There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

Semiconductor devices have become indispensable for generating electromagnetic radiation in everyday applications. Visible and infrared diode lasers are at the core of information technology, and at the other end of the spectrum, microwave and radio-frequency emitters enable wireless communications. But the terahertz region (1-10 THz; 1 THz = 10(12) Hz) between these ranges has remained largely underdeveloped, despite the identification of various possible applications--for example, chemical detection, astronomy and medical imaging. Progress in this area has been hampered by the lack of compact, low-consumption, solid-state terahertz sources. Here we report a monolithic terahertz injection laser that is based on interminiband transitions in the conduction band of a semiconductor (GaAs/AlGaAs) heterostructure. The prototype demonstrated emits a single mode at 4.4 THz, and already shows high output powers of more than 2 mW with low threshold current densities of about a few hundred A cm(-2) up to 50 K. These results are very promising for extending the present laser concept to continuous-wave and high-temperature operation, which would lead to implementation in practical photonic systems.

Terahertz semiconductor heterostructure laser

This article reviews recent theoretical and experimental advances in the fundamental understanding and active control of quantum fluids of light in nonlinear optical systems. In the presence of effective photon-photon interactions induced by the optical nonlinearity of the medium, a many-photon system can behave collectively as a quantum fluid with a number of novel features stemming from its intrinsically nonequilibrium nature. A rich variety of recently observed photon hydrodynamical effects is presented, from the superfluid flow around a defect at low speeds, to the appearance of a Mach-Cherenkov cone in a supersonic flow, to the hydrodynamic formation of topological excitations such as quantized vortices and dark solitons at the surface of large impenetrable obstacles. While the review is mostly focused on a specific class of semiconductor systems that have been extensively studied in recent years (planar semiconductor microcavities in the strong light-matter coupling regime having cavity polaritons as elementary excitations), the very concept of quantum fluids of light applies to a broad spectrum of systems, ranging from bulk nonlinear crystals, to atomic clouds embedded in optical fibers and cavities, to photonic crystal cavities, to superconducting quantum circuits based on Josephson junctions. The conclusive part of the article is devoted to a review of the future perspectives in the direction of strongly correlated photon gases and of artificial gauge fields for photons. In particular, several mechanisms to obtain efficient photon blockade are presented, together with their application to the generation of novel quantum phases.

Quantum fluids of light

We used laser interference (LIC), a combination of pulsed laser crystallization and holography, to fabricate polycrystalline silicon lines in an amorphous silicon (a-Si) film. Under appropriate conditions, the grains in the lines reach in-plane dimensions appreciably larger than the thickness of the initial a-Si film (up to 1.7 μm for a 300 nm a-Si film). Atomic force and transmission electron microscopy indicate that these large grains result from the lateral solidification of the silicon lines melted by the laser pulse. The lateral growth is well reproduced by simulations of the LIC dynamics by means of a two-dimensional melting and solidification model.

Lateral Grain Growth during the Laser Interference Crystallization of a-Si

The authors introduce an alternative approach for acousto-optical light control based on the interference of light propagating through several waveguides, each subjected to a periodic refractive index modulation induced by a surface acoustic wave. The feasibility of the concept is demonstrated by the realization of an optical switch for arbitrary time intervals with an on/off contrast ratio of 20.

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Acousto-optical multiple interference switches

We report a reflectance difference spectroscopy (RDS) study of the optical anisotropy of GaAs:(001) surface quantum wells consisting of a thin GaAs layer (3--30 nm thick) embedded between an arsenic reconstructed surface and an AlAs barrier. The RDS spectra display anisotropic contributions from the free surface and from the GaAs/AlAs interface. By comparing RDS spectra for the $c(4\ifmmode\times\else\texttimes\fi{}4)$ and $(2\ifmmode\times\else\texttimes\fi{}4)$ surface reconstructions, we separate these two contributions, and demonstrate that the anisotropy around the ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ transitions comprises a component originating from modifications of bulk states near the surface. The latter is attributed to anisotropic strains induced by the surface reconstruction. The experimental data are well described by a model for the RDS response of the multilayer structures, which also takes into account the blue energy shifts and the changes in oscillator strength of the ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ transitions induced by quantum-well confinement.

Optical anisotropy of (001)-GaAs surface quantum wells

Polycrystalline silicon on glass substrates was grown by a method based on the creation of nucleation sites using laser crystallization of amorphous silicon followed by thermal annealing at temperatures below 600 °C. Annealing induces the crystallization of the material around the seeds, eventually leading to coalescence of adjacent domains before spontaneous nucleation sets in. Micro‐Raman spectroscopy shows that the seeds experience a tensile stress, which causes a radial birefringence in the surrounding amorphous silicon, detected by optical anisotropy measurements. We conjecture that this stress facilitates the crystallization of the material around the seed upon thermal annealing.

Growth of polycrystalline silicon on glass by selective laser‐induced nucleation

The relationship between electronic carriers and hydrogen migration in a-Si:H was investigated by using secondary-ion-mass spectrometry to measure deuterium-diffusion profiles in the intrinsic (i) layer of p-i-n a-Si:H photodiodes. The carrier concentration in the i layer was controlled by varying either the temperature, or the illumination intensity, or the bias applied to the devices. It is demonstrated that hydrogen migration in a-Si:H is controlled by an electronic mechanism, and (i) is enhanced when the carrier population is increased by illumination and (ii) is suppressed when it is reduced below the thermal-equilibrium value by the application of a reverse bias to the diodes. The effect is attributed to the dependence on carrier density of the dissociation rate of hydrogen from Si-H bonds into the diffusion path consisting of interstitial sites. In addition, the migration length in the diffusion path increases under reverse bias. The enhanced migration is associated with a decrease in the effective density of traps for hydrogen in a carrier-depleted layer. The trap density under these conditions is close to the dangling-bond density, suggesting that the migration length is determined by capture into these defects. Possible mechanisms for the interaction between hydrogen migration, carriers, and defects are discussed.

Paulo V. Santos

Papers

Lateral Grain Growth during the Laser Interference Crystallization of a-Si

Acousto-optical multiple interference switches

Optical anisotropy of (001)-GaAs surface quantum wells

Growth of polycrystalline silicon on glass by selective laser‐induced nucleation

Hydrogen migration and electronic carriers in a-Si:H.