. CaNb2O6Laser media [80] CaNb2O6:Er3+ Laser active media 2011 Maxwell et al. KLN 1Laser media [88] 2013 Graça et al. EuNbO4Dielectric devices [94] 2015 Kashin et al. LiNbO3Frequency doubling [93] 1KLN: K3Li2−xNb5+xO15+2x.2SHG: second harmonic generation. Typical sapphire (Al 2 O 3 ), ruby (Al 2 O 3 :Cr), and rutile (TiO 2 ) crystals have been continually produced by this technique, improving both production efficiency and material properties, as can be deduced from work published in the last few years [ 22 , 23 , 32 , 39 , 42 , 96 – 104 ] (Table 3). Indeed, doping of sapphire with Cr 3+ and Er 3+ , or even codoping with Er 3+ /Yb 3+ , was achieved through LHPG by Seat in 2001 [ 96 ] and Seat and Sharp in 2003 [ 97 ], demonstrating the suitability of the fibers produced to be successfully employed for high-temperature sensing, as Ye et al., in 2005 [ 57 ], also similarly tested later for YAG:Cr 3+ . Likewise, Lai et al. in 2016 grew borosilicate-cladded, single-crystalline-core sapphire fibers suitable to be employed in biomedical applications, such as light sources for endoscopy [ 39 ]. It must be highlighted that Lai et al., in 2018, also demonstrated a facile and scalable approach for the transformation of a centrosymmetric sapphire ( α -Al 2 O 3 ) crystal to a large-scale 3D metamaterial (Si 4+ : γ -Al 2 O 3 ) with sub-wavelength fine structures [ 104 ]. Coming back to garnets for laser operation and revising the work mentioned up to this point, we could metaphorically denote this section as “Laser Kindergarten: laboratories in those laser parents produced laser children”. Additionally, due to their optical properties and their efficiency, it is also interesting to remark on the works focused on the growth of laser garnets based on disordered crystals like sesquioxides, fluorides, or chalcogenides (Table 4) [ 105 ]. Thus, the last 20 years have also witnessed the LFZ growth of crystals like Gd 2 O 3 doped with Yb 3+ [ 106 ], Lu 2 O 3 doped with Yb 3+ or Ho 3+ [ 50 , 72 , 106 ], Sc 2 O 3 [ 55 ], Y 2 O 3 undoped and doped with Yb 3+ , Er 3+ , or Ho 3+ [ 55 , 106 , 107 ], and Ta 2 O 5 doped with Eu 3+ [ 108 ], and fluorides like CaF 2 :Yb 3+ [ 109 , 110 ] or KY 3 F 10 :Yb 3+ [ 22 ]. It must be highlighted that fluorides have been grown under Ar atmosphere. Likewise, other complex oxides based on 2Al 2 O 3 -SiO 2 (mullite), Bi 4 Ge 3 O 12 (BGO), or Lu 2 SiO 5 (LSO) suitable to be used as scintillators have been grown in the last decade by this technique [ 111 , 112 ]. Visible light emission, highly efficient, solid-state, light-emitting materials allowing the fabrication of devices with low energy consumption, high brightness, and environmentally friendly characteristics, are presently of great interest for many researchers [ 52 ]. Thus, the LFZ or LHPG technique has allowed obtaining a wide production of phosphors and luminescent materials in the form of doped single crystals or polycrystalline and eutectic materials [ 52 , 73 , 74 , 112 – 115 ] in a fast manner. Despite this vast production, considering that this section is focused on single crystals applied as laser media Crystals 2021,11, 38 16 of 29 Grenoble, France) put in evidence recently a significant modification of the eutectic phase diagram induced by an external field for the Al2O3-YAG-ZrO2:Y system [159]. 6. Materials for Applications in Superconductivity As introduced in the previous section, texturing has been promoted in high-temperature superconducting materials, aiming to enhance superconductivity. The development of power applications based on these materials compel strong requirements on these anisotropic materials. This is more evident for the Bi-Sr-Ca-Cu-O families because of their small coherence length and a large crystallographic cell that are responsible for an extreme two-dimensional behavior. Traditional ceramic processing technologies are not adequate for obtaining bulk materials with the minimum level of performance in order to be used in large-scale electrical applications. Early works were carried out in the YBa 2 Cu 3 O x system using CO 2 lasers as heat sources [ 160 – 163 ]. These initial works showed that LFZ processing is not an adequate technique to obtain textured rods with high critical current values. Apart from the fact that a substantial evaporation takes place when Y 2 O 3 , BaCO 3, and CuO are used as precursors, it was observed that the directional solidification process was performed using a semisolid zone instead of a complete molten zone, due to the peritectic reaction that controls solidification in this material. In all the cases, growth rates lower than 40 mm/h were required to maintain a stable growth habit and the process had to be performed in controlled atmospheres, usually with oxygen partial pressures in the range between 6 and 103Pa. Different studies showed that the Bi superconducting phases were more appropriate for using LFZ as a texturing technique, because they exhibit high stability during incongruent melting and low evaporation and very low deviations from the material stoichiometry [ 160 , 164 – 168 ]. LFZ produces a conglomerate of crystals with their crystallographic c-axis perpendicular to the sample growth direction, and the a-b planes aligned in the direction of the fiber axis, improving electrical transport current flow [ 125 ]. Bi superconductors present two phases with critical temperatures above 77 K: Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212) with Tc≈80 K and Bi2Sr2Ca2Cu3O10+δ(Bi-2223) with Tc≈110 K. The Bi-2223 phase has lower stability and, despite greater interest due to its higher critical temperature, it was very complicated to obtain textured materials with this composition. Initial works [167,169,170] explored the strategy of substituting some amount of Bi by Pb and the LFZ process was performed in an 8% O 2 in Ar mixture flowing through the growth chamber. In the textured material, the main phase is the Bi-2212 phase and additional annealing is required to partially recover the Bi-2223 phase. Due to the high density of the textured materials obtained by LFZ, this annealing was longer in comparison with ceramic pellets. A second alternative was to modify the stoichiometry of the precursors aiming to enhance the formation of the Bi-2223 phase. Larrea et al., in 1994 [ 171 ], used the Bi 1.87 Pb 0.35 Sr 1.87 Ca 4 Cu 6 O y composition adding an extra amount of Ag. Costa et al. proposed the use of Bi 2 Sr 2 Ca 2 Cu 4 O 11 [ 125 , 133 ] and Miao et al., in 1997 [ 172 ], proposed precursors with different proportions of Bi-2212, Bi2Sr2CuO6(Bi-2201) and CaCuO2. Due to the higher stability of the Bi-2212 phase, an important amount of work reported was focused on this phase [ 132 , 135 , 164 , 168 , 173 – 183 ]. During these works, the Nd:YAG radiation started to be used as an alternative to the CO 2 lasers [ 168 , 175 ], observing that, with similar focusing optics, 1.06µ m radiation is more adequate to process textured Bi2212 superconductors because these oxides absorb the 10.6— µ m radiation mostly in the surface, creating larger radial temperature gradients and larger radial inhomogeneities due to constitutional supercooling. These studies [ 125 , 173 , 174 , 177 ] showed that in asgrown samples, the superconducting phase only develops for thin samples ( Φ < 250 µ m) and with very low growth rates (<5 mm/h). When the growth rate increases, growth is dominated by constitutional supercooling. Under these conditions, the first solid phases formed during solidification are Sr-Ca-Cu oxides, normally the (Sr,Ca)CuO 2 (1/1 phase) and, in some cases, the (Sr,Ca) 14 Cu 24 O 41 (14/24 phase), and the melt becomes enriched in Bi 2 O 3 , generating some superconducting grains with intergrowths of the Bi-2212 and Crystals 2021,11, 38 17 of 29 the Bi-2201 phases and an amorphous matrix of frozen bismuth-rich liquid phase between them. The growing habit of the 1/1 phase has the b-axis parallel to the growing direction and the c-axis along the shorter dimension in the transverse cross section [ 125 ] and this determines the texture of the superconducting material. Size and radial distributions of these phases are strongly dependent on the thermal gradients induced during the LFZ process and they can be controlled by adjusting the processing parameters, mainly the growth rate [ 177 ], the laser power, or the initial precursor stoichiometry [ 178 ]. The strong correlation between the effects of the different processing parameters complicates the optimization process. A Simplex optimization method was proposed [ 179 ] to process, at the same time, four growth parameters (laser power, the growth rate, and the precursor and textured rod rotation speeds) and four annealing parameters, taking as control variables to obtain high critical current values and short processing times. Main conclusions of the optimization process are that high laser powers originate drastic reductions in the T c values of the superconducting grains that grow within the central part of the rod. The second main conclusion is that a two-step annealing is needed. During the first step, at 1143 K, cation diffusion takes place to form the superconducting phase. In the second one, at 1073 K, the oxygen content required to optimize the critical temperature value is established. Critical current densities of the order of 5500 A/cm 2 at 77 K in 1-mm-diameter rods were reached after these optimization studies. Also, it was observed that critical current values decreased linearly with sample diameter, with the magnetic field generated at the sample surface being the limiting factor. Several works have analyzed the influence of modifying the composition of the Bi-2212 precursor in the final microstructure of the textured material. Results showed that moderate Ag additions, with contents up to 3 wt%, reduce porosity and increase electrical connectivity because Ag fills the holes between superconducting grains [ 180 ]. Also, the substitution of some cations has been explored. For instance, Ca substitution by Y ions [ 181 ] generates an increase in the critical temperature values but with a deterioration of the grain alignment, while substitution by Rb [ 182 ] or Cs [ 183 ] leads to an increase of the pinning force at 10 K by about 20%. Aiming to increase the level of current that can be transported by these materials, some studies have been performed, aiming to increase the cross section of the textured material. Vieira et al., in 2012 [ 130 ], analyzed the thermal gradients induced in rods with diameters between 1.75 and 2.5 mm and how they affect the phase distribution in the cross section of the sample and in the superconducting properties. Natividad et al., in 2001 [ 184 ], used LFZ to induce texture in hollow cylinders with external diameters up to 10 mm. The external part of the cross section is similar to that observed in bulk samples. These samples were used to fabricate a current lead with a coaxial configuration, showing an increase in the critical current value due to the reduction of the generated magnetic fields. Laser power limits the maximum thickness of the hollow cylinder. Angurel et al., in 2009 [ 185 ], proposed a new alternative with the aim to be able to texture rods with any diameter. In this case, samples are placed on a support that contains a series of Al 2 O 3 rods in constant rotation, on top of which superconductor cylinder preforms are placed. The proposed laser system was a diode laser and allowed to process several pieces in parallel. As it has been mentioned in previous sections, the group from Universidade de Aveiro (Costa et al.) introduced a modification in the LFZ process, called EALFZ, in which a DC electrical current is applied through the solid–liquid interface during the solidification process [ 29 , 122 , 127 , 128 , 134 , 186 , 187 ]. This modification was applied to the Bi-2212 and to the Bi-2223 phases, showing excellent results in both cases. When the electrical field is applied in the direct polarization configuration (positive pole connected to the seed) solidifications deviate from metastability and the primary solidification phase is mainly the 14/24 phase, yielding thinner and longer grains in the as-grown material and an improved texture in the superconducting rod. By contrast, when a reverse polarization is used, the dendritic microstructure disappears, generating a globular structure. Applying an Crystals 2021,11, 38 18 of 29 electrical current of 300 mA in the processing of Bi-2212/2.9 wt% Ag rods, it was possible to reach critical current values at 77 K of the order of 5800 A/cm 2 in samples of 2.3 mm in diameter, ~2.5 times higher than the values measured in similar samples processed without applying the electrical current during the solidification [ 122 ]. A review of the evolution of the superconducting families and the wavelength of the lasers used to process these materials is presented in Table 6. Table 6. Superconducting materials produced by LFZ. Year Authors Material 1Laser Ref. 1988 Feigelson et al. Bi-2212 CO2[164] 1989 Qiao et al. YBaCuO, Bi-2212 CO2[160] 1989 De la Fuente et al. Bi-2223 CO2[166] 1989 Carillo-Cabrera et al. Bi-2223 CO2[169] 1989 Gazit et al. Bi-2212 CO2[173,174] 1992 Cima et al. YBaCuO CO2[161] 1992 Snoeck et al. Bi-2223 CO2[170] 1994 Figueredo et al. YBaCuO CO2[162] 1994 Larrea et al. Bi-2223 CO2[171] 1995 De la Fuente et al. Bi-2212, Bi-2223 CO2, Nd:YAG [175] 1997 Costa et al. Bi-2223 CO2[123] 1998 Diez et al. Bi-2212, Bi-2223 Nd:YAG [176] 1998 Angurel et al. Bi-2212 Nd:YAG [177] 1999 Costa et al. Bi-2223 CO2[133] 2004 Carrasco et al. Bi-2223 Nd:YAG [29] 2004 Costa et al. Bi-2223 Nd:YAG [186] 2004 Natividad et al. Bi-2212 Nd:YAG [179] 2005 Sotelo et al. Bi-2212 Nd:YAG [180] 2015 Costa et al. Bi-2212 CO2[122] 2016 Özcelik et al. Bi-2212 Nd:YAG [181] 2020 Özcelik et al. Bi-2212 Nd:YAG [182] 1YBaCuO: YBa2Cu3O7−δ; Bi-2212: Bi2Sr2CaCu2O8+δ; Bi-2212: Bi2Sr2Ca2Cu3O10+δ. These superconducting materials were used in the development of the superconducting part of current leads [ 188 ]. In the framework of a collaboration between the INMA, the CEDEX-CIEMAT Laboratory of Applied Electromagnetics (CEDEX: Centro de Estudios y Experimentación de Obras Públicas, CIEMAT: Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas; Madrid, Spain) and the company ANTEC (Antec Inc.; Freemont, CA, USA), the company designed and fabricated a set of 600-A current leads for the LHC (Large Hadron Collider) correction magnets at CERN (European Organization for Nuclear Research) [ 189 , 190 ]. Each module consisted of four Bi-2212 textured bars, with a diameter of approximately 0.8 mm, connected in parallel. The module has to work at temperatures close to 50 K, in which the expected critical current can reach values close to 1200 A. This requirement revealed that one of the main limitations to developing technological applications with these materials was to obtain good electrical contacts to inject the electrical current. This called for an effort to deposit on the oxide surface a stable, uniform, and reproducible metal layer. Painting, sputter deposition, or electrodeposition processes from non-aqueous solvents [ 191 ] have been used. In all the cases, best results were obtained when the coating was fabricated in the as-grown material and the annealing of the superconducting rod was used to consolidate the electrical contact. Reproducible surface contact resistivity values lower than 10 −9Ω cm 2 were reached, values that allow us to use these materials in electrical applications where high transport currents are required. 7. Incongruent Melting and Volatile Materials for Additional Applications Researchers working on LFZ have shown every so often the possibility to obtain incongruent melting materials in a more or less controllable, direct, and fast way. In most cases, this was possible thanks to high temperatures provided within controlled atmo- Crystals 2021,11, 38 19 of 29 spheres and fast speeds. Lately, as was mentioned in Section 2, equipment setup advances providing high-pressure [ 23 ] or uniform radial heating by using various laser diodes [ 22 , 32 ] has allowed us to synthesize volatile (Li 2 CuO 2 , Nd 2 Mo 2 O 7 , SrRuO 3 ) and incongruent melting (BiFeO 3 , (La,Ba) 2 CuO 4 , Ba 2 Co 2 Fe 12 O 22 ) materials. For example, Schmehr et al., in 2019 [ 23 ], were able to grow volatile Li 2 CuO 2 by introducing a 80:20 Ar:O 2 atmosphere with a total pressure of 98.69 atm (100 bar) for avoiding volatility. In contrast, the absence of special atmospheres when processing an incongruent melt system like Nd 2 O 3 :SiO 2 has allowed us to synthesize unexpected materials. Thus, from stoichiometric mixtures of Nd 2 O 3 and SiO 2 processed in air, a biphasic Nd 2 SiO 5 :Nd 9.33 (SiO 4 ) 6 O 2 material has recently been produced by Rey-García et. al., observing how the dielectric character of the stoichiometric phase rules electrical properties, making it suitable for microwave dielectric devices [ 192 ]. Likewise, for growing crystals of the pyrochlore-type Nd 2 Mo 2 O 7 (NMO), suitable for Hall effect sensors, only a slight pressure of 1 atmosphere is needed, as recently reported by Kaneko and Tokura, in 2020 [ 32 ], to promote the re-evaporation of MoO 2 by rapidly increasing the temperature of the molten zone. These authors have also grown SrRuO 3 (SRO), an archetypal perovskite ferromagnetic metal, which also presents the deposit of RuO 2 near the melting point. Thus, by applying 10 atmospheres of Ar with 10% O 2 at a growth rate of 7 mm/h from a raw material with a 0.05 at% excess of Ru, a stable crystal growth without fluctuations of the material rod or the molten zone was achieved. These two volatile materials were synthetized through LDFZ technique, in which the use of various lasers allows a uniform irradiation intensity distribution on the periphery of the raw material. In addition, a vertical irradiation intensity distribution can be designed to have flat-shape or bell-shape irradiations for relaxation of residual thermal strain in the grown crystal [22,32]. The LDFZ technique has been also recently applied to produce incongruent melting materials like the Y-type hexaferrite Ba 2 Co 2 Fe 12 O 22 conventionally grown by a flux method [ 32 ]. So, single crystals of the multiferroic compound were successfully grown at 10 atmospheres and a slow growth rate of 1 mm/h from a seed crystal having the desired composition and crystal structure. Indeed, crystals of BiFeO 3 and La 2-x Ba x CuO 4 were obtained [ 22 ], as was also mentioned in Section 2, avoiding the stability problems caused by the gentle temperature gradient when growing in conventional LFZ. However, conventional LFZ has allowed us to produce other incongruent melting materials through the years [ 192 , 193 ]. For example, Andreeta et al., in 1999 [ 193 ], were able to grow single-crystal fibers of the dielectric SrHfO 3 in air from green rods. Its properties were found to be influenced by incongruent melting since, depending on the growth rate, surface inclusions and compositional variations were observed. Some years after, Tb 3 Al 5 O 12 was also successfully produced in a modified LFZ setup, adapted with four halogen lamps to reduce thermal gradients, favoring TAG crystallization [ 30 , 31 , 118 ], as mentioned earlier in Section 4. Likewise, iron aluminate garnet (Y 3 Fe 5 O 12 ) is another incongruent melting material, which was easily grown in air using conventional LFZ by Sekijima et al., in 1999 [ 119 ], and by Lim et al., in 2000 [ 194 ]. It is useful for monitoring unique magneto-optical properties in the near infrared, making it suitable as an infrared isolator, optical switch, and spatial light modulator, as well as in other sensor applications. In the beginning of the 21st century, researchers grew through the LFZ or LHPG technique other examples of incongruent materials. For example, Chen et al., in 2002 [ 195 ], successfully grew metastable BaTiO 3 crystal fibers doped with calcium oxide, namely, Ba 1−x Ca x TiO 3 (x = 0.1, 0.2). Taking advantage of their tetragonal structure, the growth from both SrTiO 3 and Ba 0.8 Ca 0.1 TiO 3 seeds was demonstrated. The growth of potassium lithium niobate (KLN) single crystal suitable for second harmonic generation with blue laser emission [ 86 – 88 ] may also be cited once more here, as an important example. 8. Future Perspectives The Laser Floating Zone (LFZ) technique reviewed in this paper, also known as LaserHeated Pedestal Growth (LHPG), could be considered as an ideal, environmentally benign Crystals 2021,11, 38 20 of 29 method to obtain high-quality single crystals within a very short time span, while minimizing the use of starting precursors and increasing overall energy efficiency with respect to other bulk-crystal growth methods. From these points of view, it is ideal to fulfill the long-sought desire to advance the understanding of fundamental physico-chemical phenomena at a fast pace. In essence, it complies perfectly with the message transmitted in a recent sentence by Schmehr et al., in 2019 [ 23 ]: “The availability of pristine single crystals is essential to the discovery of new physical phenomena in condensed matter physics”. Perhaps the latter is a good indication that LFZ is essential toward the development of an important part of the future awaiting solid-state chemistry and physics, as well as materials science and the technological developments that follow from these. Taking into account exclusively the growth of congruently melting materials in singlecrystal cylinder or fiber form, LFZ provides, to a first approximation, important advantages regarding the use of small quantities of materials, crucible-free melt containment, and fast growth rates. The quality of the crystals obtained thus far has proven sufficient for their use in demonstrators and in high-technology devices. Crystals with good optical quality for laser resonators are a good example of their demonstrated use and future potential. For example, ultra-fast mode-locking lasers producing picosecond (10 −12 s) and femtosecond (10 −15 s) high-energy pulses at multi-GHz repetition rates [ 196 ] can be applied in a wide range of applications that include nonlinear optics, telecommunications, nanomachining, medical surgery, and environmental monitoring [ 197 – 200 ]. Likewise, this method is adequate to achieve high-quality crystals of small dimensions for compact and portable instruments, as discussed in Section 4. LFZ has enabled fabrication of high-quality photonic materials suitable to be used as optical isolators, amplifiers, or modulators, waveguides by embedding into adequate cylinders, scintillators, luminescent materials, temperature sensors, electro-optics, photorefractive materials, or selective emitters for silicon photovoltaic cells, among others. More recent developments related to the use of different types of lasers, particularly diode lasers that may be deployed in various focusing configurations, have provided significant improvements on heating homogeneity and melt uniformity [ 32 ]. These affect not only the crystal quality, but potentially reduce the undesired appearance of microcracks, which make the crystal useless, or fail under further stress during use. In addition, these novel configurations enable the growth of incongruently melting phases, such as those considered in previous sections above, as well as phases that require particular atmosphere control. Examples of these include incongruently melting Ba 2 Co 2 Fe 12 O 22 , grown under O 2 atmosphere [ 32 ], and highly volatile Nd 2 Mo 2 O 7 and SrRuO 3, grown under Ar and Ar:O 2 atmospheres, respectively. An extreme case is the growth of the known volatile Li 2 CuO 2 compound, achieved within a high-pressure (HP-LFZ) apparatus [ 23 ] and an 80:20 Ar:O 2 atmosphere. Thus, the demonstrated growth of non-congruent melting [ 26 – 29 , 82 , 83 , 114 , 188 – 192 ] or volatile [ 22 , 28 , 29 , 188 ] materials in a controlled, direct, and fast manner paves the way to explore the fabrication of crystal elements for already relevant, expected, and unforeseen applications in high technology. Focusing on difficult-to-grow materials, we must highlight the recent work of Lai et al., in 2018, who studied how to design a nonlinear, hybrid, crystal-glass 3D metamaterial fiber with sub-wavelength fine structures [ 104 ]. They grew a large-scale harmonic crystal (Si 4+ : γ -Al 2 O 3 ) from a sapphire ( α -Al 2 O 3 ) crystal monolithically integrated into silica tubes. A hybrid crystal-glass metamaterial fiber was thus produced in a two-step process. Laser irradiation was followed by rapid cooling, to induce an inter-diffusion process where nucleation of large-sized harmonic crystals in anisotropic faceting was achieved. These open the possibility for attaining monolithically integrated dendrites for intracavity and resonant second harmonic generation (SHG) [104]. In the last decade, new, clean, and sustainable power-generation methods have been intensively investigated in attempts to control global warming. Innovative sustainable development is now accepted as a way of progress toward an improved environment, for which the design and development of novel materials able to be used in a plethora of Crystals 2021,11, 38 21 of 29 applications becomes an urgent necessity. In this sense, a breakthrough in the efficiency of thermoelectric (TE) materials will have an important technological and economic impact on the global energy balance. This may include novel approaches to develop advanced thermoelectric oxides, by in situ defects’ engineering and nanostructuring, promoted by controlled redox reactions. These innovative aspects can be explored by LFZ processing under strongly non-equilibrium conditions for designing desired functional thermoelectric properties [ 29 , 122 – 124 , 137 – 145 ]. Moreover, the use of an externally applied current during the growth process (EALFZ) is a powerful tool to design and control microstructure, thus to improve transport properties. Particular attention needs to be given to the fabrication of prototypes based on LFZ-processed thermoelements, considering not only the present geometry achieved for the LFZ-obtained TE materials. High-temperature superconducting ceramics have been among the most commonly studied materials with LFZ [ 29 , 122 , 125 , 133 , 160 – 162 , 164 – 182 ], probably because their anisotropic structure is ideal for directional solidification processing. As reviewed above, LFZ has demonstrated excellent microstructural transformations that result in high critical current elements in rod and tube geometries. These have been demonstrated to be competitive in several applications, particularly in superconducting current leads. These are used to connect electric power between room temperature and cryogenic temperatures, necessary for high-magnetic-field superconducting magnet operation, present for example in MRI machines, among other devices. A most relevant advancement for superconductor processing is based on the EALFZ technique [ 29 , 122 , 127 , 128 , 134 , 186 , 187 ], which provides an additional control of texture, yields excellent results in terms of critical current values, and opens new frontiers to study the fundamental phenomena that stand behind controlled diffusion during directional solidification at large. Future developments of LFZ within the superconductor materials domain should probably center at least around three aspects. First of all, on applying further and improving our understanding of electrically assisted phenomena during melting and solidification. Secondly, in exploring texturing of large-area, high-T c coatings and films, which may find a number of attractive applications, including in attractive devices driven by levitation. Thirdly, modifying precursor, heating, and solidification geometries in order to improve the scalability of the method for industrial production. A least explored aspect of LFZ entails the possibility of producing organic complex molecules in the form of crystalline fibers [ 120 , 121 ]. As suggested by the results reviewed above, a wide range of opportunities to explore bio-compatible applications may be envisioned. In addition, the production on geometries suitable to be employed, for example, in photonic devices, highly reduces the engineering, mechanical, and economical cost associated when an organic-based compound must be integrated into a commercial photonic device. These types of materials need intensive studies in order to establish a proper knowledge base for LFZ-processed organic compounds, and future efforts will determine the potential of the technique to address the intrinsic scientific and technical problems involved, particularly with respect to competing fabrication methods. Finally, a universal advantage that may always be considered for the future evolution of LFZ is, as commented earlier, its facile, low cost, and turnaround productivity of an ample collection of materials in single-crystal form and with convenient size for physical property characterization and demonstration prototypes. These would apply to wide-open disciplines of science and technology. The technique may gain relevance if consideration is given to miniaturization of apparatus and the achievement of higher mechanical precision. Together with air-cooled, small-size, and high-beam-quality lasers recently available at lower cost and high durability, LFZ may end up being incorporated into many laboratories worldwide for quick turnaround and convenient materials’ processing exploration purposes. It will certainly contribute to the advancement of both fundamental understanding of physical phenomena and developments of new devices. Crystals 2021,11, 38 22 of 29 Funding: G.F.d.l.F. and L.A.A. acknowledge support from Gobierno de Aragón “Construyendo Europa desde Aragón (research group T54_20R). This work was also developed within the scope of the projects, i3N, UIDB/50025/2020 and UIDP/50025/2020, financed by national funds through the FCT/MCTES. Acknowledgments: F.R.-G. acknowledges Aleixo, Xabier, and Diana for their comprehension despite reducing our time together. 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