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Polycrystalline materials have shown promise in advanced optical applications becauseof their unique optical and mechanical properties. As transparent polycrystalline materials become more important in optical applications, evaluation of their transmission across a wide range of wavelengths is crucial to device design. Most polycrystalline materials have non-uniform optical properties due to residual porosity, secondary phases, and/or crystalline anisotropies (e.g. birefringence). To better understand and describe transmission in these materials, an in-line transmission model is developed that incorporates reflection, scattering, and absorption losses. The model was fit onto transmission data from three polycrystalline material systems: ruby, yttria stabilized zirconia, and terbia. Parameters extracted from these fits can be used to describe wavelength dependent transmission with one simple analytical expression. The fit can also be used to decouple absorption form scattering, allowing for the extraction of important properties such as absorption coefficients. Another unique challenge to modeling transmission by transparent ceramics is modeling the scattering losses from birefringence. Even in the case of a single-phase polycrystal with negligible porosity, birefringence scattering will always be present whenever the crystal is anisotropic (non-cubic). Multiple models for birefringence scattering have been suggested in the literature that are generally successful in describing scattering loss in specific material systems. However, these models do not readily extrapolate to new material systems and their connection to birefringence scattering are never fully justified. We remedy this by deriving from Maxwell’s microscopic equations a first-principles model of birefringent scattering that is applicable to any single-phase, unaligned transparent polycrystal. This is achieved by first developing a framework for describing light propagation in dielectric solids by perturbing Maxwell’s equations These expressions are then used to develop a birefringence scattering model in ceramics. The derivation culminates in an equation which describes the birefringence scattering coefficient spectra using the single-crystal refractive index tensor and a representative polished surface micrograph. This model should be useful for designing materials and characterization. For example, by generating hypothetical transmission for a polycrystal with varying grain size or by characterizing average grain size using a transmission measurement.

Lasers are fundamental to our society and are used in an ever-increasing range of fields including manufacturing, communications, defense and medical industries. One of the main drivers in laser development is increasing laser power. This work presents the development of a new polycrystalline transparent ceramic material that shows promise as a high-power laser gain media. In addition, the development of materials specifically designed for fundamental laser-material interaction studies on lab scale as well as high energy, facility scale lasers is presented. The power deliverable by a laser scales directly with the thermal conductivity of the laser gain material. Aluminum oxide (hexagonal, Al2O3) has a higher thermal conductivity than any rare-earth host media available today thus synthesis/processing methods for obtaining high quality rare-earth doped alumina ceramics are of high interest. This work explores the impact of powder processing and densification conditions on the optical properties of ytterbium doped Al2O3 ceramics. Dopant incorporation methods and processing temperatures are explored to find the highest density ceramics possible. The first reported transparent ytterbium doped nanocrystalline alumina is characterized and discussed. The absorption and emission cross sections as well as upper state lifetime show promise for Yb:Al2O3 as a laser gain material. This work also presents target fabrication methods for high intensity laser-material interaction studies. The target materials are metals (aluminum) and semiconductors (silicon). The targets have been used in collaborative campaigns at lab and facility scale lasers at intensities up to 1014 W/cm2. In addition a study of silicon material damage as a function of intensity is presented. These experiments show that laser irradiation primarily results in melting at intensities up to 1013 W/cm2.

The development of solid-state lasers with emission in the eye safe region of 1.5 μm is interesting for scientific and industrial purposes including laser cutting/manufacturing, communications and surgery. In this work I present the processing and characterization of erbium (Er:Al2O3) and erbium, chromium co-doped (Er:Cr:Al2O3) bulk aluminum oxide for laser gain media applications. These new gain media have excellent thermomechanical properties and are promising for laser power scaling. In the first portion of this dissertation Current Activated Pressure Assisted Densification (CAPAD) is implemented to process and consolidate transparent bulk polycrystalline Er:Al2O3 ceramics via a simultaneous solid-state mixing and densification route. The materials have high transparency at the emission wavelength, and absorption lines characteristic of Er3+. Room temperature photoluminescence reveal broad emission peaks in the expected range of ~ 1.5 μm corresponding to intra-4f transitions in Er3+. The emission peaks are narrower at cryogenic temperatures (down to 10 K) revealing thermally broadened emission. In addition, ~ 1.5 μm emission has relatively long lifetimes that do not vary across a broad emission spectrum (1.5 - 1.6 μm), suggesting similar local environment of optically active Er ions responsible for the broad emission.In the second portion of this dissertation, chromium ions are co-doped with erbium into the alumina matrix, as a sensitizer in order to increase the absorption cross-sections. The ~ 1.5 μm emission corresponding to Er3+ can be accessed through the broad and deep chromium absorption bands. The result is improved absorption and longer emission lifetimes compared to the singly doped Er alumina materials. The Cr:Er:Al2O3 is promising as a tunable near infrared laser gain material that can be pumped with 0.532 μm, a wavelength commonly used for pumping Ti:Sapphire, the most successful tunable short pulsed laser. A common theme of this work is the interplay between material processing, the material properties, and the development of optical devices using bulk polycrystalline transparent ceramics.

The past decade has seen significant advances in the development and improvements to high-energy laser technologies, with improvements coming from all directions, i.e. pumping technology, cavity design, cooling methods, and improved gain media quality, etc. Regardless, the continued development of high-energy lasers and the supporting technologies remains intense. From a materials development perspective, the need for gain media with superior optical, thermal, and mechanical properties is alluring because improvements in the materials properties often translate directly to increases in device performance. Advances in powder processing and sintering/consolidation techniques, in the past two decades have produced polycrystalline ceramics with the requisite densities, transparencies, and photoluminescence properties to be viable laser gain materials. In fact, the performance of some cubic (optically isotropic) ceramics now rival and even surpass their single-crystal counterparts. In the first portion of this dissertation Current Activated Pressure Assisted Densification (CAPAD) is implemented to process and consolidate transparent bulk polycrystalline YAG and Ce:YAG ceramics via a simultaneous solid-state synthesis and densification route. The simultaneous reaction/densification during CAPAD processing results in improved densification rates and with reaction kinetics that are about 2 orders of magnitude higher when compared to traditional solid-state reaction pressureless sintering and that the higher reaction kinetics occurring during CAPAD result at much lower temperatures, (~600°C) compared to conventional reaction sintering. In the second portion of this dissertation, the increased consolidation and reaction kinetics are leveraged to develop transparent bulk polycrystalline Cr:Alumina (ruby) and rare-earth (RE), RE:Alumina into viable laser gain materials. The advantages of alumina as an optical gain media over state of the art gain materials such as YAG and laser glasses are significant; it has higher thermal conductivity, chemical inertness and higher mechanical toughness, all attributes that could lead to more stable, more powerful lasers. Despite these promising attributes, producing RE:Alumina ceramics with suitable functional properties for laser applicationshas steep processing challenges. RE:Alumina cannot be made using traditional equilibrium methods because the equilibrium solubility limits of REs in the Alumina is on the order of 10-3 to 10-4 %, not high enough to produce lasing (~10-1 %, required). In addition, alumina is birefringent which can cause severe scattering in ceramics. These obstacles are mitigated through careful powder and CAPAD processing`in order to produce ceramics with fine grain sizes that mitigate scattering and with rare-earth dopant concentrations as high as 0.5 at.% (RE:Al), orders of magnitude higher than previously reported. Results are presented that prove for the first time that bulk polycrystalline RE:Alumina is a viable laser gain media. A common theme of this work will be the interplay between material processing, the resultant material properties, and the development of optical devices using bulk polycrystalline transparent ceramics.

Limitations in single-crystal growth technology have led to the development of transparentPolycrystalline Laser Materials (PLMs) as viable alternatives towards fabricating largedimension laser gain media having engineered dopant profiles to improve the thermo-opticproperties and continue power scaling of solid-state laser systems operating in the 1 - 5 μmwavelength range. Bulk scattering due to non-uniform refractive index distribution is theprimary loss mechanism in the PLMs, resulting in the degradation of laser performance andlower output power. The objective of this dissertation was to formulate a methodology for fast,reliable and accurate identification and characterization of bulk scatter in Erbium (Er) andNeodymium (Nd) doped YAG (Yttrium Aluminum Garnet - Y3Al5O12) and Yttria (YttriumOxide - Y2O3) PLMs. Scanning Electron Microscopy, Transmission Electron Microscopy, and Confocal Laser Scanning Microscopy were used for material characterization and investigationof PLM microstructures at various stages of fabrication, thus identifying the source of refractive index inhomogeneities, which result in bulk scattering. Three optical characterizationtechniques, Transmitted Beam Wavefront Profiling (TBWP), Angle Resolved Scatter (ARS)measurements, and Schlieren Imaging, to identify bulk scatter in PLMs, are developed. TBWPwas able to directly and quickly image the distortions introduced to a propagating laser beamcaused by the presence of bulk scattering in the PLMs. ARS was able to map the distribution ofthe scattered intensity in space, and was found to be very sensitive for comparing samples. Amodified white light Schlieren imaging setup utilizing variable focusing capability, demonstratedhigh sensitivity for directly imaging local spatial variations in refractive index across andthrough the entire PLM regardless of dimensions. Finally, by comparing laser performance of0.9% Nd:YAG single crystal, high quality and low quality PLMs, with results from TBWP andSchlieren Images, the utility of characterization techniques towards identifying bulk scatter andassessing the optical quality are validated. The methods developed are applicable towards thecharacterization of any transparent material exhibiting bulk scatter.

As our understanding of ceramic processing methods for the purpose of fabricating polycrystalline optical materials has increased over the past few decades, the race is on to bring ceramic technology to markets where single crystalline materials have traditionally been used. One such market is scintillators. This Master’s thesis focuses specifically on a class of materials attractive for use as gamma-ray scintillators. These barium based halides can potentially be utilized in applications ranging from ionizing radiation detection in the field to high-energy physics experimentation. Barium bromide iodide and barium chloride single crystals have already showed high light yield, fast scintillation decay, and high energy resolution, all desirable properties for a scintillator. This work attempts to show the likelihood of moving towards polycrystalline scintillators to take advantage of the lower processing temperature, higher manufacturing output, and overall reduced cost. The experiments begin with identifying appropriate sintering conditions for hot pressed ceramics of BaBrI and BaCl2. Possible sources of optical loss in the first phase of hot pressed samples are investigated using a wide range of characterization tools. Preliminary luminescence and scintillation measurements are reported for a translucent sample of BaBrI. Recommendations are made to move toward highly transparent ceramics with scintillation properties approaching those measured in single crystal samples

Sesquioxides of yttrium, scandium, and lutetium, i.e., Y2O3, Sc2O3, and Lu2O3, have received a great deal of recent attention as potential high power solid state laser hosts. These oxides are receptive to lanthanide doping, including trivalent Er, Ho and Tm which have well known emissions at eye-safe wavelengths that can be excited using commercial diode lasers. These sesquioxides are considered superior to the more conventional yttrium aluminum garnet (YAG) due to their higher thermal conductivity, which is critical for high power laser system. Unfortunately, these oxides possess high melting temperatures, which make the growth of high purity and quality crystals using melt techniques difficult. Transparent ceramics are an attractive alternative route to laser hosts since the processing by-passes many of the challenges of refractory crystal melt growth. Moreover, transparent ceramics can possess added benefits relative to single crystals including faster production rates, the fabrication of larger sizes and composite laser structures, uniform doping concentrations, and better mechanical behavior. In order to fabricate highly transparent ceramics, the starting powders must have good dispersion and high reactivity. In this work, sesquioxide nanopowders with high sinterability were synthesized by solution precipitation techniques. For Y2O3, the nanopowders were prepared using yttrium nitrate and ammonium hydroxide with the addition of a small amount of ammonium sulfate. Doping sulfate ions was found to reduce the agglomeration of Y2O3 nanopowders. The Y2O3 nanopowders with average particle size about 40 nm were obtained by calcining at 1050 °C for 4 hours. Unfortunately, this method failed to prepare well-dispersed Sc2O3 and Lu2O3 nanopowders. For Sc2O3 and Lu2O3, the nanopowders were synthesized by using scandium or lutetium sulfate and hexamethylenetetramine (HMT). The precipitate precursors were calcined at 1100 °C for 4 hours to yielded Sc2O3 and Lu2O3 nanopowders with average particle size 40 and 60 nm, respectively. Transparent sesquioxide ceramics were successfully fabricated by vacuum sintering compacts of nanopowders at high temperature. These ceramics had relatively large grain sizes, ranging from tens to hundreds of micrometers, due to significant grain growth at the final stage of sintering. These large-grained ceramics tend to not offer significant enhancements to strength or thermal shock resistance that smaller grain-sized transparent ceramics afford. Sub-micrometer-grained highly transparent sesquioxide ceramics were fabricated using a two-step sintering process followed by hot isostatic pressing (HIP). This process yielded full densification of the sesquioxide ceramics with drastically reduced grain growth. These sub-micrometer-grained ceramics exhibited a transparency equivalent to that of single crystals in the near-infrared spectral. The microhardness and fracture toughness of transparent ceramics fabricated by this method were found to exceed those of transparent ceramics fabricated by conventional sintering. The single-crystal-like transmittance of the sub-micrometer-grained yttria ceramics in the visible and IR region with high mechanical properties is an important advancement for the use of these materials in more extreme environments, including high power laser systems where reduction of scattering and thermal shock resistance are critical.