Haim Grebel

Nonlinear optical properties of silicon nanoclusters

The nonlinear and linear optical responses of Si nanoclusters at λ=355 nm were measured. The nanoclusters were laser ablated on quartz substrates. χ(3) values as high as 2.28×10−5 esu (as measured by the Z-scan technique) and lifetime as long as 143 ns were measured for clusters of an average size of 11 nm. The optical properties were strongly correlated with the clusters’ sizes.

A statistical ultra-wideband indoor channel model and the effects of antenna directivity on path loss and multipath propagation

Ultra-wideband indoor line-of-sight frequency-domain channel measurements have been performed in the 2-6 GHz frequency band using three different transmit/receive antenna combination pairs. The effects of antenna directivity on path loss and multipath propagation in the channel are analyzed extensively for various omnidirectional and directional antenna combinations. A statistical model of the path loss in the channel is presented, in which all path loss exponents result in better than free space loss. The shadow fading experienced in the channel is shown to highly agree with the normal probability assumption. There is a significant reduction in root mean square delay spread when directional antennas are used at the transmitter and receiver or solely at the receiver with respect to an omnidirectional/omnidirectional antenna pair. Finally, a distance-dependent statistical impulse response model of the channel is presented along with simulation results that significantly concur with the measured data.

Stable hexagonal-wurtzite silicon phase by laser ablation

A stable phase of relatively large hexagonal-wurtzite silicon crystals (up to 20 μm) was directly deposited at low pressure using ultraviolet laser ablation. The films were grown on a variety of substrates at room temperature from a single crystal, cubic silicon target. Crystallites of the hexagonal-wurtzite phase of silicon were clearly identified using selected area electron diffraction. Further support for this identification was provided by confocal scanning micro-Raman spectroscopy. The deposition of hexagonal silicon films may lead to novel two-dimensional optoelectronic devices, and pave the way to studies of the electronic properties of this lower symmetry, uncommon silicon phase.

Artificial dielectrics: Nonlinear properties of Si nanoclusters formed by ion implantation in SiO2 glassy matrix

The nonlinear optical properties of Si nanoclusters formed by ion implantation into an SiO2 glassy matrix and followed by annealing have been studied at λ=532 nm and λ=355 nm by use of Z-scan and pump-probe techniques. These have been compared to the nonlinear properties of laser-ablated Si films. At relatively large intensities (>1 MW/cm2), the absolute nonlinear values for these isolated nanoclusters were comparable to those obtained for laser-ablated samples although opposite in sign. Laser-ablated samples showed a much larger effect at relatively low intensities (<1 MW/cm2), while the ion-implanted films showed almost none. Lifetime constants were in the range of 3–5 ns for all samples.

Artificial dielectrics: Nonlinear optical properties of silicon nanoclusters at λ=532 nm

The nonlinear optical responses of laser-ablated Si nanoclusters were measured at λ=532 nm. Re{χ(3)} values as high as −(1.33±0.33)×10−3 esu were measured for films that were only 200 nm thick. The response time for this nonlinearity was as short as 3.5±0.5 ns, limited by our laser pulse duration.

Nonlinear optical response of Si nanostructures in a silica matrix

We provide a systematic study on the nonlinear optical properties of silicon nanocrystals within a fused silica matrix. Nonlinear measurements at various wavelengths exhibited the role of three bands in the visible spectrum. Measurements at various laser pulse durations showed several time constants, which exhibited the role of quantum confined and surface states.

Detecting single-wall carbon nanotubes with surface-enhanced Raman scattering from metal-coated periodic structures

We report on surface-enhanced Raman scattering (SERS) from bundles of size-selected single-wall carbon nanotubes (SWCNTs) deposited on ordered arrays of silver- and gold-coated silica spheres. Selective enhancement of the Raman-active tangential and radial modes of SWCNTs is exhibited in backscattering at surface plasmon resonance conditions. Enhancement at plasmon wavelength and sub-wavelength is shown for various periodic pitches of the substrates used.

Pulsed-laser deposition of Si nanoclusters

Growth properties of thin films of Si nanoclusters that were deposited on Si wafers by use of pulsed-laser ablation are discussed. The films were characterized by Atomic Force Microscopy (AFM) and X-ray Diffraction (XRD) and FTIR spectroscopy. Large nonlinear optical absorption was measured using a Free-Electron Laser at wavelengths near the infrared absorption band centered at 9.8 μm.

Polarized Raman scattering and localized embedded strain in self-organized Si/Ge nanostructures

Using polarized Raman spectroscopy, we examine different vibrational modes (i.e., Si–Si, Si–Ge, and Ge–Ge) in Si/Ge self-organized nanostructures. Here, we present unambiguous proof that multilayers of Ge nanometer-size, “dome-shaped” islands grown on a 〈100〉 Si substrate are nearly fully relaxed and that the built-in strain field is substantially localized in the surrounding Si matrix. In contrast, multilayers with “pyramid-shaped” islands do not show observable relaxation. The large strain in the Si layers of the multilayer dome samples correlates with the greater self-organization in these structures compared to the multilayer pyramid samples.

NONLINEAR OPTICAL PROPERTIES OF A COHERENT ARRAY OF SUBMICRON SIO2 SPHERES(OPAL) EMBEDDED WITH SI NANOPARTICLES

The optical properties of a coherent array of 300 nm SiO2 spheres implanted with Si ions were shown to change nonlinearly with light intensity. These changes have been attributed to transverse confinement of the optical beam within the opal structure.