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Local doping of lithium niobate by iron diffusion: a study of photorefractive properties
Full text linkIn the last decades the electronic data transmission technology has progressively reached its performance limits and it is nowadays evident that further advances can be achieved only by all-optical signal processing systems. Thus the research in nonlinear optics have been rapidly expanding in the last twenty years, developing many applications of photonics which are now relevant for industrial and consumer markets. In particular, in electro-optic materials the phenomena based on the photorefractive effect are doubtless playing a major role in the building up of optoelectronic signal transmission and processing devices and lithium niobate (LiNbO3) is a promising material due to its high electro-optic and nonlinear optical coefficients. Moreover, lithium niobate offers incredible versatility as substrate for integrated optics, allowing to realize on the same crystal optical elements with different functions, by exploiting various microstructural technologies. This kind of devices require the capability to locally change the physical properties of the material, by doping it with the proper element only in a limited area of the substrate. In particular, it is known that by doping the lithium niobate with iron the photorefractive properties of the material are enhanced, thus to realize an integrated optical system with a photorefractive stage an iron local doping has to be performed. In this work the thermal diffusion process is exploited to realize iron locally doped lithium niobate crystals and the structural and photorefractive properties of the doped layer are studied. In particular it has been designed and built-up a new optical set-up able to investigate only a limited area of the doped layer, thus allowing to relate at each depth the examined iron concentration with the corresponding photorefractive response of the material. In this way it is possible to realize in depth-profiles of the main physical photorefractive parameters involved in the photorefractive effect and physical mechanisms never studied before can be now investigated.Negli ultimi decenni la tecnologia di trasmissione di dati elettronici ha progressivamente raggiunto i suoi limiti di prestazione ed al giorno d'oggi è evidente che ulteriori sviluppi possono essere raggiunti solo con l'utilizzo di sistemi ottici integrati. Perciò la ricerca relativa all'ottica non lineare ha avuto una rapida espansione negli ultimi ventanni, sviluppando molte applicazioni fotoniche che risultano rilevanti sia per il mercato industriale che per quello privato. In particolare, tra i materiali elettro-ottici i fenomeni che si basano sull'effetto fotorifrattivo stanno senza dubbio avendo un ruolo importante nella realizzazione di dispositivi per la trasmissione e il trattamento di segnali optoelettronici e il niobato di litio (LiNbO3) è un materiale promettente, dati i suoi alti coefficienti elettro-ottici e ottici non lineari. Inoltre il niobato di litio offre un incredibile versatilità come substrato per ottiche integrate, permettendo di realizzare sullo stesso cristallo elementi ottici con differenti funzioni, sfruttando varie tecnologie di microstrutturazione. Questo tipo di dispositivi richiede la capacità di cambiare localmente le proprietà fisiche del materiale, drogandolo con un opportuno elemento su una regione limitata del substrato. In particolare, è noto che drogando il niobato di litio con ferro le proprietà fotorifrattive del material vengono notevolmente migliorate, così per realizzare un sistema ottico integrato che presenti uno stadio fotorifrattivo si deve realizzare un drogaggio locale con ferro. In questo lavoro il processo di diffusione termica è sfruttato per realizzare cristalli di niobato di litio drogati localmente con ferro e sono studiate le proprietà strutturali e fotorifrattive dello strato drogato. In particolare è stato sviluppato e costruito un apparato ottico in grado di investigare solo un'area limitata dello strato drogato, permettendo in tal modo ad ogni profondità all'interno della zona drogata di mettere in relazione la concentrazione di ferro esaminata con la corrispondente risposta fotorifrattiva del materiale. In questo modo è possibile realizzare profili in profondità delle principali grandezze fisiche coinvolte nell'effetto fotorifrattivo e meccanismi fisici mai studiati prima possono essere ora investigati
Quantification of Iron (Fe) in Lithium Niobate by Optical Absorption
No full textA quantitative method, based solely on optical absorption, to determine the total iron (Fe) concentration in Fe : LiNbO3 is proposed. Absorption spectra of several samples doped by thermal diffusion with different concentrations and different [Fe2+]/[Fe3+] ratios show an isosbestic point at 342 nm. At this wavelength the absorption is proportional to the total Fe concentration and does not depend on the oxidation state. Thanks to the large number of samples covering a wide range of concentrations, in this work it was possible to estimate an effective absorption cross-section relating the absorbance of a given sample to its iron content. The main advantage of the proposed method is in its simplicity and the fact that the result does not depend on the reduction degree of the sample. As it is known that the absorbance of Fe:LN at another wavelength (532 nm) gives information on the amount of Fe2+ present in the sample, our method makes it possible to characterize both the total Fe amount and its reduction degree within a single optical absorption measurement
Charge sensor and particle trap based on z-cut lithium niobate
No full textThe generation of adhesive regions on a z-cut lithium niobate crystal without an additional voltage supply is demonstrated. We show that the origin of the attractive force in the respective solvent is electrophoresis, which can selectively trap charged particles in illuminated regions. Using digital holographic microscopy to measure the space-charge field in a y-cut crystal, we demonstrate the difference between electrophoretic and dielectrophoretic particle manipulation. The suggested method enables the creation of arbitrary two-dimensional patterns, circumventing restrictions originating from the crystal asymmetry. Furthermore, it allows the discrimination between charged particles of different signs, thus acting as a charge sensor
Optical control of mass ejection from ferroelectric liquid droplets: A possible tool for the actuation of complex fluids
Full text linkWe report on the optical control of the recently observed electromechanical instability of ferroelectric liquid droplets exposed to the photovoltaic field of a lithium niobate ferroelectric crystal substrate. The ferroelectric liquid is a nematic liquid crystal in which almost complete polar ordering of the molecular dipoles generates an internal macroscopic polarization locally collinear to the mean molecular long axis. Upon entering the ferroelectric phase, droplets irradiated by unfocused beam undergo an electromechanical instability and disintegrate by the explosive emission of fluid jets. We show here that the regions of jets emission can be controlled by focusing the light beam in areas close to the droplet's edge. Once emitted, the fluid jets can be walked by moving the beam up to millimeter distance from the mother droplet. Reverting the lithium niobate substrate, jets become thinner and show the tendency of being repelled by the beam instead of being attracted, thus offering an additional tool for their optical manipulation. These observations may pave the way to intriguing applications of ferroelectric nematic fluids related to manipulation, actuation, and control of soft, flexible materials
Correlation between the lattice deformation and the reduction degree of iron in lithium niobate crystals doped by thermal diffusion
No full textLocal Doping of Lithium Niobate Crystals by Iron Diffusion: Depth-resolved Study of Photorefractive Properties
No full textPhoto-Induced Electric Field Effects on Water Droplets Generated in a LiNbO3 Opto-Microfluidic Platform
Full text linkIn this work, droplets elongation as a result of the photo-induced photovoltaic electric field is presented and modeled. Moving water droplets are generated in microfluidics channels by means of a droplet-based opto-microfluidic platform integrated in lithium niobate (LN). A cross-junction is engraved on the substrate as droplets generator, on top of which a z-cut iron-doped lithium niobate (Fe:LN) crystal is placed in order to build up photo-induced electric field as a result of photovoltaic effect promoted upon suitable illumination. Droplets detection is realized by optical waveguides, designed in a Mach-Zehnder Interferometer (MZI) configuration on the same substrate. The dynamics of droplet elongation is investigated, highlighting the impact of the photovoltaic electric field on the droplet dimension and surface tension
Depth-resolved photorefractive characterization of lithium niobate doped with iron by thermal diffusion
No full textIron doping of lithium niobate crystals by thermal diffusion is a well-established technique for the realization of spatially confined photorefractive stages in integrated optical devices. In this paper we present an innovative method able to realize depth-resolved holographic measurements inside the iron-diffused layer, so that in-depth profiles of the main photorefractive parameters can be derived without the need of any waveguide. By means of this technique it is possible to achieve a better knowledge of the influence of different surface treatments on the photorefractive performances of iron-diffused layers and to link them to the results of other depth-resolved characterization techniques in the framework of microscopic models of photorefractivity
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