Vittorianna Tasco

Triple-helical nanowires by tomographic rotatory growth for chiral photonics

Three dimensional (3D) helical chiral metamaterials resulted effective in manipulating circularly polarized light in the visible-infrared for advanced nano-photonics. Their potentialities are severely limited by the lack of full rotational symmetry preventing broadband operation, high signal-to-noise ratio, and inducing high optical activity sensitivity to structure orientation. Complex intertwined 3D structures like Multiple-Helical Nanowires could overcome these limitations, allowing the achievement of several chiro-optical effects combining chirality and isotropy. Here we report 3D triple-helical nanowires, engineered by the innovative Tomographic Rotatory Growth, based on Focused Ion Beam Induced Deposition. These three dimensional nanostructures show up to 37% of circular dichroism in a broad range (500-1000 nm), with a high signal-to-noise ratio (up to 24 dB). Optical activity up to 8° only due to the circular birefringence is also shown, tracing the way towards chiral photonic devices which can be integrated in optical nanocircuits to modulate the visible light polarization.

Electroluminescence and Transmission Electron Microscopy Characterization of Reverse-Biased AlGaN/GaN Devices

Reverse-bias stress testing has been applied to a large set of more than 50 AlGaN/GaN high electron mobility transistors, which were fabricated using the same process but with different values of the AlN mole fraction and the AlGaN barrier-layer thickness, as well as different substrates (SiC and sapphire). Two sets of devices having different defect types and densities, related to the different growth conditions and the choice of nucleation layer, were also compared. When subjected to gate-drain (or gate-to-drain and source short-circuited) reverse-bias testing, all devices presented the same time-dependent failure mode, consisting of a significant increase in the gate leakage current. This failure mechanism occurred abruptly during step-stress experiments when a certain negative gate voltage, or “critical voltage,” was exceeded or, during constant voltage tests, at a certain time, defined as “time to breakdown.” Electroluminescence (EL) microscopy was systematically used to identify localized damaged areas that induced an increase of gate reverse current. This current increase was correlated with the increase of EL intensity, and significant EL emission during tests occurred only when the critical voltage was exceeded. Focused-ion-beam milling produced cross-sectional samples suitable for electron microscopy observation at the sites of failure points previously identified by EL microscopy. In high-defectivity devices, V-defects were identified that were associated with initially high gate leakage current and corresponding to EL spots already present in untreated devices. Conversely, identification of defects induced by reverse-bias testing proved to be extremely difficult, and only nanometer-size cracks or defect chains, extending vertically from the gate edges through the AlGaN/GaN heterojunction, were found. No signs of metal/semiconductor interdiffusion or extended defective areas were visible. The weak dependence on AlGaN properties, the strong process dependence, the time dependence, and the features of the localized damage identified by EL and electron microscopy suggest a multistep failure mechanism initiated by a process-induced weakness of the gate Schottky junction, which enhances current injection into pre-existing defects. As a result, further defects are generated or activated, eventually resulting in a percolation conductive path and permanent damage. A low-impedance path between the device gate and the channel is formed, increasing gate leakage current and possibly resulting in device burnout.

Programmable Extreme Chirality in the Visible by Helix-Shaped Metamaterial Platform

The capability to fully control the chiro-optical properties of metamaterials in the visible range enables a number of applications from integrated photonics to life science. To achieve this goal, a simultaneous control over complex spatial and localized structuring as well as material composition at the nanoscale is required. Here, we demonstrate how circular dichroic bands and optical rotation can be effectively and independently tailored throughout the visible regime as a function of the fundamental meta-atoms properties and of their three dimensional architecture in a the helix-shaped metamaterials. The record chiro-optical effects obtained in the visible range are accompanied by an additional control over optical efficiency, even in the plasmonic context. These achievements pave the way toward fully integrated chiral photonic devices.

The polarization response in InAs quantum dots: theoretical correlation between composition and electronic properties

III-V growth and surface conditions strongly influence the physical structure and resulting optical properties of self-assembled quantum dots (QDs). Beyond the design of a desired active optical wavelength, the polarization response of QDs is of particular interest for optical communications and quantum information science. Previous theoretical studies based on a pure InAs QD model failed to reproduce experimentally observed polarization properties. In this work, multi-million atom simulations are performed in an effort to understand the correlation between chemical composition and polarization properties of QDs. A systematic analysis of QD structural parameters leads us to propose a two-layer composition model, mimicking In segregation and In-Ga intermixing effects. This model, consistent with mostly accepted compositional findings, allows us to accurately fit the experimental PL spectra. The detailed study of QD morphology parameters presented here serves as a tool for using growth dynamics to engineer the strain field inside and around the QD structures, allowing tuning of the polarization response.

Toward Cavity Quantum Electrodynamics with Hybrid Photon Gap-Plasmon States

Combining localized surface plasmons (LSPs) and diffractive surface waves (DSWs) in metallic nanoparticle gratings leads to the emergence of collective hybrid plasmonic-photonic modes known as surface lattice resonances (SLRs). These show reduced losses and therefore a higher Q factor with respect to pure LSPs, at the price of larger volumes. Thus, they can constitute a flexible and efficient platform for light-matter interaction. However, it remains an open question if there is, in terms of the Q/V ratio, a sizable gain with respect to the uncoupled LSPs or DSWs. This is a fundamental point to shed light upon if such modes want to be exploited, for instance, for cavity quantum electrodynamic effects. Here, using aluminum nanoparticle square gratings with unit cells consisting of narrow-gap disk dimers-a geometry featuring a very small modal volume-we demonstrate that an enhancement of the Q/V ratio with respect to the pure LSP and DSW is obtained for SLRs with a well-defined degree of plasmon hybridization. Simultaneously, we report a 5× increase of the Q/V ratio for the gap-coupled LSP with respect to that of the single nanoparticle. These outcomes are experimentally probed against the Rabi splitting, resulting from the coupling between the SLR and a J-aggregated molecular dye, showing an increase of 80% with respect to the DSW-like SLR sustained by the disk LSP of the dimer. The results of this work open the way toward more efficient applications for the exploitation of excitonic nonlinearities in hybrid plasmonic platforms.

Investigation of different mechanisms of GaN growth induced on AlN and GaN nucleation layers

The evolution of GaN growth on AlN and GaN nucleation layers is compared through morphological and structural analyses, including ion beam analysis. By using AlN nucleation layer grown at high temperature, improved crystalline quality is exhibited by 300 nm thin GaN epilayers. GaN (002) x-ray rocking curve as narrow as 168 arc sec and atomic-step surface morphology characterize such a thin GaN film on AlN. Defects are strongly confined into the first 50 nm of growth, whereas a fast laterally coherent growth is observed when increasing thickness, as an effect of high temperature AlN surface morphology and Ga adatom dynamics over this template.

Sub-50-nm conjugated polymer dots by nanoprinting.

Methods for controlling, with high accuracy, the deposition of active molecular materials on surfaces play a fundamental role in the fabrication of functional devices. Different patterning techniques have been proposed for organic electronics, including optical methods, dip-pen, inkjet, and soft lithography, molecular self-assembly, nanoimprinting, and molding. However, among these approaches, only inkjet and dip-pen lithography have straightforward applications in organic pixel technology. These approaches allow one to realize matrices or arrays of separated features, although with limitations in terms of cost and serial throughput. In organic nanoand optoelectronics, a current challenging issue is the research of cheap, parallel-patterning approaches for fabricating i) addressable matrix arrangements of spatially discrete elements, and ii) high-quality emissive features with sub-100-nm size, to be employed as nanoscale light sources. High-resolution soft lithography is in principle able to fulfill these requirements. In particular, the recently developed particle replication in nonwetting templates (PRINT) method was applied to obtain particles using several materials, such as poly(ethylene glycol diacrylate), triacrylate resin, poly(lactic acid), and poly(pyrrole), possibly incorporating bioactive agents. PRINT utilizes a stamp which is not wetted by the target organic materials. If a liquid polymer is placed between a textured and a flat fluorinated surface, the organics tend to be confined within the recessed features of the mold. For this reason, in contrast to other soft lithographic methods, the presence of an

A fully integrated GaAs-based three-axis Hall magnetic sensor exploiting self-positioned strain released structures

In this work, we demonstrate a fully integrated three-axis Hall magnetic sensor by exploiting microfabrication technologies applied to a GaAs-based heterostructure. This allows us to obtain, by the same process, three mutually orthogonal sensors: an in-plane Hall sensor and two out-of-plane Hall sensors. The micromachined devices consist of a two-dimensional electron gas AlGaAs/InGaAs/GaAs multilayer which represents the sensing structure, grown on the top of an InGaAs/GaAs strained bilayer. After the release from the substrate, the strained bilayer acts as a hinge for the multilayered structure allowing the out-of-plane self-positioning of devices. Both the in-plane and out-of-plane Hall sensors show a linear response versus the magnetic field with a sensitivity for current-biased devices higher than 1000 V A−1 T−1, corresponding to an absolute sensitivity more than 0.05 V T−1 at 50 µA. Moreover, Hall voltage measurements, as a function of the mechanical angle for both in-plane and out-of-plane sensors, demonstrate the potential of such a device for measurements of the three vector components of a magnetic field.

Precise detection of circular dichroism in a cluster of nano-helices by photoacoustic measurements

Compact samples of nano-helices built by means of a focused ion beam technology with large bandwidth and high dichroism for circular polarization are promising for the construction of built-in-chip sensors, where the ideal transducer must be sufficiently confined without compromising its filtering ability. Direct all-optical measurements revealed the sample’s dichroic character with insufficient details because of scattering and diffraction interference. On the other hand, photoacoustic measurements resulted to be a possible alternative investigation, since they directly deal with absorbed power and allow to get clear evidences of the differential selection for the two opposite polarization states. Multi-level numerical simulations confirmed the experimental results, proving once again the reliability of photoacoustic technique and the versatility of this class of dichroic artificial materials.

Fabrication of GaN/AlGaN 1D photonic crystals designed for nonlinear optical applications

In this paper we present a reliable process to fabricate GaN/AlGaN one dimensional photonic crystal (1D-PhC) microcavities with nonlinear optical properties. We used a heterostructure with a GaN layer embedded between two AlGaN/GaN Distributed Bragg Reflectors on sapphire substrate, designed to generate a λ= 800 nm frequency downconverted signal (χ(2) effect) from an incident pump signal at λ= 400 nm. The heterostructure was epitaxially grown by metal organic chemical vapour deposition (MOCVD) and integrates a properly designed 1D-PhC grating, which amplifies the signal by exploiting the double effect of cavity resonance and non linear GaN enhancement. The integrated 1D-PhC microcavity was fabricate combing a high resolution e-beam writing with a deep etching technique. For the pattern transfer we used ~ 170 nm layer Cr metal etch mask obtained by means of high quality lift-off technique based on the use of bi-layer resist (PMMA/MMA). At the same time, plasma conditions have been optimized in order to achieve deeply etched structures (depth over 1 micron) with a good verticality of the sidewalls (very close to 90°). Gratings with well controlled sizes (periods of 150 nm, 230 nm and 400 nm respectively) were achieved after the pattern is transferred to the GaN/AlGaN heterostructure.