Liquid crystal cells with LiNbO<sub>3</sub>:Fe crystals as substrates, are described. The photovoltaic field generated by the substrates is able to reorient the liquid crystal director thus giving rise to a phase shift on the light propagating through the cell, as in liquid crystal light valves. The process does not require the application of an external electric field, thus being potentially useful for applications requiring a high degree of compactness. A detailed characterization of several cells based on lithium niobate crystals with different iron concentration has been carried out. The correlation between the LiNbO<sub>3</sub>:Fe characteristics and the liquid crystal reorientation is also discussed.
In micro-analytical chemistry and biology applications, optofluidic technology holds great promise for creating efficient lab-on-chip systems where higher levels of integration of different stages on the same platform is constantly addressed. Therefore, in this work the possibility of integrating opto-microfluidic functionalities in lithium niobate (LiNbO<sub>3</sub>) crystals is presented. In particular, a T-junction droplet generator is directly engraved in a LiNbO<sub>3</sub> substrate by means of laser ablation process and optical waveguides are realized in the same material by exploiting the Titanium in-diffusion approach. The coupling of these two stages as well as the realization of holographic gratings in the same substrate will allow creating new compact optical sensor prototypes, where the optical properties of the droplets constituents can be monitored.
In micro-analytical chemistry and biology applications, droplet microfluidic technology holds great promise for
efficient lab-on-chip systems where higher levels of integration of different stages on the same platform is constantly
addressed. The possibility of integration of opto-microfluidic functionalities in lithium niobate (LiNbO<sub>3</sub>) crystals is
presented. Microfluidic channels were directly engraved in a LiNbO<sub>3</sub> substrate by precision saw cutting, and
illuminated by optical waveguides integrated on the same substrate. The morphological characterization of the
microfluidic channel and the optical response of the coupled optical waveguide were tested. In particular, the results
indicate that the optical properties of the constituents dispersed in the fluid flowing in the microfluidic channel can
be monitored in situ, opening to new compact optical sensor prototypes based on droplets generation and optical
analysis of the relative constituents.
A new approach for obtaining in short time highly efficient photorefractive holographic gratings in
lithium niobate is presented. The method consists in decreasing the sample conductivity by cooling it
down to liquid nitrogen temperature. In this way the initial slope of the writing curve is largely amplified,
which makes possible to achieve in short time diffraction efficiencies one order of magnitude larger than
at room temperature in the same conditions. Simple theoretical estimates show that in this way, provided
that the proper experimental conditions are met, unit efficiency should be reachable in times much shorter
than typical ones.
In this work we study the photorefractive and electro-optical properties of Zirconium-doped congruent lithium
niobate (LN) crystals. In order to set the ground for the utilization of these crystals in nonlinear wavelengthconversion
devices, we investigate the dependence of the photorefractive properties of the crystals on dopant
concentration and incident power. In our experiments the birefringence variations induced by a 532-nm laser beam
are measured by using the Sénarmont method, in the ZrO<sub>2</sub> concentration range 0-3mol% and intensity range 155-
1800 W/cm<sup>2</sup>. In order to investigate photorefractivity at high intensities, we have also utilized the direct observation
of the distortion of the light spot transmitted by the crystal. In presence of photorefractivity, the transmitted light
spot becomes smeared and elongated along the c-axis. Our data show that the threshold ZrO<sub>2</sub> concentration can be in
the range 2.5-3mol%. Considering that the growth of large homogeneous Zr:LN crystals should be easier than for
Mg:LN, and that electrical poling of these crystals has already been demonstrated, Zr-doped LN could represent a
more convenient choice than Mg:LN for the realization of room-temperature wavelength converters.
We present the first investigation of bright screening soliton formation in Erbium doped lithium niobate grown by the Czochrlaski technique (0.7% mol.). We analyse the formation of two-dimensional spatial soliton and study its long term stability. Measurements of photovoltaic current show that presence of erbium in the lattice cause an increase of the current density. Both dynamic of soliton formation and photovoltaic measurements indicates a lower N<sub>A</sub> content in erbium doped samples compared to undoped samples.
High energy lasers processing of materials is knowing an increasing interest since it not only can make manufacturing faster, cleaner, and more accurate but also because it opens up entirely new technologies and manufacturing methods, that are simply not available by using standard techniques.
In this paper, an experimental set-up, based on pulsed KrF excimer Laser, assembled for surface patterning of mono and two-dimensional, (1D-2D), micro and submicro structures on LiNbO<sub>3</sub>(LN) crystal, will be described in detail.
The apparatus has been used to produce photonic structures in LN, both by patterning of PMMA photoresist, and by direct surface patterning through laser ablation or direct laser writing.
The structures and properties of the photonic crystals fabricated by using this apparatus have been investigated, and the preliminary results will be presented.
The realisation process and the preliminary tests on the performances of an integrated Mach-Zehnder Interferometer on LiNbO<sub>3</sub> (Lithium Niobate) substrate is presented. The microsystem has been obtained by using medium mass Ion Implantation on X-cut Lithium Niobate crystals. The interferometer is formed by integrated optical channel waveguides; the phase shift between the two optical paths has been obtained, without moving parts, by applying a suitable electric
field. The whole device is 60mm long, has a 0.5x1mm<sup>2</sup> cross section and weights a few grams. The power consumption is in the milliwatt range. In the present work results obtained in the spectral window (0.4μm-1.1μm) will be presented. The performance of the device, evaluated on standard radiation sources, demonstrates that a spectral resolution better than 0.3nm can be obtained on 400nm spectral windows. The Micro-interferometer has been tested in laboratory with a calibrated cell containing NO<sub>2</sub> gas and has demonstrated sensitivity in the ppb range if suitable optical paths are used. Its reduced dimensions and weights make these micro-systems ideal for a wide range of applications, spanning from Space Technology, Earth observation for Environment monitoring, to Safety and Security applications.
A set of samples co-doped with Er and Ag were prepared with a combined sol-gel and ion-exchange route. This multistep process sytnthesizes samples in which the silver atoms are dispersed in the matrix and/or aggregated in Ag multimers or nanoclusters. The samples exhibit a different photo-luminescence response depending on the sensitizing effect due to silver atoms. The spectroscopic properties were correlated with the structural properties investigated by extended x-ray absorption fine structure (EXAFS) spectroscopy and x-ray diffraction. The Er<sup>3+ </sup>excitation via energy transfer, obtained in a wide range of wavelengths, has been clearly related to the presence in the sample of Ag multimers.
Various material and functional properties have been measured in lithium niobate crystals (LiNbO<sub>3</sub>) 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 Li<sub>2</sub>O 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 LiNbO<sub>3</sub> 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 LiNbO<sub>3</sub>, and resulted almost isotropic. The Li-outdiffusion phenomenon has been observed and correlated wit the Ti concentration profile. A careful control on LiNbO<sub>3</sub> 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 LiNbO<SUB>3</SUB> 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 LiNbO<SUB>3</SUB> matrix is studied.