Iron-doped Lithium Niobate Research Articles

Optical fabrication of (2 + 1)-dimensional photorefractive photonic lattices by improved Billet split lens

A convenient way to optical fabricate (2 + 1)-dimensional photonic lattice microstructures was experimentally demonstrated in photorefractive crystal. An improved Billet split lens combined with a Fourier transform lens can generate large area interference of multiple plane waves. The (2 + 1)-dimensional square photonic lattice microstructures are optically induced in an iron doped lithium niobate crystal. Induced lattice microstructures are analyzed and validated using several different experimental methods. This method has good flexibility and scalability, stable equipment, and does not require complex adjustment systems. It can be used to optical fabricate more types of (2 + 1)-dimensional lattice microstructures by design and processing of improved Billet split lens reasonably.

Trifurcated Splitting of Water Droplets on Engineered Lithium Niobate Surfaces.

Controlled splitting of liquid droplets is a key function in many microfluidic applications. In recent years, various methodologies have been used to accomplish this task. Here, we present an optofluidic technique based on an engineered surface formed by coating a z-cut iron-doped lithium niobate crystal with a lubricant-infused layer, which provides a very slippery surface. Illuminating the crystal with a light spot induces surface charges of opposite signs on the two crystal faces because of the photovoltaic effect. If the light spot is sufficiently intense, millimetric water droplets placed near the illuminated spot split into two charged fragments, one fragment being trapped by the bright spot and the other moving away from it. The latter fragment does not move randomly but rather follows one of three well-defined trajectories separated by 120°, which reflect the anisotropic crystalline structure of Fe:LiNbO3. Numerical simulations explain the behavior of water droplets in the framework of the forces induced by the interplay of pyroelectric, piezoelectric, and photovoltaic effects, which originate simultaneously inside the illuminated crystal. Such a synergetic effect can provide a valuable feature in applications that require splitting and coalescence of droplets, such as chemical microreactors and biological encapsulation and screening.

Optically induced fabrication of three-dimensional photonic lattice microstructures by using cutting-combination lens

A convenient method for producing three-dimensional photonic lattice microstructures was experimentally demonstrated by using a cutting-combination lens. Three-dimensional photonic lattices are optically induced by (4 + 1) plane waves interference in an iron-doped lithium niobate photorefractive crystal. Induced three-dimensional lattice microstructures are observed and verified by planar guided wave intensity imaging and far-field diffraction pattern. This method is inexpensive and does not require complex adjustment devices. In addition, it also has flexibility and scalability. This method can be used to fabricate other more complex three-dimensional photonic lattice microstructures by designing the cutting-combination lens appropriately.

Running streams of a ferroelectric nematic liquid crystal on a lithium niobate surface

ABSTRACTSessile droplets of a ferroelectric nematic liquid crystalline material were exposed to surface electric fields produced by pyroelectric and photogalvanic (photovoltaic) effects in X-cut iron-doped lithium niobate crystals. The resulting dynamic processes were monitored by polarisation optical (video)microscopy (POM). During heating/cooling cycles, at first, the droplets change their shape from spherical to extended ellipsoidal. Then, they start to move rapidly along the surface electric field, i.e. along the crystal’s polar axis (c-axis). During this motion, several droplets merge into running streams (tendrils) extending towards the edges of the top surface area. Finally, practically all liquid crystalline material is transported from the top surface to the side surfaces of the crystal. At stabilised temperatures, laser illumination of the assembly causes dynamic processes that are localised to the illuminated area. Also, in this case, the LC droplets merge into several tendril-like formations that are preferentially oriented along the c-axis of the crystal. The pattern of tendrils fluctuates with time, but it persists as long as the illumination is present. In this case, the LC material is transported between the central and the edge regions of the illuminated area.

Light-induced dynamics of liquid-crystalline droplets on the surface of iron-doped lithium niobate crystals

We investigated the effect of a photovoltaic field generated on the surface of iron-doped lithium niobate crystals on sessile droplets of a ferroelectric nematic liquid crystalline and a standard nematic liquid crystalline material present on this surface. When such an assembly is illuminated with a laser beam, a wide range of dynamic phenomena are initiated. Droplets located outside the laser spot are dragged in the direction of the illuminated area, while droplets located inside the illuminated region tend to bridge each other and rearrange into tendril-like structures. In the ferroelectric nematic phase (NF), these processes take place via the formation of conical spikes evolving into jet streams, similar to the behavior of droplets of conventional dielectric liquids exposed to overcritical electric fields. However, in contrast to traditional liquids, the jet streams of the NF phase exhibit profound branching. In the nematic phase (N) of both the ferroelectric nematic and the standard nematic material, dynamic processes occur via smooth-edged continuous features typical for conventional liquids subjected to under-critical fields. The difference in dynamic behavior is attributed to the large increase of dielectric permittivity in the ferroelectric nematic phase with respect to the dielectric permittivity of the nematic phase.

Determination of the Dielectrophoretic Force Induced by the Photovoltaic Effect on Lithium Niobate.

The actuation of droplets on a surface is extremely relevant for microfluidic applications. In recent years, various methodologies have been used. A promising solution relies on iron-doped lithium niobate crystals that, when illuminated, generate an evanescent electric field in the surrounding space due to the photovoltaic effect. This field can be successfully exploited to control the motion of water droplets. Here, we present an experimental method to determine the attractive force exerted by the evanescent field. It consists of the analysis of the elongation of a pendant droplet and its detachment from the suspending syringe needle, caused by the illumination of an iron-doped lithium niobate crystal. We show that this interaction resembles that obtained by applying a voltage between the needle and a metallic substrate, and a quantitative investigation of these two types of actuation yields similar results. Pendant droplet tensiometry is then demonstrated to offer a simple solution for quickly mapping out the force at different distances from the crystal, generated by the photovoltaic effect and its temporal evolution, providing important quantitative data for the design and characterization of optofluidic devices based on lithium niobate crystals.

All-photonic synapse based on iron-doped lithium niobate double metal-cladding waveguides

In an artificial neural network composed of neuromorphic computing units, the resistance of the memristor can be modulated dynamically and repeatedly under external stimuli, such as electric fields, magnetic fields, and light illumination, leading to variations in local conductivity and memory effects. Here we show, using an all-photonic memristor, that the memristance arises naturally in an optical system in which solid-state ionic transport is realized under low-power external incident light. These results show an extremely stable multilevel storage weight and a high signal-to-noise ratio. In all-photonic memristors, the light signal is employed as the extra stimuli of the memristor devices to ensure a large memory window and a variation margin of multiple storage levels.

Laser inscribed waveguide optical isolators in iron-doped lithium niobate.

There is a growing need for optical isolators that do not require a magnetic field, especially for uses such as on-chip optical devices and cold atom physics. As one approach, we propose using waveguides in photorefractive materials, such as Fe:LiNbO3, as optical isolator devices due to their unique asymmetric transmission properties that allow low loss transmission in one crystal orientation and attenuation in the flipped orientation. We utilize ultrafast laser inscription to fabricate photorefractive depressed cladding waveguides in Fe:LiNbO3 along the crystal c axis to demonstrate the operation of Fe:LiNbO3 waveguide optical isolators. We show the ability to write transmission and reflection gratings into these waveguides that provide an isolation ratio of approximately 5000:1 per cm of path length.

Pyroelectric-controlled bending of a self-trapped optical beam in a photorefractive iron doped lithium niobate crystal

We present the experimental demonstration of a self-trapped optical beam bending in a photorefractive Fe-doped lithium niobate (LN:Fe) crystal controlled by the pyroelectric effect. Formation of self-trapped beams with typical [Formula: see text]50[Formula: see text][Formula: see text]m diameter and large bending of [Formula: see text]140[Formula: see text][Formula: see text]m are depicted in a 1[Formula: see text]cm length LN:Fe crystal for a laser beam at 632.8[Formula: see text]nm wavelength and 0.5[Formula: see text]mW power with a 30∘C crystal temperature change. The self-trapped beam bending is opposite to the crystal [Formula: see text]-axis. The underlying physics is elaborated and numerical simulations are performed. The long-living waveguiding channels with controlled curvilinear trajectories are promising for optical information routing.

Bulk piezo-photovoltaic effect in LiNbO3

The deformations arising in iron-doped lithium niobate crystals (LiNbO3:Fe) under laser illumination were registered by changes in the parameters of X-ray diffraction rocking curves. The piezoelectric deformation as a result of the bulk photovoltaic effect was separated from heating according to the different kinetics of these processes using a set of time-resolved X-ray diffraction techniques. The dependence of the peak shift on the diffraction order confirmed the manifestation of the bulk piezo-photovoltaic effect. Additionally, the measurements of an external electric field influence on the diffraction peaks parameters were carried out as well as the measurements of the electrophysical and bulk photovoltaic characteristics of the crystal.

Asymmetrical 1st reflection trend owing to metallization difference at ± Z-faces in indium tin oxide coated Fe-doped lithium niobate.

The charges accumulated at the interface of indium tin oxide (ITO) and iron doped lithium niobate (LN) in an extremely thin layer because of semiconductor band alignment were analyzed with ab initio theory. The formation of 2D electron gas makes the interface metallic and the excitation of surface plasmon polaritons (SPPs) possible. In experiments, diametrically opposite trends of the very first reflection (VFR) on the ± Z-faces of ITO coated Fe-LN slabs were observed and associated with the differences in metallization and the photovoltaic charge accumulation there. Microscopically, the electric environments of the two ITO/LN interfaces differ greatly owing to spontaneous polarization and photovoltaic fields, which alter the band structures and band alignment, resulting in phase gratings with a π-shift difference recorded at the two interfaces. This affects the opposite energy coupling between the SPPs and laser beams and results in the dramatically opposite trends of VFR.

Time evolution of photovoltaic fields generated by arbitrary light patterns in z-cut LiNbO3:Fe: application to optoelectronic nanoparticle manipulation.

The bulk photovoltaic effect is a phenomenon that generates high electric fields in certain ferroelectric crystals under illumination, as iron doped lithium niobate (LiNbO3:Fe). A variety of innovative applications of these electric fields require using of z-cut plates, in which the polar axis is normal to the larger crystal faces. However, the kinetics and distribution of the photovoltaic fields in this configuration have not been investigated in depth. In this work, the photovoltaic charge transport of z-cut configuration is studied through a complete finite element analysis. Light patterns commonly used for particle trapping applications are used to study the temporal evolution of the electric field developed by the crystal. Results show that photovoltaic currents perpendicular to the optical axis play a key role in the development of the final charge distribution. Moreover, there is a relevant, localized charge accumulation inside the crystal which is required to reach the saturation electric field in the whole illuminated volume. The role of crystal thickness and light absorption are analysed. It has been found that they are important to determine the time evolution of the process. The simulations are expected to be a key tool to analyse and improve photovoltaic optoelectronic tweezers.

Optical induction of two-dimensional photorefractive photonic microstructures using six wedge prisms array

We experimentally demonstrate a convenient method to fabricate two-dimensional photonic microstructures in photorefractive crystal. Three kinds of photonic microstructures have been fabricated in iron-doped lithium niobate crystal using six wedge prisms array. The experimental setup of our method does not require complicated optical alignment components. Induced two-dimensional triangular lattice photonic microstructures are analyzed and validated by plane wave guiding, far-field diffraction pattern imaging, Brillouin-zone spectroscopy, and angle-dependent transmission spectrum. The method can be easily extended to fabricate other complex photonic microstructures by designing different wedge prisms array and adjusting polarization and phase delay properly.

Optoelectronic generation of bio-aqueous femto-droplets based on the bulk photovoltaic effect.

The generation and manipulation of small aqueous droplets is an important issue for nano- and biotechnology, particularly, when using microfluidic devices. The production of very small droplets has been frequently carried out by applying intense local electric fields to the fluid, which requires power supplies and metallic electrodes. This procedure complicates the device and reduces its versatility. In this work, we present a novel and flexible, to the best of our knowledge, electrodeless optoelectronic method for the production of tiny droplets of biologically friendly aqueous fluids. Our method takes advantage of the photoinduced electric fields generated by the bulk photovoltaic effect in iron-doped lithium niobate crystals. Two substrate configurations, presenting the polar ferroelectric axis either parallel or perpendicular to the active surface, have been successfully tested. In both crystal geometries, small droplets on the femtoliter scale have been obtained, although with a different spatial distributions correlated with the symmetry of the photovoltaic fields. The overall results demonstrate the effectiveness of the optoelectronic method to produce femtoliter droplets, both with pure water and with aqueous solutions containing biological material.

Area scalable and period manipulable three-dimensional optically induced photorefractive photonic periodic crystal or aperiodic quasicrystal

Abstract We experimentally investigate the formation of area-scalable three-dimensional (3D) photonic periodic crystal and aperiodic quasicrystal in an iron doped lithium niobate photorefractive material by multi-wedge prisms assisted compact and versatile single step optical induction approach. The induced large-area lattice microstructures are analyzed and verified by plane wave guiding, Brillouin-zone spectroscopy, far field diffraction pattern imaging, angle-dependent transmission spectra. Our technique can be easily expanded for the larger area of photonic periodic crystal or aperiodic quasicrystal microstructures. We also demonstrate experimentally and theoretically the manipulation to the period of 3D photorefractive photonic lattices by engineered vertex angle of multi-wedge prisms. The intensity analyses of interference light fields show that (1) the 3D photonic lattice has not only different Z-axis longitudinal structure, but also different X–Y front structure to the corresponding 2D photonic lattice, and (2) there exist three sets of sub-lattices which are mutually nested to form the 3D triangle photonic crystal.

Optical in-situ study of the redox processes in LiNbO3: Fe crystals

Resultsare reported of an in-situ investigation of optical absorption of iron-doped lithium niobate single crystals, LiNbO3:Fe, at high temperatures. LiNbO3:Fe has been exposed to treatments in reducing (95%Ar + 5%H2) and oxidizing (O2) atmospheres at temperatures up to 1100 K. The high temperature spectra have been discussed in respect to the electronic defects giving rise to absorption. In addition, the time-dependent evolution of optical spectra upon rapid replacement of gas atmospheres has been followed at fixed wavelength. The experimental reaction kinetics is discussed in terms of oxygen vacancy diffusion accounting for the associated changes in the point defect subsystem of the material.

Theoretical Prediction of Umbilics Creation in Nematic Liquid Crystals with Positive Dielectric Anisotropy.

Optically assisted electrical generation of umbilic defects, arising in homeotropically aligned nematic liquid crystal cells and known as topological templates for the generation of optical vortices, are reported in nematic liquid crystals with positive dielectric anisotropy in detail. It is shown that nematic liquid crystals with positive dielectric anisotropy can serve as a stable and efficient medium for the optical vortex generation from both linearly and circularly polarized input Gaussian beams. Hybrid cells made from a thin layer of nematic liquid crystal confined between a photoresponsive slab of iron-doped lithium niobate and a glass plate coated with an active material, i.e., indium tin oxide, were studied. Exposure to a laser beam locally induces a photovoltaic field in the iron-doped lithium niobate substrate, which can penetrate into the liquid crystal film and induce realignment of molecules. The photovoltaic field drives charge carrier accumulation at the interface of indium tin oxide with the liquid crystal, which effectively modifies the shape and symmetry of the electric field. The photovoltaic field has a continuous radial distribution in the transverse xy-plane, weakening with increasing distance from the light irradiation center, where the electric field is normal to the cell plane. Umbilics are created as a result of the liquid crystal tendency to realign parallel to the electric field. Numerical studies of the transmitted intensity profiles in between linear polarizers reveal optical vortex pattern (of four and eight brushes) characteristics for the umbilical defects. The application of crossed circular polarizers results in annular-shaped intensity patterns as a result of spin-to-orbital angular momentum conversions, which give rise to the optical vortices.

Pulse-induced transient blue absorption related with long-lived excitonic states in iron-doped lithium niobate

Transient absorption is studied in Fe-doped lithium niobate single crystals with the goal to control and probe a blue absorption feature related with excitonic states bound to Fe$_\textrm {Li}$Li defect centers. The exciton absorption is deduced from the comparison of ns-pump, supercontinuum-probe spectra obtained in crystals with different Fe-concentration and Fe$^{2+/3+}_\textrm {Li}$Li2+/3+-ratio, at different pulse peak and photon energies as well as by signal separation taking well-known small polaron absorption bands into account. As a result, a broad-band absorption feature is deduced being characterized by an absorption cross-section of up to $\sigma ^\textrm {max}(2.85$σmax(2.85 eV$) = (4\pm 2)\cdot 10^{-22}$)=(4±2)⋅10−22 m$^{2}$2. The band peaks at about 2.85 eV and can be reconstructed by the sum of two Gaussians centered at 2.2 eV (width $\approx 0.5$≈0.5 eV) and 2.9 eV (width $\approx 0.4$≈0.4 eV), respectively. The appropriate build-up and decay properties strongly depend on the crystals’ composition as well as the incident pulse parameters. All findings are comprehensively analyzed and discussed within the model of $\textrm {Fe}^{2+}_{\textrm {Li}}-\textrm {O}^{-}-\textrm {V}_{\textrm {Li}}$FeLi2+−O−−VLi excitonic states.

A polaron approach to photorefractivity in Fe : LiNbO3

The thermally activated, incoherent hopping of small electron polarons generated by continuous illumination in iron-doped lithium niobate is simulated by a Marcus-Holstein model for which all the input parameters are known from literature. The results of the calculations are compared with a comprehensive set of data obtained from photorefractive, photogalvanic and photoconductive measurements under green light excitation on samples with different doping levels and stoichiometries in the temperature range between and room temperature. We show that the temperature and composition dependence of the photorefractive observables can be interpreted by a change in the abundance of the different hop types that a polaron performs before being captured by a deep Fe trap. Moreover, by a comparison between experimental and numerical data we obtain new insights on the initial photo-excitation part of the photorefractive process. In particular all results are consistent if a single value of the photogalvanic length is assumed for all the samples and all the temperatures. The photo-generation efficiency ϕ under green light excitation (somewhere denoted as quantum efficiency) is also estimated. It appears to decrease from 10%–15% at room temperature to about 5% at 150 K. This behavior is qualitatively interpreted in terms of a temperature-dependent re-trapping probability of the light-emitted particles from the initial Fe donor center.

Transient transmission oscillations in doped and undoped lithium niobate induced by near-infrared femtosecond pulses

Abstract