Analysis of High-Index Contrast Lithium Niobate Waveguides Fabricated by High Vacuum Proton Exchange

International audience—High-index contrast waveguides fabricated with precise control and reproducibility are of high interest for nonlinear and/or electro-optical highly efficient and compact devices for quantum and classical optical data processing. Here we present a new process to fabricate planar and channel optical waveguides on lithium niobate substrates that we called High Vacuum Proton Exchange (HiVacPE). The main purpose was to improve the reproducibility and the quality of the produced waveguides by limiting and controlling the water traces in the melt, which is used for the ionic exchange. Moreover, we discovered that, when the acidity of the bath is increased, depending on substrate orientation (Z-cut or X-cut) the waveguides are completely different in term of crystallographic properties, index profiles and nonlinearity. The best-obtained channel waveguides exhibit a refractive index contrast as high as 0.04 without any degradation of the crystal nonlinearity and state of the art propagation losses (0.16dB/cm). We have also demonstrated that the HiVacPE process allows fabricating waveguides on Z-cut substrate with high-index contrast up to 0.11 without degrading the crystal nonlinearity but high strain induced propagation losses. On top of that, we proposed an original and very useful method of analyzing waveguides with complex index profiles. This method can be used for the analysis of any waveguides whose core contains several layers

Broadband integrated beam splitter using spatial adiabatic passage

Light routing and manipulation are important aspects of integrated optics. They essentially rely on beam splitters which are at the heart of interferometric setups and active routing. The most common implementations of beam splitters suffer either from strong dispersive response (directional couplers) or tight fabrication tolerances (multimode interference couplers). In this paper we fabricate a robust and simple broadband integrated beam splitter based on lithium niobate with a splitting ratio achromatic over more than 130 nm. Our architecture is based on spatial adiabatic passage, a technique originally used to transfer entirely an optical beam from a waveguide to another one that has been shown to be remarkably robust against fabrication imperfections and wavelength dispersion. Our device shows a splitting ratio of 0.52$\pm$0.03 and 0.48$\pm$0.03 from 1500\,nm up to 1630\,nm. Furthermore, we show that suitable design enables the splitting in output beams with relative phase 0 or $\pi$. Thanks to their independence to material dispersion, these devices represent simple, elementary components to create achromatic and versatile photonic circuits

Influence of Substrate Temperature on the Properties of Ga Doped ZnO thin Films

Transparent conductive oxides, including single layer and multilayer structures, have been investigated in the last years, for transparent or semitransparent conducting electrode applications in optoelectronic devices and transparent electronics. Owing to their properties such as: high transmission in the visible region (>80%) and high electrical conductivity, they been used in applications like LEDs, laser diodes, solar cell, flat panel displays, thin films transistors etc Some alternative TCO materials including ZnO doped with III rd group elements (In, Al, Ga); TiO 2 doped with Ce, Nb, Fe; SnO 2 doped with F etc., have been investigated in the last years In order to use Ga doped ZnO thin films for different application it is very important to obtain layers with reproducible properties. Because the deposition parameters of each deposition technique can be varied, the properties of obtained thin films may differ significantly. Despite the progress made in the field, the researchers are still doing studies to establish the most convenient deposition parameters for processing Ga doped ZnO thin films with excellent optical and electrical properties. A large number of techniques were used in order to obtain ZnO:Ga thin films including pulsed laser deposition, reactive plasma deposition, RF Magnetron Sputtering, The aim of this paper is reporting some results regarding the influence of substrate temperature on the physical properties of ZnO:Ga thin films prepared by RF magnetron sputtering being analysed by XRD, SEM, Profilometry and two point method for electric properties. Experimental part Materials and methods The obtaining of thin films Ga doped ZnO thin films were deposited on glass substrates by RF Magnetron Sputtering using a 98 wt. % ZnO doped with 2 wt. % Ga 2 O 3 ceramic target of 40 mm diameter, keeping RF power constant at 50W. The substrate temperature was of 300K (sample 1), 423K (sample 2) and 523K (sample 3) respectively. The distance between target and substrate was maintained at 55 mm. For the thin film deposition, the work pressure was maintained at 1.4 mTorr in argon atmosphere. The deposition parameters are presented in table 1. Analysis methods and techniques Structural analyses of the studied films were carried out using a DRON 2 diffractometer with CuK α radiation (λ = 1.5418 A) as an X -ray source at 20 kV and 40 mA, in the 2θ =20-70° range. The films morphology was investigated by using scanning electron microscopy (SEM Tescan VEGA II LSH). The thickness of investigated films, measured using a DEKTAK profilometer, was found to be around of 150 nm. The electrical properties of studied samples were measured in a two points configuration, by using a sensitive Keithley model 6517 multimeter. For gas sensing measurements, the thin films were placed in a glass enclosure capable of controlling the different gas concentrations and working temperature. The resistance was measured in the presence of the tested gas. The gas sensing properties were investigated at various operating temperatures from 373K to 750K. As test gases were use

Spontaneous Polarization Reversal Induced by Proton Exchange in Z-Cut Lithium Niobate α-Phase Channel Waveguides

The α-phase waveguides directly produced in one fabrication step only are well known for preserving both the excellent nonlinear properties and the ferroelectric domains orientation of lithium niobate substrates. However, by using the piezoresponse force microscopy (PFM), we present a coherent study on ferroelectric dipoles switching induced by the fabrication process of α-phase waveguides on Z-cut congruent lithium niobate (CLN) substrates. The obtained results show that the proton exchange process induces a spontaneous polarization reversal and a reduction in the piezoelectric coefficient d33. The quantitative assessments of the impact of proton exchange on the piezoelectric coefficient d33 have been quantified for different fabrication parameters. By coupling systematic PFM investigation and optical characterizations of α-phase protonated regions and virgin CLN on ±Z surfaces of the samples, we find a very good agreement between index contrast (optical investigation) and d33 reduction (PFM investigations). We clearly show that the increase in the in-diffused proton concentration (increase in index contrast) in protonated zones decreases the piezoelectric coefficient d33 values. Furthermore, having a high interest in nonlinear performances of photonics devices based on PPLN substrates, we have also investigated how deep the spontaneous polarization reversal induced by proton exchange takes place inside the α-phase channel waveguides

Anomalous Angular Dispersion in Lithium Niobate One-Dimensional Waveguide Array in the Near-Infrared Wavelength Range

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Lithium niobate waveguides with high-index contrast and preserved nonlinearity fabricated by a high vacuum vapor-phase proton exchange

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