Armando Piccardi

Some properties of electrochemical systems can be properly characterized by low- and extremely low-frequency impedance measurements. The frequency range down to 1 mHz requires peculiar attention in terms of accuracy, measurement speed, portability and costs, and commercial systems are often unable to guarantee good performances without expensive or bulky solutions. Here, we first review some of the applications of electrochemical impedance spectroscopy and their requirements and then report on recent works proposing circuits and systems focused on optimizing performances, reducing costs, and increasing versatility. Circuits for generating controllable current sources are described, optimizing output impedance and measurable impedance range. Then, integrated systems are taken into account, aiming at realizing portable and versatile instruments.

Impedance measurements are crucial in a variety of applications, from the characterization of lithium batteries, microbial fuel cells, and biosensors to the study of polymers and material degradation, where strict requirements have to be met in terms of frequency bandwidth and current level. Here, we present a cost-effective compact solution for ultra-low-frequency impedance measurements, operating in a wide range, from 1 mHz to 250 kHz. Coupled to a lock-in amplifier, the designed circuit is based on a Howland current pump cascaded by a precision current divider in order to set the conversion factor at 100 nA/V, 1 μA/V, or 100 μA/V. Therefore, it is possible to generate very low-current signals to measure resistive impedances up to 100 MΩ. In addition, a feedback network is inserted to null the voltage drift induced by leakage currents and offset voltages, thus allowing the measurement of low-capacitance loads, experimentally tested down to 10 nF. Remarkably, the feedback network allows to perform measurements also in the presence of high voltage bias of the load and experimental results performed up to 60 V demonstrate the excellent stability of the designed system, thus a high voltage compliance. The proposed circuit is particularly interesting for the conditioning of both resistive and capacitive sensors and it is likely to be an effective solution for the implementation of a portable instrument for measuring signals from biosensors.

This erratum amends some errors in Opt. Lett. 43, 2296 (2018) [CrossRef] .

We demonstrated an optical method to evaluate the state of asphalt due to the presence of atmospheric agents using the measurement of the polarization/depolarization state of near infrared radiation. Different sensing geometries were studied to determine the most efficient ones in terms of performance, reliability and compactness. Our results showed that we could distinguish between a safe surface and three different dangerous surfaces, demonstrating the reliability and selectivity of the proposed approach and its suitability for implementing a sensor.

Employing several nematic liquid crystal mixtures, we investigate how the thermo-optic response of nonlinear birefringent soft-matter affects the propagation of light beams and the features of self-induced waveguides. We address the formation of optical spatial solitons and the control of their trajectories versus temperature, comparing the measurements with the expectations based on a simplified model, showing an excellent agreement. Moreover, in a guest–host mixture with an absorbing dye dopant, we study the competition between reorientational and thermal nonlinearities, demonstrating that the two processes can be adjusted independently in order to tune the soliton properties, i.e., trajectory and confinement strength. Our results are an important contribution to better comprehend the role played by material properties on linear and nonlinear beam propagation, as well as their exploitation for signal processing and addressing.

We demonstrate thermo-optic control on the propagation of optical spatial solitons in nematic liquid crystals. By varying the sample temperature, both linear and nonlinear optical properties of the reorientational material are modulated by acting on the refractive indices, the birefringence, and the elastic response. As a result, both the trajectory and transverse confinement of spatial solitons can be adjusted, demonstrating an effective means to tune and readdress self-induced optical waveguides.

We investigate topological and optical spontaneous symmetry breaking in nematic liquid crystals subject to the Fréedericksz transition. Specular nematicon states couple to mirrored distributions of the director due to symmetry breaking, with transverse velocities controlled by beam power in the strong nonlinear regime. Hysteresis in transverse velocity versus incidence angle is observed in the soliton regime.

Light-controlling-light is one of the most advanced paradigms in optical signal processing, including light-induced waveguides as well as all-optical switching and routing. Other fundamental aspects of all-optical processing are optical memories and sequential elements, which require responses depending on the evolution history of the system, such as the hysteresis stemming from optical multistability. Hereby, we report on optical bistability and hysteresis in cavityless geometries and in the presence of self-localized beams, i.e., spatial optical solitons, exploiting the nonlocal reorientational nonlinearity of nematic liquid crystals (NLC). When the optic axis of NLC is initially orthogonal to the applied electric field, the molecular dipoles start to rotate above a threshold named after the Fréedericksz transition, the latter being usually of the second order. Here, we show that such transition become of the first order via the intrinsic feedback provided by self-focusing, in turn leading to the appearance of a hysteresis loop between diffracting and self-confined beams. We report on hysteresis of the beam size versus input power, as well as hysteresis versus applied voltage at a fixed beam power. Our findings introduce a novel kind of cavity-less optical bistability with propagating light beams and disclose a novel approach to information storage based on light self-localization.