S. Vijayalakshmi

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.

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.

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.

Linear and nonlinear optical properties of single-walled carbon nanotubes within an ordered array of nanosized silica spheres

The linear and nonlinear optical properties of well-separated, single-walled carbon nanotubes were measured. The tubes were grown into the voids of an ordered array of silica spheres. Transmission of light through these tubes increased with the incident laser intensity. The nonlinear decay time of a λ=800-nm probe was measured at 320 fs when the sample was pumped with 400-nm light. Current–voltage characteristics changed upon illumination with a laser beam. Raman scattering decreased as electrical biasing increased.

Characterization of laser ablated silicon thin films

Using laser ablation, we deposited silicon layers consisting of clusters and crystalline domains onto glass, quartz, aluminum, titanium, copper, single-crystal silicon and single-crystal potassium bromide substrates. The microstructure and the morphology of the films were characterized by use of optical microscopy, laser scanning microscopy, atomic force microscopy, transmission electron microscopy, micro-Raman spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The results indicated that the deposited material was composed of microcrystalline droplets, typically 3.5 μm in diameter, separated by amorphous-like regions. The droplets were composed of crystalline material at their centers and an outer halo of nanometer-size particles.

Nonlinear optical properties of laser ablated silicon nanostructures

The role of crystallite symmetry on the nonlinear optical properties of laser ablated silicon films is discussed in this article. Laser ablated Si films exhibit a nonlinear refractive index change which may be as high as Δn=−0.5 at wavelength of λ=532 nm for films with an average thickness of 200 nm. However, the origin of this nonlinearity is not known. These films consist of large droplets comprised of hexagonal wurtzite symmetry crystallites and nanoclusters interspersed between them. Using Raman spectroscopy and linear and nonlinear optical measurements, we monitored the crystallographic symmetry of these droplets when annealed under various conditions. Based on the results we attribute the large nonlinear refraction coefficient to the hexagonal wurtzite symmetry of the crystallites, hence raising the possibility for developing very efficient nonlinear optical devices.

Self-imaging chirped holographic optical waveguides

To manipulate light propagation in optical waveguides, we have studied holographic, chirped structures within the waveguides core. The holographic structures were embedded along the wave propagation direction and extended throughout the entire guide. Various self-imaging guides have been analyzed and realized to demonstrate the effect of different structures.