Massimo De Vittorio

Nanotechnology and neuroscience : nano-electronic, photonic and mechanical neuronal interfacing

Nanotechnology and Neuroscience: nano-electronic, photonic and mechanical neuronal interfacing.- Carbon nanotubes for neuron-electrode interface with improved mechanical performance.- Nanoscale field-effect transistors for minimally invasive, high spatial resolution, and three-dimensional action potential recording.- In-cell recording and stimulation by engulfment mechanisms.-Micromachining Techniques for Realization of Three-Dimensional Microelectrode Arrays.- Focused ion beam technology as a fabrication and inspection tool in neuron interfacing.- Active Pixel Sensor Multi Electrode Array for high spatio-temporal resolution.- Multi electrode and multi transistor arrays for in vivo recording.- Optogenetics.

Comparison of Cu-gate and Ni/Au-gate GaN HEMTs large signal characteristics

In this paper a complete comparison between Copper (Cu) gate and Nickel-Gold (Ni/Au) gate passivated AlGaN/GaN High Electron Mobility Transistors (HEMTs) is presented. DC and Radio Frequency (RF) performance was compared in order to evaluate the behaviour of the two Schottky contacts in the standard HEMT structure. From the obtained data a critical drain current collapse was observed in the Cu-gate devices, with detrimental effects on the RF performance, while the Ni/Au-gate performed nicely both during pulsed I–V and RF measurements. An investigation on the drain current transients and on ID – VGS characteristics, obtained by pulsed signals showed that an acceptor trap at the Cu/AlGaN interface, with activation energy of about 0.43 eV, could be responsible for the Cu-gate HEMT poorer performance. The results suggest that a detailed investigation on surface treatments, gate metal quality and deposition methods is needed in order to fabricate Cu-gate GaN HEMTs.

1.32 μm InAs/InGaAs/GaAs quantum dot lasers operating at room temperature with low threshold current density

We report on the growth and characterization of low threshold 1.32-μm quantum dots (QDs) laser diodes. The quantum dot active region was optimised to get the highest photoluminescence emission and the lowest Full Width at Half Maximum (FWHM). From samples containing multilayer QDs and using the Limited-Area Photoluminescence (LAPL) technique we have shown that the gain of an N-layer structure is higher than N times that of a single layer. This enhancement is attributed to the increase of the quantum dot density in the upper layers and also to the use of the high growth temperature spacer layer. Broad area laser diodes were processed from the grown samples containing three layers of InAs QDs grown directly on GaAs and capped with 4-nm-thick InxGa1-xAs layer. Than measurements were performed at room temperature under pulsed excitation. The laser diodes operate at room temperature and emit between 1.29 and 1.32-μm which is beyond the strategic telecommunication wavelength. The characteristic temperature is around 80 K and very stable in the hole range of the operating temperature (from 0 to 90 °C). The internal quantum efficiency is 53% and the modal gain per QD layer was estimated to be ~ 6 cm-1. For an infinite cavity length a threshold current density of 8 A/cm2 per QD layer was obtained. From the calculation of the optical confinement of QDs, we have estimated a material gain of 1979 cm-1.

Determination of absorption and structural properties of cellulose-based hydrogel via ultrasonic pulse-echo time-of-flight approach

Biodegradable cellulose-based hydrogels are attracting increasing interest in the academic and industrial fields thanks to their high swelling capacity and reproducibility, which allow many novel applications. These properties are enabled by amplification effect of their sensitiveness on a molecular level, translated into macroscopic effects such as a change in swelling degree. The monitoring of the hydrogel state is a crucial step for understanding the response of the hydrogel to external environment. Accordingly, the major aim of this study is to exploit ultrasound to characterize the swelling and degradation of cellulose-based hydrogel with different blend of molecular weight and degree of substitutions. The ultrasonic sensor used herein relies on the determination of a Pulse-echo time of flight. This technique provides dimensional information, thanks to its capability of monitoring the thickness of the swollen/unswollen hydrogel during sorption mechanism. Furthermore, by combining these data with a rheological characterization, the degree of crosslink and its modification during multiple swelling/deswelling cycles (due to ion strength variation) has been monitored. This technique could be an effective, alternative, fast and non-destructive method for real-time hydrogel characterization.Graphical Abstract

Accurate design and modeling of χ(2) nonlinear processes in periodic waveguides by Hertzian potential method

We present in this work the scalar potential formulation of second harmonic generation process in χ(2) nonlinear analysis. This approach is intrinsically well suited to the application of the concept of circuit analysis and synthesis to nonlinear optical problems, and represents a novel alternative method in the analysis of nonlinear optical waveguide, by providing a good convergent numerical solution. The time domain modeling is applied to nonlinear waveguide with dielectric discontinuities in the hypothesis of quasi phase matching condition in order to evaluate the conversion efficiency of the second harmonic signal. With the introduction of the presented rigorous time domain method it is possible to represent the physical phenomena such as light propagation and second harmonic generation process inside a nonlinear optical device with a good convergent solution and low computational cost. Moreover, this powerful approach minimizes the numerical error of the second derivatives of the Helmholtz wave equation through the generator modeling. The novel simulation algorithm is based on nonlinear wave equations associated to the circuital approach which considers the time-domain wave propagating in nonlinear transmission lines. The transmission lines represent the propagating modes of the nonlinear optical waveguide. The application of quasi phase matching in high efficiency second harmonic generation process is analyzed in this work. In particular we model the χ(2) non linear process in an asymmetrical GaAs slab waveguide with nonlinear core and dielectric discontinuities: in the nonlinear planar waveguides a fundamental mode at λ=1.55 μm is coupled to a second-harmonic mode (λ=0.775 μm) through an appropriate nonlinear susceptibility coefficient. The novel method is also applied to three dimensional structures such as ridge waveguides.

Design and Modeling of χ(2) Second Harmonic Amplification in Circular Photonic Crystal

We analyze in this work the second harmonic amplification of χ(2) nonlinear process in membrane type GaAs circular photonic crystal. This unconventional kind of photonic crystal is well suited for the generation of whispering gallery modes due to the circular symmetric periodic pattern. The Gaussian beam of a fundamental pump signal at 1.55 μm defines a whispering gallery mode resonance and generates a second harmonic mode at 0.775 μm in the central missing hole micro-cavity. The periodic pattern and the micro-cavity are tailored and optimized in order to generate a second harmonic conversion efficiency of 50 %. We predict the resonances by an accurate 2D time domain model including χ(2) nonlinearity and also by a 3D Finite Element Method FEM. Moreover, by using a 3D membrane configuration, we predict a quality factor of the second harmonic mode of the order of 35000.

Double grating design of 3D phase matched waveguide for second harmonic χ (2) process

Theoretical analysis on second harmonic (SH) generation with phase matched grating in waveguide is presented from the viewpoint of device design. Usually high intensity sources are necessary in order to observe a SH in a χ(2) nonlinear structure. For this purpose, the novel proposed design takes into account a double grating effect which enhances the guided SH signal along the waveguide. In the presented structure two grating are considered: the first grating, considered at the interface between air and core, is designed in order to obtain an efficient SH conversion process by considering the quasi phase matching (QPM) condition; the second grating, placed at the interface between the core and the substrate region, increases the SH power along the propagation direction through the coupling with the substrate modes generated by the diffraction effect. The novelty of this work is in the combined effect of the two gratings. The grating lengths and periods are designed by considering the nonlinear coupled mode theory with the effective dielectric constant (EDC) assumption. The analysis includes three dimensional (3D) cases where phase matching is involved, in particular the model is applied to a GaAs/AlGAs waveguides with fundamental wavelength at λFU=1.55 μm and SH signal at λSH =0.775 μm.

Design criteria and 3D FEM modeling of air hole photonic crystal

This work presents a detailed numerical Finite Element Method FEM modeling for passive optical components such as photonic crystals (PhCs). The accurate modeling characterizes the PhCs structures by considering the field resonance and the radiation behavior of the periodic pattern. The frequency responses at each side of the photonic crystal are evaluated by considering the 3D periodic structure enclosed in a black box with six input/output ports. This scattering matrix approach (SMA) is useful in order to evaluate in plane and vertical PhCs the resonance of the photonic crystal. Through the analysis of all the frequency responses we characterize the passband regions and the stopband regions of the PhC slab.

Trapping mechanisms of persistent photocurrent in GaN-based MSM detectors

The trapping mechanisms at the origin of the persistent photocurrent effects in GaN-based devices have been studied on different time scales by characterizing a low barrier metal-semiconductor-metal GaN-based photodetector in the temperature range between room temperature and 500 K. The active material of the metal-semiconductor-metal device consists of a thin film of GaN grown by metal organic chemical vapour deposition. The Arrhenius plots obtained by the analysis of the decay times of the photocurrent as a function of the temperature on time scales from millisecond up to hours allowed us to calculate the activation energies of the mechanisms responsible for the persistent photocurrent. The activation energies derived from the decay times on the time scale of hours have been attributed to gallium vacancies (VGa), gallium antisites (GaN) and carbon impurities, whereas GaN excitonic resonances resulted to be responsible for the persistent photocurrent on the millisecond time scale. Finally, the influence of the decay times has been correlated with the photocurrent gain of the device, which resulted to be as high as 4.1×105 at RT and 0.85×105 at 450 K.