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

The modulation of photonic cavities by surface acoustic waves opens new possibilities for the control of photons and electrons in semiconductor nanostructures. We show that the SAW modulation combined with the strong optical fields in the cavity leads to the formation of a dynamic optical superlattice with a folded dispersion and well‐defined gaps. In addition, it allows for the optical detection of ambipolar transport of photogenerated carriers up to room temperature.

Manipulation of photons and electrons in photonic structures using surface acoustic waves

An efficient photon detector based on the ambipolar transport of photogenerated electrons and holes by surface acoustic waves (SAWs) has been investigated. In the experiments, the photogenerated electrons and holes are separated and transported in a GaAs channel and electrically detected by a lateral p–i–n junction. We show that the acoustic transport efficiency improves by using biased metallic guides along the SAW beam to create independent transport channels for electrons and holes. By optimizing the photon absorption efficiency and the amplitude of the SAW field, we demonstrated overall transport efficiencies above 85% for transport lengths exceeding 200 μ m .

Efficient photon detectors using surface acoustic waves

Phonons, the quanta of vibrations, determine equilibrium and dynamical properties of matter, their material phase and thermal and electronic transport. GHz coherent phonons can interact with and act as interconnects of a wide range of quantum systems. Harnessing and tailoring their coupling to opto-electronic excitations thus becomes a challenge for engineered materials relevant for quantum technologies. With this perspective we introduce polaromechanical metamaterials, two-dimensional arrays of $\mu$m-size zero-dimensional traps confining light-matter polariton fluids and GHz phonons. These metamaterials integrate strongly interacting phonon-polariton oscillators at the lattice sites with inter-site coupling mediated by optomechanical interactions, with remarkable consequences for the lattice dynamics. When locally perturbed by optical excitation, they respond locking the energy detuning between neighbor sites at integer multiples of the phonon quantum, evidencing synchronization blockade and a collective behaviour of the polariton and phonon fields. These results open the path for the coherent control of quantum light fluids with hypersound in a scalable platform.

Metamaterials of Fluids of Light and Sound.

Frequency gaps for zone-folded acoustic phonons in a-Si:H/a-SiNx:H superlattices were investigated by Raman and phonon transmission spectroscopy. Raman spectra in the forward scattering geometry were used to determine the magnitude and the symmetry of the modes delimiting the gaps at the center of the superlattice mini-Brillouin zone. The phonon transmission spectra show gaps associated with stop bands for phonon with mixed longitudinal and transverse character in addition to the gaps for pure transverse modes at the zone-center and zone-boundary. The position of the gaps is well explained by a description of the phonons in the elastic continuum limit.

Frequency gaps for zone-folded acoustic phonons in amorphous superlattices

On-chip quantum information processing requires controllable quantum light sources that can be operated on-demand at high-speeds and with the possibility of in-situ control of the photon emission wavelength and its optical polarization properties. Here, we report on the dynamic control of the optical emission from core-shell GaN/InGaN nanowire (NW) heterostructures using radio frequency surface acoustic waves (SAWs). The SAWs are excited on the surface of a piezoelectric lithium niobate crystal equipped with a SAW delay line onto which the NWs were mechanically transferred. Luminescent quantum dot (QD)-like exciton localization centers induced by compositional fluctuations within the InGaN nanoshell were identified using stroboscopic micro-photoluminescence (micro-PL) spectroscopy. They exhibit narrow and almost fully linearly polarized emission lines in the micro-PL spectra and a pronounced anti-bunching signature of single photon emission in the photon correlation experiments. When the nanowire is perturbed by the propagating SAW, the embedded QD is periodically strained and its excitonic transitions are modulated by the acousto-mechanical coupling, giving rise to a spectral fine-tuning within a ~1.5 meV bandwidth at the acoustic frequency of ~330 MHz. This outcome can be further combined with spectral detection filtering for temporal control of the emitted photons. The effect of the SAW piezoelectric field on the QD charge population and on the optical polarization degree is also observed. The advantage of the acousto-optoelectric over other control schemes is that it allows in-situ manipulation of the optical emission properties over a wide frequency range (up to GHz frequencies).

https://iopscience.iop.org/article/10.1088/1742-6596/1092/1/012075/pdf

Paulo V. Santos

Papers

Manipulation of photons and electrons in photonic structures using surface acoustic waves

Efficient photon detectors using surface acoustic waves

Metamaterials of Fluids of Light and Sound.

Frequency gaps for zone-folded acoustic phonons in amorphous superlattices

Surface acoustic wave modulation of single photon emission from GaN/InGaN nanowire quantum dots