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.

/pdf/spintronics-fundamentals-and-applications-2dx38n6phu.pdf

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 investigate the effect of surface acoustic waves on the atomic-like optical emission from defect centers in hexagonal boron nitride layers deposited on the surface of a LiNbO3 substrate. The dynamic strain field of the surface acoustic waves modulates the emission lines resulting in intensity variations as large as 50% and oscillations of the emission energy with an amplitude of almost 1 meV. From a systematic study of the dependence of the modulation on the acoustic wave power, we determine a hydrostatic deformation potential of about 40 meV/% for defect centers in this two-dimensional material. Furthermore, we show that the dynamic piezoelectric field of the acoustic wave could contribute to the stabilization of the optical properties of these emission centers. Our results show that surface acoustic waves are a powerful tool to modulate and control the electronic states of two-dimensional materials.

/pdf/acoustically-modulated-optical-emission-of-hexagonal-boron-3nqrb97nyi.pdf

Acoustically modulated optical emission of hexagonal boron nitride layers

Intensity and temperature dependence of the steady-state light-induced defect density in a-Si:H.

We report here on the influence of surface acoustic waves (SAWs) on the magnetization of a Mn12-acetate single crystal The crystal was mounted on the surface of a piezoelectric LiNbO3 substrate containing an interdigital transducer for the excitation of SAWs The magnetization of the crystal was measured using a rf superconducting quantum interference device with a time resolution of 1μs The piezoelectric material was excited by SAW pulses of different frequencies produced by applying microwave pulses to the transducer Our data show that molecular magnets onto the LiNbO3 surface can be used as very sensitive detectors of the SAW frequency and intensity

Acoustomagnetic pulse experiments in LiNbO3∕Mn12 hybrids

We investigate the modulation of optical phonons in semiconductor crystals by a surface acoustic wave (SAW) propagating on the crystal surface. The SAW fields induce changes on the order of ${10}^{\ensuremath{-}3}$ in the time-averaged Raman peak intensity by optical phonons in Si and GaN crystals. The SAW-induced modifications in the intensity of the Raman lines are dominated by the modulation of the longitudinal optical (LO) phonon energy by the SAW strain field. We show that while the strain field of the excited Rayleigh SAWs changes the LO phonon energy, it does not mix it with the transversal optical modes. In addition to the previous contribution, which is of a local character, the experiments give evidence for a weaker and nonlocal contribution attributed to the spatial variation of the SAW strain field. The latter activates optical modes with large wave vectors and, therefore, lower energies. The experimental results, which are well described by theoretical models for the two contributions, prove that optical phonons can be manipulated by SAWs with $\ensuremath{\mu}\mathrm{m}$ wavelengths.

Optical phonon modulation in semiconductors by surface acoustic waves

We report on the coherent control and transport of indirect exciton (IX) spins in GaAs double quantum well (DQW) nanostructures. The spin dynamics was investigated by optically generating spins using a focused, circularly polarized light spot and by probing their spatial distribution using spatially and polarization resolved photoluminescence spectroscopy. Optically injected exciton spins precess while diffusing over distances exceeding 20 {\mu}m from the excitation spot with a spatial precession frequency that depends on the spin transport direction as well as on the bias applied across the DQW structure. This behavior is attributed to the spin precession in the effective magnetic field induced by the spin-orbit interaction. From the dependence of the spin dynamics on the transport direction, bias and external magnetic fields we directly determined the Dresselhaus and Rashba spin splitting coefficients for the structure. The precession dynamics is essentially independent on the IX density, thus indicating that the long spin lifetimes are not associated with IX collective effects. The latter, together with the negligible contribution of holes to the spin dynamics, are rather attributed to spatial separation of the electron and hole wave functions by the electric field, which reduces the electron-hole exchange interaction. Coherent spin precession over long transport distances as well as the control of the spin vector using electric and magnetic fields open the way for the application of IX spins in the quantum information processing.

Paulo V. Santos

Papers

Acoustically modulated optical emission of hexagonal boron nitride layers

Intensity and temperature dependence of the steady-state light-induced defect density in a-Si:H.

Acoustomagnetic pulse experiments in LiNbO3∕Mn12 hybrids

Optical phonon modulation in semiconductors by surface acoustic waves

Coherent transport and manipulation of spins in indirect-exciton nanostructures