Surface plasmons are waves that propagate along the surface of a conductor. By altering the structure of a metal's surface, the properties of surface plasmons--in particular their interaction with light--can be tailored, which offers the potential for developing new types of photonic device. This could lead to miniaturized photonic circuits with length scales that are much smaller than those currently achieved. Surface plasmons are being explored for their potential in subwavelength optics, data storage, light generation, microscopy and bio-photonics.

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Surface plasmon subwavelength optics

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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Spintronics: Fundamentals and applications

We present experimental scattering data at microwave frequencies on a structured metamaterial that exhibits a frequency band where the effective index of refraction (n) is negative. The material consists of a two-dimensional array of repeated unit cells of copper strips and split ring resonators on interlocking strips of standard circuit board material. By measuring the scattering angle of the transmitted beam through a prism fabricated from this material, we determine the effective n, appropriate to Snell's law. These experiments directly confirm the predictions of Maxwell's equations that n is given by the negative square root of epsilon.mu for the frequencies where both the permittivity (epsilon) and the permeability (mu) are negative. Configurations of geometrical optical designs are now possible that could not be realized by positive index materials.

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Experimental Verification of a Negative Index of Refraction

The emerging field of plasmonics has yielded methods for guiding and localizing light at the nanoscale, well below the scale of the wavelength of light in free space. Now plasmonics researchers are turning their attention to photovoltaics, where design approaches based on plasmonics can be used to improve absorption in photovoltaic devices, permitting a considerable reduction in the physical thickness of solar photovoltaic absorber layers, and yielding new options for solar-cell design. In this review, we survey recent advances at the intersection of plasmonics and photovoltaics and offer an outlook on the future of solar cells based on these principles.

Plasmonics for improved photovoltaic devices

Using the freedom of design that metamaterials provide, we show how electromagnetic fields can be redirected at will and propose a design strategy. The conserved fields-electric displacement field D, magnetic induction field B, and Poynting vector B-are all displaced in a consistent manner. A simple illustration is given of the cloaking of a proscribed volume of space to exclude completely all electromagnetic fields. Our work has relevance to exotic lens design and to the cloaking of objects from electromagnetic fields.

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Controlling Electromagnetic Fields

We have measured the temperature dependence of the electron paramagnetic resonance (EPR) of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{MnO}}_{3+\mathrm{\ensuremath{\delta}}}$ for 0.0\ensuremath{\le}x\ensuremath{\le}1 at 9.2 and 35 GHz from 270 to 800 K. We vary \ensuremath{\delta} by oxygen and argon annealing. We observe that the intensity of the EPR line decreases exponentially for T above the ordering temperature ${\mathit{T}}_{\mathit{c}}$, with an activation energy which varies with x and \ensuremath{\delta}. We find that the EPR data below \ensuremath{\sim}600 K cannot be attributed to an individual Mn ion of any valency, i.e., ${\mathrm{Mn}}^{2+}$, ${\mathrm{Mn}}^{3+}$, or ${\mathrm{Mn}}^{4+}$ for all annealing conditions. The theoretical models of these ``colossal'' magnetoresistance compounds has not yet focused on the ground-state spin properties, and an interpretation of these data is therefore still lacking. We suggest that the explanation of these EPR data may provide useful tests of the correct model of the magnetization and magnetotransport properties in these compounds. The maximum EPR intensity per formula unit (for ${\mathrm{La}}_{0.67}$${\mathrm{Ca}}_{0.33}$${\mathrm{MnO}}_{3}$ air annealed, ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}270 K) corresponds to correlated moments with an effective spin of \ensuremath{\sim}30 at T=280 K. \textcopyright{} 1996 The American Physical Society.

Evidence for collective spin dynamics above the ordering temperature in La 1 − x Ca x MnO 3 + δ

We have investigated the effects of particle size and aspect ratio on the hysteresis in controlled arrays of small magnetic particles. The arrays of permalloy particles were fabricated via electron‐beam lithography. Each array consists of ∼ 106 identical, uniformly spaced particles. Hysteresis loops measured with an alternating‐gradient magnetometer for particles ∼5–0.1 μm are presented. We find an increase in the coercive force as the particle width decreases below 0.3 μm due to a change in the switching mechanism from domain‐wall nucleation and wall motion to vortex nucleation and vortex motion. A novel angular dependence of the loops is described in detail. Results from ab initio micromagnetic calculations on isolated rectangular Permalloy particles are compared, where applicable, with the measurements. We find excellent qualitative and, in selected cases, quantitative agreement between the experiments and calculations.

Hysteresis in lithographic arrays of permalloy particles: Experiment and theory (invited)

Measurement of the dynamics of the magnetization reversal in individual single domain ferromagnetic particles.

We present an experimental study of the polarization-dependent scattering of light from homogeneous and multisegment silver, gold, and nickel nanowires. The metallic nanowires are prepared within a polycarbonate membrane template by a combination of electroplating (gold and nickel) and electroless (silver) growth processes. The size range of the nanowire segments is such that surface plasmon resonances are supported, dominating the optical spectra. We characterize the light scattering properties of individual composite nanowires using an optical microscope configured for single particle spectroscopy. Because of the scattering efficiency associated with the plasmon resonance, very narrow (∼30 nm diameter) nanowires can be readily observed under white-light illumination, with the spectral characteristics of each subsection easily distinguishable. Because of their compactness, these simply prepared multiisegment plasmon resonant nanowires are capable of hosting a large number of segment sequences over a comp...

Composite Plasmon Resonant Nanowires

We present numerical simulations and microwave measurements on a loop-wire structure that acts as an effective medium exhibiting a well-defined bulk plasma frequency in the microwave regime, with an effective negative dielectric function below this plasma frequency. The dependence of this plasmonic response on the self-inductance of the constituent wire elements is made explicit. A finite structure, approximately spherical, composed of this inductive medium is studied, and reveals subwavelength surface plasmon resonances below the bulk plasma frequency.

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Sheldon Schultz

Papers

Evidence for collective spin dynamics above the ordering temperature in La 1 − x Ca x MnO 3 + δ

Hysteresis in lithographic arrays of permalloy particles: Experiment and theory (invited)

Measurement of the dynamics of the magnetization reversal in individual single domain ferromagnetic particles.

Composite Plasmon Resonant Nanowires

Loop-wire medium for investigating plasmons at microwave frequencies