Studies on optical limiting characteristics of silicon nanoparticles synthesized by laser ablation

We present synthesis of silicon nanoparticles dispersed in toluene by laser ablation and studies on their optical limiting properties with nanosecond laser pulses at 532 nm. Silicon nanoparticles in toluene show better optical limiting compared to standard optical limiter fullerene C60 in toluene. Optical limiting threshold of silicon nanoparticles is about three times less than that of C60. Detailed studies using Z-scan experiments, angle dependent scattering, intensity dependent transmission and temporal profile measurements indicate that apart from non-linear scattering, nonlinear absorption and nonlinear refraction also contribute to the optical limiting behavior of silicon nanoparticles.     

Studies of third-order optical nonlinearities and optical limiting of 2, 3-butanedione dihydrazone

2, 3-butanedione dihydrazone (BDDH) synthesized via chemical route and nonlinear optical parameters, such as nonlinear refractive index (n2), nonlinear absorption coefficient (β), third-order nonlinear optical susceptibility (χ(3)), second hyperpolarizability (γ), and optical-limiting behavior were investigated by single beam z-scan technique for different concentrations. Synthesized sample were irradiated by a Q-switched, frequency doubled Nd:YAG laser and found that n2 and β increases linearly with increasing concentration and hence γ decreases linearly. The excited-state absorption cross sections of BDDH were found to be larger than ground-state absorption cross sections, and it leads to reverse saturable absorption (RSA). The experimental results are well in agreement with the theory and also establish BDDH as one of the potential candidate materials for optical limiting at 532 nm.     

Enhanced optical limiting and nonlinear absorption properties of azoarene-appended phosphorus (V) tetratolylporphyrins

Optical limiting performance, third-order nonlinearity chi(3), and nonlinear absorption properties have been investigated in a new class of azoarene phosphorus (V) porphyrins with charge transfer (CT) states. The introduction of axial azoarene groups into the phosphorus porphyrin structure is found to reduce the limiting threshold by a factor of 2 and lead to a rise in the second hyperpolarizability by 1 order of magnitude in the picosecond time regime and by 2 orders of magnitude in the nanosecond regime. The experimental data show reverse saturation of absorption in the nanosecond time regime and a saturation of the nonlinear absorption above a fluence of 0.5 J/cm2 in the picosecond regime. The presence of the CT state reduces saturation of excited-state absorption (ESA) in the S1 --> Sn transition through the S1 --> CT transition. Faster CT --> T1 transition increases the ESA from T1 --> Tn states in the nanosecond regime. A self-consistent theoretical analysis based on rate equations is used to estimate the high-lying excited-state lifetimes and absorption cross sections from the experimental results.     

金属团簇化合物W2Ag4S8(dppf)2激发态非线性光学性质

用Z扫描方法测量了金属团簇化合物W2Ag4S8(dppf)2的非线性光学响应,发现团簇W2Ag4S8(dppf)2具有显著的反饱和吸收和自聚焦等非线性光学性质.应用激发态理论分析了团簇W2Ag4S8(dppf)2的非线性吸收和非线性折射,结果与实验数据一致.通过数值模拟获得激发态和基态吸收截面比值Ka及非线性折射度比值Kr,阐述了Ka和Kr的物理意义.确定了团簇W2Ag4S8(dppf)2的三阶极化率x(3).团簇化合物W2Ag4S8(dppf)2对脉宽为纳秒的激发脉冲限幅效果比较好.     

Continuous-wave power transmission and thermal lensing of a saturable absorber subject to excited-state absorption

Rate-equation analysis has been used in an investigation of the role of saturation and excited-state absorption in the power transmission characteristics and thermal lensing of an absorber. Use of an iterative approach gives explicit analytical results for power transmission and thermal focal length in the presence of excited-state absorption. Sample calculations indicate that pump absorption can increase or decrease with increasing incident pump power, depending on the relative strength of the excited-state absorption cross section with respect to the ground-state absorption cross section. In the case of thermal lensing, results further indicate that saturation and excited-state absorption act as two competing effects, the former reducing the strength of the thermal lens and the latter causing the opposite effect. The analytical formulas derived in this analysis should prove useful to experimentalists in determination of ground-state and excited-state absorption cross sections from experimental power transmission and lensing data.     

Spectroscopic investigation of Nd ion in LiNbO3, MgO:LiNbO3 and LiTaO3 single crystals relevant for laser applications

The polarized absorption and luminescence properties of Nd³⁺ doped isostructural LiNbO₃, MgO:LiNbO₃ and LiTaO₃ nonlinear bulk single crystals are reported. Pump-probe experiments associated with the Judd-Ofelt approach are used to estimate two types of room temperature cross sections: polarized emission cross sections of the dominant ⁴F_3/2 → ⁴I_1//2 transition near 1085 and 1093 nm and polarized excited-state absorption cross sections in the same spectral domain and in the green spectral range corresponding to self frequency doubling. Self frequency-doubling results are also given in Nd:LiNbO₃ and Nd:MgO:LiNbO₃ versus sample temperature.     

Quasi-Resonant Nonlinear Absorption for Optical Power Limiting: solgel-Processed Er(3+)-Doped Multicomponent Silica Glass

We demonstrate optical power limiting by what we believe to be a new mechanism of nonlinear absorption, which involves a quasi-resonant ground-state absorption that is either phonon assisted or assisted by the presence of defect sites (tail absorption). Such a mechanism provides high transmittance at low intensity yet optical limiting under cw conditions. The sample used was a novel solgel-processed Er(3+)-doped multicomponent silica glass. In this system the nonlinear absorption process is achieved because the resonant excited-state ((4)I(13/2) ? (4)S(3/2)) absorption cross section is larger than the quasi-resonant ground-state ((4)I(15/2) ? (4)I(9/2)) absorption cross section.     

In situ infrared aerosol spectroscopy for a variety of nerve agent simulants using flow-through photoacoustics

We present newly measured results of an ongoing experimental program established to measure optical cross sections in the mid- and long-wave infrared for a variety of chemically and biologically based aerosols. For this study we consider only chemically derived aerosols, and in particular, a group of chemical compounds often used as simulants for the detection of extremely toxic organophosphorus nerve agents. These materials include: diethyl methylphosphonate (DEMP), dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), and diethyl phthalate (DEP). As reported in a prior study [Appl. Opt. 44, 4001 (2005)], we combine two optical techniques well suited for aerosol spectroscopy [i.e., flow-through photoacoustics and Fourier transform infrared (FTIR) emission spectroscopy], to measure in situ the absolute extinction and absorption cross sections over a variety of wavelengths spanning the IR spectral region from 3 to 13 mum. Aerosol size distribution(s), particle number density, and dosimetric measurements are recorded simultaneously in order to present optical cross sections that are aerosol mass normalized, i.e., m(2)/gram. Photoacoustic results, conducted at a series of CO(2) laser lines, compare well with measured broadband FTIR spectral extinction. Both FTIR and photoacoustic data also compare well with Mie theory calculations based on measured size distributions and previously published complex indices of refraction.     

Nonlinear optical beam propagation for optical limiting

We implement numerical modeling of high-energy laser-pulse propagation through bulk nonlinear optical materials using focused beams. An executable program with a graphical user interface is made available to researchers for modeling the propagation of beams through materials much thicker than the diffraction length (up to 10(3) times longer). Ultrafast nonlinearities of the bound-electronic Kerr effect and two-photon absorption as well as time-dependent excited-state and thermal nonlinearities are taken into account. The hydrodynamic equations describing the rarefaction of the medium that is due to heating are solved to determine thermal index changes for nanosecond laser pulses. We also show how this effect can be simplified in some cases by an approximation that assumes instantaneous expansion (so-called thermal lensing approximation). Comparisons of numerical results with several Z-scan, optical limiting and beam distortion experiments are presented. Possible application to optimization of a passive optical limiter design is discussed.     

Observation of surface plasmon resonance of silver particles and enhanced third-order optical nonlinearities in AgCl doped Bi2O3-B2O3-SiO2 ternary glasses

Bi₂O₃-B₂O₃-SiO₂ ternary glasses embedded with Ag nanoparticles were prepared by introducing AgCl into the bismuthate glasses using conventional melt quenching method and characterized by several experimental techniques. Scanning electron microscopic studies indicated the formation of Ag contained nanoclusters which crack and become regular with increase of AgCl content in these composites. Optical absorption spectra of the nanocomposites showed the presence of absorption band of surface plasmon resonance (SPR) due to Ag nanoparticles at ∼600 nm. Z-scan measurement with femtosecond laser was used to investigate third-order optical nonlinearities of the nanocomposites. The results show that the nonlinear refraction γ was dramatically increased up to 30 times by the appearance of Ag nanoparticles when excited within its SPR region, while nonlinear absorption due to two-photon absorption exhibited opposite tendency or even saturated behavior. The calculation of figure of merit suggests that the Ag particle embedded Bi₂O₃-B₂O₃-SiO₂ glass composites are promising candidates for optoelectronic devices.     

Cytotoxicity and TNF-alpha secretion in RAW264.7 macrophages exposed to different fullerene derivatives

Fullerene derivatives have been reported as potential nanomedicines, however the role of surface chemical modification on the biological effects remains unclear. In this study, five kinds of water soluble C60 derivatives with different surface chemical modification, C60-(OH)20 (HFD), C60-(beta-Ala)10.1 (AFD), C60-(Lys)8.7 (KFD), C60-(Arg)8.6 (RFD) and C60-(NH(CH2)2NH2)8.8 (NFD) were synthesized. Their cytotoxicity as well as TNF-alpha secretions were evaluated in RAW264.7 macrophage cell line. The results show that no significant cytotoxicity can be observed upon 24 h exposure to C60 derivatives at less than 50 microg/mL. However, higher concentration (> 100 microg/mL) of these C60 derivatives decreases the proliferation of RAW264.7. The cytotoxicity of these fullerene derivatives is probably through the apoptosis pathway, while the extent of cytotoxicity varies with the different surface charges. Higher celluar uptake of HFD was observed in RAW264.7 cells than AFD, which correlates with the more toxic effect of HFD over AFD. The secretion of cytokine tumor necrosis factor alpha (TNF-alpha) was determined to evaluate the immunostimulating activity of these fullerene derivatives. The data show that the fullerene derivatives with negative surface charges secrete more TNF-alpha, whereas derivatives with positive charges show insignificant effect. The possible influence of various surface charge property on the observed biological effects is discussed.     

C60 fullerene derivatized nanoparticles and their application to therapeutics

Fullerenes can be formed into many new materials and devices. They have a wide range of applications in medicine, electronics, biomaterials, and energy production. An overview of the nanostructure and the physical and chemical characteristics of fullerene-drug derivatives is given. The biological behavior of fullerene derivatives shows their potential to medical application fields because C(60) is rapidly absorbed by tissues and is excreted through urinary tract and enterons, which reveals low toxicity in vitro and in vivo studies. Nanomedicine has become one of the most promising areas of nanotechnology, while many have claimed its therapeutic use against cancer, human immunodeficiency virus (HIV), and neurodegenerative disorders. Water-soluble C(60) fullerene derivatives that come from chemical modification largely enhance the biological efficacy. The blood-brain barrier (BBB) is a physical barrier composed of endothelial tight junctions that restrict the paracellular permeability. A major challenge facing neuropharmacology is to find compounds that can be delivered into the brain through the bloodstream. Fullerene C(60) was demonstratively able to cross the BBB by hybridizing a biologically active moiety dyad, which provides a promising clue as a pharmacological therapy of neural disorders.     

Synthesis and Characterisation of Some New Ferrocenyl- and Ferrocenylalkynyl-[60]Fullerene Compounds

We report the synthesis and characterisation of a series of ferrocenylfullerene compounds, and some new ferrocene derivatives required as intermediates. The new fullerene species are Fc-[60]fullerene, (8); Fc-C=C-[60]fullerene, (10); Fc-C=C-C=C-[60]fullerene, (12); and (η-C₅H₄SiBu₃)Fe(η-C₅H₄)-[60]fullerene, (14).     

Gold Nanocages: Engineering Their Structure for Biomedical Applications

The galvanic replacement reaction between a Ag template and HAuCl₄ in an aqueous solution transforms 30–200 nm Ag nanocubes into Au nanoboxes and nanocages (nanoboxes with porous walls). By controlling the molar ratio of Ag to HAuCl₄, the extinction peak of resultant structures can be continuously tuned from the blue (400 nm) to the near‐infrared (1200 nm) region of the electromagnetic spectrum. These hollow Au nanostructures are characterized by extraordinarily large cross‐sections for both absorption and scattering. Optical coherence tomography measurements indicate that the 36 nm nanocage has a scattering cross‐section of ～ 0.8 × 10^–15 m² and an absorption cross‐section of ～ 7.3 × 10^–15 m². The absorption cross‐section is more than five orders of magnitude larger than those of conventional organic dyes. Exposure of Au nanocages to a camera flash resulted in the melting and conversion of Au nanocages into spherical particles due to photothermal heating. Discrete‐dipole‐approximation calculations suggest that the magnitudes of both scattering and absorption cross‐sections of Au nanocages can be tailored by controlling their dimensions, as well as the thickness and porosity of their walls. This novel class of hollow nanostructures is expected to find use as both a contrast agent for optical imaging in early stage tumor detection and as a therapeutic agent for photothermal cancer treatment.     

Synthesis and photovoltaic properties of novel monoadducts and bisadducts based on amide methanofullerene

Four new [6,6]-phenyl-C(61) and C(71) butylsaure n-dibutyl amides (PCBDBA) with mono- and bis-adduction on C(60) and C(70) cages, respectively, have been synthesized as models to study the effect of the mono- and bis-adduction on fullerene cages on device performance when used as electron acceptors with the donor of regioregular P3HT in bulkheterojunction organic photovoltaics (BHJ-OPV). The optoelectronic, electrochemistry, and photovoltaic properties of these mono- and bis-products were fully investigated. The best device performance of these fullerene derivatives were obtained from the two monoadducts with power conversion efficiency (PCE) of 1.77% for C(60) derivative and 1.90% for C(70) derivative, respectively, which are close to PCBM's 2.43%. The results revealed the structure-function relationship among the monoadduct and bisadduct derivatives of C(60) and C(70) with the BHJ-OPV performance.     

Measurement of laser-induced refractive index change of inverted ferroelectric domain LiNbO3

Optical nonlinearities of periodically poled LiNbO(3) crystals were investigated by the single beam Z-scan technique with a continuous wave (cw) laser beam at 532 nm. The nonlinear optical absorption coefficient and refractive index change are determined to be 8.1 x 10(-6) cm/W and 2.6 x 10(-4) at 0.5 MW/cm(2) light intensity, respectively. Both sign and magnitude of the measured refractive nonlinearity are considerably different from the Z-scan results in congruent LiNbO(3). The nonlinearities in the periodically poled LiNbO(3) induced by 532 nm continuous waves are believed to be mainly due to the photorefractive effect.     

Nonlinear characteristic and optical limiting effect of oil red O azo dye in liquid and solid media

In view of a possible application in optical limiting devices for protection against laser radiation, the nonlinear optical absorption, refraction and optical limiting behavior of an organic dye, oil red O, under excitation with CW, Nd: YAG laser at 532 nm was studied. The nonlinear optical responses of the dye were studied both in solution (acetonitrile) and solid film, (methylmethacrylate [MMA]) respectively, using the single-beam Z-scan technique. The open aperture Z-scan of the solution and solid samples displayed reversible saturable absorption. The closed aperture Z-scan of the samples exhibited negative nonlinearity, which was larger in magnitude in the solid film compared to that in solution. The nonlinear refractive index was found to vary with concentration. Optical limiting characteristics of the dye at various concentrations were studied. The third-order nonlinearity of this dye is dominated by nonlinear absorption, which leads to strong optical limiting of the laser.     

Three-photon absorption and nonlinear refraction of BaMgF₄ in the ultraviolet region

The nonlinear refraction and nonlinear absorption phenomena are investigated in BaMgF₄ single crystal using the Z-scan technique in the ultraviolet region with a pulsed laser at 400?nm with 1?ps pulse duration. The remarkable nonlinear absorption behavior is identified to be three-photon absorption under the experimental conditions. In addition, both nonlinear refraction and nonlinear absorption have relatively large values and possess small anisotropy along three different crystallographic axes. The large values of nonlinear refractive index are demonstrated through the self-phase modulation effect.     

Optimization of band structure and quantum-size-effect tuning for two-photon absorption enhancement in quantum dots

The two-photon absorption, 2PA, cross sections of PbS quantum dots, QDs, are theoretically and experimentally investigated and are shown to be enhanced with increasing quantum confinement. This is in contrast to our previous results for CdSe and CdTe QDs where the reduced density of states dominated and resulted in a decrease in 2PA with a decrease in QD size. Qualitatively this trend can be understood by the highly symmetric distribution of conduction and valence band states in PbS that results in an accumulation of allowed 2PA transitions in certain spectral regions. We also measure the frequency nondegenerate 2PA cross sections that are up to five times larger than for the degenerate case. We use a k·p four-band envelope function formalism to model the increasing trend of the two-photon cross sections due to quantum confinement and also due to resonance enhancement in the nondegenerate case.