When Palladium film is exposed to hydrogen, it becomes palladium hydride. A change in the complex permittivity of the metal film results in a change of the optical properties that depends on hydrogen concentration. Ellipsometry is the technique of choice to measure the optical constants prior and during hydrogenation. Sensors are then usually designed and optimized to measure changes in transmittance or reflectance of the palladium films. Films of different thicknesses have been realized and tested to verify potential applications in hydrogen sensing by studying the optical response prior, during and after hydrogenation, to assess in particular the reversibility of the process. Within this work a deep analysis carried out by x–ray reflectance (XRR) shows that during hydrogenation the films change also their thickness, and the amount has been assessed for a specific hydrogen concentration. Ellipsometric measurements have been therefore corrected taking into account such variation to determine the optical constants. Such structural property of the palladium hydride may be exploited in surface plasmon resonance transducers, which are sensitive also to the change of the sensing film thickness during detection.
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.