**4. Highly transparent ceramics for lasers**

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. It was previously noted that ceramic samples of the highest optical quality are usually obtained using hot isostatic pressing. This is a rather expensive and complex technology. Therefore, numerous studies are being conducted to find technological solutions to avoid this operation. Here we present the results of only one of the ways to solve this problem, which is related to the use of nanopowders with a small average particle size and high uniformity of their composition within each of the nanoparticles. Let's consider the characteristics of a number of ceramics for various purposes prepared using nanopowders synthesized in a laser plume.

## **4.1 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.

First the focused broadening of the laser transition band was implemented in ceramic yttrium-aluminum garnet [39], 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 [40].

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 [41] the doping with such ions did not allow achieving the transparency necessary for high-performance generation [35]. 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 [7] 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 [42] 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 10** 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

**Figure 10.**

*The initial radiation of a semiconductor laser (*λ *= 633 nm) incident on the screen (a) and the radiation transmitted through the ceramic samples [(YbxLuyY1−x−y)2O3]1−z(ZrO2)z from the "mixed" (b) and laser (c) powders [42].*

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 [42], 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 ion 4 F3/2 ↔ <sup>4</sup> I11/2 and of the Yb3+ ion 2 F5/2 ↔ 2F7/2 (**Figure 11**). 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 (**Figure 11**, 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 [42].

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+ ion 2 F5/2 ↔ <sup>2</sup> F7/2 (**Figure 11**, 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 [42, 43].

**Figure 11.** *IR spectra of luminescence of ceramic samples activated with Nd3+ ions (left) and Yb3+ ions (right) [42].*

### *From the Laser Plume to the Laser Ceramics DOI: http://dx.doi.org/10.5772/intechopen.94464*

In the ceramics with additions of zirconium and hafnium, the trivalent Hf3+ and Zr3+ ions were found [42–44], which is confirmed by electron paramagnetic resonance spectra [44]. In the crystal, 3d104d1 Zr3+ and 4f145d1 Hf3+ 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 [42, 43]. 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 of 4 F5/2 (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 level <sup>4</sup> F3/2 (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 ransitions 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 level <sup>2</sup> F5/2 (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 the <sup>2</sup> F5/2 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 [42] 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 slope 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 [45], it was found that the laser generation band on this ceramic (**Figure 12**) practically coincides with the IR-luminescence band (**Figure 11**, 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 slope 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.

### **4.2 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 [46], and monographs [8],

**Figure 12.** *Laser generation band of 0.95[(Yb0.05Lu0.15Y0.80)2O3] + 0.05ZrO2 ceramics [45].*

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 [8] 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 [7] and MgO [47] 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.

The successful 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. Measured by the BET method 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 Å, b = 3.494 Å, c = 8.611 Å, β = 101.2°. After calcination at a temperature of 1000 °C for 30 minutes, the surface area of the powder was 25 m<sup>2</sup> /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, was 109.67 m<sup>2</sup> /g. X-ray fluorescence analysis showed that the powder consists mainly of the γ-Al2O3 phase 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

### *From the Laser Plume to the Laser Ceramics DOI: http://dx.doi.org/10.5772/intechopen.94464*

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 slope efficiency of 19% [48]. 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. **Figure 13** 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 [50] 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 (**Figure 14a**). 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.

### **Figure 13.**

*Transmission spectra of Nd:YAG single crystal (1) and ceramics (2). The inset shows a photograph of ceramics [49].*

### **Figure 14.**

*Flow-chart of the experimental setting (a) and the dependence of the generation power on the pump power (b) [50].*


### **Table 1.**

*Laser characteristics of Nd:YAG ceramics [50].*

The dependence of the output power on the pump power is shown in **Figure 14b**. 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 slope 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.

### **5. Conclusion**

The main stages and processes taking place in the preparation of high-transparent ceramics, including laser ones. The optimal conditions at which the productivity of nanopowder production is realized, depending on the thermophysical properties of the material, were found to be 10–80 g/h. It is shown that the nanoparticles obtained are weakly agglomerated, have a spherical shape and an average size of ~10 nm. A feature of such nanoparticles is the high homogeneity of the composition at a high level of doping. It is shown that the density of compacts does not depend on the method of dry pressing and is determined by pressure, although the level of residual mechanical stresses differs. Pressing was carried out at diapason of pressures of 250–300 MPa, at which compact densities were ~50%.

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 using a relatively simple technology. In particular, this approach allowed to obtain the following.

1.In samples based on Y2O3 doped with Yb2O3 and ZrO2, the slope efficiency of radiation generation can exceed 50%, and the band for smooth tuning of the radiation frequency can reach 100 nm.

2.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.
