In this work is presented a study on the surface microdomain formation in quasi-SLN Z-cut 3" crystals, with an accurate control on both the composition and on the wafering process. The UV absorption edge has been measured and correlated with the crystal composition, showing the edge shift towards shorter wavelengths. The coercive field has been measured as a function of temperature and it has been found lower in the quasi-SLN substrate if compared with the congruent crystals. The microdomain formation at wafer level can be controlled and avoided by appropriate composition choice as well as wafer mechanical and thermal treatments, and is checked by chemical etching and subsequent optical inspection. It has been found that quasi-SLN crystals with 49.82 Li2O mol% content could present microdomains formation even after the photoresist process. On the other side, quasi-SLN crystals with 49.72 Li2O mol% content seem to be more stable for both photoresist and Ti diffusion process for waveguide fabrication. A careful control on LiNbO3 composition and wafer surface quality allows one to find the proper compositional window for the realization of various advanced optical and electro-optical devices.
MicroRaman spectroscopy has been used for the surface characterization of lithium niobate (LiNbO3) crystals. 3" wafers with different Li/Nb ratio, i.e., conventional congruent (CLN) and quasi-stoichiometric LiNbO3 have been analyzed. A correlation between the width of the 150cm-1 and 870cm-1 line and the crystal composition has been found. A narrowing of the linewidths for quasi-stoichiometric crystals has been observed, showing an ordered structure, if compared with CLN. The 870cm-1 line has been used to study the surface quality of 3" Z-cut CLN crystals after the wafering process. The presence of a surface structural disorder up to 30micron has been found. Various etching methods have been employed in order to minimize both the thickness of the damaged layer and the degree of damage during the wafer slicing and polishing processes. A reliable surface stress release method has been found for optical surface finishing of LiNbO3 substrates
Various material and functional properties have been measured in lithium niobate crystals (LiNbO3) with different compositions, starting from conventional congruent composition, up to off-congruent and quasi-stoichiometric ones. The UV absorption edge has been measured and correlated with the crystal composition, showing the edge shift towards shorter wavelengths. The ferroelectric transition Curie temperatures have been determined by differential scanning calorimetry, and it increases with Li2O content in the crystal. The surface composition has been checked by micro-Raman spectroscopy. A narrowing of the linewidths has been observed for quasi-stoichiometric crystal, showing an ordered structure, if compared with congruent composition. The coercive field has been measured as a function of temperature for two different crystal compositions, and it has been found lower in the off-congruent substrate. The Ti-indiffusion process has been studied and compared in congruent and off-congruent LiNbO3 substrates by secondary ion mass spectrometry. The main diffusion process parameters have been determined. The Ti diffusion process has been found considerably slower in off-congruent substrates, if compared with conventional congruent LiNbO3, and resulted almost isotropic. The Li-outdiffusion phenomenon has been observed and correlated wit the Ti concentration profile. A careful control on LiNbO3 composition and material properties allows one to find the proper compositional window for the realization of various advanced optical and electro-optical devices.
The erbium-lithium ion exchange is presented as a method for the erbium local doping of lithium niobate crystals. Ion exchange process is performed immersing the LiNbO3 substrates in a liquid melt, containing erbium ions; due to their high mobility, the lithium ions migrate from the crystal to the melt, and are replaced by erbium ions. A systematic analysis of the doping process is performed, and the influence of the process parameters is investigated: exchange time and temperature, crystal cut direction, composition and chemical reactivity of the Er ions liquid source. By structural (X-Ray Diffraction and Rutherford Backscattering Spectrometry), compositional (Secondary Ion Mass Spectrometry) and spectroscopic techniques (optical spectroscopy and micro-luminescence), the formation of lithium deficient phases and the incorporation of the Er ions into the LiNbO3 matrix is studied.
The bulk growth of periodic poled lithium niobate (PPLN) is made by the off-centered Czochralski technique adding an impurity to the melt. The periodic domain structures are obtained with different impurities such as Er, Yb, Nd, Cr, Fe and Y. The impurity distribution along the bulk PPLN crystals has been studied to understand the formation mechanism of the periodic domain structure. The distribution coefficient of the impurities, the temperature fields and the shape of the solid-liquid interface have been found to play a key role in the PPLN formation. The cooling rate and other growth conditions control the size of the areas where the periodic domain structure appears. It has been found that independently of the impurity added to the melt the dopant concentration is constant along the periodical domain structure, while it has been observed that exists a periodical variation of the Nb concentration which is related one to one with the periodical domain structure.
An experimental and numerical investigation of active waveguides realized on lithium niobate crystals using erbium as doping element is presented. A systematic study of Er diffusion in LiNbCO3 is performed with secondary ion-mass spectrometry to determine surface concentration, penetration depth and diffusion coefficient of erbium as function of various process parameters. A software package is implemented to simulate the propagation of electromagnetic field in active waveguides using an Active Vectorial Beam Propagation Method, which permits to calculate the field evolution along the propagation axis. A comparative study of signal gain evolution, optimal device length and threshold pump power as function of erbium depth profile obtained by various doping methods is presented.
Much attention has been dedicated in the recent years to the study of suitable methodologies for preparing nonlinear optical composites, with the aim of designing all-optical switches for optoelectronics applications. The nonlinear response of glasses may be enhanced by several orders of magnitude by introducing metal nanoclusters in the glass matrix. Metal nanoclusters have been formed in light waveguides, obtained by Ag+- or Cu+-Na+ exchange process in glass, by either irradiating with low-mass ion beams or by heating in hydrogen atmosphere at temperatures varying in the range 100-250°C. Metal nanocluster-silica composites have been also obtained by the sol-gel technique. Nanocluster modifications induced by pulsed laser irradiation have been investigated and annealing behavior of nanoclusters synthesized by the sol-gel process has been studied. Composites were characterized by Secondary Ion Mass Spectrometry and Rutherford Backscattering Spectrometry, in order to determine concentration depth-profiles and by Transmission Electron Microscopy for the nanocluster detection and size evaluation. Optical analyses were performed to evidence linear and nonlinear properties.
A Critical Review is given on the use of the SIMS technique. The basic concepts of sputtering process and secondary ion production as well as quantification problems are briefly reviewed. A description of the possible operational modes and instrumental aspects are presented. Particular attention is devoted to the specific analysis of insulating samples. SIMS has been applied to the characterization of LiNbCO3 passive and active optical waveguides obtained by thermal diffusion of metals from thin film and by proton ion-exchange. Results obtained by SIMS on the study of the chemical interaction in ion-implanted silica glass are presented. The technique has also been used to measure in-depth concentration profiles of metals in soda-lime glass planar waveguides obtained by metal ion-exchange process and to correlate compositional modifications with the optical properties. The need of complementary techniques for quantification of the obtained data has been underlined.
This paper presents a review of the effects induced by ion implantation in silica glass and in SiO2 films on silicon with particular emphasis on optical modifications and new compound formation. The formation of silicon oxynitrides and nitrogen oxides in surface layers as a consequence of nitrogen implantation was investigated by using different techniques as XPS, SIMS, and optical methods.