Showing papers by "Paulo V. Santos published in 2014"

High-frequency acoustic charge transport in GaAs nanowires.

Abstract: The oscillating piezoelectric fields accompanying surface acoustic waves are able to transport charge carriers in semiconductor heterostructures. Here, we demonstrate high-frequency (above 1 GHz) acoustic charge transport in GaAs-based nanowires deposited on a piezoelectric substrate. The short wavelength of the acoustic modulation, smaller than the length of the nanowire, allows the trapping of photo-generated electrons and holes at the spatially separated energy minima and maxima of conduction and valence bands, respectively, and their transport along the nanowire with a well defined acoustic velocity towards indium-doped recombination centers.

Effects of spin-dependent interactions on polarization of bright polariton solitons.

Abstract: We report on the spin properties of bright polariton solitons supported by an external pump to compensate losses. We observe robust circularly polarized solitons when a circularly polarized pump is applied, a result attributed to phase synchronization between nondegenerate TE and TM polarized polariton modes at high momenta. For the case of a linearly polarized pump, either σ+ or σ− circularly polarized bright solitons can be switched on in a controlled way by a σ+ or σ− writing beam, respectively. This feature arises directly from the widely differing interaction strengths between co- and cross-circularly polarized polaritons. In the case of orthogonally linearly polarized pump and writing beams, the soliton emission on average is found to be unpolarized, suggesting strong spatial evolution of the soliton polarization. The observed results are in agreement with theory, which predicts stable circularly polarized solitons and unstable linearly polarized solitons.

Scalable interconnections for remote indirect exciton systems based on acoustic transport

Abstract: Excitons, quasiparticles consisting of electron-hole pairs bound by the Coulomb interaction, are a potential medium for the processing of photonic information in the solid state. Information processing via excitons requires efficient techniques for the transport and manipulation of these uncharged particles. We have carried out a detailed investigation of the transport of excitons in GaAs quantum wells by surface acoustic waves. Based on these results, we introduce here a concept for the interconnection of multiple remote exciton systems based on the long-range transport of dipolar excitons by a network of configurable interconnects driven by acoustic wave beams. By combining this network with electrostatic gates, we demonstrate an integrated exciton multiplexer capable of interconnecting, gating, and routing exciton systems separated by millimeter distances. The multiplexer provides a scalable platform for the manipulation of exciton fluids with potential applications in information processing.

Dynamics of indirect exciton transport by moving acoustic fields

Abstract: We report on the modulation of indirect excitons (IXs) as well as their transport by moving periodic potentials produced by surface acoustic waves (SAWs). The potential modulation induced by the SAW strain modifies both the band gap and the electrostatic field in the quantum wells confining the IXs, leading to changes in their energy. In addition, this potential captures and transports IXs over several hundreds of μm. While the IX packets keep to a great extent their spatial shape during transport by the moving potential, the effective transport velocity is lower than the SAW group velocity and increases with the SAW amplitude. This behavior is attributed to the capture of IXs by traps along the transport path, thereby increasing the IX transit time. The experimental results are well-reproduced by an analytical model for the interaction between trapping centers and IXs during transport.

Spin transport and spin manipulation in GaAs (110) and (111) quantum wells

Abstract: Spin dephasing via the spin–orbit interaction is a major mechanism limiting the electron spin lifetime in zincblende III–V quantum wells (QWs). QWs grown along the non-conventional crystallographic directions [111] and [110] offer new interesting perspectives for the control of spin–orbit (SO) related spin dephasing mechanisms due to the special symmetries of the SO fields in these structures. In this contribution, we show that the combination of such special symmetries with the transport of carriers by the type II modulation accompanying a surface acoustic wave allows the transport of spin polarized carriers over distances of tens of μm in GaAs(110) QWs. In the case of GaAs(111), the Rashba contribution to the SO field generated by an electric field perpendicular to the QW plane is used to compensate the Dresselhaus contribution at low temperatures, leading to spin lifetimes of up to 100 ns. The compensation mechanism is less effective at high temperatures due to nonlinear terms of the Dresselhaus contribution. Perspectives to overcome this limitation via the combination of (111) structures with the transport of spins by surface acoustic waves are discussed.

Time and spatially resolved electron spin detection in semiconductor heterostructures by magneto‐optical Kerr microscopy

Abstract: We review on time and spatially resolved two-color pump–probe magneto-optical Kerr effect (MOKE) microscopy studies of electron spins in bulk n-GaAs and GaAs (110) quantum wells (QWs) at low lattice temperatures. The influence of photocarrier heating by above-bandgap optical spin excitation on the spatially resolved magneto-optical spin detection is considered in detail. We demonstrate that a continuous-wave (cw) measurement of the local Kerr rotation at a fixed arbitrary probe wavelength in general does not correctly reveal the local spin polarization when hot electrons are present. For bulk GaAs we determine the true lateral electron spin polarization profile from cw measurements of the spatial dependence of the full excitonic Kerr rotation spectrum. For the (110) QWs, we directly obtain the electron spin diffusion coefficient from picosecond real space imaging of the time evolution of an optically excited electron spin packet, which we observe with a spectrally broad probe pulse.

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

Abstract: 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.

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

Abstract: 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.

Acoustic Carrier Transport in GaAs Nanowires

Abstract: Present semiconductor technologies allow the growth of different types of nanostructures, such as quantum wells, wires, and dots on the surface of a single semiconductor crystal. The piezoelectric field of surface acoustic waves (SAWs) propagating on the crystal surface provides an efficient mechanism for the controlled exchange of electrons and holes between these nanostructures. In this review, we explore this ability of dynamic SAW fields to demonstrate acoustically driven single-photon sources using coupled quantum wells and dots based on (Al,Ga)As (311)A material system. We address the growth of the coupled nanostructures by molecular beam epitaxy, the dynamics of the acoustic carrier transfer between them, as well as the acoustic control of recombination in quantum dots. The latter provides the basis for the operation of the acoustically driven single-photon sources, which are characterized by a low jitter and repetition frequency close to 1 GHz.

Spin control and manipulation in (111) GaAs quantum wells

Abstract: The control of spin dephasing is an essential requirement for quantum information processing using electron spins in IIIV semiconductors. GaAs quantum wells grown along the non-conventional [111] crystallographic direction are particularly interesting for spintronics due to the long spin lifetimes, which can be electrically controlled. Here, we show electron spin dynamics in (111) quantum wells by combining spatially-resolved with time-resolved photoluminescence measurements. The latter allows us to experimentally demonstrate the field induced enhancement of the spin lifetime as well as the transport of spin over several micrometers along the quantum well plane.