Highly transparent ceramics on the basis of nanopowders synthesized in laser plume. Part II. Laser ceramics
Highly transparent ceramics are more commonly used as active elements of solid-state lasers intended for various purposes, optical armor, scintillation sensors, heat and mechanically resistant windows, bulbs for high-power high-pressure lamps, wide-angle lenses, etc. In this case, the greatest difficulties occur in the laser ceramics synthesis, which require the achievement of density and transparency, close to the theoretical values. To implement these requirements, synthetic methods based on hot isostatic pressing , spark plasma consolidation , and vacuum sintering with heterovalent ions doping have been developed . The latter option is more attractive due to less exapensive and simple technology. However, this approach, with a significant concentration of additives (>1 mol%), has a significant drawback due to the release of heterovalent ions during sintering into the regions adjacent to the area’s intercrystalline boundaries. This causes the appearance of an "orange peel" , which reduces the transparency and distorts the radiation wave front upon generation . However, the possibilities of this approach in the oxide ceramics synthesis can be extended using nanopowders obtained by laser ablation, where the nanoparticles are synthesized at high temperature and rapid cooling. This ensures a high homogeneity of nanoparticles and ceramics based on them .
CERAMICS WITH DISORDERED CRYSTALLINE STRUCTURE
Such ceramics are formed by replacing matrix cations with impurity cations. This leads to a change in the local crystalline fields in the positions of the activator ions and, therefore, to broadening of the spectral lines and the gain band. This is important for reducing the duration of laser pulses in the mode-locked mode and for increasing the range of smooth tuning of the laser radiation frequency.
The focused broadening of the laser transition band was implemented in ceramic yttrium-aluminum garnet , when a part of aluminum ions was replaced by scandium ions, i. e. ion of the same valence. In this ceramic, activated by Nd3+, a laser pulse with a duration of 10 ps was obtained on its optical transitions in the 1 µm region, and when the neodymium was changed to ytterbium, it was reduced to 96 fs .
At the same time, it was found  that the greatest broadening of the gain band is achieved in the ceramics based on yttrium oxide with the introduction of heterovalent ions. However, in [4,5] the doping with such ions did not allow achieving the transparency necessary for high-performance generation . According to the authors, this was prevented by the formation of an "orange peel" due to the increased concentration of dopants near the intercrystalline boundaries. Since this class ceramic is important for the development of laser technology, we investigated its creation using two approaches.
In the first case, the traditional approach  was implemented, i. e. the ceramics were synthesized from nanopowders of simple oxides Yb2O3, Nd2O3, Y2O3, HfO2 and ZrO2, mixed in the required ratio. We refer to them as to "mixed" powders. The second approach is original  and consists in the fact that the necessary components were mixed in the preparation of a laser target, and the synthesis of nanoparticles occurred in a laser plume, i. e. at high temperature and rapid (<1 ms) cooling. Let’s refer to these powders as to "laser" ones.
Using these approaches, the samples of ceramics based on yttrium oxide with HfO2 or ZrO2 additives were prepared. The samples were 2–3 mm thick and 11 mm in diameter. Analysis of the appearance of the ceramics’ samples based on yttrium oxide, obtained by different approaches, shows that they differ insignificantly. The differences are manifested in the study of their light scattering. Figure 1 shows photographs of the initial radiation of the laser (λ = 633 nm) incident on the screen and of the radiation passing through samples of "mixed" and "laser" powders having the same chemical composition [(YbxLuyY1-x-y)2O3]1-z(ZrO2) z. It can be seen that the ceramic made of "mixed" powders possesses a large light scattering and transparency by 15–20% lower than that made of laser powders , therefore it is not yet suitable for obtaining high-performance generation. In this connection, the ceramics prepared from "laser" powders were investigated further.
Their disordered crystalline structure manifests itself in the broadening of the emission bands at laser transitions between the Stark levels of the neodymium ion4F3 / 2↔4I11 / 2 and of the Yb3+ ion2F5 / 2↔2F7 / 2 (Fig. 2). Moreover, it was found that the additives lead to a complete overlap (at a level less than 0.4 of the maximum intensity) of the contours of the two neodymium emission bands at λ = 1060 nm and 1075 nm (Fig. 2, left). This leads to the formation of a continuous emission band with a width of up to 50 nm (on the base) in the range of 1040–1090 nm. [9–11].
In the optical ceramics activated with ytterbium, the above additives also lead to broadening of the luminescence bands at λ = 1030 and 1075 nm on a laser transition between the Stark levels of the Yb3+ ion2F5 / 2↔2F7 / 2 (Fig.2, right). A complete overlap of the bands is observed at a minimum level of 0.25 of the maximum intensity with the width of the continuous band at this level reaching 100 nm on the base [11,12].
In the ceramics with additions of zirconium and hafnium, the trivalent Hf3+ and Zr3+ ions were found [11–14], which is confirmed by electron paramagnetic resonance spectra [13, 14]. In the crystal, 3d104d1Zr3+ and 4f145d1Zr3+ ions form two Stark levels: the orbital doublet (E) and the triplet (T2), with the energy gap equal to the strength of the crystal field in the positions of these ions. In yttrium oxide, these ions replace yttrium ions in two positions С2 and C3i, differing in symmetry and the strength of the crystalline field. Therefore, in the pulsed cathodoluminescence spectra of the ceramics containing zirconium or hafnium, both ions (Hf3+ and Zr3+) emit two bands each, at λ = 818 nm and 900 nm about 30 nm wide [11, 12]. Furthermore, the energy of the radiative level of the short-wave band (12225 cm‑1) of the Hf3+ and Zr3+ ions coincides with the energy of the pumping level of4F5 / 2 (12138–12436 cm‑1) of the neodymium ion, and the energy of the radiative level of the second longer wavelength band (11100 cm‑1) – with that of the upper laser level4F3 / 2 (11208–11404 cm‑1) of the Nd3+ ion. It is because of the negative influence of the Hf3+ and Zr3+ ions on the inverse population of the laser levels caused by this coincidence, that we have not obtained laser generation on the neodymium ion transitions in the ceramics with disordered crystalline structure with additions of hafnium or zirconium.
Another situation is observed for the activator Yb3+ ion. The energy of its upper laser level2F5 / 2 (10240–10673 cm‑1) is less than the energy of the radiative levels of Hf3+ and Zr3+ ions. Therefore, the Hf3+ and Zr3+ ions do not affect the population of the2F5 / 2 level of the Yb3+ ion, which allowed generation of laser radiation in disordered ceramic consisting of 0.88 [(Yb0.01Lu0.24Y0.75)2O3]+0.12ZrO2  obtained from "laser" nanopowders of a solid solution. The generation properties were investigated in a three-mirror V-shaped resonator formed by two spherical mirrors with radii of curvature of 100 mm and an output plane mirror with a transmittance of 1.2, 2.4 and 5.0%. The active element in the form of a polished ceramic disk 1.27 mm thick was installed in the resonator between spherical mirrors at the Brewster angle. Pumping was carried out through a dichroic spherical mirror with a reflection coefficient of 99.9% in the range of 1020–1100 nm and a transmittance factor of 98% in the range of 950–980 nm by a laser diode radiation with a fiber output of 9 W at a wavelength of 975 nm and a bandwidth of 3 nm. With an output mirror with a transmittance factor of 1.2, 2.4 and 5.0%, the differential efficiency was 16.5, 26.0 and 29.0% with an optical efficiency of 6.8, 7.0 and 9.5%, respectively.
Relatively low values of the laser generation parameters obtained are due to the presence of an "orange peel" in the ceramics with a high content (12 mol%) of zirconium. In the ceramic consisting of 0.95[(Yb0.05Lu0.15Y0.80)2O3]+0.05ZrO2 with a content of the sintering additive ZrO2 reduced to 5 mol%, the "orange peel" is not clearly manifested. While investigating the generation properties , it was found that the laser generation band on this ceramic (Fig.3) practically coincides with the IR-luminescence band (Fig. 2, right), its width reaches 97 nm at the base, which is currently a record value in the visible and near-IR wavelengths.
On this entire band, quasi-continuous generation with a differential efficiency equal to 49.3% and 51.2% in the band maxima at the wavelengths of 1077 and 1032 nm, respectively, was obtained. These factors provide good prospects for the development of lasers with ultrashort pulses and lasers with a wide range of smooth frequency tuning.
CERAMICS OF YTTRIUM-ALUMINUM GARNET
Taking into account the importance for the creation of technological lasers and high-scale laser systems, the great attention has been paid to YAG ceramics, doped with Nd or Yb. Extensive studies have been carried out, the results of which have been presented in a number of reviews, for example, [16, 17], and monographs , the methods for obtaining nanopowders, compaction and sintering have been developed that make it possible to synthesize samples with a transparency close to the theoretical one  and to generate a radiation with an efficiency of more than 74%. .
High-level results were obtained using both hot isostatic pressing (HIP) and vacuum sintering, but the presence of sintering additives in a mixture of nanopowders as TEOS  and MgO  was always mandatory. Using the nanopowders prepared by the laser synthesis method, we have studied the feasibility of synthesizing YAG ceramics without the use of these additives. Various approaches to the preparation of nanopowders were involved.
In the first case, Nd: YAG nanopowders were prepared directly in a laser plume. For this purpose, the laser target was pressed and sintered from Nd2O3, Y2O3 and Al2O3 micropowders in the desired ratio. To implement YAG ceramics, the required ratio was Y2O3 / Al2O3=3 / 5. However, all components have different melting points, and hence the evaporation rate. Therefore, the ratio of the components in the target was chosen experimentally. The best results were obtained when the Y2O3 content in the target exceeded the value required by stoichiometry by a factor of 1.5. In this case, the density of ceramics without the use of sintering additives was ≥99.8%, and the transparency at a wavelength of 1060 nm reached 77%. Further attempts to improve these results by selecting the target components were unsuccessful, which is apparently related to the stochastic nature of laser nanoparticle synthesis.
The following attempt to produce highly transparent YAG ceramics without the use of sintering additives was associated with the mixing of separately obtained Nd: Y2O3 and Al2O3 nanopowders in the ratio of 3 / 5. The specific surface area of the Nd: Y2O3 powder was 50.7 m2 / g. It was a solid solution based on monoclinic yttrium oxide with crystalline lattice parameters a=13.92 Е, b=3.494 Е, c=8.611 Е, β=101.2°. After calcination at a temperature of 1000 °C for 30 minutes, the surface area of the powder was 25 m2 / g for conversion to the cubic phase, i. e. the particle size increased from 12 to 49 nm. Al2O3 nanopowder was also obtained by laser evaporation of a target followed by condensation of vapors in the air stream. Its specific surface, measured by the BET method, was 109.67 m2 / g. X-ray fluorescence analysis showed that the powder consists mainly of the γ- Al2O3phase and the δ-phase content was less than 10%.
These powders were mixed in the indicated proportion in a drum mixer with an inclined rotation axis for 24 hours. Further, briquettes with a density of 20% compared to the theoretical were compacted from this mixture, which were then calcined at 1200 °C for 3 hours. As shown by X-ray fluorescence analysis, the YAG phase content in the briquettes was 96–98%. These briquettes were then milled by YSZ balls in a planetary mill for 48 hours.
The analysis of powder images after grinding showed that the agglomerates of the particles formed after calcination had an average size slightly less than 1 µm, but sometimes their size was close to 10 µm. The compacting of nanopowders into disks with a diameter of 15 mm and a thickness of 1.5–4.5 mm was carried out by the method of dry uniaxial static pressing without the use of any additives. The compacting pressure in these experiments was unchanged and was 200 MPa, which made it possible to obtain a density of 61.8%. Sintering was performed at a temperature of 1760 °C for 20 hours. The pore content in the samples was ~60 ppm, and the transparency was 83.28%. For the first time in the Nd: YAG ceramics that did not contain sintering additives, the generation was obtained with an average power of up to 4 W and a differential efficiency of 19% .
However, much better results were achieved when 0.5 wt% TEOS sintering additive was added to the nanopowder. In this case, the slightly agglomerated Nd: Y2O3 and Al2O3 nanoparticles of spherical shape with dimensions of 8–14 nm were calcined at a temperature of 900–1200 °C for transformation from the monoclinic to the cubic phase. These calcined nanopowders were weighed to ensure the Nd0.03Y2.97Al5O12 stoichiometry and mixed in a ball mill with an inclined axis of rotation in alcohol with the addition of 0.5 wt% TEOS for 48 hours.
Using the previously described approach, Nd(Yb):YAG ceramic samples were synthesized. Fig. 4 shows a photograph of a Nd: YAG ceramic sample, its transmission spectrum, and also the transmission spectrum of a single-crystal laser of the same composition, which has theoretical transparency. It can be seen that in the wavelength range of more than 450 nm, these spectra practically coincide. Compared with the above results, the optical quality of the resulting ceramic due to the presence of SiO2 was improved due to a partial reduction in agglomeration of the powder during the calcination step, inhibition of crystallite growth and pore removal due to the formation of the liquid phase, which led to reducing their content to 17 ppm. Similar results were obtained by compacting the calcined Nd:Y2O3 and Al2O3 nanopowders into compacts with a relative density of 48% and reactive sintering at 1780 °C for 20 hours.
The comparative studies of our samples and samples by Konoshima Chemical  were carried out jointly with the National Institute of Optics (Florence, Italy). They had the same composition (1 at.% Nd:YAG) and a thickness of 1.5 mm. To obtain the generation, a V-shaped resonator was used (Fig. 5a). Pumping was carried out through an end dichroic mirror having high transparency for pumping radiation and high reflection for the generated radiation and spaced from the sample by 4 mm. The distance from the end EM and the output mirror OC to the rotary mirror FM was 280 mm. The OC transmission varied between 2–20%. Pumping was carried out by rectangular pulses of a duration of 10 ms and a frequency of 12.5 Hz. Their peak power was 32 W, the radiation focusing spot was 0.8 mm.
The dependence of the output power on the pump power is shown in Fig. 5b. Similar results were obtained for the samples of Konoshima Chemical. Comparative data are given in Table 1. The best results were obtained with a transparency of the output mirror Toc = 20%, when the radiation power was Pout = 4.91 W, and the differential efficiency ηsl = 52.7%.
Thus, the introduction of a sintering additive in the form of TEOS had a significant effect on improving the characteristics of samples prepared from nanoparticles synthesized in a laser plume.
COMPOSITE (CLADDING) CERAMICS
They are necessary for creating lasers where the surface dimensions of the active element are much larger than the thickness (thin-film lasers, high-scale laser systems). In this case, the probability of "parasitic" generation along the largest path is great, which can significantly reduce the efficiency or even suppress generation. This can be avoided by connecting the active medium with the absorbing material without reflectance. It was shown in  that for a Nd: YAG active medium this material can be Cr: YAG, provided that a significant part of the chromium ions is tetravalent.
The synthesis of ceramics  was carried out according to the previously described technology. The difference is that the central part of Nd: YAG was compacted in advance at a pressure of 15–20 MPa in the form of a circle or a square. Then a mixture of (Y+Ca) / (Al+Cr) / 3 / 5 nanopowders was filled along the contour of the previously formed Nd: YAG preform, and then the entire system was compacted with uniaxial static pressure of 200 MPa. The Ca2+ ion was used as the charge compensator.
The absorption spectrum of the Cr: YAG ceramic with the Cr / Ca molar ratio (Table 2) after annealing and polishing is shown in Fig. 6. The strong absorption in the visible region is mainly due to the4А2→4T2,4T1,2T2 transitions of trivalent Cr. The4A2→2E2 transition of the Cr3+ ion has a narrow peak at 684 nm. At 800 nm, an absorption band takes place at the3B1 (3A2)→3E (3T2) transition of theCr4+ ion. The absorption cross-sections for this optical transition equal to σ=5.7·10–18 cm2 , σ=4.0·10–18 cm2 at λ = 946 nm  and σ=3.9·10–18 cm2 at λ = 914 nm  were used to estimate the concentration of Cr4+. Using the data in Fig. 6, the absorption coefficient αi(λi) was determined for the above wavelengths (i = 1, 2, 3) by the following equation,
where T(λi) is the transparency at wavelength λi, l is the thickness of the sample, R is the coefficient reflection (R = 0.085 for the sample surface and n = 1.82 for λ = 1064 nm ). The concentration of Cr4+ was determined as Ni= αi / σi. Table 2 shows the calculated values of αi and Ni for three wavelengths. Their differences, apparently, are due to the accuracy of measuring the absorption cross sections, since the dispersion of the refractive index in this wavelength range is less than 10–4. It can be seen that with an increase in the initial concentration of chromium NCr, the concentration of NCr4+ increases, but the ratio NCr4+ / NCr decreases.
Fig. 7 shows an image of composite ceramics made with a central part in the form of a circle and a square. The composition of the central part is Nd: YAG, and the cladding is 1.9 at.% Cr, 0.15 at.% Ca: YAG. To prevent the development of "parasitic" generation, the width of the cladding was determined from the equation
αi·h > g·L, (2)
from where it follows that for a cladding with the smallest α, the cladding width h should be greater than 2 mm with gain factor g = 0.45 cm‑1 and length l = 11 mm.
DIFFUSION WELDING OF LASER CERAMICS
It is difficult to create nonporous, highly transparent ceramics with a thickness of more than 5 mm at a reasonable sintering time. Therefore, in order to synthesize thicker samples used in high-power lasers, various methods of diffusion welding have been developed. The experiments in this direction are relatively few . The essence of them is reduced to the need to remove the boundaries between the welded blanks in the process of further sintering due to the recrystallization of grains. This approach differs favorably from planting into optical contact, when the boundaries between the samples in the form of pores still remain, no matter how thoroughly the contact samples are polished . For these purposes, we have developed a fairly simple method of two-stage welding of laser ceramics . At the first stage, two samples (Fig. 8) with a flatness of λ / 20 and a roughness of 50 nm were subjected to hot pressing at a temperature of 1440 °C for 2 hours at a pressure of 30 MPa. In the second stage, they were calcinated at a temperature of 1780 °C for 20 hours. Two samples of Nd: YAG ceramics with a diameter of 11 mm, a thickness of ~1.8 mm and a transparency of 82.3% and 82.0% were used in the experiments, which corresponded to α1 = 0.1080 cm‑1 and α2 = 0.1296 cm‑1.
Fig. 8 shows images of Nd: YAG ceramics before and after two-stage diffusion welding (Fig. 8a, b) and the microstructure of the interface after the first and second stages (Fig. 8c, d). It can be seen that after the welding, the borders have disappeared. The average size of the crystallites due to recrystallization increased from 48 µm to 52 µm. The transparency decreased to 81.4% due to the increase in the thickness of the welded sample. Furthermore, the porosity decreased from 31.5 ppm to 27 ppm and the absorption coefficient to 0.0985 cm‑1 due to the increase in the sintering duration.
Those ceramics are important for creating devices based on the Faraday effect: optical switches, mirrors, insulators, laser gyroscopes. Magneto-optical glasses or single crystals are manufactured using Ce, Pr, Dy or Tb oxides, with terbium oxide being the best of them . At present, terbium-gallium garnet with the Verdet coefficient characterizing the angle of rotation of the polarization plane of radiation, up to 40 rad · m‑1 · T‑1, has the greatest application for these purposes. Recently , it was possible to grow a Tb2O3 single crystal and to measure the characteristics of a sample with dimensions of 5x5x1 mm3. The transparency of this single crystal was 77%, and the Verdet coefficient 134 rad ± rad · m‑1 · T‑1. In the last decade, works on the creation of magneto-optical ceramics are being carried out actively. Here, the difficulties of creating Tb2O3 ceramics are due to the presence of 4 phases within the reach of temperatures, each phase containing different amounts of oxygen. Thus, there are 16 modifications of terbium oxide, and only one needs to be realized.
These difficulties were overcome by "imposing" the cubic phase by adding Y2O3 and using hot pressing , as well as adding ZrO2 and hot isostatic pressing . In the latter case, the best results were achieved, in particular, for (Tb0.6Y0.4)2O3 and Tb2O3 with ZrO2 additive, the transparency was 81.10% and 81.35%, respectively, and in the latter case the highest Verdet coefficient 154 rad · m‑1 · T‑1. It was shown that the Verdet coefficient is proportional to the content of Tb in the composition of ceramics.
We have investigated the possibility of obtaining highly transparent ceramics based on Tb2O3 without the use of hot pressing and hot isostatic pressing, using nanopowders synthesized in a laser plume, and the previously described technology for the production of ceramics. Fig. 9 shows the transmission spectra of three samples of ceramics based on Tb2O3 with additions of ZrO2 and Y2O3, as in . The thickness of the samples was 1 mm, and the diameter was 11 mm. Their pore content was 0.12, 0.28 and 0.45 ppm, which caused the transparency of 82.5, 81.8 and 81.5%, respectively. Measurements of the Verdet coefficient showed that its value was 113.4, 119.8 and 120.8 rad · m‑1 · T‑1 in the order of increasing transparency, which is three times higher than this parameter in the Faraday isolator based on commercial terbium-gallium garnet.
Thus, the use of nanopowders synthesized in a laser plume for the preparation of highly transparent ceramics makes it possible to increase the threshold for the formation of an "orange peel". This opens the road to the use of sesquioxides with highly disordered crystalline structure as active elements of solid-state lasers and magnetoactive elements. In particular, this approach allowed to obtain the following:
1. In samples based on Y2O3 doped with Yb2O3 and ZrO2, the differential efficiency of radiation generation can exceed 50%, and the band for smooth tuning of the radiation frequency can reach 100 nm;
2. Magnetoactive ceramics based on Tb2O3 doped with Y2O3 and ZrO2 were synthesized, with a pore content of ~0.1 ppm and the greatest transparency at the time of 82.5% and Verdet coefficient of 120.8 rad · m‑1 · T‑1, which is three times more than the analogous parameter of commercial samples of terbium-gallium garnet without the use of hot isostatic pressing;
3. Highly transparent YAG samples are prepared without the use of sintering additives, where the transparency and generation efficiency, however, is inferior to those realized when doping TEOS;
4. A method for preparation of composite (cladding) samples and diffusion welding without loss of transparency of the original discs has been developed.
The research was carried out within the framework of the theme of state task No. 0389–2016–002 (2018–2020) and with the support of the project by UB of RAS No. 18–10–2–38.
 Продолжение. Начало см. ФОТОНИКА, 2017, 7 (69), с. 52–70. В. В. Осипов, В. В. Платонов, В. А. Шитов, Р. Н. Максимов. Высокопрозрачные керамики, приготовленные на основе нанопорошков, синтезированных в лазерном факеле. Часть I: особенности получения. – DOI: 10.22184 / 1993–7296.2017.67.7.36.45.