Controlling light localization and light-matter interactions with nanoplasmonics

Nanoplasmonics is the emerging research field that studies light-matter interactions mediated by resonant excitations of surface plasmons in metallic nanostructures. It allows the manipulation of the flow of light and its interaction with matter at the nanoscale (10(-9) m). One of the most promising characteristics of plasmonic resonances is that they occur at frequencies corresponding to typical electronic excitations in matter. This leads to the appearance of strong interactions between localized surface plasmons and light emitters (such as molecules, dyes, or quantum dots) placed in the vicinity of metals. Recent advances in nanofabrication and the development of novel concepts in theoretical nanophotonics have opened the way to the design of structures aimed to reduce the lifetime and enhance the decay rate and quantum efficiency of available emitters. In this article, some of the most relevant experimental and theoretical achievements accomplished over the last several years are presented and analyzed.     

Tunable doping of a metal with molecular spins

The mutual interaction of localized magnetic moments and their interplay with itinerant conduction electrons in a solid are central to many phenomena in condensed-matter physics, including magnetic ordering and related many-body phenomena such as the Kondo effect, the Ruderman-Kittel-Kasuya-Yoshida interaction and carrier-induced ferromagnetism in diluted magnetic semiconductors. The strength and relative importance of these spin phenomena are determined by the magnitude and sign of the exchange interaction between the localized magnetic moments and also by the mean distance between them. Detailed studies of such systems require the ability to tune the mean distance between the localized magnetic moments, which is equivalent to being able to control the concentration of magnetic impurities in the host material. Here, we present a method for doping a gold film with localized magnetic moments that involves depositing a monolayer of a metal terpyridine complex onto the film. The metal ions in the complexes can be cobalt or zinc, and the concentration of magnetic impurities in the gold film can be controlled by varying the relative amounts of cobalt complexes (which carry a spin) and zinc complexes (which have zero spin). Kondo and weak localization measurements demonstrate that the magnetic impurity concentration can be systematically varied up to ～800?ppm without any sign of inter-impurity interaction. Moreover, we find no evidence for the unwanted clustering that is often produced when using alternative methods.     

Controlling light transport by using a graded photonic crystal

Light transport in a graded photonic crystal is studied using the finite-difference time-domain technique. The photonic crystal consists of a square lattice of elliptical dielectric rods. Within a frequency window, light can propagate inside the photonic crystal with the beam width nearly unchanged. The propagation direction can be easily manipulated by the structure gradient, which is achieved by gradually varying the orientation of the elliptical rods. The degree of control over the flow of light can be modulated by changing the ellipticity. This provides a promising approach to design of optical devices for spatial-beam routing.     

Long-range interaction of spins in an anisotropic metal with impurities

The influence of a periodic modulation of the maximum Josephson current on the current-voltage characteristic of the Josephson device is studied in a simulation experiment. It is shown that the pair current locks to the frequency of its modulation, which leads to the appearance of constant voltage steps in theI–V characteristic. The special case of square-wave modulation is treated also. A comparison between pair-current modulation and alternating current application is given.Supported by a grant of the Deutsche Forschungsgemeinschaft.     

Marginal behavior of a layered two-dimensional electron gas interacting with localized spins

The self-energy of a layered two-dimensional (2D) electron gas interacting with localized spins has been calculated in the Fermi liquid approach. The (2D) electron-spin interaction gives rise to a spin gap in the energy spectrum, and if we include the plasmonic excitations a marginal behavior can occur for different values of the interlayer distance. The electron-spin interaction gives as the main effect the enhancement in the energy scale of the electronic excitations.     

Refrigeration by adiabatic demagnetization of nuclear spins

A model is presented for a nuclear demagnetization cooling system in which the metallic refrigerant is distributed over a region where the magnetic field varies by a large amount. A method employing quasi-steady-state approximations is used to solve the problem in the framework of finite difference procedures. The predictions from the model are used to establish a number of design parameters in cryostats for studies of superfluid ³He and ultra-low-temperature metals physics.Work supported by the National Science Foundation under grants DMR 76-21370 and DMR76-05181.     

Helical ordering of spins in a superconductor

The coexistence region of a superconducting phase and a helical ordering of localized spins is found within an isotropic model of free electrons and localized spins. Superconducting properties of this new phase are also studied. The experimental data regarding the superconducting-magnetic transition in ternary compounds are discussed.     

Controlling cement hydration with nanoparticles

Nanoparticles have recently become a focus in the development of new accelerators for cement hydration. We have produced nanoparticles of different materials like Al₂O₃, C–S–H-phase, and quartz by different top-down and bottom-up production methods. The effect of these particles on the cement hydration was followed by heat flow calorimetry to evaluate their acceleration potential as a function of the particles material, particles size and their percentage in the cement paste. In this work we will demonstrate which particles have the best potential as accelerators for cement hydration and therefore are most suitable for further investigations. Interestingly, nanoparticles which have a retarding effect on cement hydration were also found.     

利用弹塑性变形控制碰撞与接合

根据汽车碰撞模拟试验系统中台车与吸能器之间的碰撞接合问题，用经典理论分析了两物体相互碰撞接合过程。充分利用弹性材料和塑性材料的特性，设计弹—塑性缓冲结构。采用ANSYS—DYNA3D软件对缓冲结构进行大位移变形的动态分析计算。试验证明采用这种结构，能有效控制碰撞瞬间接击力峰值。     

Stripe correlations of spins and holes in cuprate superconductors

It is argued that the stripe order of spins and holes found in La_2–x Sr _x NiO₄+ is a useful model for spin and charge correlations in La_2–x Sr _x CuO₄. A direct connection with the cuprates is now established by neutron diffraction evidence for stripe order in La_1.6–x Nd_0.4Sr _x CuO₄ withx=0.12. The experimental work has stimulated new theoretical ideas.It is a pleasure to acknowledge my collaborators, especially J. D. Axe, D. J. Buttrey, N. Ichikawa, J. E. Lorenzo, Y. Nakamura, V. Sachan, B. J. Sternlieb, P. Wochner, and S. Uchida. Work at Brookhaven is supported by the Division of Materials Sciences, U.S. Department of Energy, under Contrast No. DE-AC02-76CH00016.     

Fast polarization reversal of proton spins in solid-state targets

The method of polarization reversal of proton spins in solid-state targets, by using the effect of spin superradiance is analysed theoretically. The main aim is to find out the most accurate way of calculating the time of reversal and the final reversal polarization. Three approaches are compared: one, based on the standard Bloch equations; another, using numerical simulations for finite-particle model; and an approach based on effective equations taking into account local spin fluctuations. The latter method is shown to be the most accurate. The minimal reversal time can be of the order of 10⁻⁵ – 10⁻⁴ s. The maximal final polarization can reach 80–90%, but can never be 100% complete. The proposed method only works from negative to positive polarization.     

Diagonalization of an exchange Hamiltonian for 163He spins

An exact investigation of a finite-size quantum spin-1/2 model of bcc³He, consisting of two interpenetrating cubes with periodic boundary conditions, is performed with an exchange Hamiltonian incorporating two-, three-, and planar four-spin exchange. Although there is qualitative agreement with the mean field phase diagram, quantum effects are much larger (up to 100% in ground state energy) than with pure Heisenberg exchange. We comment on the comparison of theory and experiment in light of the present results.