Gaetano Assanto

Parametric self-trapping in the presence of randomized quasi phase matching

We report on experimental evidence of parametric spatial solitons in a quadratic crystal with randomized periodic ferroelectric poling. Two-color self-focusing via quadratic cascading overcomes the diffractive nature of both fundamental and frequency-doubled beams.

Soliton enhancement of spontaneous symmetry breaking

Spontaneous symmetry breaking (SSB) occurs when noise triggers an initially symmetric system to evolve toward one of its nonsymmetric states. Topological and optical SSB involve material reconfiguration/transition and light propagation/distribution in time or space, respectively. In anisotropic optical media, light beam propagation and distribution of the optic axis can be linked, thereby connecting topological and optical SSB. Using nonlinear soft matter, namely uniaxial liquid crystals, we report on simultaneous topological and optical SSB, showing that spatial solitons enhance the noise-driven transition of the medium from a symmetric to an asymmetric configuration, while acquiring a power-dependent transverse velocity in either of two specular directions with respect to the initial wavevector. Solitons enhance SSB by further distorting the optic axis distribution through nonlinear reorientation, resulting in power-tunable walk-off as well as hysteresis in beam refraction versus angle of incidence.

Second-harmonic generation in surface periodically poled lithium niobate waveguides: on the role of multiphoton absorption

Second-harmonic generation is investigated in lithium niobate channels realized by proton exchange and quasi-phase matched by surface periodic poling. The reduction in conversion efficiency at high powers is interpreted in terms of multiphoton absorption via two-color terms, yielding an estimate of the dominating three-photon process.

Surface periodic poling in lithium niobate and lithium tantalate

Periodic poling of lithium niobate crystals (PPLN) by means of electric field has revealed the best technique for finely tailoring PPLN structures and parameters, which play a central role in many current researches in the field of nonlinear integrated optics. Besides the most studied technique of bulk poling, recently a novel technique where domain inversion occurs just in a surface layer using photoresist or silica masks has been devised and studied. This surface periodic poling (SPP) approach is best suited when light is confined in a thin surface guiding layer or stripe, as in the case of optical waveguide devices. Also, we found that SPP respect to bulk poling offers two orders of magnitude reduction on the scale of periodicity, so that even nanostructures can be obtained provided a high resolution holographic mask writing technique is adopted. We were able to demonstrate 200 nm domain size, and also good compatibility with alpha-phase proton exchange channel waveguide fabrication. Our first experiments on lithium tantalate have also shown that the SPP technology appears to be applicable to this crystal (SPPLT), whose properties can allow to overcome limitations such as optical damage or UV absorption still present in PPLN devices. Finally, the issue of SPP compatibility with proton exchange waveguide fabrication will be addressed.

Second harmonic generation in coupled LiNbO 3 waveguides by reverse-proton exchange

We demonstrate second harmonic generation of a near-infrared pump in a nonlinearly coupled system formed by longitudinally uniform proton- and reverse-proton-exchanged LiNbO/sub 3/ planar waveguides. Phase- and mode-matched transverse electric (TE/sub 0/) frequency doubling into transverse magnetic higher order guided modes is achieved through temperature control, in agreement with the model.

Spatial optical solitons in nonlinearly coupled lithium niobate waveguides

We numerically investigate a novel type of quadratic spatial solitons in the system of surface and buried planar waveguides obtained via direct- and reverse-proton exchange in z-cut lithium niobate, respectively, and coupled by the second-order susceptibility. Through frequency doubling, transverse-electric and transverse-magnetic modes lock into a 1+1D solitary wave encompassing orthogonal field components at the two harmonic wavelengths. The phase matching scheme leads to a wavelength-comb filter with adjustable linewidths.

Frequency doubling in surface periodically poled lithium niobate waveguides: competing effects

We fabricated alpha-phase proton-exchanged (PE) lithium niobate (LN) channel waveguides quasi phase-matched (QPM) via surface periodic poling (SPP) and carried out the first experimental demonstration of second harmonic generation (SHG) in such devices. In order to quantitatively interpret the experimental data, we took into account not only the periodically-poled transverse region providing 1st-order QPM, but also the homogeneously-inverted portion of the waveguides extending below the shallow domains. By a numerical fit of the data we found that, because of the shallow non-inverted domains with depth of the order of 430nm (to be compared with a waveguide depth of 2.6mm), a cascaded process in phase-mismatch gives rise to an effective self-defocusing response which combines with the cubic nonlinearity of the material to provide the measured SHG resonance shift with power.

Proton exchange channel waveguides compatible with surface domain engineering in Lithium Niobate crystals

First experiments of proton exchange channel waveguides compatible with electric field surface periodic poling of congruent lithium niobate crystals are addressed. Picosecond nonlinear copropagating QPM-SHG measurements have been carried out on such structures.

Near-infrared wavemeter based on an array of polycrystalline Ge-on-Si photodetectors

We report on a novel solid state wavelength meter in the near infrared. The device is an array of six photodetectors based on polycrystalline germanium film evaporated on a silicon substrate and each element is a wavelength sensitive detector. We describe the design, the fabrication and the characterization of such device and we demonstrate its capability in the measurement of the wavelength of quasi- monochromatic light beams.

NANOTECHNOLOGY IN LITHIUM NIOBATE FOR INTEGRATED OPTIC FREQUENCY CONVERSION IN THE UV

In the domain of Earth Explorer satellites nanoengineered nonlinear crystals can optimize UV tunable solid-state laser converters. Lightweight sources can be based on Lithium Niobate (LN) domain engineering by electric field poling and guided wave interactions. In this Communication we report the preliminary experimental results and the very first demonstration of UltraViolet second-harmonic generation by first-order quasi-phase-matching in a surface-periodically-poled proton-exchanged LN waveguide. The pump source was a Ti-Sapphire laser with a tunability range of 700- 980 nm and a 40 GHz linewidth. We have measured UV continuous-wave light at 390 nm by means of a lock-in amplifier and of a photodiode with enhanced response in the UV. Measured conversion efficiency was about 1%W-1cm-2. QPM experiments show good agreement with theory and pave the way for a future implementation of the technique in materials less prone to photorefractive damage and wider transparency in the UV, such as Lithium Tantalate.