Lithium Niobate Samples Research Articles

Characteristics of Electromagnetic Radiation from Quartz, Lithium Niobate, and Polymethyl Methacrylate Samples Subjected to Impact Loading

Characteristics of Electromagnetic Radiation from Quartz, Lithium Niobate, and Polymethyl Methacrylate Samples Subjected to Impact Loading

Large regular arrays with submicron domains written by low-voltage e-beam on –Z cut of lithium niobate

Contact-free formation of large arrays with a uniform distribution of submicron size domains on the LiNbO3 surface is an urgent and not fully solved problem. Such arrays are widely used, including in optical quasi-phase matching devices for nonlinear frequency conversion. In this work large arrays of submicron domains were first written using low-voltage electron-beam irradiation on a –Z cut of a bulk lithium niobate sample with a grounded conductive coating. Very uniform periodic structures of submicron domains with periods of 1 μm or 1.5 μm were obtained at 5 kV and 10 kV. Surface etching was used to identify the domains within days of electron beam writing. It was found that the stability of the submicron domains became worse as the area of the irradiated squares used in the writing schemes decreased. The spatial intervals between the squares also affect the regularity of the arrays formed. When interpreting the experimental data obtained, the results of electric field distribution simulation at the corresponding writing schemes were used. Based on these results, it has been suggested that two effects competitively influence the success of large regular arrays of submicron domains formation. The electrostatic repulsion between the closest submicron domains and the convergence of electron-beam charges up to the formation of sufficiently large merging zones in which the electric field has sufficient strength. The conclusions obtained could be useful for understanding the mechanism of submicron domain formation in the injected charge zones and could contribute to the further development of electron-beam domain engineering.

Registration of a pulsed generation of electron beam in the nanosecond range under heating and cooling cycles of a lithium niobate crystal at atmospheric pressure

We describe here an experimental prototype designed for registration of nanosecond discharge current pulses with an amplitude of up to 600 mA and a negative/positive polarity in the air gap between the +Z surface of a pyroelectric lithium niobate crystal and a flat copper electrode during the crystal heating/cooling cycles. The experiments were carried out in air at atmospheric pressure using a cylindrical lithium niobate sample with a diameter of 13 mm and a thickness of 7 mm. With the temperature varying from 20 to 80 °C, there were registered over 50 discharge pulses for heating as well as for cooling stage. Their parameters testify to pyroelectric generation of pulsed electron beam with a current at maximum from 45 mA to 600 mA, duration of about 15 ns, rise time from 1 to 1.9 ns, and transferred charge from 0.95 to 5.7 nC.

The effect of contact phenomena on the measurement of electrical conductivity of reduced lithium niobate

Lithium niobate (LN) is a ferroelectric material with a wide range of applications in optics and acoustics. Annealing of LN crystals in an oxygen-free environment leads to the appearance of black coloration and the concomitant increase in electrical conductivity due to chemical reduction. The literature presents many works on the study of the electrophysical properties of reduced crystals of LN, however, the contact phenomena arising during the measurement of electrical conductivity, as well as the interaction of the electrode material with the samples under study, are practically ignored. In this paper, the effect of chromium and indium tin oxide (ITO) electrodes on the results of measurements at room temperature of electrophysical parameters of LN samples recovered at 1100 °C is investigated. It was found that significant non-linearities in the voltage characteristics (I-V curve.) at voltages less than 5V do not allow to obtain the correct values of the resistivity of NL. This leads to the need to carry out measurements at higher voltages. By the method of pulse spectroscopy, it is shown that capacitances, including those formed, probably, in the contact areas, have a strong influence on the measurement results. It is shown that the results obtained are adequately described by a model assuming the presence of contactless tanks connected in parallel to the sample’s own capacity. A possible mechanism for the formation of such containers is described, and an assumption is made about the existence of a significant density of electronic states at the “electrode - sample” interface capable of capturing charge carriers, and with increasing annealing time, the concentration of captured carriers increases.

Quantifying the coherent interaction length of second-harmonic microscopy in lithium niobate confined nanostructures

Thin-film lithium niobate (TFLN) in the form of x- or z-cut lithium-niobate-on-insulator has attracted considerable interest as a very promising and novel platform for developing integrated optoelectronic (nano)devices and exploring fundamental research. Here, we investigate the coherent interaction length lc of optical second-harmonic generation (SHG) microscopy in such samples, that are purposely prepared into a wedge shape, in order to elegantly tune the geometrical confinement from bulk thicknesses down to approximately 50 nm. SHG microscopy is a very powerful and non-invasive tool for the investigation of structural properties in the biological and solid-state sciences, especially for visualizing and analyzing ferroelectric domains and domain walls. However, unlike in bulk lithium niobate (LN), SHG microscopy in TFLN is impacted by interfacial reflections and resonant enhancement, both of which rely on film thickness and substrate material. In this paper, we show that the dominant SHG contribution measured on TFLN in backreflection is the co-propagating phase-matched SHG signal and not the counter-propagating SHG portion as is the case for bulk LN samples. Moreover, lc depends on the incident pump laser wavelength (sample dispersion) but also on the numerical aperture of the focussing objective in use. These experimental findings on x- and z-cut TFLN are excellently backed up by our advanced numerical simulations.

Структура и физико-химические свойства приповерхностных слоев оптических материалов, модифицированных путем обработки в водородной плазме

The present paper aims to investigate the structure and properties of the near-surface layers of optical materials modified by treatment in hydrogen plasma (H-plasma). For this the quartz and soda-lime glasses, lithium niobate (LN) as delivered and LN after proton exchange were used. The structure and properties of the near-surface layers of optical materials were investigated by IR spectroscopy, Raman spectroscopy, X-ray diffraction analysis, atomic force microscopy, mode spectroscopy, scanning electron microscopy, and wet chemical etching. During the treatment in H-plasma the hydrogen penetrated into the near-surface layers and caused increasing the number of defects in the structure of materials. Changes of the shape and intensity of the absorption peaks of OH–-groups and the θ/2θ- curves recorded from the LN samples processed in H-plasma were inconsequent, probably due to the thin modified layer. For the first time, a huge volume “swelling” (above 10 %) of the near-surface layers of LN and soda-lime glass after the processing in H-plasma was revealed. Also for the first time, the formation of thin strained layers on the surface of the LN after treatment in H-plasma was shown; the density reduction of the near-surface layers of the LN modified in H-plasma was described; the features of blistering and flaking were found on the surface of LN and soda-lime glass after their treatment for a long time (120–150 min). For the first time, the multilayer structure of the near-surface region of the LN was detected after treatment in H-plasma. In this paper we proposed the model of changes the structure and properties of LN after the treatment in H-plasma. The model explains the experimental results obtained in this study and previously published studies of other research groups. Due to the high concentration in the near-surface layers of optical materials after the H-plasma treatment, hydrogen predominantly forms pores and bubbles. Presumably the compounds of the Li2O–Nb2O5 system with a low Li content formed in the near-surface region of LN after the treatment in H-plasma. Modification of near-surface layers of optical materials in hydrogen plasma could be used to form elements of integrated-optical circuits, particularly the diffraction gratings.

Growth of lithium niobate and potassium niobate single crystals using the Czochralski method with liquid and ceramic charging

Lithium niobate and potassium niobate crystals of near-stoichiometric composition were grown with the help of the Czochralski method with liquid and ceramic charging. Birefringence of the grown lithium niobate sample is Δn = 1.5 at 1 THz frequency, pleochroism in absorption on 1 THz frequency makes preferable the using of the extraordinary wave because of the lower absorption coefficient (14 cm−1). The absorption coefficient for the grown crystal of potassium niobate is 24 cm−1 at 1 THz frequency.

A heterogeneously integrated silicon photonic/lithium niobate travelling wave electro-optic modulator.

Silicon photonics is a platform that enables densely integrated photonic components and systems and integration with electronic circuits. Depletion mode modulators designed on this platform suffer from a fundamental frequency response limit due to the mobility of carriers in silicon. Lithium niobate-based modulators have demonstrated high performance, but the material is difficult to process and cannot be easily integrated with other photonic components and electronics. In this manuscript, we simultaneously take advantage of the benefits of silicon photonics and the Pockels effect in lithium niobate by heterogeneously integrating silicon photonic-integrated circuits with thin-film lithium niobate samples. We demonstrate the most CMOS-compatible thin-film lithium niobate modulator to date, which has electro-optic 3 dB bandwidths of 30.6 GHz and half-wave voltages of 6.7 V×cm. These modulators are fabricated entirely in CMOS facilities, with the exception of the bonding of a thin-film lithium niobate sample post fabrication, and require no etching of lithium niobate.

Investigation of laser-induced damage and related multiphoton absorption changes in lithium niobate crystals at high repetition rate femtosecond pump

An R-on-1 laser-induced damage-threshold (LIDT) testing method was applied to test coated and uncoated magnesium oxide-doped periodically poled lithium niobate samples pumped by femtosecond Yb:KGW laser pulses at kilohertz and megahertz pulse repetition rates. LIDT values decreased by ∼1.5 and ∼38 times when increasing repetition rate from 100 to 571 kHz and to 76 MHz, respectively. We also investigated nonlinear absorption changes in lithium niobate crystals at the pulse repetition range from 60 to 600 kHz with trains consisting of 100 identical femtosecond pulses. Laser beam transmission in the crystal experienced a drop of ∼18 % from initial pulses of train to the next 40 pulses at intensities 40% to 15% lower than LIDT due to nonlinear absorption of 220 fs duration pulses at 1.03 μm.

Solitonic waveguide reflection at an electric interface.

A refractive index interface is dynamically induced in a bulk photorefractive material by biasing two adjacent regions with different electric fields, thus building up an electric wall. Effects of this interface on reflection, refraction and breathing of bright photorefractive solitons and their associated waveguides are numerically and experimentally studied as a function of the induced purely electric field gradient. Reflection and refraction efficiency depends on the amplitude and sign of the applied voltages that affect both the self-confining beam and the signals propagating inside the waveguide. Experimental tests are performed in nominally undoped lithium niobate samples.

Optical Writing of Diffraction Structures in Photorefractive Lithium Niobate by Bessel-Like Optical Fields

The evolution of the characteristics of one-dimensional phase diffraction structures during their optical induction by Bessel-like monochromatic beams in photorefractive samples of lithium niobate is studied experimentally. Both, one-dimensional (1D) and two-dimensional (2D) Bessel-like beams with different topology of 2D beam cross-sections are formed from Gaussian laser beams using the amplitude masks with rectangular and annular apertures. These almost diffraction-free light fields with wavelengths of 457 and 532 nm can change the refractive indices of photorefractive lithium niobate samples and form within them the nonlinear photonic diffraction structures.

Change nonlinear-optical response of electro-optic crystal in conditions of contributions incoherent background illumination

Influence of incoherent background on diffraction divergence of narrow light beams of He-Ne laser with wavelength of 633 nm due to refractive index changes of Fe-doped lithium niobate sample is experimentally studied. Incoherent background is produced by light-emitting diodes (LED´s) of different average wavelengths and laser diode emitting in blue region of visible. It has been experimentally demonstrated that nonlinear diffraction of coherent red beams of He-Ne laser with diameters on full width of half maximum (FWHM) near to 15–20 μm may be totally compensated by means of assistance of incoherent background with shorter average wavelengths 455–465 nm and even much lower intensity compared with that for coherent beam.

Квазиупругое и низкочастотное комбинационное рассеяние света в кристаллах ниобата лития с дефектами стехиометрии

The paper investigates quasielastic light scattering spectra at a temperature of 296 K in lithium niobate samples of various degrees of imperfection as measured by means of the acoustic quality factor. We performed a quantitative spectrum analysis in the 0--70 cm-1 frequency range for samples with different Q-factor values in a model accounting for the connection between a low-frequency optical mode of the A1(TO) symmetry type and the acoustic density of states observed in a spectrum as a result of violating the wavevector selection rule in a stoichiometrically defective crystal. The results of comparing these simulations to experimental data show that stoichiometrical defects significantly contribute to the quasielastic light scattering intensity in congruent lithium niobate crystals

Spatially Modulated Channel Waveguide Elements Optically Written in Photorefractive Lithium Niobate

Optical inducing of the channel waveguide elements with spatial modulated parameters in lithium niobate sample with Cu-doped layer is experimentally demonstrated. The channel waveguides have been formed via the point-by-point exposure at light wavelengths of 532 nm and 450 nm. It is shown that the distance between exposing light spot centers affects to the homogeneity and width of channel waveguides.

Phase diffraction patterns optically induced by laser Bessel-like beams in photorefractive lithium niobate with a doped surface layer

The evolution of the characteristics of one-dimensional phase diffraction structures during their optical induction by bessel-like monochromatic beams in photorefractive samples of lithium niobate is studied experimentally. Both, one-dimensional (1D) and two-dimensional (2D) Bessel-like beams with different topology of 2D beam cross-sections are formed from Gaussian laser beams using the amplitude masks with rectangular and annular apertures. Also, the property of reconstructing the formed bessel-like laser beams after they have passed through an obstacle has been experimentally verified. These almost diffraction-free light fields with wavelengths of 457 and 532 nm can change the refractive indices of photorefractive lithium niobate samples and form within them the nonlinear photonic diffraction structures.

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

Abstract

Intensity modulator fabricated in LiNbO3 by femtosecond laser writing

This work presents the design, development and characterization of an integrated optical modulator based on a Mach Zehnder Interferometer (MZI) recorded inside x-cut Lithium Niobate (LNB) wafers. These optical circuits were fabricated by means of femtosecond laser writing on LNB samples under planar configuration. Electro-optics modulation was achieved by adding metal electrodes on the LNB sample surface, configured as a coplanar strip layout. The latter fabrication procedure was conducted by using standard lithography and sputtering techniques from silicon platform. The MZI prototypes developed support single mode propagation at communication wavelengths (1.55 µm) and present a half wave modulation voltage, Vπ, close to 45 V measured with a bias unbalance between arms of 15 V.The MZI prototype has unique constructive features because it takes the advantages of the femtosecond laser writing. Additionally, it can be a key element of an opto-electronic device to be implemented in many systems, with high impact among others technological areas such as optical communication, sensing and control.

Quasi-Elastic Light Scattering in Congruent Lithium Niobate Crystals

Raman scattering spectra in the frequency range of 5–75 cm–1 in lithium niobate samples with various structural imperfections were obtained. It is shown that the intensity of quasi-elastic light scattering on the wing of the Rayleigh line demonstrates an exponential dependence on the concentration of large-scale defects in the samples. Calibration curves for the intensity of quasi-elastic scattering dependence at low frequencies on the acoustic quality factor are constructed.

Channel Optical Waveguides with Spatial Longitudinal Modulation of Their Parameters Induced in Photorefractive Lithium Niobate Samples

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Characterization of Thermal Energy Harvesting Using Pyroelectric Ceramics at Elevated Temperatures

Abstract Energy harvesting has drawn increasing attention due to the fast development of wireless sensors and devices. Most research has been focused on mechanical energy harvesting using piezoelectric ceramics; however, little is known on their experimental capabilities to harvest thermal energy at different temperature ranges and the impact that the temperature range has on the energy conversion efficiency. Majority of piezoelectric ceramics are pyroelectric in nature thus enabling them to couple energy between thermal and electrical domains. This paper demonstrates the use of Lithium Niobate (LNB) as a thermal energy harvesting device for high temperature applications. A custom testing setup was developed to test the LNB sample temperatures up to 225 °C. Pyroelectric coefficient of the material was characterized at different temperature ranges. Pyroelectric coefficient was found to increase with temperature, with a maximum value of −196 μC·m−2 °C−1. Power output of the sample was also characterized in different temperature ranges. A maximum value of over 20.5 μW was found when cycling the sample between 75 °C and 100 °C. Meanwhile, a maximum value of 14.8 μW was found in the 125 °C to 150 °C range. Finally, a peak value of 255 nW was found when cycling the sample in the 200 °C to 225 °C range.