Pietro Artoni

Size-scaling in optical trapping of silicon nanowires.

We investigate size-scaling in optical trapping of ultrathin silicon nanowires showing how length regulates their Brownian dynamics, optical forces, and torques. Force and torque constants are measured on nanowires of different lengths through correlation function analysis of their tracking signals. Results are compared with a full electromagnetic theory of optical trapping developed in the transition matrix framework, finding good agreement.

Strongly enhanced light trapping in a two-dimensional silicon nanowire random fractal array

We report on the unconventional optical properties exhibited by a two-dimensional array of thin Si nanowires arranged in a random fractal geometry and fabricated using an inexpensive, fast and maskless process compatible with Si technology. The structure allows for a high light-trapping efficiency across the entire visible range, attaining total reflectance values as low as 0.1% when the wavelength in the medium matches the length scale of maximum heterogeneity in the system. We show that the random fractal structure of our nanowire array is responsible for a strong in-plane multiple scattering, which is related to the material refractive index fluctuations and leads to a greatly enhanced Raman scattering and a bright photoluminescence. These strong emissions are correlated on all length scales according to the refractive index fluctuations. The relevance and the perspectives of the reported results are discussed as promising for Si-based photovoltaic and photonic applications.

Kinetics of Si and Ge nanowires growth through electron beam evaporation

Si and Ge have the same crystalline structure, and although Si-Au and Ge-Au binary alloys are thermodynamically similar (same phase diagram, with the eutectic temperature of about 360°C), in this study, it is proved that Si and Ge nanowires (NWs) growth by electron beam evaporation occurs in very different temperature ranges and fluence regimes. In particular, it is demonstrated that Ge growth occurs just above the eutectic temperature, while Si NWs growth occurs at temperature higher than the eutectic temperature, at about 450°C. Moreover, Si NWs growth requires a higher evaporated fluence before the NWs become to be visible. These differences arise in the different kinetics behaviors of these systems. The authors investigate the microscopic growth mechanisms elucidating the contribution of the adatoms diffusion as a function of the evaporated atoms direct impingement, demonstrating that adatoms play a key role in physical vapor deposition (PVD) NWs growth. The concept of incubation fluence, which is necessary for an interpretation of NWs growth in PVD growth conditions, is highlighted.

Heteroepitaxial Growth and Faceting of Ge Nanowires on Si(111) by Electron-Beam Evaporation

We demonstrated the heteroepitaxial growth of single-crystal faceted Ge nanowires (NWs) by electron-beam evaporation on top of Si(111) substrates. Despite the non-ultrahigh vacuum growth conditions, scanning electron microscope and transmission electron microscope images show that NWs have specific crystallographic growth directions ([111], [110], and [112]) and that specific surface crystallographic planes ({111} or {110}) correspond to the [110] and [112] growth directions. Moreover, we studied in detail the Ge NWs structural properties. The temperature dependence of the NW length and of the frequency of each crystallographic orientation has been elucidated. Finally, the microscopic growth mechanisms have been investigated.

A defect in myoblast fusion underlies Carey-Fineman-Ziter syndrome

Multinucleate cellular syncytial formation is a hallmark of skeletal muscle differentiation. Myomaker, encoded by Mymk (Tmem8c), is a well-conserved plasma membrane protein required for myoblast fusion to form multinucleated myotubes in mouse, chick, and zebrafish. Here, we report that autosomal recessive mutations in MYMK (OMIM 615345) cause Carey-Fineman-Ziter syndrome in humans (CFZS; OMIM 254940) by reducing but not eliminating MYMK function. We characterize MYMK-CFZS as a congenital myopathy with marked facial weakness and additional clinical and pathologic features that distinguish it from other congenital neuromuscular syndromes. We show that a heterologous cell fusion assay in vitro and allelic complementation experiments in mymk knockdown and mymkinsT/insT zebrafish in vivo can differentiate between MYMK wild type, hypomorphic and null alleles. Collectively, these data establish that MYMK activity is necessary for normal muscle development and maintenance in humans, and expand the spectrum of congenital myopathies to include cell-cell fusion deficits.

Temperature dependence and aging effects on silicon nanowires photoluminescence

In this paper we describe the luminescence properties of Si nanowires (NWs) prepared by a maskless synthesis technique, based on the Au-catalyzed wet etching of Si substrates by an aqueous solution of H(2)O(2) and HF. A strong room temperature photoluminescence (PL), centered at about 690 nm, is observed when Si NWs are optically excited. The detailed analysis of the steady-state and time-resolved PL properties of the system as a function of aging, temperature and pump power allows to demonstrate that the emission is due to the radiative recombination of quantum confined excitons. These results open the route towards novel applications of Si NWs in photonics as efficient light sources.

Simultaneous two-photon imaging of intracellular chloride concentration and pH in mouse pyramidal neurons in vivo

Significance The control of intracellular Cl− and pH plays a crucial role in several neuronal functions, and the study of these processes would be helped by tools for their noninvasive optical measurement in vivo. In this study, we have performed combined measurements of Cl− and pH of individual pyramidal neurons by means of in vivo two-photon imaging, and we provide direct experimental demonstration for the presence of the postnatal developmental shift to lower intraneuronal Cl−. Moreover, we introduce an approach for dynamic and simultaneous monitoring of intraneuronal Cl− and pH in vivo. These methods will open a window for the study of the roles of intraneuronal pH and Cl− in neuronal signaling, plasticity, and disease. Intracellular chloride ([Cl−]i) and pH (pHi) are fundamental regulators of neuronal excitability. They exert wide-ranging effects on synaptic signaling and plasticity and on development and disorders of the brain. The ideal technique to elucidate the underlying ionic mechanisms is quantitative and combined two-photon imaging of [Cl−]i and pHi, but this has never been performed at the cellular level in vivo. Here, by using a genetically encoded fluorescent sensor that includes a spectroscopic reference (an element insensitive to Cl− and pH), we show that ratiometric imaging is strongly affected by the optical properties of the brain. We have designed a method that fully corrects for this source of error. Parallel measurements of [Cl−]i and pHi at the single-cell level in the mouse cortex showed the in vivo presence of the widely discussed developmental fall in [Cl−]i and the role of the K-Cl cotransporter KCC2 in this process. Then, we introduce a dynamic two-photon excitation protocol to simultaneously determine the changes of pHi and [Cl−]i in response to hypercapnia and seizure activity.

Synchronous Bioimaging of Intracellular pH and Chloride Based on LSS Fluorescent Protein.

Ion homeostasis regulates critical physiological processes in the living cell. Intracellular chloride concentration not only contributes in setting the membrane potential of quiescent cells but it also plays a role in modulating the dynamic voltage changes during network activity. Dynamic chloride imaging demands new tools, allowing faster acquisition rates and correct accounting of concomitant pH changes. Joining a long-Stokes-shift red-fluorescent protein to a GFP variant with high sensitivity to pH and chloride, we obtained LSSmClopHensor, a genetically encoded fluorescent biosensor optimized for the simultaneous chloride and pH imaging and requiring only two excitation wavelengths (458 and 488 nm). LSSmClopHensor allowed us to monitor the dynamic changes of intracellular pH and chloride concentration during seizure like discharges in neocortical brain slices. Only cells with tightly controlled resting potential revealed a narrow distribution of chloride concentration peaking at about 5 and 8 mM, in neocortical neurons and SK-N-SH cells, respectively. We thus showed that LSSmClopHensor represents a new versatile tool for studying the dynamics of chloride and proton concentration in living systems.

Photonic Torque Microscopy of the Nonconservative Force Field for Optically Trapped Silicon Nanowires

We measure, by photonic torque microscopy, the nonconservative rotational motion arising from the transverse components of the radiation pressure on optically trapped, ultrathin silicon nanowires. Unlike spherical particles, we find that nonconservative effects have a significant influence on the nanowire dynamics in the trap. We show that the extreme shape of the trapped nanowires yields a transverse component of the radiation pressure that results in an orbital rotation of the nanowire about the trap axis. We study the resulting motion as a function of optical power and nanowire length, discussing its size-scaling behavior. These shape-dependent nonconservative effects have implications for optical force calibration and optomechanics with levitated nonspherical particles.

Arduino Due based tool to facilitate in vivo two-photon excitation microscopy.

Two-photon excitation spectroscopy is a powerful technique for the characterization of the optical properties of genetically encoded and synthetic fluorescent molecules. Excitation spectroscopy requires tuning the wavelength of the Ti:sapphire laser while carefully monitoring the delivered power. To assist laser tuning and the control of delivered power, we developed an Arduino Due based tool for the automatic acquisition of high quality spectra. This tool is portable, fast, affordable and precise. It allowed studying the impact of scattering and of blood absorption on two-photon excitation light. In this way, we determined the wavelength-dependent deformation of excitation spectra occurring in deep tissues in vivo.