Edvard Kokanyan

Reduced photorefraction in hafnium-doped single-domain and periodically poled lithium niobate crystals

Measurements of photo-induced birefringence n single domain and periodically poled lithium niobate (LN) crystals containing different non-photorefractive impurities are presented. We find that doping by HfO/sub 2/ is very effective in reducing the photorefraction.

Absorption intensities and emission cross sections of Tb3+ (4f8) in TbAlO3

Trivalent terbium absorption intensities in single-crystal TbAlO3 are analyzed using the Judd-Ofelt model to assess the crystal’s potential as a solid state laser system. The standard Judd-Ofelt model was applied to the room temperature absorption intensities of Tb3+ (4f8) to determine the phenomenological intensity parameters Ω2, Ω4, and Ω6. Seven multiplet manifolds are identified and the absorption intensities of these manifolds are least-squares fitted to the calculated intensities to obtain the intensity parameters: Ω2=40.52×10−20cm2, Ω4=8.74×10−20cm2, and Ω6=2.26×10−20cm2 in TbAlO3. These intensity parameters are then applied to determine the radiative decay rates and branching ratios of Tb3+ transitions from the D45 to the FJ′7 multiplet manifolds. Based on the results, the radiative lifetime of the excited state manifold D45 is determined from the radiative decay rates and found to be 3.5ms. The calculated lifetime is longer than the measured lifetime, reflecting the nonradiative interactions betw...

Spectroscopic analysis of the Er3+(4f11) absorption intensities in NaBi(WO4)2

A spectroscopic analysis is performed on Er3+ (4f11) ions doped in NaBi(WO4)2 (NBWO) in order to assess this material for its potential as a near-infrared laser. The Judd–Ofelt model is applied to the room-temperature absorption intensities of Er3+ (4f11) in NBWO to obtain the three phenomenological intensity parameters: Ω2=5.50×10−20 cm2, Ω4=1.00×10−20 cm2, and Ω6=0.71×10−20 cm2. The intensity parameters are then used to determine the radiative decay rates (emission probabilities), radiative lifetimes, and branching ratios for the Er3+ transitions from the excited state multiplet manifolds to the lower lying manifold states. Using the radiative decay rates for the Er3+ (4f11) transitions between the corresponding excited states and the lower lying states, the radiative lifetimes of eight excited states of Er3+ are determined in this host. Using the measured room temperature fluorescence lifetime of approximately 4.75 ms and the radiative lifetime of 5.63 ms as predicted by the Judd–Ofelt model for the 4I13/2→4I15/2 (1.52 μm) transition of Er3+ in NBWO the quantum efficiency is determined to be approximately 84% for this laser material.A spectroscopic analysis is performed on Er3+ (4f11) ions doped in NaBi(WO4)2 (NBWO) in order to assess this material for its potential as a near-infrared laser. The Judd–Ofelt model is applied to the room-temperature absorption intensities of Er3+ (4f11) in NBWO to obtain the three phenomenological intensity parameters: Ω2=5.50×10−20 cm2, Ω4=1.00×10−20 cm2, and Ω6=0.71×10−20 cm2. The intensity parameters are then used to determine the radiative decay rates (emission probabilities), radiative lifetimes, and branching ratios for the Er3+ transitions from the excited state multiplet manifolds to the lower lying manifold states. Using the radiative decay rates for the Er3+ (4f11) transitions between the corresponding excited states and the lower lying states, the radiative lifetimes of eight excited states of Er3+ are determined in this host. Using the measured room temperature fluorescence lifetime of approximately 4.75 ms and the radiative lifetime of 5.63 ms as predicted by the Judd–Ofelt model for the 4I...

Linear and nonlinear optical properties of Hafnium-doped lithium-niobate crystals

Measurements of birefringence, second-harmonic phase-matching conditions, and nonlinear coefficient d(31) are performed for a set of Hafnium-doped congruent lithium niobate (Hf:cLN) crystals as functions of dopant concentration. The data highlight that the threshold concentration, above which there is a change in the Hf incorporation mechanism, is slightly above 2mol% and that, up to this value of concentration, the efficiency of nonlinear processes is not affected by the dopant insertion. Combining these results with those already present in literature, Hf:cLN crystals appear to be very promising candidates for the development of photorefractivity-free wavelength converters working at room temperature.

Periodically poled structures in doped lithium niobate crystals

Periodically poled lithium niobate crystals doped with transition-metal ions Cr3+ and Hf4+, and rare earth ions Er3+ and Yb3+ were grown by the Czochralski method. Periodic domain structures were obtained either by rotation-induced growth striations (during off-centered growth) or by the application of a periodic electric current passed through the crystal-melt system during growth. We achieved controlled formation of domains having a width from 4 to 50 μm. The influence of dopant ion concentrations and the type of these dopant impurities were investigated relative to the formation of the periodic domain structure. Methods that involve the use of scanning electron microscopy and single-pass second harmonic generation show a high degree of domain structure uniformity over a large section (∼3 mm) of the crystals grown. Samples doped with HfO2 show reasonable photorefractive resistance, an important feature in the application of these materials in photonic devices.

Strongly sublinear growth of the photorefractive effect for increasing pump intensities in doped lithium-niobate crystals

The pump intensity dependence of the birefringence change due to the photorefractive effect in Hf-doped and Mg-doped congruent lithium niobate (LN) crystals is reported. The birefringence change is found to be very weakly dependent on the light intensity. This is in strong contrast with the behavior of undoped congruent LN, and much more similar to what happens in stoichiometric LN. The data also indicate that the minimum HfO2 concentration necessary to strongly reduce photorefractivity is lower than the value of 4 mol % indicated by previous experiments.

Phonon effects on zero-phonon transitions between Stark levels in NaBi(WO4)2:Yb3+

We report results from a modeling study of temperature-dependent line broadening and associated Debye–Waller factors (DWFs) for zero-phonon transitions between the F2J (J=7/2,5/2) manifolds of Yb3+ in NaBi(WO4)2. The Hamiltonian includes the electron-phonon interaction between the electronic states of the Yb3+ (4f13) ions and the lattice (Debye) phonon spectrum of the host crystal in a second quantization formulation. The temperature dependences of the linewidths, DWFs, and the intensities of the transitions are reported in terms of the ratio of temperature to Debye temperature (T/TD). Of particular interest is the temperature dependence of the most intense transitions at emission wavelengths of 996.6 and 1010.2 nm. These transitions are the best candidates for low-threshold stimulated emission based on measured emission cross sections and lifetimes. We find that with a decrease in temperature, an abrupt increase in radiation intensity occurs for the 1010.2 nm emission transition, suggesting that an optim...

MICRORAMAN AND PHOTOREFRACTIVITY STUDY OF HAFNIUM-DOPED LITHIUM NIOBATE CRYSTALS

We present an investigation of the properties of HfO2-doped lithium niobate crystals, in view of their possible utilization as low-photorefractivity crystals for wavelength converters operating at room temperature. MicroRaman measurements indicate that the linewidth of a specific mode can be used as a local indicator of crystal composition, and show that the grown crystals present very good uniformity. The mechanism by which the photorefractivity is strongly reduced when the HfO2 concentration is above 4 mol% is studied by combining measurements of birefringence variation, under green-light illumination, with electrical phototransport data.

Spectra and energy levels of Er3+(4f11) in NaBi(WO4)2

Absorption and fluorescence spectra of Er3+(4f11) in crystals of NaBi(WO4)2 (NBW) are reported at temperatures between 15 K and room temperature. The absorption spectra include the details of the crystal-field splitting of 11 multiplet manifolds, 2S+1LJ of Er3+(4f11), spanning the wavelength range between 350 nm and 1550 nm. The crystal-field splitting of the ground-state 4I15/2, is obtained from an analysis of the fluorescence spectrum, 4S3/2→4I15/2. Spectra are characterized by inhomogeneous broadening due to the disordered crystal structure in which different valency cations, Na+ and Bi3+, statistically fill the S4 symmetry sites. The Er3+ ions likely replace the Bi3+ ions in these sites. A quasi-center model has been chosen to interpret the crystal-field splitting of each manifold, using D2d rather than S4 symmetry as the site for the rare-earth ion in the lattice. To test the feasibility of the model, the splitting of the energy levels of Nd3+ in NBW was carried out first and compared with experiment...

Photoconductive and Electro-Optical Properties of Hf Doped Lithium Niobate Crystals

The optical and electrical properties of a set of Hf doped Lithium Niobate crystals with doping level from 2 to 8 mol% are reported. The photoconductivity is found to increase remarkably for Hf concentration equal and above 4 mol%. In contrast, the electro-optical coefficients keep almost constant with the crystal structural properties preserved. The conclusion is drawn that the decrease of the photorefractive response in Hf doped Lithium Niobate recently observed by other authors is mainly related to the increased photoconductivity.