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

Selecting input and output devices to be used in virtual walkthroughs is an important issue as it may have significant impact in usability and comfort. This paper presents a user study meant to compare the usability of two input devices used for walkthroughs in a virtual environment with a Head-Mounted Display. User performance, satisfaction, ease of use and comfort, were compared with two different input devices: a two button mouse and a joystick from a gamepad. Participants also used a desktop to perform the same tasks in order to assess if the participant groups had similar profiles. The results obtained by 45 participants suggest that both input devices have a comparable usability in the used conditions and show that participants generally performed better with the desktop; a discussion of possible causes is presented.

Comparing two input devices for virtual walkthroughs using a Head Mounted Display (HMD)

We analyze fluctuation of the layer thicknesses and its influence on the strain state of (In,Ga)As/(Al,Ga)As microtubes containing quantum well structures. In those structures a curved high-mobility two-dimensional electron gas (HM2DEG) is established. The layer thickness fluctuation studied by atomic force microscopy, x-ray scattering, and spatially resolved cathodoluminescence spectroscopy occurs on two different lateral length scales. On the shorter length scale of about 0.01 $\ensuremath{\mu}$m, we found from atomic force micrographs and the broadening of the satellite maxima in x-ray diffraction curves a very small value of the mean square roughness of 0.1 nm. However, on a longer length scale of about 1.0 $\ensuremath{\mu}$m, step bunching during epitaxial growth resulted in layer thickness inhomogeneities of up to 2 nm. The resulting fluctuation of the strain in the microtubes leads to a local variation of the chemical potential, which results in the fluctuation of the carrier density as well. This leads to a phase cancellation of the Shubnikov\char21{}de Haas oscillations in the curved HM2DEG and a reduction of the single-electron scattering time, while the electron mobility in the structures remains high. The estimated fluctuation of the carrier density agrees well with the energy fluctuation measured in the cathodoluminescence spectra of the free-electron transition of the quantum well.

Inhomogeneous microstrain in cylindrical semiconductor heterostructures and its influence on the adiabatic motion of electrons

We demonstrate a compact tunable photonic modulator driven by surface acoustic waves (SAWs) in the low GHz frequency range. The device follows a well-known Mach-Zehnder interferometer (MZI) structure with three output channels, built upon multi-mode interference (MMI) couplers. The light continuously switches paths between the central and the side channels, avoiding losses and granting a 180◦-dephasing synchronization between them. The modulator was monolithically fabricated on (Al,Ga)As, and can be used as a building block for more complex photonic functionalities. It can also be implemented in other material platforms such as Silicon or (In,Ga)P. Light modulated at multiples of the fundamental acoustic frequency can be accomplished by adjusting the applied acoustic power. An excellent agreement between theory and experiment is achieved.

/pdf/tunable-interferometers-driven-by-coherent-surface-acoustic-5fjh5t51x6.pdf

Tunable Interferometers Driven by Coherent Surface Acoustic Phonons

In this paper, photonic devices driven by surface acoustic waves and operating in the GHz frequency range are presented. The devices were designed and fabricated in (Al,Ga)As technology. In contrast to previously realized modulators, where part of the light transmission is lost due to destructive interference, in the present devices light only switches paths, avoiding losses. One of the devices presents two output channels with 180◦-dephasing synchronization. Odd multiples of the fundamental driving frequency are enabled by adjusting the applied acoustic power. A second and more complex photonic integrated device, based on the acoustic modulation of tunable Arrayed Waveguide Gratings, is also proposed.

Photonic Mach-Zehnder modulators driven by surface acoustic waves in AlGaAs technology

We discuss the formation of a tunable one-dimensional photonic band
gap structure through the modulation of the resonance frequency of
an optical microcavity by a surface acoustic wave (SAW). The
microcavity consists of a λ/2 GaAs layer bounded by
AlAs/GaAs Bragg mirrors. The SAW periodically modulates the optical
thickness of the cavity layer, leading to a light dispersion
relation folded within a mini-Brillouin zone (MBZ) defined by
|k x |≤ π/λ SAW (k x denotes the photon wave vector component along the SAW propagation direction x-with-caret). In reflection and diffraction experiments, we observe photon modes bounding the gaps in the center and at the boundary of the MBZ as well as a renormalization of the optical energies. Furthermore, the width of the energy gaps can be tuned by changing the acoustic power densities. The experimental results are in good agreement with a simple model for the dispersion in the presence of SAWs. We show the application of acoustically tunable microcavities in efficient optical on/off switches and modulators as well as a tunable cavity operating at 1.3μm.

Paulo V. Santos

Papers

Comparing two input devices for virtual walkthroughs using a Head Mounted Display (HMD)

Inhomogeneous microstrain in cylindrical semiconductor heterostructures and its influence on the adiabatic motion of electrons

Tunable Interferometers Driven by Coherent Surface Acoustic Phonons

Photonic Mach-Zehnder modulators driven by surface acoustic waves in AlGaAs technology

Acoustically tunable photonic band gap structures