Recently, the U.S. Army Research Laboratory (ARL) has focused increased attention on the development of transparent armor material systems for a variety of applications. Future combat and non-combat environments will require lightweight, threat adjustable, multifunctional, and affordable armor. Current glass/polycarbonate technologies are not expected to meet the increased requirements. Results over the past few years indicate that the use of transparent crystalline ceramics greatly improve the performance of a system. These results coupled with recent processing and manufacturing advances have revitalized the interest in using transparent ceramics for armor systems. The materials currently under investigation at ARL are magnesium aluminate spinel (MgAl₂O₄), aluminum oxynitride spinel (AlON), single crystal sapphire (Al₂O₃), glasses, and glass-ceramics. The polymers under investigation are polycarbonate (PC) and polyurethane (PU). An overview of current ARL efforts in these areas, including the motivation for using transparent ceramics, the requirements, the potential applications, and the ongoing processing research will be reviewed.

For the past several decades, the Army has been interested in materials transparent to visible and infrared wavelengths for use in armor, IR windows and sensor windows. Future requirements for transparent armor are systems that can defeat greater threats without increased weight and thickness and minimal optical distortion. The Army Research Laboratory is developing transparent armor systems to increase the performance of new windows. Aluminum oxynitride spinel and single-crystal sapphire are two of the ceramic candidates for advanced transparent armor applications.

The Sapphire Statistical Characterization and Risk Reduction Program tested approximately 1500 4-point flexure bars with different crystal orientations at different temperatures to establish a mechanical strength database for engineering design. Sapphire coupons were selected to represent surfaces on two different missile windows and a missile dome. Sapphire was obtained from the same suppliers used for the windows or dome and, as much as possible, coupons were fabricated in the same manner as the corresponding part of the window or dome. Perhaps the most interesting result was that sapphire from one fabricator was 50% stronger than sapphire made to the same specifications from the same blanks by another fabricator. In laser heating tests, sapphire performed better than predicted from flexure tests. When a compliant layer of graphite was used as a pad between the test specimens and the load fixture, sapphire in which the principal axis of tension and compression was parallel to the c-axis increased in apparent strength by a factor of 2 - 3. Strengths of other crystal orientations were not significantly affected by the graphite pads, but the incidence of twinning at 883 K was reduced by graphite.

Flexure strength testing of single crystal sapphire was conducted in the 500 - 600 degree(s)C temperature range using thin sheets of Grafoil^TM to reduce failures due to the contact stresses at the loading points. Load-point failures occur often in high temperature testing of sapphire when twinning mechanisms activate above 500 degree(s)C. Bend bars of three different orientations were tested, and the flexure strength was found to be strongly dependent on the orientation type both in Grafoil and non-Grafoil tests. Use of Grafoil increases measured flexure strength up to the factor of three due to the reduction of contact stress. The effect of using Grafoil in mechanical testing of sapphire, including flexure strength, compressive strength and biaxial flexure strength testing is discussed.

Various ways to increase the 600C strength of sapphire were explored, including heat treatments, doping and improvement of fabrication techniques. Different grinding and polishing procedures were performed on sapphire disks and compression specimens. Measured strength showed correlation with fabrication procedures, with less aggressive, multi-step processes resulting in the highest strength. Simple heat treatments in oxidizing atmosphere significantly improve both compressive and biaxial flexure strength of sapphire. MgO doping was found to be very effective in increasing the compressive strength of sapphire when combined with the heat treatment.

With the objective of producing affordable large area windows, high strength edge bonds between individual sapphire components have been developed. Several bonding methods have been demonstrated, with bond fracture strengths ranging from 100 - 200 MPa. The directed energy process, which yields the strongest bonds, has produced bonded sapphire components 600 mm long and 3 mm thick with a 75 mm wide bond line. When polished, the bonded windows show no degradation in transmittance or transmitted wavefront quality. The processes have recently been scaled up to 355 m wide, 10 mm thick bonds lines and multipane window blanks. In addition, doubly-curved bonded sapphire components have been produced and polished with excellent results, using singly curved sapphire components for the individual panes. The edge bonding approaches shows promise for fabricating affordable sapphire windows up to 750 mm diameter.

The optical and mechanical properties of polycrystalline MgAl₂O₄ spinel make this material of interest for transparent armor and for window and dome applications in the 0.3 micrometers to 5.5 micrometers range. Spinel was briefly produced commercially, and qualified for a range of dome and window applications in the early 1990's. Since 1993 however, there has been no commercial producer and consequently the interest in the application of spinel has waned. This paper summarizes development efforts by Technology Assessment and Transfer (TA&T) to fabricate transparent spinel with high optical quality for both transparent armor, and a selection of window and dome applications. A cooperative research and development agreement between TA&T and the US Army Research Laboratory is focused at optimizing processing parameters to maximize strength and transparency while minimizing the costs for fabrication by the hot-press/HIP approach. Present interest is in fabricating large armor panels of spinel up to 15 inches square and 0.5 inches thick, and in the fabrication of thinner windows and domes with the view to establishing TA&T as a commercial supplier of spinel in the near future.

Thin films of amorphous and crystalline tungsten trioxide were deposited onto glass covered with indium tin oxide, by reactive dc magnetron sputtering. The samples were intercalated with Li ions in 1M LiClO₄ in propylene carbonate. The infrared reflectance was measured ex-situ at wavelengths between 2 and 50 micrometers for Li_xWO₃ of different thickness, d, and at different Li/W ratios, x. The absorption bands (i.e. the dips in the reflectance spectra) of a thick amorphous film (d equals 1450 nm) get stronger up to the threshold value x equals 0.17. Above this value, the amplitude of the oscillations in the reflectance spectra decreases, which resembles the behavior of a thick crystalline film (d equals 1270 nm). The latter seems to have a lower threshold value, around x equals 0.08 and the spectra at large x are flatter than the ones for the amorphous film.

We, here, present the study on the crystalline behavior of TiO₂ in the presence of SiO₂ and its optical property at low SiO₂ content region from x equals 0 and 0.12 in the (1-x)TiO₂-xSiO₂ system. From X-ray diffraction, it is observed that just a small addition of SiO₂ into TiO₂-SiO₂ films has made a rutile peak completely vanish. Anatase peaks were decreased in intensity and broadened with the increase in added SiO₂ content. Besides, anatase peak was shifted to the large d-spacing and its means that Si ions have incorporated into TiO₂. However, the anatase peak shift was saturated above 8 mol%. These facts confirmed that incorporated Si ions inhibited the phase transition between the rutile and the anatase and the crystallization of anatase is also suppressed with SiO₂ addition. The anatase band frequency (approximately 140 cm^-1) shift to the higher one in Raman spectra and the appearance of the band due to Si-O-Ti vibration in IR spectra indicates clearly that the SiO₂ incorporation happens as SiO₂ is added in films. The surface topography observed by SEM and AFM shows that the surface becomes smooth and the TiO₂ crystallite become smaller with increasing SiO₂ content. UV-Visible transmittance spectra showed that all films have good transparency up to about 90% in visible region.

Glasses were prepared from the system xLa₂O₃(DOT)(1-x)P₂O₅, where 0.00 <EQ x <EQ 0.30 (mole fraction), and densities, refractive indices and glass transition temperatures were determined. Inflections in the property/composition trends were observed at x approximately 0.15 La₂O₃. Raman spectra indicate that the phosphate network evolves from a cross- linked ultraphosphate to chain-like metaphosphate with increasing x. The property and spectral trends are interpreted with respect to the La-coordination environment, which appears to change from isolated La-polyhedra to a La-O-La subnetwork when x >= 0.15.

Conditions for preparing samples of Yb³⁺-doped fluoroaluminate glasses of high optical quality ((Delta) n -6), their structure, and optical properties have been studied while varying the phosphate (5 to 40 mol%) and the Yb³⁺ content (5 to 12 X 10²⁰ ions/cm³). Refractive index and dispersion increase the higher the phosphate content. Adding Nb₂O₅ to the glass composition or substituting phosphate by sulfate increases the non-linear refractive index, due to an enhanced asymmetry of the Yb³⁺ sites resulting in larger cross sections for absorption and emission. Depending on melt composition the fluorescence lifetime of the ²F_5/2 laser level drops from 2.0 and 1.1 ms when phosphate content is increased but it shows no concentration quenching at higher Yb³⁺-doping levels. Glasses exhibit a very low OH content with an absorption of less than 0.1 cm^-1 at around 3000 cm^-1.

Following initiation at absorbing surface flaws, laser-induced damage ((lambda) equals 0.35 micrometers ) on polished fused-silica surfaces continues to grow with subsequent laser pulses. The growth process is believed to be associated with a plasma-induced transformation of the silica to a highly absorbing state. A number of optical spectroscopy techniques have been used to characterize the nature of the laser-modified silica. Increases in the concentration of defect states is suggested by the spectral characteristics of the emission. Photoluminescence spectroscopy detects three characteristic emission peaks within damage sites when illuminated at 355 nm. Two of the peaks are likely due to the well-known non-bridging oxygen hole centers (NBOHC; broken Si-O bonds) and oxygen deficiency centers. The third, and dominant, peak at 565 nm has not been clearly identified, but may be associated with small clusters of Si atoms. Raman spectroscopy suggests densification which is associated with decreases in characteristic ring sizes within the amorphous silica structure. A 3D model for the nature of laser- damage sites in silica is being developed. This model will provide the basis for the development of strategies to slow or stop the rate of damage growth.

As compared to other optical glasses, when fused silica (SiO2) is ground or polished, it has some unique features, such as low surface microroughness and subsurface damage, low grinding- induced residual stresses, and low removal rates. It also has higher hardness. On the other hand, fused silica's Young's modulus and fracture toughness are in the middle range of the optical glasses. So, compared to other optical glasses, fused silica behaves differently under compressive loads. When subjected to sufficiently large compressive stresses, fused silica glass will densify permanently. It is also noticed that shear will facilitate densification. We model this behavior by a simple constitutive law including both shear flow and densification. The constitutive model was used to interpret nanoindentation tests on fused silica via 3D finite element simulations of indentations of Berkovich indenter. This work extends our previous work on axisymmetric (2D) indentation.

Using conventional materials like fused silica and sapphire for critical window components in a high-power laser system can lead to intolerable thermal distortions and optical path difference effects. A new oxyfluoride glass is being developed which has the unique property of possessing a negative thermo-optic coefficient (dn/dT) in the near- and mid-wave infrared. Specifically, the refractive index (n) of oxyfluoride glass decreases as the temperature increases. The distortions caused by thermal expansion of the glass during laser irradiation are partly offset by the negative dn/dT. This paper specifically addresses optical properties and surface finishing of oxyfluoride glass compared to fused silica. Normarski micrographs and surface profiles were measured to inspect the surface quality since smooth surfaces are essential for suppressing surface scattering and absorption. The refractive index and thermo-optic coefficient were measured using null polarimetry near the Brewster angle. Low dn/dT is required for laser windows. Transmittance spectra were measured to deduce the extinction coefficient by comparing with the transmittance calculated from the refractive index and to screen for unwanted absorption from contaminants including hydrocarbon oils, polishing residue, and water or -OH groups. Total integrated scattering was measured for both surface and bulk scattering. All measurements were done on 1.0- and 1.5-inch-diameter witness samples.

The optical properties of glasses are traditionally characterized out to 1 micron in most catalogs, yet, the useful spectral range can extend out to as far as 4 microns. For this reason, the temperature and frequency dependent infrared properties of the Schott glasses Suprasil 300, Ultran 20, SK4, SF6, F2, BK7, UBK7 and KzFSN4 have been measured. This is accomplished by broadband transmittance and reflectance measurements as a function of temperature from room temperature up to 600 degree(s)C. The infrared absorption edge of these glasses is analyzed and modeled with a theoretical multiphoton model and oscillator model. The refractive index of these glasses in the infrared region is estimated from the measurements and a Sellmeier model extending into the infrared is now obtained.

Strong photoluminescence was observed from nominally pure lanthanum aluminate crystals and related materials with perovskite structure in a visible and ultraviolet wavelength regions. Single crystals of RAlO₃ (R equals La, Y, Nd, Sm) were grown with a floating-zone method in reduced atmosphere. Photoluminescence from as-grown LaAlO₃ single crystals was composed of a fast ultraviolet component and a broad visible emission. From the lifetime measurement, absorption spectra, excitation spectrum, and comparison between samples as-grown and annealed in oxygen, we assigned the visible component to the emission from excited state formed at oxygen defect sites because the emission was strongly suppressed with oxidation. On the other hand, the UV emission was assigned to band-to-band emission or emission from polaron pairs. Similar two components were observed in YAlO₃ crystals at longer wavelength region than in LaAlO₃. Fast UV emission was observed also in NdAlO₃ and SmAlO₃. Because it is possible to make some perovskite crystals electrically conductive by varying the concentration of dopants, the combination of luminescence and electrical conductivity in perovskite oxides make them auspicious materials for novel optoelectronic devices. Our results show the possibility of light-emitting devices made of perovskite aluminate crystals.

The National Ignition Facility will require hundreds of very large single crystals (boules) of KDP and KD*P for the amplifier and frequency conversion optics. Rapid growth now routinely produces 250 - 300 kg boules of KDP. Technical hurdles overcome during the past year include inclusion formation and spurious nucleation. Areas of continued interest are control of asymmetry and aspect ratio.

Flat panel displays play an important role as the visual interface for today's electronic devices (Notebook computers, PDA's, pagers, mobile phones, etc.). Liquid Crystal Display's are dominating the market. While for higher resolution displays active matrix displays like Thin Film Transistor LCD's are used, portable devices are mainly using Super Twisted Nematic (STN) displays. Based on the application, STN displays for mobile applications require thinner glass substrates with improved surface quality at a lower cost. The requirements and trends for STN glass substrates are identified and discussed. Different glass manufacturing processes are used today for the manufacture of these substrates. Advantages and disadvantages of the different glass substrate types are presented and discussed.

BaO-Ga₂O₃-GeO₂ (BGG) glasses have the desired properties for window applications in the 0.5 - 5 micrometers wavelength region. BGG glasses are low cost alternatives to the currently used window materials and are being scaled-up to large sizes for various Department of Defense applications. Fabrication of a high optical quality 6.5' X 8.5' X 0.5' rectangular BGG glass window has already been demonstrated. A transmitted wave front error of about (lambda) /15 at 632 nm has been achieved in polished BGG glasses. Although the mechanical properties of BGG glasses are acceptable for various window applications, it is demonstrated here that they can be further improved significantly either by the glass-ceramization process or by forming a composite. Neither process adds any significant cost to the final window material. The glass composite transmits in the 0.5 - 5 micrometers region while the crystallite size in the glass-ceramic currently limits its transmission to the 2 - 5 micrometers region.

Rates of dehydroxylation of two Nd-doped metaphosphate laser glasses (LG-770 and LHG-8) are measured and modeled. Glass melts ranging in size from 100 g to 2.8 kg were bubbled with O₂ containing various H₂O partial pressures (P_{H(subscript 2}O)) and with O₂/Cl₂ mixtures at temperatures ranging from 925 - 1300 degree(s)C. The OH content in the glass was measured by monitoring the OH absorption at 3.333 micrometers at various bubbling times. The OH removal by inert gas bubbling (e.g. O₂ bubbling) is governed by the transport (diffusion) of OH to the glass liquid/vapor interface and by the chemical equilibrium between OH at the surface and H₂O in the gas phase. The equilibrium OH content in glass melts bubbled with O₂ containing different P_{H(subscript 2}O) varies as P_{H(subscript 2}O)^1/2.

Fluorozirconate and fluoroaluminate glasses doped with 0, 1.5 and 3.0 mol% NdF₃ were made under a variety of processing conditions ranging from dry (less than 5 ppm H₂O) and oxygen- free using ultra-pure raw materials to an open laboratory furnace with less pure raw materials. Even under the optimum dryness and purity conditions 20 g batches of fluoroaluminate glass could not be made crystal-free whereas fluorozirconate glass could. However a blue shift in the ⁴I_9/2-⁴I_13/2 fluorescence band was noted for the fluoroaluminate glass compared to the fluorozirconate as had been reported in other publications along with a blue shift in the same band for glasses made under drier conditions. The environment around the neodymium was examined using EXAFS with the Nd L_II edge for the glasses and dry and hydrated NdF₃. Hydrated NdF₃ has then first peak in the EXAFS Fourier transform at shorter distance with a broader FWHM. Fluoroaluminate glasses exhibited the same trends when made in more hydrated conditions. This would be consistent with at least partial coordination of the Nd by OH.

This paper analyzes all factors affecting the transmittance of compound glass optical fibers. By studying pre-treatment of raw materials, optical structure design of compound glass fibers, glass level control and temperature field control of the double crucible, a proper production process and all parameters to be controlled for making compound glass optical fibers have been established. Finally, the optical transmittance of optical fibers made by the established process has shown about 10% increase.

Interferometric measurements are often used to assess the homogeneity of optical glass. In order to determine the uniformity of refractive index of optical glass due to chemical fluctuations within the glass, residual stress from thermal processing must be low. We provide data showing the effects of residual stress, resulting from the annealing process, on the transmitted wavefront used to grade optical glass quality.

The laser cutting technology utilizes a laser beam to apply heat to the glass. At first, the glass is heated locally by a CO₂ laser beam. The next step is a defined cooling. On account of the quick heating followed by a fast cooling, tensions are caused within the glass. The consequence will be a vertical and smooth fracture along the cutting edge where the tension was induced without any edge chips or micro cracks. Owing these characteristics of the cutting edge, the mechanical strain capacity is increased considerably compared to conventional scribed and broken edges. Post processing like grinding will be reduced or will even become unnecessary. The production costs will be considerably reduced whereas the production quality will be improved simultaneously by the new laser cutting process in connection with the appropriate machines.

By the sol-gel processing, silver ions in presence of stabilizing function (3-thiocyanatopropyl)triethoxysilane are reduced by heating gels at 180 C for several times in air atmosphere. The spectroscopic Uv-Vis observations, confirm silver nanoparticles presence with peak maximum around 350 nm. The optical properties of the metallic particles are observed at room temperature as function of time, and the absorption spectra practically do not change, which indicated they are trapped and stabilized within the fine porous silica cage. Mie theory calculations, considering the mean free path effect of the conduction electrons, are compatible with experimental spectra, indicating homogeneity in size and form of the metallic nanoparticles. Smithard correlation curve, between half width height (W_1/2) of the optical absorption and the particle diameter 2r, predict silver particles size between 4 and 10 nm, during composite heating. Activation energy was measured and compared with previous data on similar systems and the probable reduction process are discussed.

Fabrication and application of periodic grating have been spreading widely in optical systems. In this paper, we report a single step UV imprinting of sinusoidal surface grating on photosensitive hybrid sol-gel SiO₂/TiO₂ glasses by a holography technique utilizing a rotary mirror stage. The synthesis of the hybrid sol-gel glass film is introduced. Characteristics of the sol-gel glass including refractive index and film thickness are discussed and experimental results are presented as a function of baking temperature and UV exposure time respectively.

The UV-photosensitivity effect in germania-doped optical waveguides has become an important area of research because of the ease and utility of making Bragg gratings. In this study we report on the finding of a large UV-induced refractive index change in a conventionally melted an alkali-alumino-boro-germano- silicate composition that has been loaded with molecular hydrogen. The exposure was done with either CW 244-nm light, or a pulsed KrF excimer laser at 248-nm. A modulated refractive index of the order of 2-3 X 10^-4 has been measured.

We have investigated bulk and coated optical elements as well as UV detectors and CCD cameras in long term exposure experiments using 193 nm and 157 nm continuously pulsed laser irradiation at high fluence levels up to 10 mJ/cm² and at repetition rates of 1 kHz. The irradiation is performed in nitrogen purged exposure chambers. The samples accumulate several billion of laser shots up to total dose values above 100 kJ/cm². In this report we present some recent results on optical elements and UV- detectors each of several different suppliers which are illuminated by more than 2.4 billion shots of 157 nm laser radiation. The original and final properties are compared.

Storage Ring Free Electron Laser are attractive and full of promise tunable and powerful laser sources for the UV range. Concerning the optical cavity, the relatively small gain obtained in the UV calls for the necessity to use high reflectivity multilayer mirrors with reliable longevity in synchrotron environment. It is also crucial to limit their absorption in order to optimize the extracted power required for most of the applications. Indeed, the front mirror of the laser cavity receives not only the first harmonic where the lasers operates but all the synchrotron radiation emitted by the undulator: a wide spectrum extending towards X rays. These short wavelengths are responsible for the mirror degradation which results from changes in the coating materials (high induced absorption, color centers, heating...) as well as from carbon contamination due to cracked hydrocarbons originating from the residual vacuum atmosphere. Deposition technologies which allow the manufacture of very dense oxide coatings with low absorption and high reflectivity in UV spectral region were optimized and characterized for this purpose. We report here degradation studies performed on UV mirrors for Storage Ring Free Electron Lasers down to wavelengths as short as 200 nm.

In order to improve the degradation stability of dielectric mirrors for the use in UV-Free Electron Laser optical cavities a comparative study of the properties of SiO₂, Al₂O₃, and HfO₂ single layers was performed which was addressed to grow very dense films with minimum absorption in the spectral range from 200 nm to 300 nm. The films have been deposited by low loss reactive electron beam evaporation, by ion assisted deposition using a Mark II ion source, and by plasma ion assisted deposition using the APS source. Optical and structural properties of the samples have been studied by spectral photometry, infrared spectroscopy, x-ray diffraction and - reflectometry, as well as by investigation of the surface morphology. The interaction of UV radiation with photon energies close to the band gap was studied. For HfO₂ single layer, LIDT at 248 nm were determined in the 1-on-1 and the 1000-to-1 test mode in dependence on the deposition technology and the film thickness.

Damage to the eye can result from unexpectedly low levels of light, particularly at the shorter wavelengths. UV emitting lasers such as Excimer (248 nm, 308 nm, 351 nm) and HeCd lasers (325 nm, 354 nm) are increasingly used in various applications including: medical, research (spectroscopy), stereolithography and semiconductor industries. Therefore, protection against harmful UV light is needed, but more importantly, development of a material that will react to detect the presence of UV light while completely protecting the wearer, is of great interest. Such materials have been developed with the use of lightweight clear plastics such as acrylics by incorporation of UV reactive photochromic compounds into a polymeric matrix.

The development of photon devices requires the materials with high optical response speed. A type of composite thin films composed of nano-scale Ag particles and barium oxide medium was fabricated. There were two methods in the formation of the thin films: single metallic layer method and triple metallic layer method. The thickness of the thin films in different formation methods was approximately 150 nm or 300 nm, while, Ag particles had an average diameter of 20 nm or 15 nm. A pump-probe method was used for measuring the transient change of optical transmissivity of thin films which were stimulated by ultrashort laser pulses of duration 150 fs and wavelength 647 nm. The Ag-BaO thin films presented that the transmissivity increased rapidly.

This paper has found for the first time that in the case of indirect up-conversion sensitization, the proportion of the up- conversion luminous intensity of (formula available in paper) under direct sensitization to 7.50 X 10^-2 (or 1/13.3) under indirect sensitization. This paper has analyzed the physics understanding of the upconversion phenomenon penetratingly. It is found that spontaneous radiation A and multiphoton non-radiate relaxation W_n does act simultaneously.

Phosphate glasses containing metallic copper nanoparticles were prepared by melt-quenching and post heat treatment method. The average size, size-distribution, concentration, and dispersion of metallic copper nanoparticles in phosphate glasses were investigated by using both spectrophotometer and transmission electron microscope. The time-temperature-deposition diagram for the glass dispersed with metallic copper nanoparticles was constructed based on the particle size and dispersion of metallic copper nanoparticles in phosphate glasses.

Vacuum ultraviolet (VUV) transmission spectra of aluminium fluoroberyllate glasses, MgF₂-CaF₂-SrF₂-AlF₃- BeF₂ (MCSA-Be) and MgF₂-CaF₂-SrF₂-AlF₃- YF₃-BeF₂ (MCSAY-Be), were measured in a wavelength region from 130 to 240 nm. The influence of YF₃ and BeF₂ on the thermal stabilization against crystallization and VUV transmittance was investigated. With increasing the BeF₂ content, crystallization tendencies decreased in the MCSA-Be and MCSAY-Be glasses. The MCSA-20Be glass composition exhibited the shortest wavelength of UV absorption edge (approximately 147 nm), and the highest transmittance at 157 nm (60% for 2 mm sample thickness). The transmittance at 157 nm and the slope (steepness) of the absorption edge were found to be significantly influenced by the trace amount of Fe²⁺.

The broad photoluminescent emission from Ce:Sr_0.6Ba_0.4Nb₂O₆ has been observed at 550 nm with excitation in the blue. Photoluminescent excitation spectrum has revealed a trivalent cerium absorption band that is assumed to be a 4f-5d transition. Temperature dependence of the photoluminescence spectra and its lifetimes in 15 - 365 K showed the existence of two different trap centers in the material. We have also investigated the photoluminescence during two-wave mixing experiment to study trap centers.

A heavily magnesium (5.0 mol%) and erbium (2.0 mol%) codoped lithium niobate crystal has been grown using Czochralski method. The crystal is highly transparent and has strong resistance to photorefractive damage. Spectroscopic study showed that the crystal exhibits the typical f-f transitions of erbium ions. Photoluminescence at 1.54 micrometers was measured under excitations at 514 nm from an Ar⁺ laser and 980 nm from a GaAs laser diode. The temporal evolution of the 1.54 micrometers emission showed a single-exponential decay with a lifetime of approximately 5 ms. Upconversion property of the crystal under 980 nm excitation was also studied and the transient spectrum of the green emission at 548 nm also yields a single-exponential decay with a lifetime of approximately 35 microsecond(s) . The results have shown that clustering and pairing of Er ions could be effectively avoided by magnesium codoping.