Thermal and long-term stability of fast-ion-irradiation-induced refractive index changes in lithium niobate crystals

Irradiation of congruently melting, nominally undoped lithium niobate crystals (LiNbO₃) with high-energy, low-mass ³He ions, which are transmitted through the crystal, causes large and stable changes of the refractive index. In the irradiated regions, the extraordinary index is increased whereas the ordinary index is diminished. The decay of these changes upon annealing treatments up to 400°C is investigated, exhibiting a strongly non-monoexponential decay behaviour. Long-term measurements of the refractive index changes yield no pattern erasure on a timescale of four years.     

Radiation-damage-assisted ferroelectric domain structuring in magnesium-doped lithium niobate

Irradiation of 5% magnesium-doped lithium niobate crystals (LiNbO₃:Mg) with high-energy, low-mass ³He ions, which are transmitted through the crystal, changes the domain reversal properties of the material. This enables easier domain engineering compared to non-irradiated material and assists the formation of small-sized periodically poled domains in LiNbO₃:Mg. Periodic domain structures exhibiting a width of ≈520 nm are obtained in radiation-damaged sections of the crystals. The ferroelectric poling behavior between irradiated and non-treated material is compared.     

Fabrication of embedded waveguides in lithium-niobate crystals by radiation damage

Irradiation of lithium-niobate crystals (LiNbO₃) with fast, high-energy ³He ions changes the refractive index in the interaction region where the ions speed through the material. Thus an inhomogeneous flux density profile can be used for a tailored modification of the optical properties of LiNbO₃ crystals, without employing ion implantation. A new method to fabricate embedded, polarization sensitive channel waveguides in LiNbO₃ utilizing accelerated ³He ions with an energy of 40 MeV is demonstrated. PACS 78.20; 42.82     

Ultraviolet to infrared refractive indices of tetragonal double tungstate and double molybdate laser crystals

Sellmeier parameters of thirteen tetragonal (space group $I\overline{4}$ ) double tungstate and double molybdate laser crystals with M⁺T³⁺(X⁶⁺O₄)₂ composition have been calculated using the room temperature refractive indices determined from the ultraviolet band gap of the crystals to ????2 ??m. All considered crystals are uniaxial but only crystals with Bi in their composition show a significant birefringence (??n>10^?2). The refractive index value increases for the sequence T³⁺=La, Y, Gd, Lu, and Bi independently of the M⁺ and X⁶⁺ cation pair. Implications for the design of laser waveguides and laser pulse dispersion are discussed.     

Analysis of the refractive indices of lithium niobate at high temperatures

The temperature dependence of the refractive indices of lithium niobate crystals of stoichiometric composition is analyzed. It is shown that, in the region of the ferroelectric phase, the electronic dipole polarizability of oxygen ions significantly depends on the crystal temperature. The difference in the refractive indices Δn = n _e ? n _o in the paraelectric phase at λ = 1200 nm is determined to be 0.036 ± 0.003.     

Diffraction grating fabrication in lithium niobate and KDP crystals with femtosecond laser pulses

We report studies of the modifications induced inside lithium niobate and KDP crystals by Yb:KGW ultrafast laser pulses, having 300-fs pulse duration and operating at 100-kHz repetition rate. By focusing the laser beam with a 0.42 numerical-aperture objective, we have recorded homogeneous volume Bragg gratings in the bulk of the niobate crystal that showed excellent diffraction efficiencies, reaching up to 87%, and remained permanent after thermal annealing for one hour at 150°C. The refractive index modification level was found to be 0.002 in lithium niobate. The results show that lithium niobate is a very promising crystal candidate for microphotonics applications. On the contrary, in the KDP crystal no smoothly modified refractive index zones were created.     

Giant Birefringence of Lithium Niobate Crystals in the Terahertz Region

Terahertz time-domaln spectroscopy （THz-TDS） is used to study the spectral response of lithium niobate crystals （LiNbO3） in the far infrared region. The optical constants are derived from the measured complex refractive index. A giant birefringence is observed in this material, and the average refractive-index difference between the ordinary wave and the extraordinary wave, no - ne, can reach up to about 1.6. Such a large birefrlngence is attributed to the different p honon modes orAl（z） and E （ x , y ）. This unusual property makes LiNbO3 a promising material to be used as a functional material in the terahertz region, e.g. employed as wave-plates and polarization separators.     

Swift ion beam irradiation on lithium niobate single crystals

Single crystals of lithium niobate were irradiated by 50 MeV Li³⁺ion irradiation at various ion fluences. The irradiated single crystals were subjected to various studies such as X-ray diffractometry, UV–VIS–NIR analysis and refractive index measurements. Atomic force microscopy patterns were recorded to study the surface changes after irradiation. Micro-Raman measurements were also recorded for irradiated specimens and the result of the measurements are discussed in detail.     

Ultraviolet nanosecond laser-assisted micro-modifications in lithium niobate monitored by Nd<Superscript>3+</Superscript> luminescence

This work reports on the microstructural modifications produced by nanosecond ultraviolet ablation in neodymium doped lithium niobate crystals. The neodymium ions have been used as optical probes to determine the extension and nature of the modified bulk material. From micro-luminescence experiments we have been able to determine the spatial distribution of the UV ablation induced material densification, local disorder and defect creation. Results have been compared to those previously obtained from femtosecond irradiated lithium niobate crystals. PACS 42.55.Rz; 42.62.Fi; 42.70.-a     

Manifestation of the specific structural features of lithium niobate single crystals of different composition in their Raman spectra

The Raman spectra of nominally pure lithium niobate single crystals of congruent, close to stoichiometric, and stoichiometric compositions and congruent lithium niobate single crystals doped with Gd³⁺, Y³⁺, and Mg²⁺ ions have been investigated. Weak lines whose widths anomalously decrease with an increase in cation sublattice disordering at a change in the single crystal composition were found for the first time. These lines may be indicative of fine ordering processes involving structural units of the cation sublattice, as a result of which this sublattice is disordered as a whole.     

New approach to analyzing and evaluating cross sections for partial photoneutron reactions

The presence of substantial systematic discrepancies between the results of different experiments devoted to determining cross sections for partial photoneutron reactions??first of all, (??, n), (??, 2n), and (??, 3n) reactions??is a strong motivation for studying the reliability and authenticity of these data and for developing methods for taking into account and removing the discrepancies in question. In order to solve the first problem, we introduce objective absolute criteria involving transitional photoneutron-multiplicity functions F ₁, F ₂, F ₃, ??; by definition, their values cannot exceed 1.0, 0.5, 0.33, ??, respectively. With the aim of solving the second problem, we propose a new experimental-theoretical approach. In this approach, reaction cross sections are evaluated by simultaneously employing experimental data on the cross section for the total photoneutron yield, ?? ^expt(??, xn) = ?? ^expt(??, n) + 2?? ^expt(??, 2n) + 3?? ^expt(??, 3n) + ??, which are free from drawbacks plaguing experimental methods for sorting neutrons in multiplicity, and the results obtained by calculating the functions F _theor ¹ , F _theor ² , F _theor ³ , ?? on the basis of the modern model of photonuclear reactions. The reliability and authenticity of data on the cross sections for (??, n), (??, 2n), and (??, 3n) partial reactions???? ^eval(??, in) = F _i ^theor ?? ^expt(??, xn)??were evaluated for the ⁹⁰Zr, ¹¹⁵In, ^{112,114,116,117,118,119,120,122,124}Sn, ¹⁵⁹Tb, and ¹⁹⁷Au nuclei.     

Neutron-rich hypernuclei in the chiral soliton model

The binding energies of neutron-rich strangeness S = ?1 hypernuclei are estimated in the chiral soliton approach using the bound state rigid oscillator version of the SU(3) quantization model. Additional binding of strange hypernuclei in comparison with nonstrange neutron-rich nuclei takes place at not large values of atomic (baryon) numbers, A = B ?? ??10. This effect becomes stronger with increasing isospin of nuclides, and for the ??nuclear variant?? of the model with rescaled Skyrme constant e. Binding energies of _?? ⁸ He and recently discovered _?? ⁶ H satisfactorily agree with data. Hypernuclei _?? ⁷ H, _?? ⁹ He are predicted to be bound stronger in comparison with their nonstrange analogues ⁷H, ⁹He; hypernuclei _?? ¹⁰ Li, _?? ¹¹ LI, _?? ¹² Be, _?? ¹³ Be, etc. are bound stronger in the nuclear variant of the model.     

Role of nanosecond processes in the formation of domains in ferroelectric crystals

The contribution from pyroelectric and photovoltaic effects to the formation of light-induced domains in crystals of oxide ferroelectrics of lithium niobate type upon irradiation by nanosecond focused laser beams is considered. It is established that inverted domains are formed in irradiated regions under the combined effect of generated pyroelectric and photovoltaic fields with a sharp increase in temperature. It is shown that the most stable domains are formed in lithium niobate samples containing Fe²⁺ and Fe³⁺ impurity ions.     

Experimental Study of Photon Induced Reactions on 3He and 4He at Low Energies

Data are reported for the photodisintegration cross section of the reaction ³He(??, p)²H at ten energies between 7.0 and 16.0?MeV. Very preliminary data are presented for the reaction ⁴He(??, p)³H between 22.0 and 29.5?MeV in 0.5?MeV energy steps, and for the reaction ⁴He(??, n)³He at three energies around 28.0?MeV. High-pressure He/Xe gas scintillators served as target and detector. Our data are in better agreement with recent theoretical calculations than the majority of the existing data for all three reactions, but differ significantly from recent data taken with a mono-energetic photon beam and a time-projection chamber.     

Doping mechanism of optical-damage-resistant ions in lithium niobate crystals

The doping mechanism of optical-damage-resistant ions (Mg²⁺, Zn²⁺, In³⁺, Sc³⁺, Hf⁴⁺, and Zr⁴⁺) in the lithium niobate crystallographic frame is quantitatively studied from the chemical bond viewpoint. Calculated results show that optical-damage-resistant ions have a strong interaction with the lithium niobate matrix, which is quantitatively evaluated by the deviation between normal and calculated valence states and the global instability index. All optical-damage-resistant ions first substitute Nb_Li and then Li ions, they change their dopant occupancies from Li to Nb sites at the same global instability index value 0.1055. On the basis of such a quantitative interaction, the doping mechanism of these ions is finally derived. Furthermore, a criterion in searching for new optical-damage-resistant ions is also proposed.     

Modeling and Characterization of Border Domain Effects in Periodically Poled Lithium Niobate Crystals Grown by the Off-center Czochralski Technique

We report on the optical characterization of periodically poled lithium niobate crystals grown through the Czochralski off-center technique. The optical diffraction technique pointed out that refractive index changes occur at the border domain. Their effect on the second harmonic generation is therefore investigated by numerical analysis in order to predict the conversion efficiency of the grown crystals.     

Growth,characterization and upconversion properties of erbium-doped potassium lithium tantalate niobate single crystals under 975 nm laser excitation

Potassium lithium tantalate niobate single crystals doped with erbium ions are grown by top-seeded solution growth method. The crystals are characterized by X-ray diffraction and differential thermal analysis. The refractive indices of the crystal are measured using ellipsometry method and fitted by Sellmeier equation. The as-grown crystals are tetragonal phase tungsten bronze-type structure with Curie temperature of 271.3 °C. Characteristic Er^3 + absorption bands are observed from 350 to 1100 nm in ultraviolet–visible-near infrared absorption spectra. These crystals emit brightly green and red upconversion fluorescence under 975 nm LD laser excitation, and the steady state upconversion spectra are obtained at room temperature. The red emission intensity increases as the erbium ions concentration increases in crystals. Processes of excited state absorption and energy transfer are responsible for upconversion luminescence. The emission intensities are quadratic dependences on pump power from pump power dependence analyses and deduction of transition rate equation model.     

Glauber Dynamics for the Mean-Field Potts Model

We study Glauber dynamics for the mean-field (Curie-Weiss) Potts model with q??3 states and show that it undergoes a critical slowdown at an inverse-temperature ?? _s (q) strictly lower than the critical ?? _c (q) for uniqueness of the thermodynamic limit. The dynamical critical ?? _s (q) is the spinodal point marking the onset of metastability. We prove that when ??<?? _s (q) the mixing time is asymptotically C(??,q)nlogn and the dynamics exhibits the cutoff phenomena, a sharp transition in mixing, with a window of order n. At ??=?? _s (q) the dynamics no longer exhibits cutoff and its mixing obeys a power-law of order n ^4/3. For ??>?? _s (q) the mixing time is exponentially large in n. Furthermore, as ?????? _s with n, the mixing time interpolates smoothly from subcritical to critical behavior, with the latter reached at a scaling window of O(n ^?2/3) around ?? _s . These results form the first complete analysis of mixing around the critical dynamical temperature??including the critical power law??for a model with a first order phase transition.     

Grating spacing dependence of nonvolatile holographic recording with arbitrary charge transport lengths

Grating spacing dependence of nonvolatile holographic recording in doubly doped lithium niobate crystals is theoretically investigated allowing arbitrary charge transport lengths. It is shown that the nonvolatile refractive index modulation initially increases with increasing grating spacing, then a saturation behavior arises because of the dominant bulk photovoltaic effect. Although different charge transport length results in different nonvolatile refractive index modulation, the grating spacing dependence of nonvolatile holographic recording obeys almost the same rules for arbitrary charge transport lengths. The experimental results obtained by recording nonvolatile holograms in LiNbO₃:Cu:Ce crystals with different grating spacing are consistent with the theoretical analyses.     

Planar waveguides in calcium barium niobate fabricated by MeV He ion implantation

A planar optical waveguide was formed in calcium barium niobate (CBN) crystal by 2.8-MeV He-ion implantation with a dose of 1.0×10¹⁶ ions/cm² at room temperature. The prism-coupling method was used to take dark mode measurements at both 633 nm and 1539 nm. The refractive-index profile (n_o and n_e) of the He-implanted CBN waveguide was analyzed with the reflectivity calculation method. The results show that the MeV He implantation results in a decrease in refractive index in barriers for both n_o (4.1%) and n_e (3.1%), but for n_e there is an increase in the waveguide region. The intensity profile of the guide mode and waveguide loss were obtained by end-fire coupling. PACS 42.79.Gn; 61.80.Jh