In this work we investigate, by first-principles calculations, the structural, electronic and optical properties of: (1) oxygenated silicon-based nanoclusters of different sizes in regime of multiple oxidation at the surface, and (2) hydrogenated Si nanoclusters (H-Si-nc) in their ground and excited state configurations. Structural relaxations have been fully taken into account in all cases through total energy pseudopotential calculations within density functional theory. In the first case we have varied systematically the number of Si=O bonds at the cluster surface and found a nonlinear reduction of the energy gap with the Si=O bond number. A saturation limit is reached, which allows us to provide a consistent interpretation of the photoluminescence (PL) redshift observed in oxidized porous silicon samples. Our results help also to explain some very recent findings on the single silicon quantum dot photoluminescence bandwidth. In the second case, after a preliminary study of the clusters stability, the properties of the ground and excited states have been compared varying the cluster dimensions from 1 to 29 Si atoms. Ab-initio calculations of the Stokes shift as a function of the cluster dimension will be presented. A structural model linked to the four level scheme recently invoked to explain the experimental outcomes relative to the observed optical gain in Si-nc embedded in a SiO₂ matrix will be also suggested.

The work function of nano Porous Silicon (PS) has been studied by the kelvin probe method as a function of the exposure to different gaseous species. Characterisation has been performed n dark and in presence of sub band and supra band gap light Surface Photovoltage (SPV)measurements. Traces of ammonia and nitrogen dioxide change drastically the shape of SPV as a function of photon energy:light induces transitions from and to surface states produced by gas adsorption. The results foresee the possibility to improve semiconductor sensor selectivity by using monochromatic light at well defined frequency able to activate/deactivate surface states where species are adsorbed

Surface-enhanced optical third-harmonic generation (THG)is observed in silver island films. The THG intensity from 2-D array of silver nanoparticles is enhanced by two orders of magnitude and the enhancement is attributed to the local field resonance at third-harmonic wavelength mediated by excitation of the local surface lasmons.

In recent years, a large amount of activity has been carried out searching new materials for second order non-linear optics. Nanostructured materials composed of a host matrix and optical active nanocrystals form a new part of this research field. Our present work consists in "doping" SiO₂ matrices with 5 to 30% mass of LiIO₃ nanocrystals. Indeed, α-LiIO₃ is a compound known to be very efficient for second harmonic generation (SHG). This nanocomposite glass has been developped using sol-gel technics. Bulk samples have been first studied showing nanocrystallites with size ranging from 50 nm to 400 nm and second harmonic generation (SHG). Then thin layers have been elaborated using dip coating and spin coating technics. Being given that LiIO₃ crystals present structurally a strong dipolar moment, non-linear optical properties could be enhanced using Corona discharges to orientate nanocrystals. This paper relates the fabrication process and the structural and optical characterizations of bulk and thin layer material.

Stimulated emission depletion (STED) population and polarisation dynamics following two-photon excitation are investigated for rhodamine 6G in ethylene glycol. Time resolved fluorescence intensity and polarisation measurements were made using picosecond time-correlated single photon counting (TCSPC). Cross-sections for the stimulated transition were measured between 614nm (2.32x10^-16 cm²) and 663.5nm (6.05x10^-17 cm²), ground state vibrational lifetimes were found to vary between 314fs and 467fs. A collinear (180°) excitation-detection geometry was employed to investigate re-polarisation of the excited state array yielding fluorescence anisotropies above the two-photon limit. The circumvention of single-photon selection rules is demonstrated allowing the measurement of higher order parameters and correlation functions that are wholly inaccessible to 'conventional' (spontaneous) time resolved fluorescence techniques.

In the framework of the semi-empirical version of TDHF approach, the spectral parameters, linear and higher polarizabilities (in particular, for THG and EFISH) are calculated for free C₆₀ molecule and crystal, and for closed-shell anions (C₆₀)^-2,(C₆₀)^-4,(C₆₀)^-6 in fullerides. It is demonstrated that the direct calculation of polarizability for the neutral molecule in crystals and similar calculation by means of the Lorentz factors gives the equivalent results while for HP the direct calculation includes higher field effects resulting in the approximately 5% increase of HP as compared to the Lorentz type calculation.

Photoluminescence spectroscopy of porous silicon photonic crystal microcavities is studied by the far-field and near-field robes using the apertureless scanning near-field optical microscope. Narrow microcavity mode with the spectral width of 10nm in far-field spectra and broad photoluminescence peak with the spectral width of 50nm in near-field spectra of microcavity samples is observed. It has been studied some correlations between near-and far-field spectra of porous silicon structures.

The combination of liquid crystals (LCs) and reconfigurable nanoparticulate networks results in most versatile materials for controlling light beams. These material systems can be used for developing multi-functional reconfigurable photonics and opto-electronics components and spatial light modulators with ultimate light modulating capabilities. We review here our results in laser recording of one and two-dimensional diffraction gratings and gratings with variable pitch. Nonlinear optical properties of LC with nanoparticulate internal networks and diffraction gratings laser-recorded in these materials are discussed. Nanoparticulate networks are capable of stabilizing the thermodynamic relaxation of photoinduced cis-isomers of molecules in photosensitive azobenzene LCs used as host for the nanoparticulate network leading to bistability of the phase state (anisotropic and isotropic) of the material, and reversible all-optical switching between those states.

We report on a medium exhibiting extremely efficient light scattering properties: a liquid network formed in a porous matrix. Liquid fragments confined in the solid matrix result in a random fluctuation of the dielectric function and act as scattering objects for photons. The optical scattering efficiency is defined by the filling factor of the liquid in the pores and its dielectric constant. The spectral dependence of the scattering length of photons indicates that the phenomenon is governed by a Mie-type scattering mechanism. The degree of the dielectric disorder of the medium, i.e. the level of opacity is tunable by the ambient vapor pressure of the dielectric substance. In the strongest scattering regime the scattering length of photons is found to be in the micrometer range. By incorporation of dye molecules in the voids of the porous layer a system exhibiting optical gain is realized. In the multiple scattering regime the optical path of diffusively propagating photons is enhanced and light amplification through stimulated emission occurs: a strong intensity enhancement of the dye emission accompanied by significant spectral narrowing is observed above the excitation threshold for a layer being in the opalescence state.

The feasibility of manipulating the single molecule absorption-emission cycle using picosecond stimulated emission depletion (STED) is investigated using a stochastic computer simulation. In the simulation the molecule is subjected to repeated excitation and depletion events using time delayed pairs of excitation (PUMP) and depletion (DUMP) pulses derived from a high repetition rate pulsed laser system. The model is used to demonstrate that a significant and even substantial reduction in the occurrence of 'dark states' in the fluorescence emission can be achieved using stimulated emission depletion. Variation in the PUMP-DUMP window allows precise control of the fluorescence yield with substantial increases in the fluorescence intensity observed at early PUMP-DUMP delays.

Nanophotonics, dealing with optical science and technology at nanoscale, is an exciting new frontier, which provides numerous opportunities both for fundamental research and new applications of photonics. The Institute for Lasers, Photonics and Biophotonics at Buffalo has a comprehensive multidisciplinary program in Nanophotonics funded by the United States Department of Defense. This program focuses on three major areas of Nanophotonics: (i) interactions involving nanoscale confined radiation, (ii) use of nanoscale photoexcitation for nanofabrication and (iii) design and control of excitation dynamics in nanostructured optical materials. Selected examples of our accomplishments in nanophotonics are presented here which illustrate some of the opportunities.

Preferential sequestering of surface modified metal/semiconductor nanocrystals within microphase separated block copolymer domains holds the promise for engineering large-scale polymer based photonic materials. In my talk I want to review block copolymers as a material platform for photonic crystal engineering as well as the prospects of metallodielectric photonic materials based on metal nanocrystal/block copolymer composites. The effect of metal nanocrystal additives on the optical properties of the composite is found to be determined by: (1) changes in the optical properties of individual nanocrystals due to the spatial confinement of the free electrons by the crystal boundary and (2) by collective effects resulting from the particle size-dependent morphology of the nanocrystals within the polymer domains. The particle core size, the polymer domain spacing as well as the particle surface chemistry are shown to determine three distinct morphological types in particle/block copolymer composites. A detailed comparison between morphological studies and theoretical predictions will be presented that aim to better understand and control morphologies of structured cluster matter in order to tailor optical and mechanical properties of new photonic materials.

This presentation focuses on the synthesis and characterization of luminescent silicon nanoparticles that have potential as components of hybrid inorganic/organic materials for photonic and biophotonic applications. In our lab, silicon nanoparticles with bright visible photoluminescence are being prepared by a new combined vapor-phase and solution-phase process, using only inexpensive commodity chemicals. CO₂ laser-induced pyrolysis of silane is used to produce Si nanoparticles at high rates (20 to 200 mg/hour). Particles with an average diameter as small as 5 nm can be prepared directly by this method. Etching these particles with mixtures of hydrofluoric acid (HF) and nitric acid (HNO₃) reduces the size and passivates the surface of these particles such that they exhibit bright visible luminescence at room temperature. The wavelength of maximum photoluminescence (PL) intensity can be controlled from above 800 nm to below 500 nm by controlling the etching time and conditions. Particles with blue and green emission are prepared by rapid thermal oxidation of orange-emitting particles. These particles have exciting potential applications in optoelectronics, display technology, chemical sensing, biological imaging, and other areas. The availability of relatively large quantities of these particles is allowing us to begin to functionalize particles for these applications, as well as to study the optical, electronic, and surface chemical properties of them. All of these potential applications require inorganic/organic hybrid materials, in the sense that the nanoparticles must have their surfaces coated with organic molecules that mediate the interaction of the particles with the polymeric or biological host matrix. The particle synthesis methods, photoluminescence measurements on the particles, the stability of the photoluminescence properties with time, chemical quenching of photoluminescence, and functionalization of the particles for incorporation into different organic matrices or for specific interaction with small molecules or biomolecules are discussed in the context of applications to photonics and biophotonics.

This paper describes the development of using individual micro and nano meter-sized vesicles as delivery vessels to functionally map the distribution of cell surface proteins at the level of single cells. The formation of different sizes of vesicles from tens of nanometers to a few micrometers in diameter that contain the desired molecules is addressed. An optical trap is used to manipulate the loaded vesicle to specific cell morphology of interest, and a pulsed UV laser is used to photo-release the stimuli onto the cell membrane. Carbachol activated cellular calcium flux, upon binding to muscarinic acetylcholine receptors, is studied by this method, and the potential of using this method for the functional mapping of localized proteins on the cell surface membrane is discussed.

This work was aimed at fabrication of a three-dimensional magento-photonic crystal. In our experiments LATEX spheres with diameter of about 200 nm were coated with Fe₃O4 fine particle. The diameter of Fe₃O4 fine particles is about 30 nm when ration of Fe³⁺ / Fe²⁺ (r) = 3% and pH = 8.6. The diameter of these particles decreases about 10 nm by increasing up to r=20% in the same pH region. It was found that a decrease of the coated particles size can be made as low as the size of 5% of LATEX spheres diameter. Therefore, it may use these particles for preparation a three-dimensional magento-dielectric structure.

Optical properties of silicon and indium phosphide nanoparticles with emission throughout the visible wavelength range are presented. The peak emission wavelength of these nanoparticles is controlled by the reaction time and by post-growth etching treatments. Ultrafast spectroscopy is used to determine the photoluminescence lifetime in order to correlate the spectral response with the structural and chemical characterization of these nanoparticles. The measured lifetimes are used to identify surfactant, surface, and core nanoparticle emission. The nanoparticles exhibit efficient emission that is quenched when embedded within particular polymeric matrices.

The synthesis and the emission properties of colloidal YVO₄:Ln nanocrystals (8nm in diameter) are investigated, where Ln = Eu, Nd, Yb, Er. The nanoparticles synthesized here are constituted of a (Y,Ln)VO₄ crystalline core and stabilized by a lanthanide-citrate complexing shell. Surface derivatization of the particles can be achieved through the controlled growth of a silicate shell using a functionalized silane, with elimination of the citrate shell. The luminescence of the Eu³⁺ -doped YVO₄ colloids is studied in details and compared to the bulk material. Chemical treatments are achieved in order to explain the observed differences. Improvement of the emission quantum yield after the transfer of the colloidal particles into D₂O shows that surface OH groups act as efficient quenchers of the Eu³⁺ emission. The growth of a silica shell around the particles decreases the optimum europium concentration, showing that energy transfers within the nanoparticles are limited by the quenching of the excited states of the vanadate ions. Moreover, site selective excitation spectroscopy seems to prove the coexistence of core sites and surface sites for the europium ions. Finally, colloidal nanoparticles exhibit an emission yield of about 25%, which appears already suitable for some applications.

A new class of fluorophores has been identified that can be imaged at the single-molecule level and offer additional beneficial properties such as a significant ground state dipole moment, moderate hyperpolarizability, and sensitivity to local rigidity. These molecules contain an amine donor and a dicyanodihydrofuran (DCDHF) acceptor linked by a conjugated unit (benzene, thiophene, alkene, styrene, etc.) and were originally designed to deliver both high polarizability anisotropy and dipole moment as nonlinear optical chromophores for photorefractive applications. Surprisingly, we have found that these molecules are also well-suited for single-molecule fluorescence imaging in polymers and other reasonably rigid environments. We report the bulk (ensemble) and single-molecule photophysical properties measured for six dyes in this new class of single-molecule reporters, with absorption maxima ranging from 486 to 614 nm.

The optical properties of nano-scale zinc oxide/PMMA composites were investigated. Films were produced by spin coating and cast coating dispersions of nanoparticle/PMMA organic solutions. This allows for the preparation of nanocomposite films with controlled thickness containing up to 22 vol. % homogeneously dispersed nanofillers. It was found that the refractive index of the composite films scaled linearly with volume fraction of nanofillers. The films were transparent in the visible light range and strongly absorbing in the UV range. When the films were loaded with a high content of such semiconductor nanoparticles, the surface resistivity decreased enough for the coatings to be used for antistatic applications.

Silicone polymeric materials are being developed that will allow the hybrid integration of tunable functionality provided by polymer dispersed liquid crystal, PDLC, and continuous phase liquid crystal materials on planar silica-on-silicon and planar polymer light circuits. The advantages of this approach are ease of integration, the possibility for reduced power consumption, and therefore a reduction of the overall cost for component manufacturing and operation. A successful demonstration of a low loss approach to hybrid integration of polymers and liquid crystals is presented. The challenges for successful integration and acceptance will be discussed. New liquid crystal materials are being developed specifically for this application.

Two-photon polymerization (2PP) of photosensitive inorganic-organic hybrid polymers (ORMOCERs, developed at the Frauenhofer Institut fuer Silicatforschung) is demonstrated as a very promising approach for the fabrication of complicated three-dimensional micro- and nanostructures. These materials are produced by sol-gel synthesis with molecular level mixing of different components. It is remarkable that properties of the hybrid polymers can be tuned from those that are characteristic for organic polymers to those that are similar to inorganic glasses. They have negative resist behaviour and can be used as storage-stable, liquid photo-polymerizable resins. When Ti:sapphire femtosecond laser pulses are tightly focused into the volume of this resin (which is transparent in the infrared) they can initiate two-photon polymerization process transferring liquid into solid state. This process is confined to a highly localized area at the focal point due to the quadratic dependence of the two-photon absorption rate on the laser intensity. When the laser focus is moved through the resin in three dimensions, the polymerization occurs along the trace of the focus. This allows to fabricate any computer-generated 3D structure by direct laser "recording" into the volume of the ORMOCER. The non-irradiated liquid resin can be dissolved in alcohol leaving the polymerized copy of the computer model. Compared to conventional photo-lithography which is a planar processing, two-photon polymerization is a real three-dimensional volume microfabrication technique. This technology can be used for rapid prototyping and low-cost fabrication of artificial micro- and nanostructured components which are required for different applications in optics, medicine, and biology. Numerous examples such as photonic crystals, micromechanical and microoptical devices are presented.

Computer generated holograms are used to create doughnut beams. We use negative-tone inorganic-organic hybrid SiO₂: TiO₂ glasses to fabricate computer-generated holograms. This sol-gel material enjoys an advantage over materials used in the conventional photoresist-based fabrication techniques in terms of a single-step etching-free process. The doughnut beams may be useful in making laser traps for precision measurements and for Bose-Einstein condensation.