This special edition is comprised of a selected number of papers presented at the 10th Laser Ceramics Symposium in Daejon, South Korea in December 2013. In what follows, we summarize the 11 contributions of this special feature according to three topic areas, namely (i) Processing of ceramics, (ii) Special Lasers and (iii) Applications.

The first topic area can be separated into two groups, specifically, “processing of isotropic ceramics” and “processing of anisotropic ceramics”. There are four papers in the first group and two in the second group, respectively. Collectively, they describe the impact of processing conditions on the morphology and transparency of isotropic and anisotropic ceramics. Three of the papers also demonstrate laser oscillation, highlighting the exceptional optical quality of the ceramics. The first of these papers in the first group is by Chretien et al [1] and compares the influence of a post-sintering hot isostatic pressing (HIP) treatment on reactively sintered Nd:YAG ceramics. During conventional sintering, pores and grain boundaries separate at the final stage of the vacuum sintering step leading to the formation of intragranular porosity which cannot be eliminated by longer sintering treatment. This work demonstrates that fine-grained transparent Nd:YAG can be fabricated by vacuum sintering with an additional post-HIP treatment. Post-HIP allows less silica sintering aid content thus drastically limiting grain growth in Nd:YAG ceramics. Under these conditions, the separation between pores and grain boundaries is no longer observed and higher optical quality ceramics can be obtained. In the second paper, Kim et al [2] address a major issue associated with agglomeration in powders that leads to trapped porosity and high optical scattering losses in ceramics. They demonstrate that jet milling is a very effective technique to break up large agglomerates in ceramic powders and without cross-contamination. They use this approach to fabricate homogeneous, uniform, and highly transparent ceramic samples from jet-milled Yb:Lu₂O₃ powder. The transmission is very close to the theoretical limit, demonstrating the viability of their approach. Optimization and scale-up of their process would enable high-power laser applications of the sesquioxide Yb:Lu₂O₃, of interest due to its high thermal conductivity. The third paper by Fu et al [3], highlights the detrimental role that color centers play on the optical performance, especially laser oscillation, in their Nd:YAG ceramics derived from solid state reactive sintering. They demonstrate that absorption losses associated with color centers due to oxygen vacancies and transition metal ion impurities can be reduced by post-annealing of Nd:YAG ceramics. In their particular samples, post annealing at 1450°C for >5 hours appears to represent the best conditions for reducing loss and obtaining the highest lasing efficiency. This work helps to provide a better understanding of the importance of annealing to improve the laser performance. Yang et al [4] used a similar approach of post-annealing at 1400°C for 15 hours to remove internal stresses and eliminate oxygen vacancies in their Ho:YAG and Ho:LuAG transparent ceramics. The ceramics were fabricated by a reactive sintering method under vacuum and exhibited homogeneous grains with average grain size of about 10 μm. The samples were of excellent optical quality, exhibiting high transmission and without any signs of trapped porosity. The samples exhibited lasing at around 2.1 μm with reasonable efficiencies. This wavelength is often termed “eye-safer” and is especially important for high power lasers since it leads to significantly less collateral eye damage than corresponding lasers at 1 μm.

The second group consists of two papers that address the challenge of making transparent ceramics from anisotropic materials and avoiding the optical scattering due to the inherent birefringence. Currently, there are two possible methods used to reduce the effects of birefringence in anisotropic laser ceramics: by generating nanostructured grains through a fast sintering consolidation process or by achieving an orientated texture through the application of a high magnetic field. Wu [5] demonstrates fabrication of transparent Yb:Sr₅(PO₄)₃F (Yb:SFAP) ceramic by generating 150 nm sized grains through a fast sintering consolidation process exploiting electric field assisted hot pressing. The critical parameter is that the densification rate is faster than the rate of grain growth during sintering. Even though the Yb: SFAP is an anisotropic crystal material with hexagonal symmetry, the small grained ceramic exhibits low optical scattering and consequently good IR optical transmission. Optimization of this process should lead to near theoretical transmission in the visible region. In the second approach, Sato et al [6] demonstrate fabrication of highly transparent Yb:Ca₅(PO₄)₃F (Yb:FAP) ceramics by slip casting under a rotational magnetic field of 1.4T, followed by vacuum sintering and hot isostatic pressing in the absence of a field. The magnetic field orients the submicron sized crystals, micro-domains, during the slip casting process and then they grow in that preferred direction during the sintering process. They also show that magnetic anisotropy enhancement of non-magnetic crystals by spin-orbit interactions in rare-earth doped crystals enables orientation control of the hard magnetization axis of grains in laser grade ceramics. They go on to demonstrate lasing in their unique material. Optimization of both these processes could potentially provide a path forward for laser oscillation in ceramics made from anisotropic crystals that possess superior thermal, optical, and mechanical properties compared with their cubic counterparts and enable the use of these types of materials in many applications including laser fusion. The approach also has impact for all ceramics, as smaller grain size can lead to higher strength ceramics.

The second topic area of “Special Lasers” consists of three papers highlighting the unique advantages of ceramic materials in high power laser applications, exploiting thin disks, novel geometries and nonlinear effects, respectively. While thin disk lasers have been demonstrated using YAG, further enhancement in laser performance requires the use of materials with a combination of both high thermal conductivity and high rare earth dopant concentration. Understanding these requirements, Nakao et al [7] demonstrate CW laser operation of thin-disk lasers made from Yb-doped Lu-based oxide ceramics. Since the atomic mass of Lu and Yb are very similar, the thermal conductivities of Yb³⁺-doped Lu-based materials decrease only moderately and these materials exhibit high thermal conductivity even at high doping levels [8,9]. They prepare ceramic samples of 3 at.% Yb:Lu₂O₃ and 10 at.% Yb:LuAG with 3 μm grain size using vacuum sintering of co-precipitated nano-powder. Their ceramic thin disks demonstrate laser oscillation with high efficiencies, the best results being obtained for those samples bonded to a heat sink using a special soldering technique. Tokita et al [10] demonstrate a multi-total-reflection-active-mirror (multi-TRAM) design for high-average-power and high-pulse-energy lasers. The multi-TRAM is a monolithic ceramic with one undoped YAG tetragonal prism and three thin Yb:YAG active layers. Unlike a single TRAM design, the multi-TRAM design allows the gain in each of the active layers to be lowered, thereby decreasing parasitic amplified spontaneous emission (ASE) and improving energy storage efficiency while keeping the overall gain constant. They report the first demonstration of laser amplification of 10-ns laser pulses to sub-joule energy (500 mJ at a 10-Hz repetition rate) in a cryogenic multi-pass amplifier using the multi-TRAM architecture. This architecture has great potential for scaling to higher energies. Kong et al [11] present the recent progress on the development of the Kumgang laser, a high energy and high repetition rate laser. A major focus is the coherent beam combination with stimulated Brillouin scattering phase conjugation mirrors (SBS-PCMs). The coherent beam combining technique utilizes a small gain medium that can be cooled rapidly, allowing it to operate at a high repetition rate with a high output energy level. Furthermore, the phase-conjugation property of the SBS-PCM prevents the degradation of the amplified beam quality. In this paper, the authors successfully demonstrated the single-frequency hybrid master oscillator power amplifier (MOPA) front end and Nd:YAG rod power amplifier of the Kumgang laser. The front end produces a 10 kHz pulse laser beam which has a 8.5 ns pulse width and a 95 MHz line-width. The power amplifier amplifies the laser beams to 200 W (20 mJ @ 10 kHz / 8.5 ns) with an input power of 5.1 W (0.51 mJ @ 10 kHz / 8.5 ns). With this front end and power amplifier, the Kumgang laser will be completed by the middle of 2015. Once completed, the Kumgang laser will be utilized as a new laser machining technology in the future to cut micro-SD RAM cards using a hologram, without the need for the scanning of a focused laser beam by a focusing lens.

The third topic area pertains to applications and consists of two papers that highlight unique bulk and surface properties of advanced ceramics. Starobor et al [12] evaluate the bulk properties of ceramics for application as Faraday isolators (FI) to prevent optical feedback into a laser cavity. Currently, terbium gallium garnet (TGG) crystals are almost the only material for FI for lasers with high average power. However, crystal sizes are limited to <40 mm [4x], so they cannot be used for large-apertures. The present materials for large-aperture FIs are made of magneto-optical glasses whose heat conductivity, Verdet’s constant and other characteristics are inferior to those of crystals [13]. In this paper, the authors compare magneto-optical TGG, TAG and Ce:TAG ceramics that are potential candidates for large-aperture Faraday isolators and recommend further improvement of growth technology and use of doping for increasing Verdet’s constant and reducing thermally induced distortions. Busse et al [14] describe the direct nano-patterning on the surface of an optical element to achieve reduced Fresnel reflections as an attractive alternative to traditional AR coatings. Unlike coatings, the anti-reflective surface structures (ARSS) processing does not involve applying additional materials on the surface of the optics, which often results in coating delamination under thermal cycling and laser damage to the coating at lower thresholds than the window. In contrast, state-of-the-art processing has resulted in antireflective performance of ARSS comparable to that of the traditional AR coatings, while adding significant advantages such as higher laser damage thresholds, large acceptance angles and ease of cleaning, since there is no foreign material on the surface. They demonstrate these advantages on fused silica windows, lenses and fibers, and spinel ceramics. Remarkably high pulsed laser damage thresholds of 100 J/cm² at 1.06 µm were measured for fused silica windows up to 10 cm diameter. Spinel ceramic samples with ARSS showed damage thresholds more than two times higher than that for spinel with traditional AR coatings. These results significantly highlight the important utility of these AR surface structures as a highly attractive alternative to traditional AR coatings for practical applications including high-energy laser optics and windows.

The editors believe that the future of optical ceramics is strong and lies in its diversity and depth to address basic scientific problems within the context of applied practical challenges and limitations. We see great headway being made across the board in optical ceramics from passive optics to active optics, including rare-earth-doped lasers at eye-safer wavelengths. The editors note that the papers for this special issue represent authors from nine countries. We thank the authors for their outstanding work and encourage researchers to continue to develop exciting theoretical and applied research in the field of optical ceramics. We would like to express our gratitude to all authors and reviewers for their efforts in improving the manuscripts during the review process. We also thank David Hagan, Editor-in-Chief of Optical Materials Express, for his support and encouragement of this feature issue and the OSA journal staff for their excellent support during the review and production processes.