Effect of electron beam writing parameters for ferroelectric domain structuring LiNbO3:Nd

We have successfully applied the versatile technique of direct electron beam writing to produce column shaped 180° micro-domains on a solid state laser crystal: Nd³⁺ doped LiNbO₃. The effect of the different parameters used during the electron lithography process has been analyzed on the morphology and on the optical properties of the inverted domains. Namely, we have studied the effect of the value of the accelerating voltage of the electrons and the effect of the electronic charge deposited onto the sample. By means of optical micrography and computer simulation we show that the accelerating voltage has a major effect on the diameter of inverted domains. On the other hand, the value of the electronic charge not only affects the domain size but it has also an additional and interesting effect on the optical spectroscopy of Nd³⁺ ions. Using low temperature site selective optical spectroscopy we demonstrate the possibility of acting selectively on the crystal field of the optically active Nd³⁺ centers in LiNbO₃ by means of the different electronic doses used to reverse the polarization of the matrix. The work constitutes an approach towards the control of the properties of reversal ferroelectric domains structures prepared by direct electron beam lithography and towards understanding the role of optically active defects on the switching behaviour in ferroelectric crystals.     

Ridge waveguide laser in Nd:LiNbO3 by Zn-diffusion and femtosecond-laser structuring

Ridge waveguide lasers have been fabricated on Nd³⁺ doped LiNbO₃ crystals. The fs-laser writing technique was used to define ridge structures on a gradient-index planar waveguide fabricated by Zn-diffusion. This planar waveguide was formed in a z-cut LiNbO₃ substrate homogeneously doped with a 0.23% of Nd³⁺ ions. To obtain lateral light confinement, the surface was then micromachined using a multiplexed femtosecond laser writing beam, forming the ridge structures. By butting two mirrors at the channel waveguide end-facets, forming a waveguide laser cavity, TM-polarized laser action at 1085 nm was achieved by end-fire TM-pumping at 815 nm. The waveguide laser shows a threshold of 31 mW, with a 7% of slope efficiency.     

Normally-off characteristics of LiNbO3/AlGaN/GaN ferroelectric field-effect transistor

With the help of MgO mask layer, LiNbO₃ (LN) ferroelectric films were etched effectively using wet etching method and LN/AlGaN/GaN ferroelectric field-effect transistors (FFETs) were fabricated. The electrical properties of the FFETs were studied. Due to the ferroelectric polarization nature of LN films, normally-off characteristics with a turn-on voltage of about + 1.0 V were exhibited in the device. The operation mechanisms of the LN/AlGaN/GaN FFET devices were proposed by the numerical calculations of the electronic band structure and charge distribution.     

Enhanced Charge Transfer by Passivation Layer in 3DOM Ferroelectric Heterojunction for Water Oxidation in HCO3−/CO2 System

Photoelectrochemical carbon dioxide conversion to fuels such as carbon monoxide, methanol, and ethylene exhibits great potential to solve energy issues. Unfortunately, CO₂ conversion efficiency is still low due to violent charge recombination at the photoanode. Herein, a novel 3D macroporous ferroelectric heterojunction composed of BiFeO₃ and LiNbO₃ is developed by a template‐assisted sol–gel method, aiming at facilitating charge transfer kinetics. As expected, a tremendous enhancement of photocurrent density (300 times vs bare planar BiFeO₃ film) and charge transfer efficiency (up to 76%) is obtained in the HCO₃^?/CO₂ system without any cocatalyst. The photoelectrochemical performance is switchable by poling to form a depolarization electric field. Photoelectrochemical impedance spectroscopy reveals that the charge transfer resistance decreases due to the synergistic effect of BiFeO₃ 3D macroporous skeleton and LiNbO₃ passivation layer by tuning surface states. These results suggest a novel strategy for enhancing photoelectrochemical water oxidation as the anodic reaction of CO₂ reduction.     

Multiplicity of europium centers in doped stoichiometric crystals of lithium niobate

The optical spectra of europium-doped stoichiometric lithium niobate (LiNbO₃:Eu³⁺) crystals have been studied using combined excitation-luminescence spectroscopy in the range of ⁵D₀ → ⁷F₁, ⁷F₀ optical transitions. Analysis of the results shows that Eu³⁺ ions can occupy 14 energetically nonequivalent positions in the LiNbO₃ crystal lattice. This multiplicity of impurity centers is related to possible variants of the incorporation of Eu³⁺ ions into the LiNbO₃ crystal lattice and the compensation of excess charge. Energy positions of the ⁵D₀ level and the lowest sublevel of the ⁷F₁ Stark multiplet are determined for Eu³⁺ centers of all 14 types.     

Tunable and highly reproducible surface-enhanced Raman scattering substrates made from large-scale nanoparticle arrays based on periodically poled LiNbO3 templates

Abstract

This work describes novel surface-enhanced Raman scattering (SERS) substrates based on ferroelectric periodically poled LiNbO₃ templates. The templates comprise silver nanoparticles (AgNPs), the size and position of which are tailored by ferroelectric lithography. The substrate has uniform and large sampling areas that show SERS effective with excellent signal reproducibility, for which the fabrication protocol is advantageous in its simplicity. We demonstrate ferroelectric-based SERS substrates with particle sizes ranging from 30 to 70 nm and present tunable SERS effect from Raman active 4-mercaptopyridine molecules attached to AgNPs when excited by a laser source at 514 nm.     

Efficient Bi → Nd energy transfer in Gd2O3:Bi,Nd

Bi³⁺,Nd³⁺ co-doped Gd₂O₃ were prepared by solid state reaction and the optical properties were investigated. The results show that the near-infrared emission of Nd³⁺ ions is significantly enhanced by the introducing of Bi³⁺ in co-doped samples. An efficient energy transfer from Bi³⁺ to Nd³⁺ corresponds to the near-infrared emission enhancement. The energy transfer efficiency reaches 64.1% for the sample with the strongest near-infrared emission, which has the optimized doping concentrations of 0.5% for Bi³⁺ and 2% for Nd³⁺. The interesting optical properties make Bi³⁺,Nd³⁺ co-doped Gd₂O₃ promising as the luminescent down-conversion layers in front of c-Si solar cells to enhance the performance of the solar cells.     

Electrical Tunability of Domain Wall Conductivity in LiNbO3 Thin Films

Domain wall nanoelectronics is a rapidly evolving field, which explores the diverse electronic properties of the ferroelectric domain walls for application in low‐dimensional electronic systems. One of the most prominent features of the ferroelectric domain walls is their electrical conductivity. Here, using a combination of scanning probe and scanning transmission electron microscopy, the mechanism of the tunable conducting behavior of the domain walls in the sub‐micrometer thick films of the technologically important ferroelectric LiNbO₃ is explored. It is found that the electric bias generates stable domains with strongly inclined domain boundaries with the inclination angle reaching 20° with respect to the polar axis. The head‐to‐head domain boundaries exhibit high conductance, which can be modulated by application of the sub‐coercive voltage. Electron microscopy visualization of the electrically written domains and piezoresponse force microscopy imaging of the very same domains reveals that the gradual and reversible transition between the conducting and insulating states of the domain walls results from the electrically induced wall bending near the sample surface. The observed modulation of the wall conductance is corroborated by the phase‐field modeling. The results open a possibility for exploiting the conducting domain walls as the electrically controllable functional elements in the multilevel logic nanoelectronics devices.     

Varied roles of Pb in transition-metal PbMO3 perovskites (M = Ti,V, Cr,Mn, Fe,Ni, Ru)

Different structural chemistries resulting from the Pb²⁺ lone-pair electrons in the PbMO₃ perovskites are reviewed. The Pb²⁺ lone-pair electrons enhance the ferroelectric transition temperature in PbTiO₃, stabilize vanadyl formation in PbVO₃, and induce a disproportionation reaction of Cr^IV in PbCrO₃. A Pb²⁺ + Ni^IV = Pb⁴⁺ + Ni^II reaction in PbNiO₃ stabilizes the LiNbO₃ structure at ambient pressure, but an A-site Pb⁴⁺ in an orthorhombic perovskite PbNiO₃ is stabilized at modest pressures at room temperature. In PbMnO₃, a ferroelectric displacement due to the lone pair electron effect is minimized by the spin–spin exchange interaction and the strong octahedral site preference of the Mn^IV/III cation. PbRuO₃ is converted under pressure from the defective pyrochlore to the orthorhombic (Pbnm) perovskite structure where Pb–Ru interactions via a common O −2p orbital stabilize at low temperature a metallic Imma phase at ambient pressure. Above P_c a covalent Pb–Ru bond is formed by Pb²⁺ + Ru^IV = Pb⁴⁺ + Ru^II electron sharing.     

Optical homogeneity and photorefractive properties of stoichiometric and congruent lithium niobate crystals grown using charges of different origins

A stoichiometric lithium niobate crystal (LiNbO_{3 st}) and congruent lithium niobate crystals grown from a charge prepared using cyclohexanone as an extractant (LiNbO_{3 cong}(CHN)) and a charge prepared using cyclohexanone and carboxylic acid dimethylamides as extractants (LiNbO_{3 cong}(CHN + A)) have been characterized by photoinduced light scattering and laser conoscopy. The results demonstrate that the LiNbO_{3 cong}(CHN + A) crystals are rather homogeneous along their growth direction and possess good optical properties, similar to those of the LiNbO_{3 cong}(CHN) crystal. At the same time, the LiNbO_{3 cong}(CHN + A) crystal offers significantly better electro-optical properties (r _e = 29.3 pm/V).     

Photoluminescence in Nd-doped V2O5

Based on the melt-quenching method, V₂O₅ was doped with Nd³⁺ ions. Photoluminescent properties due to electronic transitions of Nd³⁺ were found. In spite of the used method in the fabrication of this material, which consisted in a thermal shock from 1073 to 100 K, all the characterization techniques indicated that V₂O₅ has a crystalline orthorhombic phase and is acting as a host matrix for the Nd³⁺ ions. For instance, in the X-ray diffraction patterns, only well-defined peaks associated to V₂O₅ were detected. By Raman spectroscopy, vibrational modes related to V₂O₅ were observed. An E _g  = 2.15 eV was determined from the optical absorption spectrum for this doped material. Scanning electron microscopy images recorded on a fresh fracture show that this material is formed by lamellar plates. The elemental semi quantitative analysis indicates that the doping level with Nd³⁺ was of the order of 2.0 ± 0.2 at.%. Dark conductivity measurements yielded values in the 10^?6–10^?12 (Ω cm)^?1 range. Finally, by micro photoluminescence spectroscopy, the {⁴F_5/2, ⁴F_3/2} → ⁴I_9/2 electronic transitions related to Nd³⁺ ions were observed.     

A TEM study of ordering in the perovskite,Pb(Sc1/2Ta1/2)O3

Ordering behaviour and the factors which influence the ordering have been studied in both single crystals and hot-pressed ceramics of Pb(Sc_1/2Ta_1/2)O₃ using TEM techniques, including low-temperature microscopy. The characteristics of the structurally-ordered domains are described and also the observation by electron diffraction of new superlattice reflections and diffuse scattering in both the paraelectric and ferroelectric phases. The superlattice reflections are interpreted in terms of Pb²⁺ cation displacements. The non-interaction of ordered domains and ferroelectric domains is explained within this model.     

Spectral noise analysis of single- and double-pass Er3+ - doped LiNbO3 waveguide amplifiers

Abstract

In this paper we report the derivation and evaluation of the spectral optical gain, spectral noise figure, amplified spontaneous emission photon number and signal-to-noise ratio in single- and double-pass configurations for an Er³⁺-doped LiNbO₃ waveguide amplifier pumped near 1484nm. The theoretical analysis was made using the small-gain approximation in the unsaturated regime. The simulations show the evolution of the above-mentioned parameters under various pumping regimes and waveguide lengths. The results obtained show how the model can be used for the design of Er³⁺-doped LiNbO₃ waveguide amplifiers.     

Effect of inert gas on pulsed laser deposition of rare earth doped fluoride thin films

The realization of crystalline sub micrometric films of Nd³⁺:LiYF₄ fluorides on pure LiYF₄ substrates by pulsed laser deposition is reported. The films were obtained by laser ablation with 355 nm photons of a bulk LiYF₄ crystal doped with Nd³⁺ ions at 1.5% atomic concentration. Both low and high laser ablation fluency regimes were checked. The fundamental contribution of He inside a vacuum chamber to obtain crystalline Nd³⁺:LiYF₄ films was demonstrated. The optical characteristics of the films were analyzed via recording the emission spectra following IR excitation, polarized both with E || and E ⊥ to the substrate c-axis. Lifetime measurements of the fundamental Nd³⁺ ion transition in the films were also performed. All the results were compared with those obtained in the Nd³⁺:LiYF₄ bulk crystal under the same experimental conditions. The scanning electron microscope pictures of the depositions gave indications about the film surface morphology.     

Optical emission from LiNbO3(:Fe)/porous Si structures

Using pulsed laser deposition, a layer of LiNbO₃(:Fe) film with a thickness of 180 nm was coated onto porous Si (PS) stored in air for 1 year. A photoluminescence (PL) band enhanced about three times was observed at ∼625 nm with a same peak position as that of the stored PS. It is revealed that a new PL excitation band occurs at ∼363 nm, which is nearly equal to the fundamental optical absorption edge position of LiNbO₃. The X-ray diffraction results disclose that the enhancement of the PL intensity is closely related to formation of a textured LiNbO₃ film. Based on spectral analysis, we attribute the enhanced PL to optical transition in the E′ defect centers localized at the surfaces of PS nanocrystals, whereas the photoexcited carriers mainly come from the coated LiNbO₃ film. This kind of LiNbO₃(:Fe)/PS structures is expected to have important applications in modern ferroelectric optoelectronics.     

Nd3+ doped CAS glasses: A thermo-optical and spectroscopic investigation

Previous works have showed that calcium aluminosilicate (CAS) glasses, when prepared under vacuum conditions, are good candidates for solid state laser medium hosts and optical devices due to their appropriated thermal, optical and mechanical properties. These promising results led us to investigate the thermo-optical properties and emission spectra as a function of temperature in Nd³⁺ doped CAS glasses. Temperature changes in optical systems can cause structural modifications to the host, as well as other effects, such as emission quenching, or self-focalization. In this work, two series of CAS glasses, doped up to 5 wt.% Nd₂O₃, were prepared and characterized. Measurements of thermal coefficient of optical path length (dS/dT) and emission were performed on both series of Nd³⁺ doped CAS. In addition, measurements of optical absorption coefficient and emission lifetime were carried out. The results are discussed in terms of temperature dependence of these properties and Nd₂O₃ content. Comparisons with other glasses, such as LSCAS (low-silica calcium aluminosilicate) are also presented.     

Ce dopant segregation in Y3Al5O12 optical ceramics

Ce³⁺-doped YAG garnet optical ceramic have been sintered at the Shanghai Institute of Ceramics in China to characterize dopant distribution in optical ceramics by combining optical spectroscopy and two spatially resolved techniques as imaging confocal microscopy and transmission electron microscopy. A strong Ce³⁺ segregation and spatial variations of content between grains and grain boundaries has been confirmed by quantitative data obtained by TEM microscopy. This observation is another evidence of the inhomogeneous Ce³⁺ distribution across grain and grain boundaries in optical ceramics comparable to that of Nd³⁺ ions in YAG ceramics. These results correlate well with low segregation coefficients of Nd³⁺ and Ce³⁺ observed in the garnet crystals grown from the melt and/or flux.     

Optical response and surface morphology of crystalline Nd-doped fluoride films grown on monocrystalline LiYF4 substrates by pulsed laser deposition

The realization of crystalline films of Nd³⁺:YF₃ and Nd³⁺:LiYF₄ on a monocrystalline LiYF₄ substrate by pulsed laser deposition is reported. The films were obtained by laser ablation with 355 nm photons of a bulk LiYF₄ crystal doped with Nd³⁺ ions at 1.5% atomic concentration in the presence of different ablation/deposition parameters. The films optical characteristics, analyzed via laser induced polarized fluorescence spectroscopy upon IR excitation, are presented. Lifetime measurements of the fundamental Nd³⁺ ion transition in the film were also performed. All these results were compared with those obtained in the Nd³⁺:LiYF₄ bulk crystal. The surface morphology of the depositions was analyzed via a scanning electron microscope. When the production of the deposition took place in high vacuum (1 × 10^− 4 Pa) and the substrate temperature was 750 °C, the grown film was Nd³⁺:YF₃. A 1 Pa controlled atmosphere of He in the ablation chamber and a substrate temperature of 650 °C favoured the growth of a Nd³⁺:LiYF₄ film. In the latter case the film showed also a smoother surface.     

Structure and optical homogeneity of LiNbO3〈Mg〉 crystals grown from different charges

This paper reports a comparative study of the homogeneity of LiNbO₃〈Mg〉 (≥5 mol % Mg) crystals grown from a charge synthesized using Nb₂O₅〈Mg〉 prepared through homogeneous magnesium doping of a reextract in the Nb₂O₅ extraction step, and LiNbO₃〈Mg〉 crystals prepared through direct doping of a growth charge. The LiNbO₃〈Mg〉 crystals were characterized by optical microscopy and Raman spectroscopy. The results demonstrate that the LiNbO₃〈Mg〉 crystals prepared using homogeneous doping are more structurally uniform.     

Properties of neodymium-doped Ca<Subscript>0.15</Subscript>Sr<Subscript>1.85</Subscript>Bi<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>18</Subscript> ferroelectric ceramics

Bismuth-layered compound Ca_0.15Sr_1.85Bi_4−xNd_xTi₅O₁₈ (CSBNT, x = 0–0.25) ferroelectric ceramics samples were prepared by solid-state reaction method. The effects of Nd³⁺ doping on their ferroelectric and dielectric properties were investigated. The remnant polarization Pr of CSBNT ceramics increases at beginning then decreases with increasing of Nd³⁺ doping level, and a maximum Pr value of 9.6 μC/cm² at x = 0.05 was detected with a coercive field Ec = 80.2 kV/cm. Nd³⁺ dopant not only decreases the Curie temperature linearly, but also the dielectric constant (ε_r) and dielectric loss tangent (tan δ). The magnitudes of ε_r and tan δ at the frequency of 100 kHz are estimated to be 164 and 0.0083 at room temperature, respectively.