Congruent LiNbO3 Research Articles

Multiphoton-induced luminescence contrast between antiparallel ferroelectric domains in Mg-doped LiNbO3

We report on differentiating antiparallel ferroelectric domains in congruent Mg-doped LiNbO3 (Mg:LNO) single crystals through a multiphoton photoluminescence technique. Sample illumination with femtosecond laser pulses at λ = 790 nm results in a broad multiphoton emission spectrum revealing a domain contrast of >3% between virgin and inverted domains. The contrast decreases via annealing and shows an exponential decay in the temperature range from 80 to 150 °C. Our findings give clear ground of a thermally induced structural change by surpassing a specific activation energy. Hence, the reported contrast dynamics must be closely connected to the thermal activation of charged defects, which dramatically alters the internal bias field of these defects. This explanation is also supported when using single crystal LNO of different Mg doping levels showing much lower multiphoton effects for a < 5% Mg concentration. Based on this effect of multiphoton luminescence, it becomes easy to microscopically monitor and quantify virgin and switched domains in LNO and other samples.

Raman Studies of Photorefractive Lithium Niobate Single Crystals

A study of Raman spectra of lithium niobate single crystals: stoichiometric, ostensibly pure congruent LiNbO3, and congruent LiNbO3 containing admixture of yttrium is reported. Raman bands observed in spectra excited by visible laser radiation are thoroughly compared to the same bands excited by infrared laser radiation. The broadening of Raman bands excited by radiation of shorter wavelengths is attributed to photo-refraction induced by visible radiation (514 nm) while being absent under infrared radiation (1064 nm). Attributing the effect of photo-refraction to radiation-induced structural changes in the illuminated part of the crystal lattice the authors interpret the observed broadening of Raman bands as a result of structural disorder caused by the exciting radiation.

The Cation Sublattice Ordering in the Ferroelectric LiNbO3:Zn Single Crystals

It has been revealed for the first time by Raman spectroscopy that in LiNbO3:Zn crystal with Zn2+ concentration range 0.03–0.94 mol.% the ordering of the main doping cations and vacant octahedrons along the polar axis is better than in the undoped crystal of congruent composition. Thus, the configuration of oxygen octahedrons is closer to ideal. The line widths of the Raman spectra in this concentration range are narrower than in congruent LiNbO3 crystals and closer to the line width in stoichiometric crystals with the best known cation ordering. As far as Zn2+ concentration is small, the growth technique changes very slightly compared to the growth of the pure LiNbO3 crystals.

Diode-pumped 1988-nm Tm:YAP laser mode-locked by intracavity second-harmonic generation in periodically poled LiNbO_3

We report a diode-pumped intracavity second-harmonic generation mode-locked solid-state Tm:YAP laser operating at 1988 nm using a periodically poled congruent LiNbO3 as the nonlinear crystal. The threshold of continuous wave mode locking is 11.6 W. The maximum output power is 1.67 W, while the shortest pulse obtained is 4.7 ps at a repetition rate of 97.09 MHz.

Third column electro-optical coefficients of zirconium-doped congruent lithium niobate crystals

All coefficients of the third-column of the unclamped and clamped electro-optic tensor of zirconium-doped congruent LiNbO3 single crystals, in the concentration range up to 2.5mol%, are determined as function of the dopant concentration by direct techniques based on interferometric and Sénarmont optical arrangements at the wavelength of 633nm and at room temperature. It is found that all the unclamped and clamped electro-optic coefficients are relatively constant, except for the sample doped with 2mol% of zirconium. From the obtained experimental values, the acoustic contribution is then deduced and found constant over all the studied samples. Finally, the electro-optic behavior of LN:Zr as function of the dopant concentration was confirmed by dielectric characterizations.

Backward frequency doubling of near infrared picosecond pulses

We report on backward second-harmonic generation using ps laser pulses in congruent lithium niobate with 3.2 µm periodic poling. Three resonant peaks were measured between 1530 and 1730 nm, corresponding to 16th, 17th and 18th quasi-phase-matching orders in the backward configuration, with a conversion efficiency of 4.75 x 10(-5%)/W for the 16th order. We could also discriminate the contributions from inverted domains randomized in duty-cycle.

Terahertz time-domain spectroscopy of congruent LiNbO3 and LiTaO3 crystals

The complex dielectric constants of congruent LiNbO3 and LiTaO3 single crystals in a THz frequency region have been determined directly by using broadband terahertz time-domain spectroscopy (THz-TDS) with reflection configuration. We have successfully extracted the accurate complex dielectric constants by using corrected phase information derived from the transmission measurement results. The lowest E(TO1) phonon modes of both crystals have been observed clearly. On the other hand, additional bands at approximately 2.5 THz below the lowest A1(TO1) mode frequency have been observed. These bands have also been reported by Raman and hyper-Raman spectroscopy and attributed to intrinsic defects in congruent crystals. According to the experimental results that these additional bands were observed at almost the same frequency on Raman, hyper-Raman, and THz-TDS, these bands are suggested to be assigned as a first-order scattering process.

Tantalum and niobium diffusion in single crystalline lithium niobate

Abstract LiNbO3 and LiTaO3 are isomorphous and Nb and Ta have the same valence electron configuration and the same ionic radii. This suggests the use of Ta as a tracer to probe the self-diffusion of Nb in LiNbO3. The diffusion system consisted of a 20 nm layer of LiTaO3 sputter deposited on top of (i) a congruent LiNbO3 single crystal, i.e. (48.3 ± 0.1) mol% Li2O, and on top of (ii) a VTE processed LiNbO3 single crystal with nearly stoichiometric composition, i.e. (49.9 ± 0.1) mol% Li2O. The diffusion anneals (1000 °C ≤ T ≤ 1100 °C) were performed under a constant oxygen partial pressure of 200 mbar. From the resulting SIMS depth profiles of tantalum a constant diffusivity was extracted which can be assumed to reflect the niobium self-diffusivity in LiNbO3. For sub-stoichiometric LiNbO3 the joint discussion of this work and of literature data on the basis of the generally accepted defect model, 4[NbLi4•] = [VLi′], suggests Nb transport in the Li sublattice. For hyper-stoichiometric LiNbO3 the defect model 5 V Nb 5 ′ = L i i • is derived from the Li3NbO4/LiNbO3 solution reaction of the VTE process designed to obtain Li2O-rich LiNbO3 in accordance with the Li2O–Nb2O5 phase diagram. This model is supported by theoretical calculations of the defect formation energy. Interestingly, the migration enthalpy for the Li vacancy mediated transport of the anti-site defect NbLi4• in the Li sublattice of congruent, i.e. sub-stoichiometric, LiNbO3 is similar (within a ± 10% error) to the one derived for the Nb vacancy mediated transport of Nb in the Nb sublattice of hyper-stoichiometric LiNbO3, i.e. about 3 eV. The significant discrepancies between our results and some earlier literature data can be consistently rationalised if the experimental procedures of those studies are carefully analysed.

Notice of Retraction: Strip Waveguide With High Diffusion-Doped Concentration

We report a Ti:Er:LiNbO3 strip waveguide with high diffusion-doped surface Er3+ concentration. The waveguide was fabricated with a technological process in sequence of preparation of noncongruent, Li-deficient LiNbO3 substrate by performing Li-poor vapor transport equilibration treatment on a congruent Z-cut LiNbO3 plate, diffusion of 40-nm-thick Er metal film, and fabrication of 8-μm-wide Ti-diffused strip waveguide. The waveguide retains the LiNbO3 phase and shows the waveguiding characteristics similar to the conventional Ti:LiNbO3 waveguide. Secondary ion mass spectrometry study shows that the Er3+ diffusion reservoir was exhausted and the profile is Gaussian with a surface concentration two times larger than that of the conventional Ti:Er:LiNbO3 waveguide. The waveguide shows stable 1547-nm small-signal enhancement under the 1480-nm pumping without serious optical damage observed, and a 5-dB signal enhancement is obtained for the available coupled pump power of only 90 mW. A saturated net gain as much as 5 dB/cm is predicted theoretically.

Micro-Raman spectroscopic visualization of lattice vibrations and strain in He^+- implanted single-crystal LiNbO_3

Scanning micro-Raman spectroscopy has been utilized to image and investigate strain in He+-implanted congruent LiNbO3 samples. By using abruptly patterned implanted samples, we show that the spatial two-dimensional mapping of the Raman spectral peaks can be used to image the strain distribution and determine its absolute magnitude. We demonstrate that both short- and long-range length-scale in-plane and out-of-plane strain and stress states can be determined using the secular equations of phonon-deformation-potential theory. We also show that two-dimensional Raman imaging can be used to visualize the relaxation of strain in the crystal during low-temperature annealing.

Upconversion for enlarging solar spectrum response in near-stoichiometric and congruent Er:LiNbO3 crystals

To increase the photovoltaic efficiency of solar cell, the visible luminescence produced at the excitation of the near infrared laser light in near-stoichiometric and congruent LiNbO3 crystals heavily doped with Er3+ ions (Er:NSLN and Er:CLN, respectively) were investigated. An enhancement of the green upconversion emission observed in Er:NSLN crystal was attributed to the inefficient cross relaxation processes of 2H11/2+4I13/2→4I11/2+4F9/2 and 4F7/2+4I11/2→4F9/2+4F9/2. The Er content in the crystal was measured by an inductively coupled plasma mass spectrometry (ICP-MS). The OH− absorption and UV–vis-near infrared absorption spectra indicated that the Er3+ cluster sites (ErLi2+–ErNb2-) were dissociated in Er:NSLN crystal. The strong green upconversion emission produced by Er:NSLN crystal will be beneficial for improving the practical performance of solar cells.

Highly efficient terahertz pulse generation by optical rectification in stoichiometric and cryo-cooled congruent lithium niobate

We report, and review in detail, experiments resulting in a record 3.7% optical-to-terahertz (THz) conversion efficiency by optical rectification (OR) in cryogenically cooled congruent lithium niobate (cLN) using a near-optimal 680 fs pump pulse at 1030 nm. In addition, we report a record conversion efficiency of 1.7% at room temperature using stoichiometric lithium niobate (sLN) which results in 21.8 μJ of THz energy from a 1.2 mJ optical pulse. Electro-optical sampling measurements reveal the THz pulses to be single-cycle and centered at 0.45 THz. The experimentally measured efficiency, THz waveform, and THz spectrum are in good agreement with theoretical calculations. Finally, spatial beam profile measurements are also provided. To our knowledge, these results represent an order of magnitude improvement in efficiency of THz generation by OR in lithium niobate over previous results.

Lithium diffusion in congruent LiNbO3 single crystals at low temperatures probed by neutron reflectometry

The self-diffusion of lithium in congruent LiNbO3 single crystals was investigated at low temperatures between 379 and 523 K by neutron reflectometry. From measurements on (6)LiNbO3 (amorphous film)/(nat)LiNbO3 (single crystal) samples, Li self-diffusivities were determined in single crystals down to extremely low values of 1 × 10(-25) m(2) s(-1) on small length scales of 1-10 nm. The measured diffusivities are in excellent agreement with (extrapolated) literature data obtained by experiments based on Secondary Ion Mass Spectrometry and Impedance Spectroscopy. The tracer diffusivities can be described by a single Arrhenius line over ten orders of magnitude with an activation enthalpy of 1.33 eV, which corresponds to the migration energy of a single Li vacancy. A deviation from the Arrhenius behaviour at low temperatures, e.g., due to defect cluster formation is not observed.

Notice of Retraction: Optical-Damage-Resistant Highly Er$^{\bf {3+}}$ -Doped Ti:Er:LiNbO$_{\bf 3}$ Strip Waveguide

Retracted.

Electro-optic and dielectric properties of Zirconium-doped congruent lithium–niobate crystals

Measurements of electro-optic and dielectric properties in Zirconium (Zr)-doped lithium niobate (LN:Zr) crystals are performed as functions of the dopant concentration in the range 0.0-2.5 mol%. The clamped and unclamped electro-optic coefficients r 222 of Zr-doped LN and the corresponding dielectric permittivity as well, have been experimentally determined and compared with the results obtained in undoped congruent LN crystals. We show that the electro-optic and dielectric properties present a kink around 2 mol% of zirconium which seems to be the threshold concentration required to strongly reduce the photorefractive effect. All reported results confirm that the LN:Zr is a very promising candidate for several non linear devices.

Nonlinear optical waveguides fabricated in Mg-doped LiNbO3 by swift heavy ion irradiation: anomalous photorefractive damage behavior

Using swift heavy fluorine ion irradiation, we have successfully fabricated optical waveguides in Mg-doped LiNbO3 substrates. A systematic characterization of these structures has been carried out including refractive index profiles, propagation losses, nonlinear coefficients, and, specially, photorefractive optical damage. Step-like refractive index profiles with Δne ≈ 0.1 and Δno ≈ 0.2, propagation losses lower than 0.5 dB/cm and high nonlinear optical coefficients similar to those of the substrate have been obtained. Unexpectedly, the photorefractive damage is only moderately reduced with regard to the one presented in congruent LiNbO3 waveguides. Specifically, light intensity damage thresholds Ith are only a factor 2 higher at RT and a factor 4 at 90 °C with regard to undoped waveguides. At this latter temperature, a remarkably high Ith = 30.000 W/cm2 is reached. A final discussion on the observed anomalous optical damage behavior induced by swift heavy ion irradiation is also included.

Influence of Factors on the Growth of the Er3+/Tm3+-Codoped LiNbO3 Crystal by Er3+/Tm3+ Codiffusion

The influence of factors on the growth of the Er3+/Tm3+-codoped LiNbO3 (LN) crystal by codiffusion of stacked Er and Tm metal thin films was studied. These factors include the thicknesses and the coating sequence of the Er and Tm metal films, and the growth is characterized by the diffusivity and solubility. To achieve the goal, Er3+/Tm3+-codoped LN crystals were grown by codiffusion in air at 1130 °C of stacked Er and Tm metal thin films coated onto the surface of Z-cut congruent LN plates. The metal films have different thicknesses and coating sequences. After the growth, the Er3+ and Tm3+ profiles were analyzed by secondary ion mass spectrometry. The Er3+/Tm3+ diffusivity and surface concentration were obtained from the measured profiles. The diffusivity is influenced by neither the thickness nor the coating sequence of metal films. The solubility consists of Er3+ and Tm3+ components, and the two components equal the respective concentrations in the diffusion reservoir and change with the initial metal...

Effect of Mg doping on the growth aspects, crystalline perfection, and optical and thermal properties of congruent LiNbO3single crystals

Mg-doped congruent lithium niobate single crystals were grown by the Czochralski technique. High-quality single crystals were grown using a novel seeding technique in a resistive heating furnace. Analysis of crystalline perfection carried out by a multi-crystal X-ray diffractometer revealed that the grown crystals do not contain any structural grain boundaries but do contain point defects. The transmission characteristics showed an enhancement of band gap with an increase in Mg concentration. Conoscopy patterns revealed that the grown crystals are homogeneous and the incorporation of Mg into the lattice does not affect the optical sign (negative uniaxial) of the crystal. The refractive index measurements carried out using a prism coupler showed an increase in the optical birefringence (Δn), while the refractive index was found to decrease with the increase in doping concentration. Further, thermal conductivity was found to decrease with Mg incorporation in the lattice owing to phonon scattering from the Mg ions and, as a consequence, at high concentrations (>4 mol%) crack formation occurred. However, optimization of growth conditions reveals that a slower pulling rate leads to crack-free lithium niobate crystals even at 6 mol% Mg doping.

Upconversion emissions induced by 1550 nm in near-stoichiometric Er:LiNbO_3 crystal

To increase the photovoltaic efficiency of solar cells, green and red upconversion (UC) emissions produced at an excitation of 1550 nm were investigated. A drastically enhanced red UC emission was observed in near-stoichiometric LiNbO(3) crystal heavily doped with Er(3+) ions (Er:NSLN). Raman spectra showed that the maximum phonon energy shifted from 631 cm(-1) in Er(3+)-doped congruent LiNbO(3) (Er:CLN) to 871 cm(-1) in Er:NSLN crystal. The time decay of the (4)S(3/2)→(4)I(15/2) transition suggested that the Er(3+) cluster sites (Er(Li)(2+)-Er(Nb)(2-)) were dissociated in the Er:NSLN crystal. The visible UC luminescence converted from near infrared at the wavelength of 1550 nm was important to enlarge the solar spectrum response of solar cell.

Relation of Refractive Index to Ti-Concentration in Near-Stoichiometric ${\rm Ti{:}LiNbO}_{3}$ Waveguide

Multi-mode near-stoichiometric (NS) Ti:LiNbO3 planar waveguide was fabricated by co-work of Li-rich vapor transport equilibration and in-diffusion of Ti-film on an initially congruent LiNbO3 substrate. The Ti-concentration is profiled by secondary ion mass spectrometry. The refractive index profile is constructed from measured mode indices and correlated with the Ti-concentration profile. The results show that the index change and Ti-concentration follow an exponential relationship with a power index 0.75/0.49 for the ordinary/extraordinary ray. The relationship is different from that of either conventional congruent waveguide or homogeneously Ti-doped NS bulk material or NS waveguide fabricated by direct Ti-diffusion in an NS substrate.