N. Argiolas

Effect of low dose high energy O3+ implantation on refractive index and linear electro-optic properties in X-cut LiNbO3: Planar optical waveguide formation and characterization

X-cut LiNbO3 crystals were implanted at room temperature by 5.0 MeV O3+ ions with doses ranging from 1.0×1014 to 6.0×1014 O/cm2. Secondary ion mass spectrometry profiles of atomic species migration as well as damage profiles by the Rutherford backscattering channeling technique and refractive index variation were investigated as a function of dose and subsequent annealing conditions. Two different kinds of damage produced by oxygen implantation were seen: near-surface damage correlated to electronic stopping, which causes an increase of the extraordinary refractive index, and end-of-ion range damage generated by collision cascades, which decreases the extraordinary refractive index values. The different nature of the two kinds of damage is also seen by the different temperature conditions needed for recovery. Low loss planar optical waveguides were obtained and characterized by the prism coupling technique.

Damage effects produced in the near-surface region of x-cut LiNbO3 by low dose, high energy implantation of nitrogen, oxygen, and fluorine ions

The damage effects produced in the near-surface region of x-cut LiNbO3 by low dose, high energy implantation of carbon, nitrogen, oxygen, and fluorine ions are investigated as a function of the dose and substrate temperature during the implant process. The damage profiles were obtained by the Rutherford backscattering RBS-channeling technique, whereas the compositional profiles were performed by secondary ion mass spectrometry. The experimental results showed that the mechanisms governing the damage formation at the surface are strongly connected to the interaction of defects produced when the electronic energy loss exceeds a given threshold close to 220 eV/A. In particular, we observed a damage pileup compatible with a growth of three-dimensional defect clusters.

On the dynamics of the damage growth in 5 MeV oxygen-implanted lithium niobate

The damage induced by 5 MeV oxygen ion implantation in x-cut congruent LiNbO3 has been investigated by Rutherford backscattering spectrometry channeling technique. The dynamics of the damage growth has been described by an analytical formula considering the separate contributions of nuclear and electronic energy deposition. It has been hypothesized that the nuclear damage provides the localization of the energy released to the electronic subsystem necessary for the conversion into atomic displacements. The strong influence of the preexisting defects on the damage pileup, foreseen by the analytical formula, has been experimentally verified by pre-implanting the samples with 500 keV oxygen ions. It has been shown that a subsequent 5 MeV oxygen implantation step gives rise to an impressive damage accumulation, eventually leading to the total amorphization of the surface, even at moderate fluences.

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

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.

Structural and optical properties of zirconium doped lithium niobate crystals

Zirconium doped lithium niobate is a promising candidate as a substrate for nonlinear optical applications, since it does not suffer from the so-called “optical damage.” In order to optimize this aspect, the proper Zr concentration has be used, hence the precise determination of the so-called “threshold concentration,” i.e., the concentration above which the photorefractive effect is markedly reduced, is of great importance. In this work, we prepared by Czochralski growth a series of Zr-doped lithium niobate crystals with various Zr content and studied them using structural (high-resolution x-ray diffraction) and optical (birefringence) measurements as a function of the dopant content in the melt. Both the approaches pointed out a marked change in the crystal characteristics for a Zr concentration between 1.5 and 2 mol %, a value which is identified as the threshold concentration.

Luminescence-induced photorefractive spatial solitons

We report the observation of spatial confinement of a pump beam into a photorefractive solitonic channel induced by luminescence [luminescence induced spatial soliton (LISS)]. Trapped beams have been obtained in erbium doped lithium niobate crystals at concentrations as high as 0.7 mol % of erbium. By pumping at 980 nm, erbium ions emit photons at 550 nm by two-step absorption, wavelength which can be absorbed by lithium niobate and originates the photorefractive effect. The luminescence at 550 nm generates at the same time the solitonic channel and the background illumination reaching a steady-state soliton regime.

The r33 electro-optic coefficient of Er:LiNbO3

By exploiting a Mach–Zehnder interferometer, the r33 electro-optic coefficient of erbium-bulk-doped lithium niobate crystals grown by the Czochralski technique was measured depending on the erbium content. The r33 coefficient decreases with the erbium concentration. Such a trend has been connected with both the reduction of the lattice parameter, measured by high resolution x-ray diffraction, and the increase of lattice defects.

Lithium niobate crystals doped with iron by thermal diffusion: Relation between lattice deformation and reduction degree

We report, to our knowledge for the first time, on the experimental observation that the maximum lattice deformation induced at the surface of iron doped lithium niobate crystal by thermal diffusion depends on both the Fe concentration and the reduction degree of the doped layer itself. By exploiting a simple linear model, we suggest a description of this experimental evidence and we point out a procedure that allows the characterization of the in-depth profile of the [Fe2+]/[Fe3+] ratio.

Self-confined beams in erbium-doped lithium niobate

We experimentally investigate the formation of self-confined beams in lithium niobate crystals which are doped in the bulk with erbium. Samples have been grown by the Czochralski method with erbium nominal concentrations varying in the range 0.0–0.7 mol% in the melt. The speeds of beam confinement depend on the applied bias and on the beam intensity, as expected, but on the doping level as well. The erbium incorporation influences both the electro-optic coefficient and the photovoltaic field. A very general dependence of the confined beam waist on the refractive index change was experimentally derived, valid now for every lithium niobate crystal, independent on its growing procedure or doping.

Integrated opto-microfluidics platforms in lithium niobate crystals for sensing applications

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 (LiNbO3) crystals is presented. Microfluidic channels were directly engraved in a LiNbO3 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.