High-melting-point rare earth oxides, due to their advantages of high thermal conductivity, high mechanical strength, low phonon energy, and high cation site density, exhibit characteristics such as large segregation coefficients, strong electron-phonon coupling, and multiple lattice sites. These properties have made them a hot topic of research in high-power, ultrafast, and infrared laser crystals. However, their high-melting-points, particularly for sesquioxides (~2 450 ℃), pose significant challenges for crystal growth. Though being reported decades ago, the development of sesquioxide crystals remains in its early stage. In recent years, breakthroughs in various crystal growth techniques have been achieved, leading to a diversification of rare earth ion-doped laser research. This paper reviews the advantages of high-melting-point rare earth oxides, recent advancements in crystal growth, and the progress in laser performance across the infrared 1, 2 and 3 m wavelength bands.

The development direction of advanced solid-state lasers is higher output power and better beam quality. Rare earth ion doped yttrium aluminum garnet transparent ceramics are preferred materials for solid-state laser gain media, but thermal effects during the pumping process deteriorate the beam quality and hinder further increase in output power. The heat generation and dissipation of laser ceramics with complex configurations are more uniform during the pumping process, which can significantly reduce thermal effects. Compared with traditional preparation methods, 3D printing is able to form more complex design structures, suppress thermal effects, and achieve integration and miniaturisation of multi-module devices, driving lasers towards a wider range of applications. In this paper, several traditional methods for the preparation of laser ceramics with complex configurations are introduced firstly, and the advantages and limitations are analyzed. Then, the research status and problems in 3D printing yttrium aluminum garnet-based laser ceramics are systematically reviewed. Common 3D printing methods include direct ink writing, stereolithography, digital light processing, material jet printing, two-photon polymerization and micro-continuous liquid interface printing. Among them, there are interlayer defects in the samples printed by stereolithography and digital light processing, resulting in severe scattering losses. Material jetting printing combined with dry pressing can be used to manufacture planar waveguides. Two-photon polymerization and micro-continuous liquid interface printing are suitable for the fabrication of highly complex structures at micrometer scales. Direct ink writing not only enables the fabrication of laser ceramics for the first time, but also has the most extensive relevant research and is still the most promising method for the preparation of laser ceramics with complex configuration. At the end of this paper, the current problems and research prospects of yttrium aluminum garnet-based laser ceramics prepared by 3D printing are summarized.

Er3+-doped CaYAlO4 crystals with the doping concentration (atomic fraction) of 15% and 30% were successfully grown by Czochralski method. The polarized spectral and laser properties of Er∶CaYAlO4 crystals were studied. The polarized absorption spectra, polarized emission spectra and fluorescence decay curves were measured at room temperature. The disorder of CaYAlO4 crystal results in broadband characteristics of absorption and emission spectra. The emission cross section of 15% Er∶CaYAlO4 is determined to be 1.22×10-20 cm2 at 2 712 nm for polarization and 0.71×10-20 cm2 at 2 754 nm for polarization, with a full width at half maximum (FWHM) of 152 and 167 nm, respectively. As Er3+ concentration increases, fluorescence lifetime of 4I11/2 and 4I13/2 energy levels decreases in different degrees, and lifetime ratio (lower/upper) greatly reduces. The energy transfer upconversion ETU1 not only suppresses self-termination effect but also improves quantum efficiency of 2.7 m radiative transition. Under 974 nm LD excitation, laser property for output couple mirror with a transmittance of 1.5% is best. The maximum output power is 358 mW at 2 720 nm with a slope efficiency of 15.4% in c-cut 15% Er∶CaYAlO4 crystal.

Nd3+-doped Sr0.7La0.3Mg0.3Al11.7O19 (Nd∶ASL) single crystal fibers with different Nd3+ concentration were successfully grown by the micro-pulling-down (-PD) method. The crystal structure, polarized spectroscopic properties and continuous wave laser performance were studied. The Judd-Ofelt parameters 2, 4 and 6 are obtained to be 1.093×10-20, 2.285×10-20 and 2.117×10-20 cm2, respectively. The fluorescence lifetime of 0.5%, 1.0%, 3.0% and 5.0% Nd∶ASL single crystal fibers are 375, 398, 384 and 363 s, respectively. A continuous wave laser is obtained by pumping with a commercial laser diode of 800 nm emission wavelength. The maximum output power is 1.08 W at 1 055 nm with a slope efficiency of 18.5%. All the results indicate that Nd3+-doped ASL single crystal fiber grown by the -PD method has potential as a laser gain medium for laser system.

Rare-earth ions doped sesquioxide ceramics exhibit notable advantages, including high thermal conductivity, low phonon energy, and high laser damage threshold, thereby demonstrating significant potential for solid-state laser applications. Powders with high purity and low agglomeration were synthesized by chemical co-precipitation method, then 5% (atomic fraction) Yb∶Lu2O3 transparent ceramics with high density and high transparency were obtained through oxygen pre-sintering and hot isostatic pressing (HIP) sintering. The samples have an average grain size of less than 1 m, and exhibit an in-line transmittance of above 81% at 1 100 nm. The absorption cross-section at 974 nm is ~0.97×10-20 cm2, and the emission cross-section at 1.08 m is ~0.39×10-20 cm2. Using a 976 nm laser diode as the pump source, a continuous-wave (CW) laser output at 1 080.1 nm is achieved from the fabricated Yb∶Lu2O3 ceramics at room temperature, with a output power of 11.67 W and slope efficiency of 55.3%.

Er∶Lu2O3 transparent ceramics are one of the candidate solid-state gain media for high efficiency and high power laser in the near and mid-infrared bands due to the advantages of high thermal conductivity, high thermal shock resistance, good spectral properties and low preparation temperature. Exploring the influence of Er3+ doping concentration (atomic number fraction) on the preparation and properties of Er∶Lu2O3 transparent ceramics for the related laser application is crucial for achieving high-quality Er∶Lu2O3 ceramics. In this paper, Er∶Lu2O3 transparent ceramics were fabricated using vacuum pre-sintering combined with hot isostatic pressing (HIP) post-treatment from the nano-powders synthesized by the coprecipitation method. The results show that the phases of all Er∶Lu2O3 nano-powders calcined at 1 100 ℃ with different Er3+ concentrations are almost consistent with that of the pure cubic Lu2O3 and the 10% Er∶Lu2O3 nano-powder shows better dispersibility. The average grain sizes of 0.3% and 10% Er∶Lu2O3 ceramics pre-sintered at 1 500 ℃ are 0.86 and 0.87 m with the relative densities of 95.1% and 96.2%, respectively. The corresponding pre-sintered ceramics were HIP-ed at 1 750 ℃ show the in-line transmittance of 71.6% and 78.0% (thickness 2.9 mm) at 2 400 nm, respectively. Especially, the in-line transmittance of the 0.3% Er∶Lu2O3 ceramics sharply decreases in the short wave range due to the numerous remained residual pores. The absorption and emission spectra of 0.3% and 10% Er∶Lu2O3 transparent ceramics were calculated. 0.3% Er∶Lu2O3 ceramics possesses higher absorption and emission cross-sections around the band centered at 1.55 m.

Using high-purity commercial oxide powders as raw materials, the layered composite ceramic green bodies that meet the design requirements were obtained by dry pressing combined with cold isostatic pressing. Two-layered and three-layered composite YAG/10% Yb∶YAG (atomic number fraction) transparent ceramics with high optical quality were successfully fabricated by vacuum pre-sintering at 1 750 ℃ for 30 h combined with hot isostatic pressing at 1 750 ℃ for 3 h under 200 MPa Ar. The optical transparency, microstructure and elemental distribution of the composite ceramics were analyzed. The results show that, the in-line transmittance values of two-layered and three-layered composite YAG/Yb∶YAG transparent ceramics with the thickness of 4 mm are 83.6% and 84.1% at 1 100 nm, respectively. Both YAG and Yb∶YAG ceramic regions have dense microstructure, and the average grain sizes are 29.0 and 34.5 m, respectively. The element analysis of surface scanning images and linear scan images for the central section reveal that the boundary between YAG and Yb∶YAG ceramic region is straight, with Yb element mainly existing in the Yb∶YAG ceramic region, indicating that the actual structure of the layer-structured transparent ceramics is consistent with the initial design.

Cr4+∶YAG crystal materials have been proved to be an ideal passive Q-switched material in recent years. Using MgO and CaO as sintering aid and charge compensation agent, 0.25% Cr4+∶YAG (molar concentration) transparent ceramics with high optical quality were successfully prepared by low-temperature vacuum sintering combined with hot isostatic pressing (HIP) sintering. Effect of MgO, CaO, and MgO-CaO co-added divalent oxides on the transmittance, absorption spectra, and fluorescent properties of Cr4+∶YAG ceramics were investigated. It is found that, compared with CaO and MgO-CaO series, MgO exhibits the most significant effect on the densification of Cr4+∶YAG ceramics when the vacuum-sintering temperature is 1 550 ℃. Especially when the molar concentration of MgO is 0.25%, the relative density reaches 98%. After HIP sintering at 1 680 ℃, the corresponding in-line transmittance of the densified Cr4+∶YAG ceramics reaches as high as 84.4% at the wavelength of 1 400 nm. Moreover, by annealing at 1 400 ℃ for 20 h, the absorption coefficient of Cr4+ at 1 030 nm reaches the highest value of 3.0 cm-1, though some Cr3+ still remain in the Cr4+∶YAG ceramics.

As a typical garnet transparent ceramic used in high-energy solid-state laser systems, terbium aluminum garnet (Tb3Al5O12, TAG) magneto-optic ceramics have become a focal point of research due to their excellent magneto-optic properties and high-power performance. This paper focuses on TAG magneto-optic ceramics prepared by vacuum solid-state reaction, with particular emphasis on the optimization of sintering aids and lattice doping modifications. By comparison, the most suitable additives for TAG ceramics were identified. The study highlights the longstanding challenge in improving the quality of TAG ceramics, which arises from the reliance on large doses of sintering aids and the resulting compositional inhomogeneity caused by sintering aid residues. To address these issues, we present our latest research, which involves co-doping the lattice with Mg2+ and Si4+. This approach enables the preparation of higher optical quality TAG ceramics via a one-step vacuum solid-state sintering process, without reducing the absolute amount of MgO and TEOS. The best sample achieve a transmittance of 82.48% at 1 064 nm, which, to our knowledge, is the highest value reported using a similar sintering method based on domestic powders. This lattice doping strategy, rather than the traditional mass-ratio addition of sintering aids, offers a promising new approach for further improving the quality of garnet transparent ceramics used in high-energy laser systems.

Nd∶TSAG magneto-optical crystal with the size of 50 mm×100 mm were grown by Czochralski method for the first time. The structure, spectra, magneto-optical, and thermal properties of the crystal were studied in detail. Spectral parameters, such as oscillator strength, line strength, intensity parameter t, transition probability between 4F3/2→4IJ energy levels, radiation lifetime, and fluorescence branching ratio were calculated. The doping concentration of Nd∶TSAG polycrystalline samples was optimized, and the best doping concentration of Nd3+(atomic fraction) is 1%. The results of X-ray diffraction analysis of crystal and powder show that the crystal has a pure phase and good crystal quality. The separation coefficient of Nd3+ in TSAG crystal is 0.64. In the visible and near infrared bands, the transmittance of Nd∶TSAG crystals is over 80%, except for the characteristic absorption of Tb3+ and Nd3+. With 808 nm excitation, the strongest fluorescence peak of Nd∶TSAG is around 1 070 nm. The refractive indices of Nd∶TSAG crystals at 473, 532, 632.8, and 1 064 nm are 1.925 9, 1.915 3, 1.903 8, and 1.885 2, respectively. The four-parameter Sellmeier refractive index equation is fitted. The Verdet constants of Nd∶TSAG measured by extinction method at 405, 532, 635, and 1 064 nm are 599.8, 202.8, 152.3, and 46.8 rad·m-1·T-1, respectively, which are higher than that of TGG crystals. At room temperature, the thermal conductivity of Nd∶TSAG crystals is 3.95 W/(m·K). The results show that Nd∶TSAG is an excellent magneto-optical crystal and a potential excellent laser crystal. Meanwhile, it is promising to realize the synergistic effect of magneto-optic and laser.

In this paper, the effects of over-doping of Nd3+, Ho3+, Dy3+ and Ce3+ rare earth ions on the preparation and related properties of TSAG ceramics were investigated, and the results show that over-doping of rare earth ions has a certain effect on the micro-defects and transmittance of TSAG ceramics. The existence of a small number of fine particles which are not in the secondary phase on the grain surface, led to the decrease of the transmittance of the ceramic samples. The Verdet constant of rare earth ion over-doped TSAG ceramics is better than that of pure phase TSAG ceramics at the same test wavelength. In addition, YAG/TSAG-based bilayer composite structural ceramics were explored. The YAG/TSAG and YAG/2.0% Ce∶TSAG bilayer composite structural ceramics were successfully prepared, with sub-micron thickness transition layer formed on the contact surface, and the transmittance of the YAG/TSAG ceramics at 1 064 nm reaches 71.0%. The successful preparation of YAG/TSAG-based composite ceramics broadens the application prospects of TSAG materials in high-power lasers.

Lanthanide-doped lead zirconate titanate (PLZT) transparent ceramics have significant potential for application in modern optical communication and high energy laser technology due to their exceptional optical transparency and electro-optical effects. However, the electro-optical (E-O) properties of PLZT transparent ceramics are highly sensitive to changes of ambient temperature, which presents a significant challenge in their application in E-O modulators across a wide temperature range. The objective of this study was to investigate the effects of temperature on the ferroelectric, dielectric and E-O properties of PLZT (10/65/35) transparent ceramics, and optimize the material E-O properties and their temperature stability by regulating the distribution of the key doping element La3+ in micro regions. A series of PLZT ceramic samples with pure perovskite structures were obtained by a conventional hot-press sintering process in oxygen atmosphere. The resulting microstructures are uniform and dense, and the optical transmittances of the samples are 52%~60% at a wavelength of 632.8 nm. The broadened dielectric peaks of PLZT ceramics indicate the relaxation characteristics of the materials, and it is promoted as La3+ content increasing, and enhance obviously by the enlarged La3+ fluctuation in micro-regions in the 0-3 and 2-2 composite PLZT ceramics with a more broadened dielectric peak and higher dielectric constant. The ferroelectric hysteresis loops (P-E) of the PLZT ceramics all exhibit a gradual decrease in maximum polarization value (Pmax), residual polarization value (Pr), and coercivity field strength (Ec) with the temperature increasing. Pmax values of 0-3 and 2-2 composite PLZT (10/65/35) ceramics are obviously higher than that of the well-distributed PLZT (10/65/35) ceramics, and their equivalent secondary E-O coefficients are significantly increased near the temperature of diffuse phase transition.

In this study, 0.96(K0.48Na0.52)(Nb0.95-xTa0.05Sbx)O3-0.04Bi0.5(Na0.82K0.18)0.5ZrO3 (0.96KNNTSx-0.04BNKZ, x=0.03, 0.04, 0.06, 0.07) lead-free transparent piezoelectric ceramics were prepared, and the effects of Sb doping levels on their piezoelectric properties and optical transparency were examined. All samples exhibit a perovskite structure. With increasing Sb content, both piezoelectric performance and transparency improve. Notably, at x=0.04, the piezoelectric coefficient reaches 240 pC/N, with an optical transmittance of 31.5% at 780 nm. This study demonstrates that compositional design can achieve enhanced transparency and high piezoelectric performance in KNN-based lead-free ceramics, offering significant potential for applications in optoacoustic imaging and transparent transducers.

Spinel-type Zn1.1Ga1.8Ge0.1O4 is an excellent host material for dielectric and long-persistent luminescent ceramics. It is helpful to expand application when the transparent ceramics is fabricated. In this paper, the pressureless sintering kinetics of Zn1.1Ga1.8Ge0.1O4 ceramics were studied. The results show that both densification and grain growth are controlled by the grain boundary diffusion of O2-. The activation energy for densification in the intermediate sintering stage is determined as (426±35) kJ/mol, while the activation energy for grain growth in the final sintering stage is calculated as (439±14) kJ/mol. When the combination of sintering parameters is optimized to enter into the so-called “sintering window”, the favorable microstructure of the pressureless-sintered body is obtained. After hot isostatic pressed at high temperature, Zn1.1Ga1.8Ge0.1O4 transparent ceramics with optical transmission ranging from 0.3 m to 9 m and in-line transmittance of 81.2% at 2.5 m are prepared.

The preparation of a well-dispersed, high solids content suspension is the foundation for colloidal forming methods to achieve ceramic green bodies with a uniformly microstructure, and meet the application requirements of various optical window materials. This article aims to enhance the dispersion stability of Y2O3 suspension by utilizing the synergistic effect of two dispersants, namely triammonium citrate (TAC) and ammonium polyacrylate (A40). The influence of the mixing ratio of two dispersants on the stability of Y2O3 suspension and the microstructure of formed green bodies were mainly investigated, and the advantages of the dual dispersant system in improving the solid content and stability of Y2O3 suspension were determined. The results show that the solid content of Y2O3 suspension could reach 38% (volume fraction) under the condition of TAC to A40 mass ratio of 7∶3. A uniformly microstructure and densely packed particle was achieved. Y2O3 transparent ceramics with a diameter of 9 mm and a thickness of 1 mm were obtained by vacuum sintering at 1 800 ℃, with a transmittance of 70.34% at 800 nm.

Y2O3 transparent ceramics have broad applications including solid-state lasers, transparent windows, etc., owing to its excellent optical and physical-chemical properties. The dispersity of the powder among the fabrication process of transparent ceramics has attracted much attention from researchers for decades. In this paper, domestic commercial nanopowder was adopted as the starting material, and the dispersion of the powder was improved through mechanical force combined with HF acid-washing. Y2O3 transparent ceramics with high transmittance were achieved by slip casting and vacuum sintering. The results show that ultrasonic combined with HF acid-washing is an effective way to improve the dispersibility and sinterability of the powder, the D50 of the powder reduces from 3.5 m to 1.4 m. The transmittance of the Y2O3 ceramic at 400 nm is improved from 65.5% to 71.5% for the sample after ultrasonic combined with HF acid-washing process. The results lay a solid foundation of preparation technology and theoretical basis for the application of Y2O3 transparent ceramics on solid-state lasers and transparent window materials.

Y3+∶CaF2 powders were synthesized by chemical coprecipitation method, and transparent ceramics were fabricated by vacuum hot pressing sintering method. Characteristics of the as-synthesized powders were investigated by XRD, SEM and TEM techniques, the relationship between microstructure and transmittance of transparent ceramics were also analyzed. Crystal structures of as-synthesized Y3+∶CaF2 powders belong to single fluorite phase. With the Y3+ doping content increases, the average particles sizes decrease and the lattice constants increase from 5.452 1 to 5.465 5 . The 5% Y3+∶CaF2 transparent ceramics sintered at 900 ℃ for 3 h exhibits the highest transmittance, and the transmittance at 500 and 1 500 nm are 73.1% and 86.4%, respectively. Microstructure of ceramics are modified by the Y3+ doping content and sintering temperatures. For a low growth velocity, it is not easy to eliminate the residual pores, while for a higher growth velocity, residual pores are easily embedded within grains. Both cases will reduce the transmittance of transparent ceramics. This work provides a reference for the preparation of high-quality composite CaF2 transparent ceramics.

Using Y(NO3)3·6H2O as yttrium source, nano-sized Y2O3 powder was obtained by calcining the precursor prepared by Pechini method. This Y2O3 powder was then used as a sintering additive to fabricate transparent AlON ceramics via pressureless sintering at 1 880 ℃ for 2.5 h. The effects of precursor calcination temperature on the composition, morphology and particle size of Y2O3 powder, as well as on the transparency of AlON ceramics were thoroughly studied. The results indicate that increasing calcination temperature of the precursor can promote the decomposition of organic compounds, but it also leads to the growth of Y2O3 particles. Residual organic compounds and excessive growth of Y2O3 powders are unfavorable for fabrication of AlON ceramics with high transmittance and high relative density. The nano-sized Y2O3 powder prepared by calcining the precursor at 900 ℃ for 2 h is favorable for the fabrication of highly transparent AlON ceramics via rapid pressureless sintering. The infrared transmittance of the obtained AlON ceramics reaches up to 83.4% at ~3 750 nm.

With the increase of laser power, high-energy laser systems have increasingly demanding requirements for laser exit aperture windows. Magnesium aluminate spinel (MgAl2O4) transparent ceramics exhibit excellent optical, thermal, and mechanical properties, making them widely used in infrared guidance windows, high Mach aircraft radomes, and windows for optoelectronic equipment in extreme environments. However, there are relatively few reports on their use as laser exit aperture windows. This article briefly introduces the current state of research and development of magnesium aluminate spinel transparent ceramics for laser exit aperture window applications. Focusing on the most critical performance aspects for this application, the absorption coefficient of magnesium aluminate spinel transparent ceramics currently used in transparent bulletproof armor and aircraft windows are tested. The purity of magnesium aluminate spinel powders prepared by the bimetallic alcoholate method and the magnesium aluminate spinel transparent ceramics produced using hot pressing combined with hot isostatic pressing techniques are analyzed. Furthermore, the factors affecting the absorption coefficient of the ceramics are discussed.