Roberto Luigi Oliveri

Exploiting the Optical Quadratic Nonlinearity of Zinc-Blende Semiconductors for Guided-Wave Terahertz Generation: A Material Comparison

We present a detailed analysis and comparison of dielectric waveguides made of CdTe, GaP, GaAs and InP for modal phase matched optical difference frequency generation (DFG) in the terahertz domain. From the form of the DFG equations, we derived the definition of a very general figure of merit (FOM). In turn, this FOM enabled us to compare different configurations, by taking into account linear and nonlinear susceptibility dispersion, terahertz absorption, and a rigorous evaluation of the waveguide modes properties. The most efficient waveguides found with this procedure are predicted to approach the quantum efficiency limit with input optical power in the order of kWs.

Continuous-wave backward frequency doubling in periodically poled lithium niobate

We report on backward second-harmonic-generation in bulk periodically poled congruent lithium niobate with a 3.2 μm period. A tunable continuous-wave Ti:sapphire laser allowed us exciting two resonant quasi-phase-matching orders in the backward configuration. The resonances were also resolved by temperature tuning and interpolated with standard theory to extract relevant information on the sample.

Random quasi-phase-matched second-harmonic generation in periodically poled lithium tantalate

We observe second harmonic generation via random quasi-phase-matching in a 2.0 mum periodically poled, 1-cm-long, z-cut lithium tantalate. Away from resonance, the harmonic output profiles exhibit a characteristic pattern stemming from a stochastic domain distribution and a quadratic growth with the fundamental excitation, as well as a broadband spectral response. The results are in good agreement with a simple model and numerical simulations in the undepleted regime, assuming an anisotropic spread of the random nonlinear component.

Universal Charts for Optical Difference Frequency Generation in the Terahertz Domain

We present a universal and rigorous approach to study difference frequency generation in the terahertz domain, especially when reaching for the quantum efficiency limit. Through the definition of a suitable figure of merit, we have been able to keep the number of degrees of freedom to a minimum, in order to draw up suitably normalized charts, that enable to predict the optical-to-terahertz conversion efficiency of any efficient system based on wave propagation in quadratic nonlinear materials. The predictions of our approach take into account the effects of both terahertz absorption and optical pump depletion, and are found to be in good agreement with the best experimental results reported to date. This enabled also to estimate the d 22 nonlinear coefficient of high quality GaSe.

Features of randomized electric-field assisted domain inversion in lithium tantalate.

We report on bulk and guided-wave second-harmonic generation via random Quasi-Phase-Matching in Lithium Tantalate. By acquiring the far-field profiles at several wavelengths, we extract statistical information on the distribution of the quadratic nonlinearity as well as its average period, both at the surface and in the bulk of the sample. By investigating the distribution in the two regions we demonstrate a non-invasive approach to the study of poling dynamics.

High resolution x-ray investigation of periodically poled lithium tantalate crystals with short periodicity

Domain engineering technology in lithium tantalate is a well studied approach for nonlinear optical applications. However, for several cases of interest, the realization of short period structures (<2 μm) is required, which make their characterization difficult with standard techniques. In this work, we show that high resolution x-ray diffraction is a convenient approach for the characterization of such structures, allowing us to obtain in a nondestructive fashion information such as the average domain period, the domain wall inclination, and the overall structure quality.

Nanostructured electrochemical devices for sensing, energy conversion and storage

Nanostructured materials are attracting growing interest for improving performance of devices and systems of large technological interest. In this work, the principal results about the use of nanostructured materials in the field of electrochemical energy storage, electrochemical water splitting, and electrochemical sensing are presented. Nanostructures were fabricated with two different techniques. One of these was the electrodeposition of the desired material inside the channels of a porous support acting as template. The other one was based on displacement reaction induced by galvanic contact between metals with different electrochemical nobility. In the present work, a commercial polycarbonate membrane was used as template. In the field of the electrochemical energy storage, the attention was focused on lead-acid battery, and it has been found that nanostructured morphology enhances the active mass utilization up to about 80%, with consequent increase of specific energy and cycling rates to unattainable values for the commercial battery. Nanostructured Ni-IrO2 composite electrodes showed valuable catalytic activity for water oxidation. By comparison with other Ni-based electrocatalyst, this electrode appears as the most promising anode for electrochemical water splitting in alkaline cells. Also in the field of sensing, the nanostructured materials fabricated by displacement reaction showed performance of high interest. Some new results about the use of copper nanowires for H2O2 sensing will be showed, evidencing better performance in comparison with copper thin film. In this work, we will show that nanostructured electrodes are very promising candidate to form different electrochemical setups that operate more efficiently comparing to device with flat electrode materials.

The role of nonlinear optical absorption in narrowband difference-frequency terahertz-wave generation

We present a general analysis of the influence of nonlinear optical absorption on terahertz generation via optical-difference frequency generation when reaching for the quantum conversion efficiency limit. By casting the equations governing the process in a suitably normalized form, including either two-photon- or three-photon absorption terms, we have been able to plot universal charts for phase-matched optical-to-terahertz conversion for different values of the nonlinear absorption coefficients. We apply our analysis to some experiments reported to date in order to understand to what extent multiphoton absorption could have played a role and also to predict the maximum achievable conversion efficiency at higher peak pump intensities.

Ultraviolet quasi-phase-matched second harmonic generation in surface periodically poled lithium niobate optical waveguides

The compatibility of low concentration (α-phase) proton exchange channel waveguides with electric field surface periodic poling of congruent lithium niobate (SPPLN) crystals has been experimentally demonstrated. With such waveguides, we obtained ultraviolet second harmonic generation (SHG) by first order quasi-phase-matching (QPM), a result made possible by the fabrication, on Z-cut LN crystals, of periodic structures with a pitch down to 750 nm. Nonlinear copropagating QPM-SHG measurements have been carried out on such structures. 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 a photodiode with an enhanced response in the UV. The measured conversion efficiency was about 1% W−1 cm−2.

Nanostructured lead acid battery for electric vehicles applications

This paper presents an innovative lead acid battery, based on nanostructured active materials. Both charging time and specific energy are greatly enhanced in comparison with commercial lead acid battery. Starting from the extremely valuable performances of the nanostructured battery, also a circuital model, for application in electric vehicle traction, has been specifically developed. The circuital model has demonstrated that an enhanced nanostructured battery allows an increase of traveled distance by electric vehicles.