Two-dimensional array self-assembled quantum dot sub-diffraction waveguides with low loss and low crosstalk

We model and demonstrate the behavior of two-dimensional (2D) self-assembled quantum dot (QD) sub-diffraction waveguides. By pumping the gain-enabled semiconductor nanoparticles and introducing a signal light, energy coupling of stimulated photons from the QDs enables light transmission along the waveguide. Monte Carlo simulation with randomized inter-dot separation reveals that the optical gain necessary for unity transfer is 3.1 × 10(7)?m(-1) for a 2D (2?μm length by 500?nm width) array compared to 11.6 × 10(7)?m(-1) for a 1D (2?μm length) given 8?nm diameter quantum dots. The theoretical results are borne out in experiments on 2D arrays by measurement of negligible crosstalk component with as little as 200?nm waveguide separation and is indicative of near-field optical coupling behavior. The transmission loss for 500?nm wide structures is determined to be close to 3?dB/4?μm, whereas that for 100?nm width is 3?dB/2.3?μm. Accordingly, higher pump power and gain would be necessary on the narrower device to create similar throughput. Considering existing nanoscale propagation methods, which commonly use negative dielectric materials, our waveguide shows an improved loss characteristic with comparable or smaller dimensions. Thus, the application of QDs to nanophotonic waveguiding represents a promising path towards ultra-high density photonic integrated circuits.     

Finite size effect on hydrogen gas sensing performance in single Pd nanowires

We present the hydrogen sensing performance of individual Pd nanowires grown by electrodeposition into nanochannels of anodized aluminum oxide (AAO) templates investigated as a function of the nanowire diameter. Four-terminal devices based on individual Pd nanowires were found to successfully detect hydrogen gas (H(2)). Our experimental results show that the H(2) sensing sensitivity increases and the response time decreases with decreasing diameter of Pd nanowires with d = 400, 200, 80 and 20?nm, due to the high surface-to-volume ratio and short diffusion paths, respectively. This is in qualitatively good agreement with simulated results obtained from a theoretical model based on a combination of the rate equation and diffusion equation.     

Bioconjugated superstructures of CdTe nanowires and nanoparticles: multistep cascade Förster resonance energy transfer and energy channeling

Nanoparticle/nanowire assemblies with a degree of radial organization were prepared around luminescent semiconducting CdTe nanowires using bioconjugation with streptavidin and D-biotin linkers. Red-emitting nanowires (6.62 +/- 1.55 nm diameter, 512 +/- 119 nm length) and green-emitting nanoparticles (3.2 +/- 0.7 nm diameter) were surface-modified with biotin, while orange-emitting nanoparticles (4.1 +/- 1.2 nm diameter) were decorated with streptavidin. CdTe nanocrystals produced two fuzzy layers around the nanowires in which the diameter of CdTe nanoparticles decreased with the distance from the nanowire axis. F?rster resonance energy transfer (FRET) from the outside layer of nanoparticles to the central nanowire was observed for nanowires conjugated with 4.1 nm CdTe. Addition of 3.2 nm CdTe resulted in a red-orange-green optical progression with band gaps of CdTe decreasing toward the axis of the superstructure. In this case, 4-fold luminescence enhancement of the nanowire luminescence was observed and was attributed to multistep FRET. This observation indicated the accumulation of photogenerated excitons in the cascade terminal. A simple model of multiconjugated superstructure with cascade energy transfer is developed and used to describe and understand the experimental data. The experimental data and theoretical model suggest the possibility of utilization of the prepared superstructures with radial symmetry in several classes of optoelectronic devices including nanomaterials for energy collection. They can also be a convenient model object for the investigation of methods of energy funneling in nanoscale assemblies.     

Reflectivity spectra of NaNO<Subscript>2</Subscript>-infiltrated synthetic opal

We analyze the reflectivity spectra of synthetic opal crystals (sphere diameters of 200, 240, and 290 nm) infiltrated with ferroelectric sodium nitrite (NaNO₂) after preannealing in air or argon. The reflectivity spectra of the opal samples show a band corresponding to the photonic band gap. Its position and shape strongly depend on the sphere diameter and annealing conditions. The spectra of the sodium-nitrite-infiltrated opal samples preannealed in air differ markedly from those of the samples preannealed in argon. The experimentally determined band gap position as a function of sphere diameter is compared to calculation results. Our data demonstrate that argon preannealing ensures effective infiltration of molten sodium nitrite into the pores of synthetic opal.     

Fiber designs with significantly reduced nonlinearity for very long distance transmission

A class of low-nonlinearity dispersion-shifted fibers based on depressed-core multistep index profiles is investigated. A systematic approach for designing these fibers in which a reference W-index profile is used to initiate the design ispresented. Transmission properties, including effective area, mode-field diameter, dispersion, dispersion slope, and cutoff wavelength, are evaluated for several design examples. The effects of varying fiber dimensions and indices on effective area and mode-field diameter are assessed. It is shown that there is a trade-off between these two properties and, generally, larger effective areas are associated with larger mode-field diameters. Dispersion-shifted single-mode fiber designs with effective areas of from 78 to 210 mum(2) and the corresponding mode-field diameter of from 8.94 to 14.94 mum, dispersion less than 0.07 ps/nm km, and dispersion slope of approximately 0.05 ps/nm(2) km are presented.     

Variations of the Effective Magnetostriction with Annealing Conditions for Nanocrystalline Magnetic Alloys

A theoretical model of effective magnetostriction for nanocrystalline magnetic alloys was proposed considering the effects of impurity phases. The dependence of effective magnetostriction on the volume fraction of different components determined by annealing conditions was analyzed. Moreover, the phenomenon that the effective magnetostriction has a valley value which can??t be explained previously was interpreted perfectly by using this model. The results of theoretical analysis are in excellent agreement with existing experimental data.     

Sizing nanoparticles by time-of-flight spectrometry

Time-of-flight (TOF) spectrometry utilizing scattered light detection of particles passing discrete detection locations in an accelerating gas jet is used for rapid measurement of highly-resolved size distributions of dry powder, gas- or liquid-borne particles having diameter > 300 nm. Extension of the measurement range of this technique to include nanoparticles (< 100 nm diameter) is investigated using a theoretical model to predict the reduction in minimum, scattered-light-detectable, particle size, a limit which restricts present TOF instruments to particles above 300 nm diameter. A single set of preliminary measured data are compared with the theoretical-model predictions. Together, these results indicate that a TOF spectrometer can provide rapid, highly-resolved particle size distribution measurement of nanoparticle powders and suspensions down to ≈ 50 nm diameter.     

纳米晶TiO2的形貌对其荧光性能影响的研究

利用溶剂热合成法成功制备出了形貌为准球形、棒形、球棒混合型和菱形,粒径在50nm以下、尺寸均一的锐钛矿型TiO2纳米晶,对合成出的纳米晶TiO2用荧光光谱,紫外/可见光吸收光谱进行光学性能表征,结果表明,TiO2纳米晶在330 nm的激发光下,分别在345 nm、363nm、380nm和402 nm处存在4个发光峰位。在实验中,首次发现和证实了理论计算出的锐钛矿型TiO2纳米晶的两种直接跃迁发光,分别对应为X（1b）→X（2b）（345 nm）和X（1b）→X（1a）（363 nm）,主要因为油酸改变了TiO2纳米晶的{001}晶面族晶面的表面态。TiO2纳米晶的紫外吸收峰位于229 nm,且与其形貌无关;禁带宽度的计算值接近其理论值3.2 eV。     

A multi-scale framework for effective elastic properties of porous materials

This paper presents a statistical micromechanics-based multi-scale material modeling framework to predict the effective elastic moduli of porous materials. The present formulation differs from most of the existing theoretical models in that the interaction effects among the pores are directly accounted for by considering the pair-wise interaction and the statistical information of pore distribution is included by applying the ensemble volume averaging process. The theory of average fields is employed to derive the stress and strain concentration factor tensors that relate the local average fields to the global averages. Closed-form and analytical explicit expressions for the effective elastic moduli of porous materials are obtained in terms of the mechanical properties of the matrix material and porosity. The dependence of effective elastic properties on the porosity is investigated. Comparison of our theoretical prediction with the results of the published experimental data and other existing theoretical models is performed to illustrate the predictive capability of the proposed framework for porous materials.     

Nanosize and vitality: TiO2 nanotube diameter directs cell fate

We generated, on titanium surfaces, self-assembled layers of vertically oriented TiO2 nanotubes with defined diameters between 15 and 100 nm and show that adhesion, spreading, growth, and differentiation of mesenchymal stem cells are critically dependent on the tube diameter. A spacing less than 30 nm with a maximum at 15 nm provided an effective length scale for accelerated integrin clustering/focal contact formation and strongly enhances cellular activities compared to smooth TiO2 surfaces. Cell adhesion and spreading were severely impaired on nanotube layers with a tube diameter larger than 50 nm, resulting in dramatically reduced cellular activity and a high extent of programmed cell death. Thus, on a TiO2 nanotube surface, a lateral spacing geometry with openings of 30-50 nm represents a critical borderline for cell fate.     

超纯水中杂质对光子相关谱法测量纳米颗粒粒径的影响

为了研究光子相关光谱法测粒技术中,溶液介质中的杂质对纳米颗粒有效直径和分散度测量准确性的影响,获取并分析了3种超纯水蒸发后残留杂质的显微图像,讨论了上述3种水对标称直径为960、340、90、60nm的颗粒物测定结果的影响。研究结果表明,水中杂质会增大光子平均计数率,以及增大100nm以下单分散颗粒的有效直径和分散度的测量结果;对于自身分散度较大的颗粒,水质对分散度测量结果的影响不明显。     

Lattice Boltzmann modeling of the effective elastic property for ZrB2-based composites

This paper introduces a numerical method for predicting the effective elastic module of ZrB₂-based composites, which includes a random generation-growth method for generating microstructures of ZrB₂-based composites based on existing statistical macroscopic geometrical characteristics and a highly efficient lattice Boltzmann algorithm for solving the governing equations on the multiphase microstructures. The effective elastic module of random ZrB₂-based materials is analyzed with different parameters by the present method, including effects of component size and material anisotropy. The simulation results indicate this numerical method’s effectiveness and robustness by comparing the predictions with experimental data and other theoretical models.     

Lattice Boltzmann modeling of the effective elastic property for ZrB<Subscript>2</Subscript>-based composites

This paper introduces a numerical method for predicting the effective elastic module of ZrB₂-based composites, which includes a random generation-growth method for generating microstructures of ZrB₂-based composites based on existing statistical macroscopic geometrical characteristics and a highly efficient lattice Boltzmann algorithm for solving the governing equations on the multiphase microstructures. The effective elastic module of random ZrB₂-based materials is analyzed with different parameters by the present method, including effects of component size and material anisotropy. The simulation results indicate this numerical method’s effectiveness and robustness by comparing the predictions with experimental data and other theoretical models.     

Solid solubility limit in alloying nanoparticles

In order to gain a better thermodynamic understanding of the phase diagram of alloying nanoparticles, we establish a size-dependent solid solubility model of binary metallic systems to elucidate the anomalous solid solubility in nanometre-sized alloying particles. It is found that a diameter of 20?nm seems a threshold value of the size of alloying nanoparticles for the unusual solid solubility, i.e.?the solubility is greatly promoted with decreasing grain size when the size of alloying nanoparticles is less than 20?nm. Taking the Pb-Sn system as an example, we show that the theoretical predictions are consistent with experimental data.     

Patterning of periodic nano-cavities on PEDOT-PSS using nanosphere-assisted near-field optical enhancement and laser interference lithography

A simple approach for creating periodic nano-cavities and periodic stripes of nano-cavity arrays on poly (3,4-ethylene dioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) thin films using a combination of optical near-field enhancement through self-assembled silica nanospheres and laser interference lithography is presented. Monolayers of close-packed silica nanospheres (800, 600, and 430 nm in diameter) are self-assembled on 2 μm thick PEDOT-PSS electropolymerized films and are subsequently irradiated with 10 ns pulses of 355 nm wavelength laser light. Over areas spanning 2 cm(2), circular nano-cavities with central holes of size 50-200 nm and surrounding craters of size 100-400 nm are formed in the PEDOT-PSS films directly underneath the nanospheres due to strong enhancement (11-18 fold) of the incident light in the near-field, which is confirmed through Mie scattering theory. Predictions from theoretical simulations examining the combined effects of near-field enhancement and interference are in good agreement with the experimental results. The results illustrate the versatility of the described technique for creating nano-cavity arrays or nano-pores in PEDOT-PSS over large areas with designed periodicity and hole size.     

Photoluminescence efficiency and size distribution of self assembled ge dots on porous TiO2

For Ge nanodots approximately 20 nm in diameter grown by annealing a thin amorphous Ge layer deposited by molecular beam epitaxy on a mesoporous TiO2 layer on Si(001), photoluminescence (PL) was observed as a wide near-infrared band near 800 meV. Using a tight binding theoretical model, the energy-dependent PL spectrum was transformed into a dependence on dot size. The average dot size determined the peak energy of the PL band and its shape depended on the size distribution, including bandgap enlargement due to quantum confinement. Combining the dot sample PL with an established dependence of emission efficiency on dot diameter, it was possible to derive a dot size distribution and compare it with results obtained independently from atomic force microscopy.     

Optical absorption characteristic of highly ordered and dense two-dimensional array of silicon nanodiscs

We created a two-dimensional array of sub-10 nm Si-nanodiscs (Si-NDs), i.e. a 2D array of Si-NDs, with a highly ordered arrangement and dense NDs by using a new top-down technique comprising advanced damage-free neutral-beam (NB) etching and a bio-template (iron oxide core) as a uniform sub-10 nm etching mask. The bandgap energy (E(g)) of the fabricated 2D array of Si-NDs can be simply controlled from 2.2 to 1.3 eV by changing the ND thickness from 2 to 12 nm. Due to weak quantum confinement existing in the diameter direction resulting from the sub-10 nm Si-ND diameter, even though the thickness of the Si-ND is much larger than the Bohr radius of Si, E(g) is still larger than the 1.1 eV E(g) of bulk Si. Si-ND not only has wide controllable E(g) but also a high absorption coefficient due to quantum confinement in three dimensions. This new technique is a promising candidate for developing new nanostructures and could be integrated into the fabrication of nanoelectronic devices.     

Nanoscale ferroelectric information storage based on scanning nonlinear dielectric microscopy

An investigation of ultrahigh-density ferroelectric data storage based on scanning nonlinear dielectric microscopy (SNDM) is described. For the purpose of obtaining fundamental knowledge on high-density ferroelectric data storage, several experiments on nanodomain formation in a lithium tantalate (LiTaO3) single crystal were conducted. Through domain engineering, a domain dot array with an areal density of 1.5 Tbit/inch2 was formed on congruent LiTaO3 (CLT). Sub-nanosecond (500 psec) domain switching speed also has been achieved. Next, actual information storage is demonstrated at a density of 1 Tbit/inch2. Finally, it is described that application of a very small dc offset voltage is very effective in accelerating the domain switching speed and in stabilizing the reversed nano-domain dots. Applying this offset application technique, we formed a smallest artificial nano-domain single dot of 5.1 nm in diameter and artificial nano-domain dot-array with a memory density of 10.1 Tbit/inch2 and a bit spacing of 8.0 nm, representing the highest memory density for rewritable data storage reported to date.     

Determination of Young’s modulus in polyester-Al2O3 and epoxy-Al2O3 nanocomposites using the Digital Image Correlation method

Young’s modulus of nano-composite systems composed of unsaturated polyester and epoxy resins with alumina nanoparticles of different sizes has been experimentally estimated. The nanoparticles used were spherical alpha-Al₂O₃ having 30-40 and 200 nm in diameter. Young’s modulus was estimated using an inverse problem that is solved by means of the classical Levenberg-Marquardt technique. A cantilever beam under bending was used in the experiments and the experimental procedure was performed using the Digital Image Correlation method, which is a well-established optical-numerical method for estimating full-field displacement. Experimental results indicate that Young’s modulus increases with increasing nanoparticle volume fraction. Finally, the estimated Young’s moduli were compared with classical theoretical models, showing that the experimental results are in agreement with literature data.     

Two-colour strong-field ionization and dissociation of H2 using 780 and 390 nm femtosecond pulses

Abstract

We report the observation of a substantial enhancement in H₂ ⁺ and H⁺ ion production when the two-colour (780 and 390 nm) pulses are superposed. A strong dependence of ion yield on the relative directions of the polarization of the two fields is observed. The experimental results are discussed and compared with existing theoretical calculations.