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 demonstrated coherent control of a quantum two-level system based on two-electron spin states in a double quantum dot, allowing state preparation, coherent manipulation, and projective readout. These techniques are based on rapid electrical control of the exchange interaction. Separating and later recombining a singlet spin state provided a measurement of the spin dephasing time, T2*, of E10 nanoseconds, limited by hyperfine interactions with the gallium arsenide host nuclei. Rabi oscillations of two-electron spin states were demonstrated, and spin-echo pulse sequences were used to suppress hyperfine-induced dephasing. Using these quantum control techniques, a coherence time for two-electron spin states exceeding 1 microsecond was observed.

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Coherent manipulation of coupled electron spins in semiconductor quantum dots.

The canonical example of a quantum-mechanical two-level system is spin. The simplest picture of spin is a magnetic moment pointing up or down. The full quantum properties of spin become apparent in phenomena such as superpositions of spin states, entanglement among spins, and quantum measurements. Many of these phenomena have been observed in experiments performed on ensembles of particles with spin. Only in recent years have systems been realized in which individual electrons can be trapped and their quantum properties can be studied, thus avoiding unnecessary ensemble averaging. This review describes experiments performed with quantum dots, which are nanometer-scale boxes defined in a semiconductor host material. Quantum dots can hold a precise but tunable number of electron spins starting with 0, 1, 2, etc. Electrical contacts can be made for charge transport measurements and electrostatic gates can be used for controlling the dot potential. This system provides virtually full control over individual electrons. This new, enabling technology is stimulating research on individual spins. This review describes the physics of spins in quantum dots containing one or two electrons, from an experimentalist’s viewpoint. Various methods for extracting spin properties from experiment are presented, restricted exclusively to electrical measurements. Furthermore, experimental techniques are discussed that allow for 1 the rotation of an electron spin into a superposition of up and down, 2 the measurement of the quantum state of an individual spin, and 3 the control of the interaction between two neighboring spins by the Heisenberg exchange interaction. Finally, the physics of the relevant relaxation and dephasing mechanisms is reviewed and experimental results are compared with theories for spin-orbit and hyperfine interactions. All these subjects are directly relevant for the fields of quantum information processing and spintronics with single spins i.e., single spintronics.

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Spins in few-electron quantum dots

High-volume information-processing and communications devices are at present based on semiconductor devices, whereas information-storage devices rely on multilayers of magnetic metals and insulators. Switching within information-processing devices is performed by the controlled motion of small pools of charge, whereas in the magnetic storage devices information storage and retrieval is performed by reorienting magnetic domains (although charge motion is often used for the final stage of readout). Semiconductor spintronics offers a possible direction towards the development of hybrid devices that could perform all three of these operations, logic, communications and storage, within the same materials technology. By taking advantage of spin coherence it also may sidestep some limitations on information manipulation previously thought to be fundamental. This article focuses on advances towards these goals in the past decade, during which experimental progress has been extraordinary.

Challenges for semiconductor spintronics

Nuclear Dynamic Polarization by Optical Electronic Saturation and Optical Pumping in Semiconductors

In a semiconductor, absorption of circularly polarized light (optical pumping) leads to spin-oriented photoelectrons. In this situation, the nuclei of the crystal are dynamically polarized through their hyperfine interaction with the electronic spins. Consequently the electrons experience a hyperfine magnetic field due to the oriented nuclei which may reach several kilogauss. When this large nuclear field is driven obliquely with respect to the direction of the exciting light, the precession of the electronic spins around the nuclear field leads to a decrease of the electronic polarization along the light excitation: it is a nuclear Hanle effect. This work is an experimental and theoretical study of these effects in weak external magnetic fields, of the order of the local field which characterizes the nuclear spin-spin interactions (a few gauss). Large nuclear fields are obtained at 77\ifmmode^\circ\else\textdegree\fi{}K in strongly doped and compensated $p$-type GaAs samples. We present a model which includes the different effects of the hyperfine coupling when there is a nuclear spin temperature among all the nuclei of the sample: Dynamic polarization, nuclear field, but also, existence of an electronic field acting on the nuclei. We show that a small external field is able to drive the large nuclear field acting on the electrons; consequently the electronic polarization is very sensitive to external fields which are too small to have a direct effect on the electronic spin motion. We study experimentally the variation of the electronic polarization with the direction and magnitude of a small external magnetic field, by measuring the circular polarization of the luminescence light. The experimental results are in quantitative agreement with the theoretical predictions. The usual Hanle Lorentzian depolarization curve is strongly modified in low fields and $W$-like singularities appear around zero field. The experimental values of the average electronic and nuclear fields are in reasonable agreement with theoretical evaluations. These nuclear effects may strongly alter the measurement of the Hanle linewidth in standard optical pumping experiments.

Low field electron-nuclear spin coupling in gallium arsenide under optical pumping conditions

We have measured by optical-pumping methods the spin-relaxation time ${T}_{1}$ of photocreated conduction electrons in $p$-type GaAs (${N}_{A}=4\ifmmode\times\else\texttimes\fi{}{10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) as a function of temperature. To analyze our results we present a detailed discussion of the possible relaxation mechanisms in $p$-type semiconductors. The electronic spin relaxation may originate from: (i) the splitting of the conduction band, (ii) the spin-orbit interaction, (iii) the hyperfine interaction with nuclei of the host crystal, and (iv) the exchange interaction between electrons and holes. The spin-relaxation time is given in each case as a function of experimentally attainable parameters and permits one to obtain a numerical result for all usual III-V compounds. It is established that in doped $p$-type GaAs the exchange interaction with the holes is the dominant relaxation mechanism at low temperatures, the other mechanisms being too weak by several orders of magnitude. For higher temperatures ($T\ensuremath{\ge}100$ \ifmmode^\circ\else\textdegree\fi{}K), the relaxation due to the ${k}^{3}$ splitting of the conduction band may become predominant for ${N}_{A}\ensuremath{\lesssim}{10}^{+7}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. The theory is compared with experimental data available in the literature and with our experiments. From our measurements we obtain in GaAs the value of the exchange splitting of $1s$ exciton ${\ensuremath{\Delta}}_{x.1s}\ensuremath{\simeq}0.1$ meV. We also show that the observed spin depolarization at high kinetic energy (200-300 meV) can be explained by the splitting of the conduction band, taking into account the energy relaxation by optical-phonon scattering.

Spin relaxation of photoelectrons in p -type gallium arsenide

Magneto-optical properties of noble-metal-ferromagnetic-metal multilayer thin films have been investigated in the total reflection condition. The experimental and theoretical study of a Au/Co/Au model structure demonstrates that the resonant coupling of the $p$ component of the light electric field with the gold surface plasmons results in an enhancement of the magneto-optical response of the system, both in the reflected light and in the evanescent mode.

Magneto-optical Effects Enhanced by Surface Plasmons in Metallic Multilayer Films.

The magneto-optical properties of noble-metal--ferromagnetic-metal multilayer thin films have been investigated as a function of the incidence angle, including the total reflection range, in the polar, longitudinal, and equatorial geometries, and for different values of the photon energy in the near-infrared and visible spectrum. The experimental and theoretical results are obtained on a Au/Co/Au model system. They demonstrate that the resonant coupling of the p component of the light electric field with the gold surface plasmon, which occurs in the total reflection range, yields a strong enhancement of the magneto-optical response and signal-to-noise ratio of the system for the three magnetization directions. This resonant coupling and the resulting enhancement of the relevant magneto-optical quantities are achieved for any photon energy in the near infrared and visible range simply by tuning the incidence angle. The efficiency of this enhancement effect is shown to increase towards the infrared region of the spectrum following the rise of the quality factor of the surface plasmon resonance.

Georges Lampel

Papers

Nuclear Dynamic Polarization by Optical Electronic Saturation and Optical Pumping in Semiconductors

Low field electron-nuclear spin coupling in gallium arsenide under optical pumping conditions

Spin relaxation of photoelectrons in p -type gallium arsenide

Magneto-optical Effects Enhanced by Surface Plasmons in Metallic Multilayer Films.

Surface-enhanced magneto-optics in metallic multilayer films