Ultrafast nonlinear optical response of a single gold nanorod near its surface plasmon resonance

The ultrafast optical nonlinearity of an optically characterized single gold nanorod is investigated around its surface plasmon resonance, by combining a far-field spatial modulation technique with a high sensitivity pump-probe setup. The spectrally and temporally dependent response is quantitatively interpreted in terms of the bulklike optical nonlinearity enhanced by the plasmonic effect. The plasmon resonance dynamics is shown to be mostly governed by nonequilibrium electron and phonon processes. Their contributions to the nonlinear optical response of a single metal nano-object are elucidated, and the latter is connected to the nonlinearities of ensembles.     

Tens of thousands‐fold upconversion luminescence enhancement induced by a single gold nanorod

Upconversion nanoparticles (UCNPs) have gained increasing attention for their wide applications in bioimaging, displays and photovoltaics. However, low efficiency has been an ongoing challenge for further developments. In this work, it is proposed that the ultrasmall size of UCNPs is essential for achieving large enhancement factors and experimentally demonstrated with 4‐nm UCNPs. A strategy of plasmonic dual resonance is proposed in which two distinct localized surface plasmon resonance (LSPR) peaks of gold nanorods (GNRs) were designed to perfectly match both the excitation and emission light wavelength of UCNPs. Combining the excitation enhancement and Purcell effect, a huge enhancement factor of tens of thousands‐fold is stochastically demonstrated for single UCNPs in solution. The largest overall enhancement region is close to the end of a GNR but not in its central part. The excitation enhancement (up to three orders of magnitude) and the emission enhancement (larger than one order of magnitude) induced by the Purcell effect are experimentally demonstrated separately. This study provides insight into how to achieve a very large upconversion enhancement factor with surface plasmons and will catalyze development of UCNPs’ extensive applications.

     

Spectral degeneracy breaking of the plasmon resonance of single metal nanoparticles by nanoscale near-field photopolymerization

We report on controlled nanoscale photopolymerization triggered by enhanced near fields of silver nanoparticles excited close to their dipolar plasmon resonance. By anisotropic polymerization, symmetry of the refractive index of the surrounding medium was broken: C infinity v symmetry turned to C2v symmetry. This allowed for spectral degeneracy breaking in particles plasmon resonance whose apparent peak became continuously tunable with the incident polarization. From the spectral peak, we deduced the refractive-index ellipsoid fabricated around the particles. In addition to this control of optical properties of metal nanoparticles, this method opens new routes for nanoscale photochemistry and provides a new way of quantification of the magnitude of near fields of localized surface plasmons.     

Core-excited resonance enhancement in the two-photon complete fragmentation of helium

A time-dependent close-coupling method is used to calculate, for the first time, fully differential cross sections for the complete fragmentation of helium by two photons. Surprising differences in the magnitude of the total-integral cross sections are found in comparisons with other calculations. These differences are found to be due to a core-excited resonance enhancement of the two-photon process for both single and double ionization. These calculations provide theoretical support for ground-breaking measurements expected to be obtained from free-electron laser experiments in the near future.     

Room-temperature strong coupling between dipolar plasmon resonance in single gold nanorod and two-dimensional excitons in monolayer WSe_2

All-solid-state strong coupling systems with large vacuum Rabi splitting energy have great potential applications in future quantum information technologies, such as quantum manipulations, quantum information storage and processing,and ultrafast optical switches. Monolayer transition metal dichalcogenides(TMDs) have recently been explored as excellent candidates for the observation of solid-state strong coupling phenomena. In this work, from both experimental and theoretical aspects, we explored the strong coupling effect by integrating an individual plasmonic gold nanorod into the monolayer tungsten diselenide(WSe_2). Evident anti-crossing behavior was observed from the coupled energy diagram at room temperature; a Rabi splitting energy of 98 meV was extracted.     

Theoretical investigation of plasmonic enhancement of silica-coated gold nanorod on molecular fluorescence

The plasmonic enhancement or quenching effects of a silica-coated gold nanorod (GNR@SiO₂) on the fluorescence of a molecule doped in the silica layer are studied using the multiple multipole method. The enhancement factors (EF) of a GNR with a typical aspect ratio of 3 coated by a 13 nm silica layer upon the fluorescence of a molecule embedded at different locations with various orientations irradiated by a plane wave are analyzed, particularly at the longitudinal surface plasmon resonance (SPR) of GNR. The numerical results show that the EF of a GNR@SiO₂ on the fluorescence is sensitive to the molecular location and orientation. Furthermore, an effective EF (EEF), which is an average of EF over all possible orientations at a specific location, is calculated. According to EEF, the proximities of the end-caps of a GNR are strong enhancing zones. In contrast, the waist area is the weak zone. Moreover, a bigger GNR (a=10 nm) possesses a higher EEF than a smaller one (a=7 nm) for the same aspect ratio and the molecular relative location. Hence, a strong enhancement on the fluorescence is obtained using bigger GNR, if the molecule is near the end-cup and the dipole orientation is along the long axis. On the contrary, the consequence could be quenching, if the molecule is near the waist of a small GNR. The Stokes shift of fluorescence can also affect the EF, except the excitation wavelength.     

Preparation of a novel ferrofluidic photoresist for two-photon photopolymerization technique

We present a novel route for the preparation of ferrofluidic photoresist compatible with two-photon photopolymerization (TPP). To get a homogeneous ferrofluidic photoresit, the compatibility of photoresist and magnetic materials has been improved. Monodispersed Fe₃O₄ nanoparticles synthesized via thermal decomposition of iron precursor were stabilized by 6-(methacryloyloxy) hexanoic acid (a kind of acrylate-based monomer). A ferrofluidic photoresist was prepared by doping the modified Fe₃O₄ nanoparticles in acrylate-based resin. In this way, the dispersibility of nanoparticles in photoresist was enhanced significantly. As a representative example, a precise magnetic micron-sized spring was created. In the test of the magnetic response, the sensitivity of magnetic microspring was improved remarkably due to the optimization of the ferrofluidic photoresist. When the intensity of external magnetic field reached a value of 1500 Gs, the deformation rate of the microspring would get to 2.25, indicating the compatibility of the ferrofluidic photoresist in microfabrication.     

Temperature measurement in a single patterned gold nanorod cluster using laser-induced fluorescence

The ability of photon to thermal conversion on wet chemically synthesized gold nanorods (GNRs) is a unique advantage to explore specific local heating. In this study, we demonstrate the thermal response of a single patterned GNR cluster in aqueous solution under near infrared irradiation. To improve the properties of GNRs, such as solubility, we describe the initiated chemical vapor deposition method by the interaction of poly(2-hydroxyethyl methacrylate). A laser-induced fluorescence technique was utilized as a potential and non-intrusive way to measure the temperature field in and around the highly localized GNR cluster. The correlation between fluorescence intensities and temperature was investigated with two dyes by controlling the near infrared laser intensities to heat up the GNRs. Using this technique, we observed highly localized temperature rise in the GNR cluster and heat transfer to the surrounding medium during the laser irradiation.     

Plasmonic effects in near-field optical transmission enhancement through a single bowtie-shaped aperture

In this paper, the enhanced optical transmission through a special type of aperture of a bowtie shape is investigated through near-field imaging and finite-difference numerical analysis. Under linear polarizations in two orthogonal directions, the optical near fields of the bowtie aperture and comparable square and rectangular apertures made in gold and chromium thin films are measured and compared. The bowtie aperture is able to provide a nanometer-sized optical spot when the incident light is polarized across the bowtie gap and delivers a considerable amount of light. Localized surface plasmons are clearly observed in the near-field images for both bowtie and rectangular apertures in gold, but invisible in chromium. Finite-difference time-domain calculations reveal that, depending on the polarization of the incident light, the unique optical properties of the bowtie aperture are a result of either the optical waveguide and the coupled surface plasmon polariton modes existing in the bowtie gap or the coupling between the two open arms of the bowtie aperture. PACS 81.07.-b; 07.79.Fc; 71.36.+c; 78.66.Bz; 42.79.Gn; 42.79.Vb     

Polymerization of butadiene on nanoparticles’ surfaces and formation of metal/polymer nanocomposites

In this paper, we demonstrate that laser vaporization of metals in the presence of a small concentration of butadiene vapor leads to the polymerization of butadiene and incorporation of the metal nanoparticles within the polymer matrix. The metal nanocomposites are characterized by electron microscopy, X-ray diffraction and EDX. The results from high pressure mass spectrometry indicate that multiple additions of butadiene molecules on the metal cations Fe⁺, Ni⁺ and Pt⁺, generated by laser vaporization, take place at room temperature thus providing an efficient means of initiating further polymerization reactions. The Pt⁺ reactions show extensive fragmentations and elimination steps generating hydrocarbon ions. The laser vaporization/polymerization method provides the ability to encapsulate several different metals or metal oxides which undoubtedly will play a significant role in tuning the various properties of the polymer composites.     

Effect of surface modification on dielectric and magnetic properties of metal powder/polymer nanocomposites

Metal nanopowder (Co and Fe)/polymer composites, both with and without surface modification by behenic acid, were fabricated and their dielectric and magnetic properties were measured at 1 GHz to study the effect of surface modification on the electromagnetic properties. The relative permittivity and the real part of the permeability of the composites with surface modified powders were higher than those with unmodified powders. Related dielectric losses remained at almost the same level, but magnetic losses were somewhat increased. The increase of relative permittivity could result from the increased volume fraction of interphase with a slightly higher relative permittivity at the particle/polymer interface than that of the bulk polymer. The increase in the real part of the permeability may be caused by suppression of the induced demagnetizing field due to suppression of eddy currents by a better particle distribution and a decrease in effective agglomerate size because of the surface modification.     

有机分子第一超极化率色散效应和双光子共振增强理论研究

采用含时Hartree-Fock和多态求和方法计算了半花菁衍生物生色团的第一超极化率色散效应. 采用二能级模型研究了第一超极化率的双光子共振增强特征. 研究发现，第一激发态对半花菁非线性光学性质起决定性作用，随着入射光波长向短波方向移动，二次谐波产生β(-2ω;ω,ω)不断增大并且在950nm附近出现双光子共振效应，理论结果与实验结果较好相符. 此外，共振条件下的二能级模型需要考虑激发态弛豫效应. 研究结果为实验测量和实际应用提供了理论参考. 关键词： 分子非线性光学 超极化率 色散效应 共振增强     

Tuning the microstructure of pulsed laser deposited polymer–metal nanocomposites

Pulsed laser deposition was used to grow complex polymer–metal nanocomposites consisting of Ag, Cu and Nb clusters embedded in a poly(methyl-methacrylate) (PMMA) matrix. During deposition at room temperature, the size and amount of the metal clusters can be tuned in different ways. First, it is controlled by the number of laser pulses hitting the respective targets. In the case of Ag, a bimodal size distribution of the clusters is observed, induced by total coalescence and secondary nucleation processes. For Cu and Nb, much smaller clusters and higher cluster densities are obtained due to a stronger reactivity with the polymer and thus a lower diffusivity in the PMMA. Additionally, the microstructure of the Ag clusters is affected by the degree of cross linking of the polymer and can be influenced by pre-deposition of nucleation seeds in the form of small Cu clusters. PACS 68.55.-a; 81.15.Fg; 82.35.Np     

Ferromagnetic resonance and relaxation in single crystal metal films

We review the results of oblique-angle ferromagnetic resonance experiments in single-crystal metal films, and compare them with recent calculations for oblique-angle resonance based on isotropic metals and on anisotropic insulators with magnetoelastic interactions.Supported in part by the National Science Foundation.We are please to acknowledge the contributions of our colleagues J. L. Bleustein, T. Kobayashi, and C. Vittoria.     

Plasmonic nanostructures for local field enhancement in the UV region

In this work we describe an ultraviolet subwavelength focusing in plasmonic nanostructures. A system which provides a 20–25 times local field enhancement at a wavelength of 350 nm is proposed. This system represents a metalized V-shaped groove in a surface of a dielectric medium. Subwavelength focusing is achieved by a plasmon wave propagation along the surface of metal film and by the transfer of electromagnetic field through the dielectric medium. The influence of system parameters on a local field enhancement is investigated. A simplified model that allows for determining the geometric parameters of an optimized resonator is proposed.     

Log-pile photonic crystal of CdS–polymer nanocomposites fabricated by combination of two-photon polymerization and in situ synthesis

A log-pile photonic crystal of CdS nanoparticles–polymer nanocomposites was successfully fabricated by a novel method combining the two-photon polymerization technique and in situ synthesis of CdS nanoparticles in a polymer matrix. The photonic band gap of the three-dimensional (3D) log-pile photonic crystal is confirmed and becomes more effective for CdS nanoparticles–polymer nanocomposites than polymer doped with Cd²⁺ ions, because the nanocomposites possess a higher refractive index than the polymer. The proposed concept in the new fabrication method for a 3D microstructure of polymer nanocomposites should be of critical importance in providing a general methodology for functionalization of materials via functional nanocomposites used in the field of laser microstructure fabrication. PACS 42.70.Qs; 78.66.Sq; 81.40.Tv; 82.35.Np; 82.50.Pt     

Plasmonic effects in composite metal nanostructures for sensing applications

We have investigated numerically the plasmonic effect on a two-dimensional periodic array of metallic nanostructures. The unit cell of the array has an Ag nanosphere and nanorod pair formed in a single structure. Three-dimensional finite element method is used for the study on the sensing performance within the optical spectra. The study takes into account the influences of the structural and material parameters, the rotational angle of the metal nanostructure, the number of metal nanostructure per unit cell, and the localized surface plasmon resonances. The proposed nanostructures function as a refractive index sensor with a sensitivity of 400 nm/RIU (RIU is the refractive index unit), showing the characteristics of low transmittance (T?=?3.90%), high absorptance (A?=?94.5%), and near-zero reflectance (R?=?0.15%), could be achieved by a triangular arrangement of nanostructures within a unit cell. We also show how the tailoring of the structural parameters relates to the specific sensing schematics of the sensor.

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Graphical abstract x-y sectional plane of electric field intensity, electric force lines (pink lines), energy flows (green arrows) and surface charge density of type 2, corresponding to the surrounding testing medium of (a) n=1.00 and (b) n=1.33 around the PMNSs.

     

Formation dynamics of bipolaron in a metal/polymer/metal structure

Charge injection process from metal electrode to a nondegenerate polymer in a metal/ polymer/ metal structure has been investigated by using a nonadiabatic dynamic method. We demonstrate that the dynamical formation of a bipolaron sensitively depends on the strength of applied electric field, the work function of metal electrode, and the contact between the polymer and the electrode. For a given bias applied to one of the electrode (V₀) and coupling between the electrode and the polymer (t^′), such as V₀=0.79 eV and t^′=1 eV, the charge injection process depending on the electric field can be divided into the following three cases: (1) in the absence of the electric field, only one electron tunnels into the polymer to form a polaron near the middle of the polymer chain; (2) at low electric fields, two electrons transfer into the polymer chain to form a bipolaron; (3) at higher electric fields, bipolaron can not be formed in the polymer chain, electrons are transferred from the left electrode to right electrode through the polymer one by one accompanying with small irregular lattice deformations.