In the present study, the absorbance induced in Zinc germanium diphosphide, ZnGeP2 (ZGP) crystals and new Sn-doped ZGP crystals has been investigated under irradiation with high energy electrons. Online and in situ experiments were performed in the range 720-1100 nm. The spectra of the induced absorbance are constituted by a "tail" and the maximum appears to be located at wavelengths lower than those of the investigated spectral range. A fraction of the induced absorbance disappears within one minute after the electron beam is switched off, revealing the presence of metastable effects. A low and a high fluence contribution to the induced absorbance have been observed at 800 nm. The first reaches a limit value and seems to be affected by the Sn doping or by the absence of post-growth annealing, while the high fluence contribution does not feature a saturation in the investigated range and does not depend on the sample type.

An experimental study on how to reduce the content of singly ionized zinc vacancy (V − Zn ) in ZnGeP 2 crystals to improve their optical quality is presented. Their electron paramagnetic resonance signal has been studied in samples treated in different ways. The data provide a scheme of the reductions that can be obtained by the different methods or by their combinations. The adding of Sn during the synthesis phase helps the production of large‐size single crystals and the reduction of the V − Zn content. Thermal treatment and electron irradiation are then needed for further reduction. At the fluence of 2 × 10 17 e cm −2 , the V − Zn decrease does not reach a minimum value, so with higher fluences, additional reduction of V − Zn content is expected.

Transparent Y2O3-MgO nanocomposite ceramic has been processed for mid-IR window applications. The powder was synthesized by a sol-gel route. The final temperature of this process has an impact on the crystallite size and crystallization ratio. A homogeneous powder with around 10 nm crystallite size was made with a final tem-perature of 600 degrees C. The powder was then sintered by the SPS technique performing a two-step sintering process at 1200 degrees C/50 MPa instead of a conventional bearing. A fully dense ceramic (>99%) with an average grain diameter of 150 nm (compared to 350 nm with one-stage sintering) was obtained. After HIP at 400 MPa and annealing in air for 100 h post-treatments, the IR transmittance in the 3-5 mu m wavelength range exceeds 80% for a thickness of 1 mm. Transparency loss at 5 mu m is less than 10% at 1000 degrees C, which is more suitable for the IR's band II than materials such as sapphire, spinel or AlON.

Transparent ceramics are materials of choice for high temperature IR window applications: they are a compromise between transparency, thermal shock resistance and processing costs. Alumina, AlON and spinel-type ceramics have been developed for the 3-5 μm atmospheric transparency band. Nevertheless, these compounds show a degradation of their optical properties (transparency and emissivity) at high temperature (above 500°C), limiting their use in that wavelength range. MgO and Y2O3 have a broader transparency window up to 9 μm and they are transparent enough in the 3-5 μm range even at high temperature but their thermomechanical resistance is weak. Authors have combined these compounds into a nanocomposite ceramic to improve this while keeping good IR transparency. To reach such properties, porosity ratio must be close to zero and the average grain size must stay as small as possible (< 200 nm). Throughout this study, Pechini’s esterification sol-gel route was chosen in order to process the Y2O3-MgO nanocomposite powder. Then, a two-step sintering and low temperature profiles (700‡C) were performed by the Spark Plasma Sintering technique. Post treatments, air annealing and Hot Isostatic Pressing at 400MPa, improved the quality of the ceramics. Finally, structural, microstructural and optical characterizations were carried out. Samples with different nanostructures were obtained. The best samples have average grain diameters below 200 nm with almost no porosity. Good mid-IR transparency, up to 80% for a thickness of 1 mm, was obtained in band II. The material showed a loss of transparency at 5μm below 15% at 1000°C.

Directed Energy Deposition of the commercial intermetallic Ti-48Al-2Cr-2Nb alloy was investigated. The CLAD® process is dependent on multiple parameters, which were successfully optimised through several experiments, including series of beads, small blocks, and massive blocks, under argon atmosphere. The use of adapted temperature management leads to massive blocks manufacturing that bear no apparent macroscopic defects, such as cracks, which are generally observed in this brittle material due to strong temperature cycling during the manufacturing. The microstructure and geometrical parameters were characterised by scanning electron microscopy (SEM). This process generates an ultra-fine and anisotropic microstructure, which is restored to a homogeneous duplex microstructure by a subsequent heat-treatment. Mechanical characterisation is in progress and will be used to validate the soundness of the materials produced in these conditions.

Single crystal quality is a key issue for optical applications. Indeed, in optical frequency conversion processes, defects in single crystals can drastically decrease the conversion yield. The study of the quality of an AgGaGeS4 single crystal is presented in this work. Scanning Electron Microscopy (SEM) combined with Energy Dispersive X-Ray Spectroscopy (EDS) was used to perform a chemical analysis mapping of a large size single crystal cut (surface 26 x 20 mm(2)). Chemical inhomogeneity was found along the crystal growth axes and confirmed by optical characterization showing laser beam perturbations. Compounds volatility, lack of melt homogenization and instability of crystallization front might explain this chemical inhomogeneity. Solutions to improve the crystal growth process and enhance the crystal's quality are finally proposed.

During the last decades, mid-infrared (3–12 μm) laser sources using crystals with nonlinear optical properties have attracted a particular attention due to their potential applications in different fields such as optical counter-measures and remote chemical sensing. As transparency of common oxide crystals is limited to about 4 μm, many researches have been focused on chalcopyrite compounds because of their wide transparency in the mid-IR range and their strong nonlinear optical coefficients. There is a need of such crystals able to convert efficiently the near infrared wavelengths from commercially available lasers into wavelengths higher than 5 μm through Optical Parametric Oscillators (OPO) systems. This paper presents a new compound: ZnGa2Se4 (ZGSe). Measurements made on dense polycrystals show a high thermal conductivity value in comparison with other selenides (2.9 W m−1 K−1), a wide transparency from visible to beyond the band III of the mid IR (0.6 μm–17 μm) and a coefficient of thermal expansion inferior to 10−5 K−1. These results are very encouraging and improvements of the growth process are now undertaken to obtain single crystals with the required optical quality.

In this study, LMD-CLAD® process (Direct Laser Additive manufacturing) is developed for alumina and Al2O3-Y2O3-ZrO2 ternary eutectic compositions. Powder flowability, laser-material interaction and thermal gradient control have been investigated. Powder granules of aforementioned compositions have been designed by spray-drying. Particle size distribution, Hall funnel test and SEM observations have been performed. Flowability has been improved by 20% in order to match with the LMD-CLAD® process by adjusting their density, size and surface quality. Otherwise, optical absorption of the ceramics has been increased up to 90% thanks to the addition of doping ions. With such a flowability improvement, laser powder deposition tests were successful and enabled us to investigate the effect of laser parameters and thermal environment on deposited beads state.

The splitting of the electronic levels under the action of the crystal field enables laser action with a quasi-three-level scheme. Consequently, the spectroscopic characteristics and laser properties of ytterbium are particularly host-dependent. The performance of ytterbium lasers is therefore not fundamentally limited by the quasi-three-level nature of the laser scheme but more classically by thermal issues. Indeed, the fraction of absorbed power deposited as heat generates a temperature gradient inside the crystal, which in turn causes the crystal to be under stress, and ultimately could lead to fracture. This property leads to locally variable crystal field around the dopant ions and the linewidths of the electronic transitions for the ytterbium cations are found to be broader in these so called "disordered" crystals than in "ordered" laser hosts.

In the transparent ceramics processing, the green body elaboration step is probably the most critical one. Among the known techniques, wet shaping processes are particularly interesting because they enable the particles to find an optimum position on their own. Nevertheless, the presence of water molecules leads to drying issues. During the water removal, its concentration gradient induces cracks limiting the sample size: laboratory samples are generally less damaged because of their small size but upscaling the samples for industrial applications lead to an increasing cracking probability. Thanks to the drying step optimization, large size spinel samples were obtained.

Transparent yttrium oxide (Y2O3) ceramics were processed by several densifications steps without any doping species. The green bodies were obtained by the aqueous way and sintered at high temperature under vacuum and then under high pressure. We studied the effects of different sintering cycles and air annealing at different steps of the process on the density and the grain growth. We also focused on the reaction between yttria ceramics and BN-coated graphite crucible which occurs during HIP. We noted that a low heating rate and two annealing steps are necessary to improve our samples’ transparency.

A fluid dynamics analysis and solution homogenization in a Bridgman-Stockbarger furnace was studied. We aimed to optimize the use of the furnace to elaborate single crystals for non-linear laser applications in the mid infrared. This paper describes the study of the influence of different rotation profiles on the homogenization for improved crystal growth.

AgGaGeS4 compound (AGGS) is a promising nonlinear material for mid-IR applications. The different steps of this materials processing are presented. The chemical synthesis of polycrystals and the single crystal growth process are described. Compounds volatility can induce stoichiometry deviation and reduce the quality of obtained single crystals. Nevertheless, 28 mm diameter and 70 mm length single crystals have been grown by Bridgman-Stockbarger method, cut and polished AGGS crystal is obtained. The crystal has good homogeneity and absorption coefficient of less than 0.1 cm(-1) in the 0.5-11.5 mu m range which make it usable in nonlinear devices.

We present our recent progress in the synthesis, growth and post-treatment of low-absorption ZnGeP2. The current residual absorption level at 2 μm is of 0.15 cm-1 without resorting to electron irradiation. We also present our first results on nonlinear frequency conversion and related laser-induced damage threshold measurements.

Mid infrared (3-12 µm) laser sources are essential for applications such as optical counter-measures (missile jamming) for military and civil aircraft security or remote chemical sensing. The principle of those sources is based on non linear effect: a short wavelength laser emission (1 or 2 µm) is converted into two long wavelength laser emissions, with complementary energies, inside a non linear crystal in an Optical Parametric Oscillator (OPO). The first critical parameter for our applications is the transparency of the crystals in the mid-IR range. However, classical non linear oxide crystals, which have a transparency limited to 4 µm due to multiphonon absorptions, are not appropriate. Thus, to go beyond this limit, non oxide materials are required like phosphides, selenides sulfides… Moreover, other important parameters are the crystal’s resistance to high power and its non linear coefficients which have a direct influence on the energy conversion efficiency. ZnGeP 2 (ZGP) presents a good compromise between these properties and is a ternary compound of choice considering the applications cited above. In this work, the high quality ZGP single crystal processing will be presented. It is divided into three main steps which will be precisely described: i) The chemical synthesis from high purity elements Zn, Ge and P which is carried out in a special designed furnace. The main difficulty of this step is to complete the chemical reaction while avoiding the reactor explosion which can occur because of the high vapor pressure of volatile compounds. ii) The crystal growth which is performed using the vertical Bridgman method. iii) Annealing post-treatments which are necessary to reduce the residual absorption around 2 µm. The absorption coefficient at this wavelength, which is the pumping wavelength for a ZGP based OPO, has to be as low as possible to increase the energy conversion efficiency. Crack free ZGP single crystals were grown following our process (Figure 1). Structural and optical quality characterizations will be presented and discussed. A very low absorption coefficient

Laboratory-scaled transparent samples have already been obtained, but upsizing the samples remains a technological challenge. Indeed, every step of the process can lead to defects that hinder optical properties. Among all of the processing steps, drying is one of the most critical, especially for large samples with complicated shapes. It can induce micro-cracks that enlarge during sintering. Optimizing this step has thus become a necessity to obtain valid samples for industrial applications. We developed a finite element method (FEM) simulation model from a simple drying profile performed on laboratory-scaled cylindrical samples. Then, we applied this model to larger industrial-scaled samples, to optimize their drying process. Finally, we obtained crack-free 75mm diameter sintered samples, which become highly transparent after the appropriate hot isostatic pressing thermal treatment.

AgGaGeS4 (AGGS) is a promising nonlinear crystal for mid-IR laser applications which could satisfy the lack of materials able to convert a 1.064 μm pump signal (Nd:YAG laser) to wavelengths higher than 4 μm, up to 11 μm. The processing steps of this material are presented in this study. The key issue of AGGS crystal processing is the control of decomposition at high temperature due to the high volatility of GeS2. This study presents solutions to obtain high quality single crystals. AGGS ingots with 28 mm diameter and 70 mm length were grown by the Bridgman–Stockbarger method. The crystals have good homogeneity and high transparency in the 0.5–11.5 μm spectral range, making them suitable for optical experiments. The influence of GeS2 volatility on melt stoichiometry during the AgGaGeS4 processing is outlined, and solutions to improve the crystals quality are presented.

AgGaGeS4 is a promising non linear crystal for mid-IR laser applications. One presents the two steps of the material preparation, the synthesis of polycrystals and the crystal growth using the Bridgman-Stockbarger technique.

We present the processing of transparent spinel polycrystalline samples with large size (80 mm in diameter), specially the green body drying step. Optical transmissions and thermal conductivities are also presented.