Project description
The main objective of the project is to obtain and characterize crystalline materials dedicated for lasers and magneto-optical applications (fluorides and oxides) within and with the development of a Centre of Excellence in the field of crystal growth at WUT.
Objectives 1., 2. and 3.
Obtaining crystals for laser (Tm3+ and Tb3+ – doped fluorides, Yb3++ Er3+ doped sesquioxides) and magneto-optical (KR3F10, RE2O3, RE = Y, Tb) applications
Experimental determination of growth parameters. The crystals will be grown in WUT crystal growth facilities using Bridgman and Czochralski furnaces. Experiments will be performed to find: the temperature gradient in the growth zone, set-up design (number of heat shields and their positions in the growth set-up), position of crucible in the heater, pulling speed, rotation speed, growth rates.
Optimization of crystal growth process using numerical modelling. Time-dependent 3D computations will be carried out using the STHAMAS3D software (developed by Prof. Vizman). Temperature field and interface shape will be obtained and compared with the experimental results.
Growth of various concentrations of Tm3+ and Tb3+ – doped fluoride (CaF2, BaF2), KRE3F10, RE2O3 (RE = Y, Tb) and Yb3+ / Er3+ – double doped sesquioxides (Y2O3, Gd2O3, Lu2O3) crystals using vertical Bridgman and flux methods. All the crystals will be grown at WUT: the fluorides in two Bridgman type installations (one developed in house and one made by GERO GmbH) and the sesquioxides by flux method in CZ installation (type Cyberstar 05-03). Processing processes will take place using a crystal cutting and polishing machines, type Buehler MINIMET-1000 and Buehler ISOMET. At least 3 different concentrations of dopants will be considered for each type of host.
Objective 4.
Characterization of structural defects of grown crystals for laser and magneto-optical applications
X-ray Diffraction (XRD) Analysis (using a BRUKER D8 Advance Diffractometer at WUT facility) and full structural Rietveld and classical lattice parameter refinements to check the crystalline structure will be performed. Laue back-scattering analysis on crystals (budgeted in the acquisition list) will be performed in order to determine the orientation of single crystals necessary for cutting, polishing a surface or for doing measurements. After each growth experiment, a bunch of samples will be tested in order to feed the crystal growth considerations back.
Studies on the structural defects (i.e. dislocations) using chemical etching method will be made on all crystals using an optical microscope equipped with an image capture and processing module (type OMNIMET) in order to evaluate the crystal quality, and in order to be later correlated with the laser efficiency and magneto-optical properties of the materials.
Chemical analysis and study of dopant ions distribution along the crystals. The dopant distribution along the crystals will be determined using EPMA, GDMS analysis (a specific budget it is planned for these types of analysis) and using the optical absorption method. From the distribution of the absorption coefficient along the crystal, using the Scheil formula, we will obtain the effective segregation coefficient for each of the ions.
Dielectric spectra of crystals will provide information about charge compensating defect type (e.g. the F- ion, in various positions together with the RE3+ ion is an electric dipole, considered as a defect). Correlated with the optical absorption measurement, the type and number of charge compensating defects will be determined (e.g. possible positions of the Er3+ ions in the host lattice and how the charge compensation take place). Measurements will be performed using a RLC Meter type ZM2355, NF Corporation, Japan.
DFT and DFT + eDMFT studies for structural analysis (dopant distribution, defects-dislocations) will allow us to perform structural relaxations using numerical methods to distinguish energetically between interstitial/substitution impurities and their effects on the local structure. Theoretical calculations/interpretations of XANES and phonon driven diffuse scattering in order to understand local distortions induced by the impurities will be made.
Objective 5.
Characterization of spectroscopic properties of fluoride and sesquioxides grown crystals for laser applications
Optical absorption spectra measurements in UV-VIS-IR range will be recorded by a Shimadzu 1650PC spectrophotometer and will provide information about the nature and site symmetry (e.g. Oh,C3v, C4v, clusters) of the dopant dissolved in host (the arrangement of the RE ions). Absorption spectroscopy will be also used to quantify the amount of dopant in the host.
Emission, excitation and lifetime measurements. For the first time will evaluate the laser potential for the double doped (Yb3+ / Er3+) sesquioxides crystals (obtained by the flux method using a borate-based solvent) as promising UV-VIS laser materials. Also the luminescence properties of Tm3+ and Tb3+ – doped fluorides (CaF2, BaF2) will be studied, determining which matrix is more suitable. For (Yb3+ / Er3+) doped-sesquioxides (with Y, Gd, or Lu) under 980 nm near-infrared (NIR) excitation, UC emission bands in the 410-660 nm range are to be expected. On the other hand, for CaF2 and BaF2 fluorides doped with RE ions (Tb3+, Tm3+), emission bands are expected in the VIS range at around 480 – 680 nm for pumping wavelength of 380 – 485 nm for Tb3+ and in SWIR range at 1500 nm, 2300 nm by 800 nm pumping for Tm3+. The emission spectra of all doped crystals will be recorded using an existing Perkin Elmer LS55 Spectrofluorimeter but also specific budget was planned in order to buy a much more efficient and versatile modular equipment that which will allow lifetime measurements. An important direction that we intend to achieve is to find the dopant concentration for which the laser properties in the desired spectral region are the best.
Judd-Ofelt (J-O) analysis will be applied to calculate the spectroscopic parameters from the optical absorption spectra. The obtained results will be compared with the experimental luminescence spectra. We expect to prove the UV-VIS emission for all the RE3+ doped crystals, and to evaluate their laser potential calculating the emission cross-section, quantum efficiency and optical gain parameter.
DFT and DFT+eDMFT studies for spectroscopic analysis (optical properties, interaction of the correlated f states with the host) will be used to perform electronic calculations to understand the interactions between the host and the correlated f electrons and their influence on the J-O analysis.
Objective 6.
Characterization of magneto-optical properties of KRE3F10 crystals and RE2O3 (R = Y, Tb) solid solutions
Magneto-optic effect will be characterized by measurement of Faraday rotation and Verdet constant calculations with respect to the Tb3+ content. Verdet constant calculations from Faraday rotation measurements, FTIR measurement for absorption aabs due to OH- inclusions, magnetic susceptibility measurements (for Tb4+ content estimation from experimental effective magnetic moment), optical homogeneity measurements as a function of temperature as well as high power laser resilience experiment will be performed by international collaboration (France, LICB-Dijon).
Ground state properties using DFT + eDMFT (state-of-the-art method in studying f electron systems) will be studied by calculations of the electronic and optical properties for the KRE3F10 using the state-of-the-art DFT + eDMFT method, where we treat the strong electronic correlations between the f electrons dynamically. We plan to validate our theoretical method with the experimental results obtained in this study and then check the effects of substituting K/F ions in order to predict compounds with improved magneto-optical properties.