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Influence of growth and heat treatment conditions on lasing properties of ZrO2-Y2O3-Ho2O3 crystals
Optical Materials 99 (2020) 109611
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Influence of growth and heat treatment conditions on lasing properties of ZrO2-Y2O3-Ho2O3 crystals Sergey A. Artemov a, Mikhail A. Borik b, Tatyana V. Volkova a, Mikhail V. Gerasimov a, Alexey V. Kulebyakin b, Elena E. Lomonova b, Filipp O. Milovich c, Valentina A. Myzina b, Polina A. Ryabochkina a, *, Nataliya Yu. Tabachkova c a b c
N.P. Ogarev Mordovia State University, Bolshevistskaya Street, 68, Saransk, 430005, Russia A.M. Prokhorov General Physics Institute, Russian Academy of Sciences, Vavilova Street, 38, Moscow, 119991, Russia National University of Science and Technology «MISIS», Leninskiy Prospect, 4, Moscow, 119049, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: Zirconia based single crystals Single crystal growth Optical defects Solid state lasers
The influence of growth and subsequent heat treatment conditions on the parameters of two-micron lasing in (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals at the 5I7→5I8 transition of Ho3þ ions has been investigated. We show that the lasing properties of the material of the active element can be improved by reduction of the growth rate which leads to an increase in the optical homogeneity of the single crystals and by annealing of the crystals after growth which relieves thermoelastic residual stresses.
1. Introduction Technologies of high optical quality crystals doped with rare-earth (RE) and transition ions for use in laser physics were actively devel oped in the 1970–80s. The search for new laser materials in the 1970s led to the development of direct high-frequency melting technology in a cold container. This technology allows growing single crystals with a high melting point from the melt in the air [1–3]. The most successful achievement of the technology of direct high-frequency melting in a cold container was the synthesis of ZrO2 based crystals with a melting point of about 3000 оС [3]. On the basis of this technology, industrial pro duction of zirconia based single crystals developed in many countries world over [1,2]. ZrO2 based single crystals are solid solutions which contain a stabi lizing oxide that ensures the preservation of the fluorite cubic structure upon cooling to room temperature [2]. Yttrium oxide is most often used as stabilizing oxide; it is also possible to use oxides of alkaline-earth and rare-earth elements for this purpose. Yttria-stabilized zirconia crystals are isotropic optical media with a broad spectral transmittance region (250–7500 nm); they have high hardness comparable to that of aluminum oxide (8.5–9 of Mohs’ scale). The spectroscopic properties of these crystals are quite diverse due to the
possibility of varying the solid solution composition in a wide range and introducing a large number of activating impurities. The possibility of obtaining lasing in yttria-stabilized zirconia crystals doped with RE ions by means of lamp pumping was reported [4]. It should be noted that, due to the low thermal conductivity of stabilized zirconia crystals [5], they did not receive wide use as active media of lamp-pumped solid-state lasers. The development and general use of resonant semiconductor laser pumping eased the requirements to the thermomechanical characteris tics of the material. For this reason the interest to the study of the lasing properties in ZrO2 based cubic crystals grew again [6–9] with an eye to obtaining tunable lasing and ultrashort laser pulses in these crystals due to the presence of wide luminescence bands of RE ions which originate from the disorder in their crystal structure. So far we have studied the lasing characteristics of (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals and obtained continuous lasing at the 5I7→5I8 transition of Ho3þ ions under LiYF4: Tm laser pumping [7,8] and under thulium fiber laser pumping [9]. Also, tunable lasing was obtained in (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals in the 2056–2168 nm range [8]. Generation experiments the results of which were presented earlier [7,8] were carried out for (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals
* Corresponding author. E-mail addresses: [email protected] (S.A. Artemov), [email protected] (M.A. Borik), [email protected] (T.V. Volkova), [email protected] (M.V. Gerasimov), [email protected] (A.V. Kulebyakin), [email protected] (E.E. Lomonova), [email protected] (F.O. Milovich), vamyzina@lst. gpi.ru (V.A. Myzina), [email protected] (P.A. Ryabochkina), [email protected] (N.Yu. Tabachkova). https://doi.org/10.1016/j.optmat.2019.109611 Received 19 August 2019; Received in revised form 6 December 2019; Accepted 8 December 2019 Available online 25 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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grown by direct high-frequency melting technology in a cold container in a Kristall-407 plant with a 130 mm diameter cold crucible at a growth rate of 10 mm/h in air atmosphere. However, it is well-known [1–3] that the synthesis of ZrO2 based crystals by directional crystallization in a cold container can introduce growth defects leading to optical inhomogeneity in the crystals. As a rule, these defects are growth bands, cellular structures and residual thermal stresses. In turn, optical inhomogeneity in crystals can cause significant losses in lasing. Reducing the speed of crucible lowering when growing ZrO2 based cubic crystals increases the optical homogeneity of the crystals, de creases their number and increases the volume of individual single crystals in the ingot [1–3]. High residual thermal stresses are typical of single crystals grown by directional melt crystallization in a cold container using direct high-frequency heating [1–3]. This is caused by a decrease in the conductivity of dielectric crystals during cooling after growing. This does not allow one to anneal single crystals directly during the growth in a cold container. The rate of crystal cooling in a cold container depends on the total ingot weight of the crystallized melt [1–3]. During the generation experiments with (ZrO2)0.86(Y2O3)0.136( Ho2O3)0.004 crystals the results of which were presented earlier [7–9], it was found that at the pump into different regions of the active element the energy characteristics of lasing may vary. In our opinion, this is due to the optical inhomogeneity of ZrO2–Y2O3–Ho2O3 crystals, i.e., the presence of growth bands in them [1–3]. It should be noted that a detailed study of the effect of optical in homogeneity in ZrO2–Y2O3–Ho2O3 crystals grown under various tech nological conditions on their lasing characteristics has not yet been carried out. However, the study of the causes of optical defects and their elimi nation in laser crystals, including through heat treatment, is an impor tant task since this provides an increase in the efficiency of lasing in solid-state lasers based on them [10–17]. The aim of this work was to study the influence of optical defects in ZrO2–Y2O3–Ho2O3 crystals on their lasing characteristics and to develop recommendations for optimizing the conditions of obtaining these crystals without these defects.
Optical inhomogeneity in the crystals associated with growth con ditions was studied with optical microscopy methods. Residual thermal stresses in the as-grown crystals were controlled using the birefringence based polarization optical method, both after growth and after anneal ing which is necessary for residual thermal stress removal. Optical in homogeneity was studied using an AxioImager Z2 Vario (Carl Zeiss) optical microscope by two methods: the polarization method in trans mitted linearly polarized light and the differential interference contrast method (C-DIC) in reflected circularly polarized light [18]. The homo geneity of optical anisotropy in the volume of the active elements was studied in transmitted linearly polarized light. The C-DIC method was used to study the homogeneity of the refractive index distribution. The chemical composition of the as-grown crystals was studied with a JEOL 5910 LV scanning electron microscope equipped with an AZte cENERGY energy dispersive analytical system (Oxford Instruments). The crystal structure was studied using transmission electron mi croscopy at a 200 kV accelerating voltage with a JEM-2100 microscope. The specimens were prepared by ion etching in PIPS II (GATAN, En gland) setup. For the generation experiments, 3 � 20 mm sized active elements were cut out from Crystals 1 and 2 in the direction along the growth axis. The optical scheme of the laser used for the generation experiment with the active elements cut from Crystals 1 and 2 is shown in Fig. 1. The laser resonator was formed by a flat input mirror with a trans mission coefficient of 93% at a pump wavelength of 1910 nm, T ¼ 0.1% and a lasing wavelength of 2130 nm, and a spherical output mirror with a curvature radius of 150 mm and the transmittance T ¼ 6% at the lasing wavelength. For lasing experiments the active elements were wrapped in indium foil and placed in a copper holder. Antireflective coating for the lasing wavelength was not applied to the ends of the active elements. Pumping to the 5I7 level of Ho3þ ions was carried out with a LiYF4: Tm laser at 1910 nm and a maximum output power of 14.5 W. The pump radiation was focused onto the active element using a four-lens objec tive. The pump spot diameter was 400 μm. 3. Results and discussion If the lowering speed of the cold container with the melt relative to the inductor was 10 mm/h, the number of as-grown single crystals in the ingot of crystallized melt was 20–30, while for a lowering speed of 3 mm/h the number of single crystals reduced to 7–8 large crystals. Crystals of the series grown at a container lowering speed of 10 mm/ h are designated hereafter as Crystals 1, and those for a speed of 3 mm/h
2. Experimental The test (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals were grown by directional melt crystallization with direct high-frequency heating in a cold container in a Kristall-407 plant with a crucible diameter of 130 mm at crucible lowering speeds of 10 mm/h and 3 mm/h. The crystals grown at a 10 mm/h rate are denoted in the text as Crystals 1, and those grown at a 3 mm/h rate, as Crystals 2. The weight of the melted material for growing crystals was 5 kg. To establish phase equilibrium between the melt and the solid phase in the water-cooled crucible, we kept the melt for 1–2 h under stable conditions. Directional melt crystallization was carried out by lowering the container with the melt relative to the inductor at speeds of 10 and 3 mm/h. Crystallization began in the lower part of the cold crucible at solid phase crystalline grains (skull). As a result of degeneration caused by the multiple nucleating crystals, a small part of them survived and an ingot formed consisting of columnar single crystals. The crystallization process was controlled by the parameters of the high-frequency generator [1–3]. After crystallization completion the crystalline ingot cooling process was monitored by measuring the sur face temperature of the upper heat shield with a Gulton 900–1999 ra diation pyrometer (above 1000 � C) and a Pt/Pt–Rh thermocouple (from 1000 � C to 500 � C). The cooling rate of the ingot from the melt tem perature to 1000 � C was 200 � C/min, and then to 500 � C, ~30 � C/min. Stress relief annealing of the crystals and the active elements was carried out in a pit type resistance vacuum furnace 1.2,5/25 with mo lybdenum heaters at 2100 � C in 10 3–10 4 mmHg vacuum with a 3 h exposure and a temperature decrease at a rate of 80 � C/h.
Fig. 1. The optical scheme of the laser on (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals pumped by YLF:Tm laser radiation. 2
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as Crystals 2. Photographs of Crystals 1 and 2 are shown in Fig. 2. The distribution of the solid solution components in Crystals 1 and 2 was studied by energy dispersive analysis. Within the accuracy of the measurement method, the solid solution composition was uniform in different areas of the crystals and corresponded to the composition of the charge. The results of transmission electron microscopy studies indicate that inclusions and visible defects are not observed in Crystals 1 and 2. For example, Fig. 3 shows high resolution TEM image and EDX analysis data for Crystal 1. The optical homogeneity of Crystals 1 and 2 grown under different conditions was studied. Fig. 4 shows orthoscopic images of the active element cut from Crystal 1 obtained with an optical microscope using two methods. One method was polarization in crossed nicols in trans mitted light for the Y direction along the growth axis and for the X di rection perpendicular to the growth axis. The other was the C-DIC method in reflected light for the X direction perpendicular to the growth axis. For laser elements cut from Crystals 1, polarization microscopy study in two mutually perpendicular directions (Fig. 4 a, b) revealed the absence of complete extinction of cubic crystals in polarized light in crossed nicols. Rotation of the microscope stage revealed a wavy nature of extinction in the crystal, i.e., different parts of the crystal were extinguished in different positions. This fact indicates the presence of optical anisotropy in the crystals, i.e., polarization planes have different orientations in different crystal parts. This is typical of deformed (stressed) crystals [19]. Stresses have thermal character in these crystals. Optical inhomogeneity occurs also in Crystals 1, showing itself in the form of growth bands which are clearly detected in reflected light with the C-DIC method (Fig. 4c). As shown earlier [1–3], growth bands forming during the growth of zirconia-based crystals by direct high-frequency melting in a cold container are caused by fluctuations in the composition of the single crystals during crystallization. These composition fluctuations are due to concentration supercooling caused by impurities with a low effective distribution coefficient. These impurities can be either uncontrolled impurities in the raw oxides (for example, impurities of silicon, titanium, etc.) or impurities that make up the solid solution. Growth bands are almost absent in Crystals 2 grown at a speed of 3 mm/h. Fig. 5 shows orthoscopic images of the active element cut from Crystal 2 which clearly shows not only extinction inhomogeneity of the active element but also the presence of local inhomogeneity (indicated by the black arrow) caused by a high stress concentration (large
Fig. 3. High resolution TEM image and EDX analysis data for Crystal 1 grown at a cold container lowering speed of 10 mm/h.
compared to Crystals 1) or violation of the crystal integrity. A decrease in the crystallization rate to 3 mm/h leads to a decrease in the number of the as-grown single crystals [1–3]. A decrease in the number of the single crystals and an increase in their individual volume for the same weight of crystallized melt increase stress in the single crystals as compared to single crystals that have a smaller size but are present in a greater number in the ingot [19]. In the latter case, stresses are relieved along the boundaries of the single crystals, and the ingot of the crystallized melt after cooling is fragmented into separate single crystals. Therefore in order to relieve internal thermoelastic stresses in the active element cut from Crystal 2 we subjected it to heat treatment the conditions of which are described above. Fig. 6 shows orthoscopic optical microscope images of the active element cut from as-annealed Crystal 2. Comparative analysis of the images presented in Figs. 5 and 6 sug gests that extinction inhomogeneity in the active element cut from Crystal 2 after heat treatment is expressed but weakly. The extinction inhomogeneity is the most weakly expressed in the X direction perpendicular to the growth axis (Fig. 6b). During the generation experiment for the active elements cut from Crystals 1, lasing was obtained at the 5I7→5I8 transition of Ho3þ ions with a lasing wavelength of 2030 nm. Lasing was not obtained (for the same generation experiment conditions) in active elements cut from Crystals 2. After heat treatment of the active element cut from Crystal 2, a generation experiment was carried out again under the conditions described above. In the course of the experiment lasing was obtained at the 5I7→5I8 transition of Ho3þ ions. For comparative analysis a generation experiment was carried out for the active element cut from Crystal 1 after heat treatment. The dependence of the output lasing power on the pump power absorbed in the active element cut from Crystal 2 after heat treatment and the respective dependences for the active element cut from Crystal 1 without heat treatment and after heat treatment are shown in Fig. 7. As can be seen from Fig. 7, these dependences differ slightly for the active element cut from Crystal 1 without heat treatment and for the active element cut from Crystal 2 after heat treatment. The lasing threshold and differential efficiency for the laser with the
Fig. 2. Photographs of (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals grown at different cold container lowering speeds: (a) 10 mm/h (Crystals 1), (b) 3 mm/h (Crystals 2). 3
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Fig. 4. Orthoscopic images of the active element cut from single Crystal 1: (a) in crossed nicols in transmitted light for the Y direction along the growth axis, (b) for the X direction perpendicular to the growth axis and (c) in reflected light for the X direction perpendicular to the growth axis.
these experiments are similar to those presented above. Thus, the absence of lasing in the active elements cut from as-grown Crystals 2 was caused by significant thermoelastic stresses which develop if the growth rate decreases. The presence of small residual stresses in the active element cut from Crystal 2 after heat treatment is due to the fact that the time of vacuum annealing chosen by us might be insufficient. It is desirable to increase the exposure time in order to completely remove thermoelastic stresses. Nevertheless, it was possible to obtain lasing in the sample even at this degree of stress reduction. This confirms the assumption that the absence of lasing in the active element cut from Crystal 2 without heat treatment was caused by high postgrowth thermoelastic stresses in the crystal. It can be clearly seen from Fig. 8 that the local inhomogeneities present in the active element before heat treatment were persisted also after heat treatment. This being apparently caused by a high concen tration of stresses or violation of the crystal integrity (Fig. 5b). It is most likely that these “cracks” were introduced as a result of cutting the active element cut from Crystal 2 which was more stressed after growth as compared with Crystal 1. The optical microscopy studies of active elements cut from Crystals 1 and 2 which have the same composition but were grown at different rates (10 mm/h and 3 mm/h) showed that the optical defects of different nature dominate in them. In Crystals 1, the predominant optical defects are “growth bands”. In Crystals 2, “growth bands” are almost absent but there are strong thermoelastic stresses which cause significant lasing losses in these crystals. Complex studies of the optical and lasing properties of active ele ments cut from Crystals 1 and 2 revealed the following. The “growth bands” in the active elements cut from Crystals 1, the residual stresses after heat treatment and the local defects in the active elements cut from stressed Crystals 2 have approximately the same effect on the output energy characteristics of lasers on these active elements. However, it
Fig. 5. Orthoscopic images of the active element cut from Crystal 2 (without heat treatment) in crossed nicols in transmitted light (a) for the Y direction along the growth axis and (b) for the X direction perpendicular to the growth axis. The black arrow indicates local inhomogeneity.
active element cut from Crystal 1 without heat treatment were 1.8 W and 33%, respectively. The corresponding values for the laser with the active element cut from Crystal 2 were 1.6 W and 29%, respectively. Thus, it can be seen that the laser energy characteristics obtained in generation experiments prove to be almost the same for the active ele ments cut from Crystals 1 and 2. However, the active element cut from Crystal 1 after heat treatment is characterized by a lower lasing threshold and an increase in the differential efficiency. These parame ters are 1.4 W and 40%, respectively. It should be noted that the generational experiments were carried out for a series of active elements cut from Crystals 1 and 2. The results of
Fig. 6. Orthoscopic images of the active element cut from as-annealed Crystal 2 in crossed nicols in transmitted light (a) for the Y direction along the growth axis, (b) for the X direction perpendicular to the growth axis and (c) in reflected light for the X direction perpendicular to the growth axis. 4
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addition, high thermal inertia of large melt volumes during crystalliza tion prevents temperature fluctuations at the crystallization front caused by random fluctuations in the input power during direct high-frequency heating [1,2]. This in turn prevents discontinuities of the growth rate at the crystallization front and contributes to an increase in the optical homogeneity of single crystals. Before cutting active elements from single crystals it is advisable to subject them to high-temperature annealing in order to relieve residual thermal stresses. 4. Conclusions Thus, we studied the influence of growth process conditions (growth rate) and subsequent heat treatment on the optical inhomogeneity of (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals which significantly affects lasing characteristics. We found that local thermoelastic stresses have a greater influence on the lasing characteristics of the crystals than “growth bands”. Recommendations are provided concerning the choice of process conditions for obtaining high optical quality ZrO2–Y2O3 crystals doped with RE ions aiming to obtain efficient lasing in them.
Fig. 7. Output lasing power at the 5I7→5I8 transition of Ho3þ ions vs the absorbed pump power for the active elements cut from Crystal 1 without heat treatment (■) and after heat treatment (●), and in the active elements cut from Crystal 2 (after heat treatment) (▴).
Author contribution Sergey A. Artemov, generation experiments. Mikhail A. Borik, crystal growth. Tatyana V. Volkova, study of optical properties. Mikhail V. Gerasimov, study of optical properties. Alexey V. Kulebyakin, crystal growth. Elena E. Lomonova, crystal growth, writing the manuscript. Filipp O. Milovich, study of crystals by the energy dispersive analysis. Valentina A. Myzina, annealing of crystals. Polina A. Ryabochkina, statement of the problem, writing the manuscript. Nataliya Yu. Tabachkova, study of crystals by the transmission electron microscopy. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Russian Foundation for Basic Research, Grant No. 18-29-20039, and the Ministry of Education and Science of the Russian Federation (Project 1.4926.2017/6.7).
Fig. 8. Orthoscopic image of the active element cut from Crystal 2 (after heat treatment) obtained using an optical microscope with the polarization method in crossed nicols in transmitted light for the X direction perpendicular to the growth axis.
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was found that heat treatment of the active element cut from Crystal 1 which was grown at a rate of 10 mm/h improves its lasing properties. The results of these studies allow us to make recommendations regarding the process conditions of synthesizing ZrO2–Y2O3 crystals doped with RE ions aiming to obtain efficient lasing in them. A decrease in the growth rate contributes to an increase in the optical homogeneity of single crystals due to a reduction in the density of the “growth bands”. However, in order to avoid the formation of significant thermoelastic stresses, it is advisable to grow crystals in large diameter containers which allow fusing up to 60–100 kg of material (for a cold container diameter of 400 mm) or up to 600–800 kg (for a cold container diameter of 700 mm) [1–3]. This will lead to a significant decrease in the cooling rate after growth and hence to a reduction in residual thermal stresses. Single crystals grown from larger melt volumes have higher optical homogeneity [2,3] due to a more efficient displacement of impurities which cause fluctuations in the growth rate at the crystallization front, since their concentration gradients increase with melt volume. In 5
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Influence of growth and heat treatment conditions on lasing properties of ZrO2-Y2O3-Ho2O3 crystals Sergey A. Artemov a, Mikhail A. Borik b, Tatyana V. Volkova a, Mikhail V. Gerasimov a, Alexey V. Kulebyakin b, Elena E. Lomonova b, Filipp O. Milovich c, Valentina A. Myzina b, Polina A. Ryabochkina a, *, Nataliya Yu. Tabachkova c a b c
N.P. Ogarev Mordovia State University, Bolshevistskaya Street, 68, Saransk, 430005, Russia A.M. Prokhorov General Physics Institute, Russian Academy of Sciences, Vavilova Street, 38, Moscow, 119991, Russia National University of Science and Technology «MISIS», Leninskiy Prospect, 4, Moscow, 119049, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: Zirconia based single crystals Single crystal growth Optical defects Solid state lasers
The influence of growth and subsequent heat treatment conditions on the parameters of two-micron lasing in (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals at the 5I7→5I8 transition of Ho3þ ions has been investigated. We show that the lasing properties of the material of the active element can be improved by reduction of the growth rate which leads to an increase in the optical homogeneity of the single crystals and by annealing of the crystals after growth which relieves thermoelastic residual stresses.
1. Introduction Technologies of high optical quality crystals doped with rare-earth (RE) and transition ions for use in laser physics were actively devel oped in the 1970–80s. The search for new laser materials in the 1970s led to the development of direct high-frequency melting technology in a cold container. This technology allows growing single crystals with a high melting point from the melt in the air [1–3]. The most successful achievement of the technology of direct high-frequency melting in a cold container was the synthesis of ZrO2 based crystals with a melting point of about 3000 оС [3]. On the basis of this technology, industrial pro duction of zirconia based single crystals developed in many countries world over [1,2]. ZrO2 based single crystals are solid solutions which contain a stabi lizing oxide that ensures the preservation of the fluorite cubic structure upon cooling to room temperature [2]. Yttrium oxide is most often used as stabilizing oxide; it is also possible to use oxides of alkaline-earth and rare-earth elements for this purpose. Yttria-stabilized zirconia crystals are isotropic optical media with a broad spectral transmittance region (250–7500 nm); they have high hardness comparable to that of aluminum oxide (8.5–9 of Mohs’ scale). The spectroscopic properties of these crystals are quite diverse due to the
possibility of varying the solid solution composition in a wide range and introducing a large number of activating impurities. The possibility of obtaining lasing in yttria-stabilized zirconia crystals doped with RE ions by means of lamp pumping was reported [4]. It should be noted that, due to the low thermal conductivity of stabilized zirconia crystals [5], they did not receive wide use as active media of lamp-pumped solid-state lasers. The development and general use of resonant semiconductor laser pumping eased the requirements to the thermomechanical characteris tics of the material. For this reason the interest to the study of the lasing properties in ZrO2 based cubic crystals grew again [6–9] with an eye to obtaining tunable lasing and ultrashort laser pulses in these crystals due to the presence of wide luminescence bands of RE ions which originate from the disorder in their crystal structure. So far we have studied the lasing characteristics of (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals and obtained continuous lasing at the 5I7→5I8 transition of Ho3þ ions under LiYF4: Tm laser pumping [7,8] and under thulium fiber laser pumping [9]. Also, tunable lasing was obtained in (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals in the 2056–2168 nm range [8]. Generation experiments the results of which were presented earlier [7,8] were carried out for (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals
* Corresponding author. E-mail addresses: [email protected] (S.A. Artemov), [email protected] (M.A. Borik), [email protected] (T.V. Volkova), [email protected] (M.V. Gerasimov), [email protected] (A.V. Kulebyakin), [email protected] (E.E. Lomonova), [email protected] (F.O. Milovich), vamyzina@lst. gpi.ru (V.A. Myzina), [email protected] (P.A. Ryabochkina), [email protected] (N.Yu. Tabachkova). https://doi.org/10.1016/j.optmat.2019.109611 Received 19 August 2019; Received in revised form 6 December 2019; Accepted 8 December 2019 Available online 25 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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grown by direct high-frequency melting technology in a cold container in a Kristall-407 plant with a 130 mm diameter cold crucible at a growth rate of 10 mm/h in air atmosphere. However, it is well-known [1–3] that the synthesis of ZrO2 based crystals by directional crystallization in a cold container can introduce growth defects leading to optical inhomogeneity in the crystals. As a rule, these defects are growth bands, cellular structures and residual thermal stresses. In turn, optical inhomogeneity in crystals can cause significant losses in lasing. Reducing the speed of crucible lowering when growing ZrO2 based cubic crystals increases the optical homogeneity of the crystals, de creases their number and increases the volume of individual single crystals in the ingot [1–3]. High residual thermal stresses are typical of single crystals grown by directional melt crystallization in a cold container using direct high-frequency heating [1–3]. This is caused by a decrease in the conductivity of dielectric crystals during cooling after growing. This does not allow one to anneal single crystals directly during the growth in a cold container. The rate of crystal cooling in a cold container depends on the total ingot weight of the crystallized melt [1–3]. During the generation experiments with (ZrO2)0.86(Y2O3)0.136( Ho2O3)0.004 crystals the results of which were presented earlier [7–9], it was found that at the pump into different regions of the active element the energy characteristics of lasing may vary. In our opinion, this is due to the optical inhomogeneity of ZrO2–Y2O3–Ho2O3 crystals, i.e., the presence of growth bands in them [1–3]. It should be noted that a detailed study of the effect of optical in homogeneity in ZrO2–Y2O3–Ho2O3 crystals grown under various tech nological conditions on their lasing characteristics has not yet been carried out. However, the study of the causes of optical defects and their elimi nation in laser crystals, including through heat treatment, is an impor tant task since this provides an increase in the efficiency of lasing in solid-state lasers based on them [10–17]. The aim of this work was to study the influence of optical defects in ZrO2–Y2O3–Ho2O3 crystals on their lasing characteristics and to develop recommendations for optimizing the conditions of obtaining these crystals without these defects.
Optical inhomogeneity in the crystals associated with growth con ditions was studied with optical microscopy methods. Residual thermal stresses in the as-grown crystals were controlled using the birefringence based polarization optical method, both after growth and after anneal ing which is necessary for residual thermal stress removal. Optical in homogeneity was studied using an AxioImager Z2 Vario (Carl Zeiss) optical microscope by two methods: the polarization method in trans mitted linearly polarized light and the differential interference contrast method (C-DIC) in reflected circularly polarized light [18]. The homo geneity of optical anisotropy in the volume of the active elements was studied in transmitted linearly polarized light. The C-DIC method was used to study the homogeneity of the refractive index distribution. The chemical composition of the as-grown crystals was studied with a JEOL 5910 LV scanning electron microscope equipped with an AZte cENERGY energy dispersive analytical system (Oxford Instruments). The crystal structure was studied using transmission electron mi croscopy at a 200 kV accelerating voltage with a JEM-2100 microscope. The specimens were prepared by ion etching in PIPS II (GATAN, En gland) setup. For the generation experiments, 3 � 20 mm sized active elements were cut out from Crystals 1 and 2 in the direction along the growth axis. The optical scheme of the laser used for the generation experiment with the active elements cut from Crystals 1 and 2 is shown in Fig. 1. The laser resonator was formed by a flat input mirror with a trans mission coefficient of 93% at a pump wavelength of 1910 nm, T ¼ 0.1% and a lasing wavelength of 2130 nm, and a spherical output mirror with a curvature radius of 150 mm and the transmittance T ¼ 6% at the lasing wavelength. For lasing experiments the active elements were wrapped in indium foil and placed in a copper holder. Antireflective coating for the lasing wavelength was not applied to the ends of the active elements. Pumping to the 5I7 level of Ho3þ ions was carried out with a LiYF4: Tm laser at 1910 nm and a maximum output power of 14.5 W. The pump radiation was focused onto the active element using a four-lens objec tive. The pump spot diameter was 400 μm. 3. Results and discussion If the lowering speed of the cold container with the melt relative to the inductor was 10 mm/h, the number of as-grown single crystals in the ingot of crystallized melt was 20–30, while for a lowering speed of 3 mm/h the number of single crystals reduced to 7–8 large crystals. Crystals of the series grown at a container lowering speed of 10 mm/ h are designated hereafter as Crystals 1, and those for a speed of 3 mm/h
2. Experimental The test (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals were grown by directional melt crystallization with direct high-frequency heating in a cold container in a Kristall-407 plant with a crucible diameter of 130 mm at crucible lowering speeds of 10 mm/h and 3 mm/h. The crystals grown at a 10 mm/h rate are denoted in the text as Crystals 1, and those grown at a 3 mm/h rate, as Crystals 2. The weight of the melted material for growing crystals was 5 kg. To establish phase equilibrium between the melt and the solid phase in the water-cooled crucible, we kept the melt for 1–2 h under stable conditions. Directional melt crystallization was carried out by lowering the container with the melt relative to the inductor at speeds of 10 and 3 mm/h. Crystallization began in the lower part of the cold crucible at solid phase crystalline grains (skull). As a result of degeneration caused by the multiple nucleating crystals, a small part of them survived and an ingot formed consisting of columnar single crystals. The crystallization process was controlled by the parameters of the high-frequency generator [1–3]. After crystallization completion the crystalline ingot cooling process was monitored by measuring the sur face temperature of the upper heat shield with a Gulton 900–1999 ra diation pyrometer (above 1000 � C) and a Pt/Pt–Rh thermocouple (from 1000 � C to 500 � C). The cooling rate of the ingot from the melt tem perature to 1000 � C was 200 � C/min, and then to 500 � C, ~30 � C/min. Stress relief annealing of the crystals and the active elements was carried out in a pit type resistance vacuum furnace 1.2,5/25 with mo lybdenum heaters at 2100 � C in 10 3–10 4 mmHg vacuum with a 3 h exposure and a temperature decrease at a rate of 80 � C/h.
Fig. 1. The optical scheme of the laser on (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals pumped by YLF:Tm laser radiation. 2
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as Crystals 2. Photographs of Crystals 1 and 2 are shown in Fig. 2. The distribution of the solid solution components in Crystals 1 and 2 was studied by energy dispersive analysis. Within the accuracy of the measurement method, the solid solution composition was uniform in different areas of the crystals and corresponded to the composition of the charge. The results of transmission electron microscopy studies indicate that inclusions and visible defects are not observed in Crystals 1 and 2. For example, Fig. 3 shows high resolution TEM image and EDX analysis data for Crystal 1. The optical homogeneity of Crystals 1 and 2 grown under different conditions was studied. Fig. 4 shows orthoscopic images of the active element cut from Crystal 1 obtained with an optical microscope using two methods. One method was polarization in crossed nicols in trans mitted light for the Y direction along the growth axis and for the X di rection perpendicular to the growth axis. The other was the C-DIC method in reflected light for the X direction perpendicular to the growth axis. For laser elements cut from Crystals 1, polarization microscopy study in two mutually perpendicular directions (Fig. 4 a, b) revealed the absence of complete extinction of cubic crystals in polarized light in crossed nicols. Rotation of the microscope stage revealed a wavy nature of extinction in the crystal, i.e., different parts of the crystal were extinguished in different positions. This fact indicates the presence of optical anisotropy in the crystals, i.e., polarization planes have different orientations in different crystal parts. This is typical of deformed (stressed) crystals [19]. Stresses have thermal character in these crystals. Optical inhomogeneity occurs also in Crystals 1, showing itself in the form of growth bands which are clearly detected in reflected light with the C-DIC method (Fig. 4c). As shown earlier [1–3], growth bands forming during the growth of zirconia-based crystals by direct high-frequency melting in a cold container are caused by fluctuations in the composition of the single crystals during crystallization. These composition fluctuations are due to concentration supercooling caused by impurities with a low effective distribution coefficient. These impurities can be either uncontrolled impurities in the raw oxides (for example, impurities of silicon, titanium, etc.) or impurities that make up the solid solution. Growth bands are almost absent in Crystals 2 grown at a speed of 3 mm/h. Fig. 5 shows orthoscopic images of the active element cut from Crystal 2 which clearly shows not only extinction inhomogeneity of the active element but also the presence of local inhomogeneity (indicated by the black arrow) caused by a high stress concentration (large
Fig. 3. High resolution TEM image and EDX analysis data for Crystal 1 grown at a cold container lowering speed of 10 mm/h.
compared to Crystals 1) or violation of the crystal integrity. A decrease in the crystallization rate to 3 mm/h leads to a decrease in the number of the as-grown single crystals [1–3]. A decrease in the number of the single crystals and an increase in their individual volume for the same weight of crystallized melt increase stress in the single crystals as compared to single crystals that have a smaller size but are present in a greater number in the ingot [19]. In the latter case, stresses are relieved along the boundaries of the single crystals, and the ingot of the crystallized melt after cooling is fragmented into separate single crystals. Therefore in order to relieve internal thermoelastic stresses in the active element cut from Crystal 2 we subjected it to heat treatment the conditions of which are described above. Fig. 6 shows orthoscopic optical microscope images of the active element cut from as-annealed Crystal 2. Comparative analysis of the images presented in Figs. 5 and 6 sug gests that extinction inhomogeneity in the active element cut from Crystal 2 after heat treatment is expressed but weakly. The extinction inhomogeneity is the most weakly expressed in the X direction perpendicular to the growth axis (Fig. 6b). During the generation experiment for the active elements cut from Crystals 1, lasing was obtained at the 5I7→5I8 transition of Ho3þ ions with a lasing wavelength of 2030 nm. Lasing was not obtained (for the same generation experiment conditions) in active elements cut from Crystals 2. After heat treatment of the active element cut from Crystal 2, a generation experiment was carried out again under the conditions described above. In the course of the experiment lasing was obtained at the 5I7→5I8 transition of Ho3þ ions. For comparative analysis a generation experiment was carried out for the active element cut from Crystal 1 after heat treatment. The dependence of the output lasing power on the pump power absorbed in the active element cut from Crystal 2 after heat treatment and the respective dependences for the active element cut from Crystal 1 without heat treatment and after heat treatment are shown in Fig. 7. As can be seen from Fig. 7, these dependences differ slightly for the active element cut from Crystal 1 without heat treatment and for the active element cut from Crystal 2 after heat treatment. The lasing threshold and differential efficiency for the laser with the
Fig. 2. Photographs of (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals grown at different cold container lowering speeds: (a) 10 mm/h (Crystals 1), (b) 3 mm/h (Crystals 2). 3
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Fig. 4. Orthoscopic images of the active element cut from single Crystal 1: (a) in crossed nicols in transmitted light for the Y direction along the growth axis, (b) for the X direction perpendicular to the growth axis and (c) in reflected light for the X direction perpendicular to the growth axis.
these experiments are similar to those presented above. Thus, the absence of lasing in the active elements cut from as-grown Crystals 2 was caused by significant thermoelastic stresses which develop if the growth rate decreases. The presence of small residual stresses in the active element cut from Crystal 2 after heat treatment is due to the fact that the time of vacuum annealing chosen by us might be insufficient. It is desirable to increase the exposure time in order to completely remove thermoelastic stresses. Nevertheless, it was possible to obtain lasing in the sample even at this degree of stress reduction. This confirms the assumption that the absence of lasing in the active element cut from Crystal 2 without heat treatment was caused by high postgrowth thermoelastic stresses in the crystal. It can be clearly seen from Fig. 8 that the local inhomogeneities present in the active element before heat treatment were persisted also after heat treatment. This being apparently caused by a high concen tration of stresses or violation of the crystal integrity (Fig. 5b). It is most likely that these “cracks” were introduced as a result of cutting the active element cut from Crystal 2 which was more stressed after growth as compared with Crystal 1. The optical microscopy studies of active elements cut from Crystals 1 and 2 which have the same composition but were grown at different rates (10 mm/h and 3 mm/h) showed that the optical defects of different nature dominate in them. In Crystals 1, the predominant optical defects are “growth bands”. In Crystals 2, “growth bands” are almost absent but there are strong thermoelastic stresses which cause significant lasing losses in these crystals. Complex studies of the optical and lasing properties of active ele ments cut from Crystals 1 and 2 revealed the following. The “growth bands” in the active elements cut from Crystals 1, the residual stresses after heat treatment and the local defects in the active elements cut from stressed Crystals 2 have approximately the same effect on the output energy characteristics of lasers on these active elements. However, it
Fig. 5. Orthoscopic images of the active element cut from Crystal 2 (without heat treatment) in crossed nicols in transmitted light (a) for the Y direction along the growth axis and (b) for the X direction perpendicular to the growth axis. The black arrow indicates local inhomogeneity.
active element cut from Crystal 1 without heat treatment were 1.8 W and 33%, respectively. The corresponding values for the laser with the active element cut from Crystal 2 were 1.6 W and 29%, respectively. Thus, it can be seen that the laser energy characteristics obtained in generation experiments prove to be almost the same for the active ele ments cut from Crystals 1 and 2. However, the active element cut from Crystal 1 after heat treatment is characterized by a lower lasing threshold and an increase in the differential efficiency. These parame ters are 1.4 W and 40%, respectively. It should be noted that the generational experiments were carried out for a series of active elements cut from Crystals 1 and 2. The results of
Fig. 6. Orthoscopic images of the active element cut from as-annealed Crystal 2 in crossed nicols in transmitted light (a) for the Y direction along the growth axis, (b) for the X direction perpendicular to the growth axis and (c) in reflected light for the X direction perpendicular to the growth axis. 4
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addition, high thermal inertia of large melt volumes during crystalliza tion prevents temperature fluctuations at the crystallization front caused by random fluctuations in the input power during direct high-frequency heating [1,2]. This in turn prevents discontinuities of the growth rate at the crystallization front and contributes to an increase in the optical homogeneity of single crystals. Before cutting active elements from single crystals it is advisable to subject them to high-temperature annealing in order to relieve residual thermal stresses. 4. Conclusions Thus, we studied the influence of growth process conditions (growth rate) and subsequent heat treatment on the optical inhomogeneity of (ZrO2)0.86(Y2O3)0.136(Ho2O3)0.004 crystals which significantly affects lasing characteristics. We found that local thermoelastic stresses have a greater influence on the lasing characteristics of the crystals than “growth bands”. Recommendations are provided concerning the choice of process conditions for obtaining high optical quality ZrO2–Y2O3 crystals doped with RE ions aiming to obtain efficient lasing in them.
Fig. 7. Output lasing power at the 5I7→5I8 transition of Ho3þ ions vs the absorbed pump power for the active elements cut from Crystal 1 without heat treatment (■) and after heat treatment (●), and in the active elements cut from Crystal 2 (after heat treatment) (▴).
Author contribution Sergey A. Artemov, generation experiments. Mikhail A. Borik, crystal growth. Tatyana V. Volkova, study of optical properties. Mikhail V. Gerasimov, study of optical properties. Alexey V. Kulebyakin, crystal growth. Elena E. Lomonova, crystal growth, writing the manuscript. Filipp O. Milovich, study of crystals by the energy dispersive analysis. Valentina A. Myzina, annealing of crystals. Polina A. Ryabochkina, statement of the problem, writing the manuscript. Nataliya Yu. Tabachkova, study of crystals by the transmission electron microscopy. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Russian Foundation for Basic Research, Grant No. 18-29-20039, and the Ministry of Education and Science of the Russian Federation (Project 1.4926.2017/6.7).
Fig. 8. Orthoscopic image of the active element cut from Crystal 2 (after heat treatment) obtained using an optical microscope with the polarization method in crossed nicols in transmitted light for the X direction perpendicular to the growth axis.
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