Bulk LiNbO3 Crystal Research Articles

Broadband heterodyne electro-optic sampling using a lithium niobate ridge-waveguide

Abstract We demonstrated the bandwidth broadening of terahertz waves detected by heterodyne electro-optical sampling by implementing a ridge waveguide structure in a lithium niobate (LiNbO3) crystal. Such an approach effectively reduces absorption loss, eases the phase matching condition and enhances the nonlinear interaction length through the optical confinement effect. As a result, we have more than doubled the bandwidth and improved the signal-to-noise ratio compared with an equivalent approach in a bulk LiNbO3 crystal. Heterodyne electro-optic sampling in a ridged-waveguide structure is only marginally dependent on the probe beam wavelength, suggesting its potential as a versatile method for broadband terahertz detection.

Efficient second- and higher-order harmonic generation from LiNbO3 metasurfaces.

Lithium niobate (LiNbO3) is a material that has drawn great interest in nonlinear optics because of its large nonlinear susceptibility and wide transparency window. However, for complex nonlinear processes such as high-harmonic generation (HHG), which involves frequency conversion over a wide frequency range, it can be extremely challenging for a bulk LiNbO3 crystal to fulfill the phase-matching conditions. LiNbO3 metasurfaces with resonantly enhanced nonlinear light-matter interaction at the nanoscale may circumvent such an issue. Here, we experimentally demonstrate efficient second-harmonic generation (SHG) and HHG from a LiNbO3 metasurface enhanced by guided-mode resonance. We observe a high normalized SHG efficiency of 5.1 × 10-5 cm2 GW-1. Moreover, with the alleviated above-gap absorption of the material, we demonstrate HHG up to the 7th order with the shortest generated wavelength of 226 nm. This work may provide a pathway towards compact coherent white-light sources with frequency spanning into the deep ultraviolet region for applications in spectroscopy and imaging.

Bound modes in the continuum in integrated photonic LiNbO3 waveguides: are they always beneficial?

We discuss several types of integrated photonic LiNbO3 waveguides supporting propagation of modes which can be classified as bound states in the continuum (BICs). The key properties leading to the existence of BICs (or quasi-BICs) considered here are the material anisotropy, the waveguide birefringence, or the combination of both. Typical examples are titanium diffused and proton exchanged waveguides in bulk LiNbO3 crystals and recently proposed dielectric-loaded waveguides on LiNbO3 thin films. Proton exchanged waveguides in thin film LiNbO3 are considered, too. These waveguide structures are discussed from the point of view of their benefit for applications, especially in electro-optic devices.

Thin Patterned Lithium Niobate Films by Parallel Additive Capillary Stamping of Aqueous Precursor Solutions

Thin micropatterned lithium niobate (LiNbO3) layers may be used for photonic components, actuators, and data storage devices because LiNbO3 exhibits nonlinear optical properties as well as anisotropic polarizability and ferroelectric behavior. Commonly, thin micropatterned LiNbO3 layers are integrated into device architectures by complex manufacturing algorithms including direct wafer bonding, mechanochemical wafer thinning or mechanical cleavage of thin LiNbO3 layers from bulk LiNbO3 crystals, as well as lithographic pattering of and/or pattern transfer into the thin LiNbO3 layers. The high‐throughput generation of thin microstructured LiNbO3 layers by parallel additive capillary stamping of environmentally friendly aqueous LiNbO3 precursor solutions with topographically patterned porous polymer stamps is reported. The precursor solutions contain the cheap, commercially available compounds lithium acetate and niobium oxalate hydrate, which are simply dissolved in water as received. In this way, rough surfaces not suitable for layer transfer methods involving direct wafer bonding, such as the surfaces of indium tin oxide (ITO) substrates, are functionalized with microstructured LiNbO3 layers. Microstructured holey 100 nm‐thick LiNbO3 films showing uniform second‐harmonic generation (SHG) except at the positions of the holes are obtained. Orthogonal substrate formation is demonstrated by electrodeposition of gold into the holes, which increases the SHG output 5.4 times.

Direct combination of carbon structure with optoelectronics crystal: thermal behavior of implanted carbon in lithium niobate crystal at near surface

The thermal motion mechanism of carbon (C) in lithium niobate (LiNbO3) crystal was briefly studied, which provides experience and direction for the experimental parameters of graphene production by direct implantation of carbon ions into LiNbO3. Thin-film LiNbO3 crystal and bulk LiNbO3 crystal of z-cut were direct implanted by carbon ions with a dose of 1.14 × 1016 cm−2 and then annealed at different temperatures, thin-film LiNbO3 at 700 °C and bulk LiNbO3 at 900 °C. The experimental conditions and parameters of ion implantation and annealing were all the same except the annealing temperature. The samples were characterized by RBS, XRD, EDS and Raman spectrum. The results show that during annealing, the implanted carbon ions aggregate into clusters while moving towards the surface. This behavior prevents the carbon ions from precipitating on the LiNbO3 surface, which is not conducive to the production of graphene. The formation of graphene on LiNbO3 surface by direct ion implantation can only occur when implanted C dose and annealing temperature lie in a specific range.

Mid-infrared upconversion imaging using femtosecond pulses

Mid-infrared (mid-IR) imaging and spectroscopic techniques have been rapidly evolving in recent years, primarily due to a multitude of applications within diverse fields such as biomedical imaging, chemical sensing, and food quality inspection. Mid-IR upconversion detection is a promising tool for exploiting some of these applications. In this paper, various characteristics of mid-IR upconversion imaging in the femtosecond regime are investigated using a 4f imaging setup. A fraction of the 100 fs, 80 MHz output from a Ti:sapphire laser is used to synchronously pump an optical parametric oscillator, generating 200 fs mid-IR pulses tunable across the 2.7–4.0 μm wavelength range. The signal-carrying mid-IR pulses are detected by upconversion with the remaining fraction of the original pump beam inside a bulk LiNbO3 crystal, generating an upconverted field in the visible/near-IR range, enabling silicon-based CCD detection. Using the same pump source for generation and detection ensures temporal overlap of pulses inside the nonlinear crystal used for upconversion, thus resulting in high conversion efficiency even in a single-pass configuration. A theory is developed to calculate relevant acceptance parameters, considering the large spectral bandwidths and the reduced interaction length due to group velocity mismatch, both associated with ultrashort pulses. Furthermore, the resolution of this ultrashort-pulsed upconversion imaging system is described. It is demonstrated that the increase in acceptance bandwidth leads to increased blurring in the upconverted images. The presented theory is consistent with experimental observations.

Spatial distribution of optical coloration in single crystalline LiNbO3 after high-temperature H2/air treatments

The paper presents results of investigations of optical absorption spatial changes in congruent bulk LiNbO3 crystals in 300–800 nm spectral region along Х, Y, Z directions caused by high-temperature (400–700 оС) annealing in Н2 and, subsequently, in air. As it is shown, the coloration/discoloration processes are anisotropic: a maximal depth of coloration is observed for Z direction and a minimal one – for X direction. The reversibility of optical properties with respect to the annealing atmosphere changes as well as results of investigations by SIMS and IR spectroscopy methods suggest that the oxygen loss/incorporation. processes play a major role in the coloration/discoloration. The additional absorption spectrum caused by reduction consists of three bands, whose nature is associated with different point defects: 350–410 nm – defects in anion sublattice, 500–520 nm – bipolarons; 700–750 nm – bound polarons. Individual contributions of the bands to the additional absorption spectra depend on the annealing temperature and on the distance from the crystal surface. During reduction lithium oxide is leaving the crystal which leads to the formation of Li2O-depleted structural phase LiNb3O8 in near-surface regions. Experimentally obtained distribution of depth-dependent additional absorption in reduced crystals was successfully approximated basing on the oxygen vacancies diffusion model and solution of differential equations set describing a quasichemical reaction of bipolarons formation due to the oxygen loss in LiNbO3 crystal. On the other hand, this model does not allow to describe accurately the depth-dependent discoloration after oxidizing annealing of previously reduced lithium niobate crystals.

Spatial distribution of LiNbO3 single crystals optical properties changes after redox high-temperature treatment

The changes in optical properties of bulk LiNbO3 crystals were investigated depending on the diffusion depth. For this purpose the samples were isochronously annealed in hydrogen, subsequently the reduced samples were isochronously annealed in air. The investigations were held in different crystallographic directions. The experimentally determined distribution of depth-dependent additional absorption in reduced crystals was approximated basing on the oxygen vacancies diffusion model and solution of differential equations set describing the quasichemical reaction of bipolarons formation due to the oxygen loss in LiNbO3 crystal.

Lithium niobate nanoparticle-coated Y-coupler optical fiber for enhanced electro-optic sensitivity.

Single crystals of lithium niobate (LiNbO3), possessing high birefringence and anisotropic properties have been explored, for a long time, to harness their excellent electro-optic properties. However, their nanoforms are comparatively less explored. In this context, dielectric constant and polarization (P) versus electric-field (E) characteristics of LiNbO3 nanomaterials have been studied. A nonideal P-E loop and a dielectric constant of 20 at the onset of 1 kHz were seen. The electro-optic sensitivity was found to be 4 times as compared to the bulk LiNbO3 crystals. The results are attributed to oxygen vacancies, antisite defects, and grain boundary effects in an already congruent structural matrix of LiNbO3.

Micro-spectroscopic characterization of ferroelectric domain structures in Yb^3+:LiNbO_3 prepared by electron beam writing

Sub-micrometer inverted domains in Yb3+ doped bulk LiNbO3 crystals are reported by using direct electron beam writing (DEBW) as a tool to reverse the spontaneous polarization in a two dimensional geometry. The effect of electron bombardment within the domain inversion process is analyzed at the micrometer scale by combining spatially resolved confocal Raman, Fluorescence and Second Harmonic Generation (SHG) imaging techniques. The obtained results not only confirms the feasibility of DEBW on the inversion procedure -the linear and nonlinear optical properties of the system remain unaltered after the process, but they also show that the slight structural changes associated with the polarization reversal in LiNbO3 are independent on the employed switching mechanism. The possibility of obtaining complementary non-destructive spectroscopic images of domains is also shown. Together, the results highlight the outstanding opportunities offered by confocal spectroscopy as a non invasive tool to probe the interaction between intrinsic defects and ferroelectric domain reversal structures in LiNbO3. Additionally, they provide valuable information to further decrease the size and distance between adjacent inverted domains in a solid state system in which, IR laser action, self-frequency conversion processes and laser tunability have been demonstrated.

Writing and probing light-induced waveguides thanks to an endlessly single-mode photonic crystal fiber

We demonstrate writing and probing of light-induced waveguides in photorefractive bulk LiNbO3 crystal using an endlessly single-mode photonic crystal fiber. The optical waveguides are written at visible wavelengths by slightly raising the ferroelectric crystal temperature to benefit from the pyroelectric-driven photorefractive effect and the guiding properties are investigated at telecom wavelengths using the same photonic crystal fiber. End butt coupling with this photonic crystal fiber enables writing and probing of optical waveguides due to the self-alignment properties of spatial solitons.

Terahertz-wave surface-emitted difference-frequency generation without quasi-phase-matching technique

A scheme of terahertz (THz)-wave surface-emitted difference-frequency generation (SEDFG), which lacks the drawbacks associated with the usage of periodically orientation-inverted structures, is proposed. It is shown that both material birefringence of the bulk LiNbO(3) crystal and modal birefringence of GaAs/AlAs waveguide are sufficient to obtain SEDFG up to a frequency of approximately 3THz. The simplicity of the proposed scheme, along with the fact that there is a much smaller THz-wave decay in nonlinear crystal, makes it a good candidate for the practical realization of efficient THz generation. The use of a GaAs waveguide with an oxidized AlAs layer is proposed for enhanced THz-wave SEDFG in the vicinity of the GaAs polariton resonance at 8THz.

Optical High DC Voltage Sensor Based on White-Light Interferometry

This letter reports an optical system for high dc voltage measurement utilizing a bulk LiNbO3 crystal with specialized-orientation as a sensing element. White-light interferometry is employed to encode the electric voltage information in the optical spectrum instead of intensity. Measurement of dc voltage from 0 to 9 kV with resolution of 2.3 V has been experimentally demonstrated

Depth-resolved analysis of ferroelectric domain structures in bulk LiNbO3 crystals by scanning force microscopy

Ferroelectric domains were written in congruently melting lithium niobate crystals (LiNbO3) by electrical field poling using a mask with circular openings. For the surface sensitive detection of the domains, we used scanning force microscopy. Thinning the crystal step-by-step reveals that the domains evolve from circular shape at the very surface to hexagonal shape at a depth of around 30μm, demonstrating the impact of the crystal symmetry on the domain shape.

High‐efficiency diffraction modulation of light by strain‐optic effect of piezoelectrically induced standing acoustic waves in bulk LiNbO3 crystal

Abstract100% modulation of light was achieved by the strain‐optic effect of the piezoelectrically induced standing acoustic waves in a bulk LiNbO3 crystal. The modulation index as high as 4 was attained. As a result, in a single pass a continuous wave light has been modulated resulting in transmission peaks of width less than 2 ns at a repetition rate of 100 MHz.