Showing papers by "Marco Bazzan published in 2011"

Zirconium-doped lithium niobate: photorefractive and electro-optical properties as a function of dopant concentration

Abstract: Measurements of refractive indices, electro-optic coefficients and photorefractivity are performed for a set of Zirconium-doped congruent lithium niobate (Zr:LN) crystals as functions of the dopant concentration in the range 0.0-3.0 mol%. The photorefractive properties are studied by measuring the green-light induced birefringence change and by direct observation of the transmitted-beam distortion. The index of refraction data show that the threshold concentration, above which there is a change in the Zr incorporation mechanism, is about 2.0 mol%, but photorefractivity results suggest that the concentration of ZrO2 required to strongly reduce the photorefractive effect is somewhat larger than the 2.0 mol% “threshold” concentration derived from index-of-refraction data. The electro-optic coefficients are little influenced by Zr-doping. All the reported results confirm that Zr:LN is a very promising candidate for the realization of efficient electro-optic and all-optical nonlinear devices working at room temperature.

Quantification of Iron (Fe) in Lithium Niobate by Optical Absorption

Abstract: A 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.

Steering of an ultrarelativistic proton beam by a bent germanium crystal

Abstract: Curved crystals, thanks to the electrostatic potential generated by the coherent atomic structure, may deflect ultrarelativistic charged particles by means of channeling and volume reflection effects. Most of the experimental knowledge about these phenomena was gathered with Si crystals, though the performance could be improved by using materials with a larger atomic number. In this letter, we investigate planar and axial channeling and volume reflection in a high quality Ge short strip crystal. All the effects are demonstrated to occur in agreement with theoretical expectations, which take into account the stronger confinement potential for an ideal Ge crystal.

Iron doping of lithium niobate by thermal diffusion from thin film: study of the treatment effect

Abstract: Thermal diffusion from thin film is one of the most widespread approaches to prepare iron doped regions in lithium niobate with limited size for photorefractive applications. In this work, we investigate the doping process with the aim of determining the best process conditions giving a doped region with the characteristics required for photorefractive applications. Six samples were prepared by changing the atmosphere employed in the diffusion treatment in order to obtain different combination of diffusion profiles and reduction degrees and also to check the effect of employing a wet atmosphere. The compositional, optical, and structural properties are then extensively characterized by combining Secondary ion Mass Spectrometry, UV, visible and IR spectrophotometry, High Resolution X-Rays Diffraction, and Micro-Raman Spectroscopy. Moreover, the sample topography was checked by Atomic Force Microscopy. An analysis of all our data shows that the best results are obtained performing a double step process, i.e. diffusion in oxidizing atmosphere and subsequent reduction at lower temperature in an hydrogen-containing atmosphere.

Diffusion of iron in lithium niobate: a secondary ion mass spectrometry study

Abstract: Iron-doped X-cut lithium niobate crystals were prepared by means of thermal diffusion from thin film varying in a systematic way the process parameters such as temperature and diffusion duration. Secondary Ion Mass Spectrometry was exploited to characterize the iron in-depth profiles. The evolution of the composition of the Fe thin film in the range between 600°C and 800°C was studied, and the diffusion coefficient at different temperatures in the range between 900°C and 1050°C and the activation energy of the diffusion process were estimated.

Strain modification of AlGaN layers using swift heavy ions

Abstract: Epitaxial AlGaN/GaN layers grown by molecular beam epitaxy (MBE) on SiC substrates were irradiated with 150 MeV Ag ions at a fluence of 5×1012 ions/cm2 The samples used in this study are 50 nm Al02Ga08N/1 nm AlN/1 μ m GaN/01 μ m AlN grown on SI 4H-SiC Rutherford backscattering spectrometry/channeling strain measurements were carried out on off-normal axis of irradiated and unirradiated samples In an as-grown sample, AlGaN layer is partially relaxed with a small tensile strain After irradiation, this strain increases by 022% in AlGaN layer Incident ion energy dependence of dechanneling parameter shows E 1/2 dependence, which corresponds to the dislocations Defect densities were calculated from the E 1/2 graph As a result of irradiation, the defect density increased on both GaN and AlGaN layers The effect of irradiation induced-damages are analyzed as a function of material properties Observed results from different characterization techniques such as RBS/channeling, high-resolution XRD and AFM

Photorefractivity of zirconium-doped lithium niobate

Abstract: 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 Senarmont method, in the ZrO 2 concentration range 0-3mol% and intensity range 155- 1800 W/cm 2 . 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 2 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.

Photorefractivity of zirconium-doped lithium niobate crystals

Abstract: Polarization-independent all-optical wavelength converters (AOWCs) enable dynamic signal routing, wavelength reuse, path protection, and restoration.1 These features will be of utmost importance in future generation, high bit-rate optical communication networks. An efficient AWOC—operating on a 100Gb/s phase-modulated polarization-multiplexed signal at 1550nm—was recently realized by exploiting the cascade of two nonlinear optical processes in a lithium niobate (LN) waveguide.2 This cascading technique for wavelength conversion is only effective if the LN crystals are prepared with alternating up and down domains (i.e., periodical poling) of appropriate periodicity. Currently, LN crystals offer the best performance in nonlinear devices, but their applicability is limited by the photorefractive effect, which is a change of refractive index under strong illumination. The standard growth process of LN produces lithium-deficient (i.e., congruent) crystals. The presence of defects in the congruent crystals gives rise to photorefractivity, which is detrimental to the efficiency of nonlinear interactions. To avoid this, one can operate the LN-based wavelength converters at temperatures above 100iC.2 Since this is impossible for most in-field applications, crystals with negligible photorefractivity must be developed. Here, we explore the use of zirconium-doped congruent LN crystals (Zr:LN) for AOWC and other optical devices.3 Photorefractivity can be reduced by employing stoichiometric LN or magnesium ion (Mg2C)-doped LN.4 However, neither approach is fully satisfactory because it is often difficult to grow large crystals of high optical quality. Also, there are Figure 1. Birefringence variation (i n) induced on lithium niobate crystals, as a function of the zirconium (Zr) dioxide doping concentration, at pump-beam intensities of 300 ( ), 600 (N), 900 ( ), and 1200 ( ) W/cm2.

Intensity-dependent photorefractivity of Zirconium-doped lithium niobate crystals

Abstract: The pump intensity dependence of photorefractive effect in Zr-doped Lithium-Niobate crystals is investigated. Photorefractivity grows linearly with light intensity in the undoped crystal, whereas it saturates when doping concentration exceeds 2mol%.

Photorefractivity, electro-optical coefficients and refractive indices of Zr-doped LiNbO 3 crystals

Abstract: Lithium niobate (LN) is an extremely interesting material for the realization of optical devices and circuits, thanks to its good optical and electro-optical properties. At the state of the art, the use of LN crystals for nonlinear optical functions is limited, despite the very large d33 nonlinear coefficient exhibited by this crystal, because LN is severely affected by the so called “optical damage” due to the photorefractive effect [1]. When a high intensity, spatially inhomogeneous, visible light beam impinges on the LN crystal, a significant and non uniform electric field is produced in the crystal, because of the low crystal photoconductivity, and this yields (through the electro-optic effect) a non-uniform modification of the crystal refractive indices. As a consequence, the efficiency of nonlinear interactions in LN can be greatly reduced, and it becomes necessary to operate the device at high temperature (in some cases even > 200°C), in order to get rid of photorefractivity. In order to operate the nonlinear device at room temperature (or close to it), it is possible to use LN crystals doped with a small quantity of appropriate elements able to significantly increase crystal photoconductivity, like Mg, In, or Sc [1], but it is difficult to obtain high-quality crystals when LN is doped with such ions. Recently it was shown that by using tetravalent ions, like Hf or Zr, the dopant concentration required to significantly reduce the crystal photorefractivity is considerably lower than that required for bivalent and trivalent dopants, even if some uncertainty about the exact required concentration is still present due to the discrepancies between the values reported in the literature. The low dopant-concentration, in combination with the fact that the segregation coefficient of the Zr ion is reported to be very close to 1, should allow growing more easily large homogeneous high-optical-quality Zr:LN crystals.