Showing papers by "Paulo V. Santos published in 2018"

Interaction of surface acoustic waves with electronic excitations in graphene

Abstract: This article reviews the main theoretical and experimental advances regarding the interaction between surface acoustic waves (SAWs) and electronic excitations in graphene. The coupling of the graphene electron gas to the SAW piezoelectric field can modify the propagation properties of the SAW, and even amplify the intensity of SAWs traveling along the graphene layer. Conversely, the periodic electric and strain fields of the SAW can be used to modify the graphene Dirac cone and to couple light into graphene plasmons. Finally, SAWs can generate acousto-electric currents in graphene. These increase linearly with the SAW frequency and power but, in contrast to conventional currents, they depend non-monotonously on the graphene electric conductivity. Most of these functionalities have been reported in graphene transferred to the surface of strong piezoelectric insulators. The recent observation of acousto-electric currents in epitaxial graphene on SiC opens the way to the large-scale fabrication of graphene-based acousto-electric devices patterned directly on a semi-insulating wafer.

Luminescent Defects in a Few-Layer h -BN Film Grown by Molecular Beam Epitaxy

Abstract: Defects in hexagonal boron nitride ($h$-BN) are sources of single photons, even at room temperature, and creating $h$-BN films by molecular beam epitaxy (MBE) is promising for applications, as this allows the deposition of $h$-BN on various substrates. The authors show that $h$-BN films grown by MBE do contain defects that emit in both the ultraviolet and visible spectral ranges, and are localized within multilayer islands that form at the nucleation centers of the film. These luminescent defects have real potential as quantum light sources in optoelectronic devices based on epitaxial combination of dissimilar two-dimensional materials.

Interaction of surface acoustic waves with electronic excitations in graphene

Abstract: This article reviews the main theoretical and experimental advances regarding the interaction between surface acoustic waves (SAWs) and electronic excitations in graphene. The coupling of the graphene electron gas to the SAW piezoelectric field can modify the propagation properties of the SAW, and even amplify the intensity of SAWs traveling along the graphene layer. Conversely, the periodic electric and strain fields of the SAW can be used to modify the graphene Dirac cone and to couple light into graphene plasmons. Finally, SAWs can generate acousto-electric currents in graphene. These increase linearly with the SAW frequency and power but, in contrast to conventional currents, they depend non-monotonously on the graphene electric conductivity. Most of these functionalities have been reported in graphene transferred to the surface of strong piezoelectric insulators. The recent observation of acousto-electric currents in epitaxial graphene on SiC opens the way to the large-scale fabrication of graphene-based acousto-electric devices patterned directly on a semi-insulating wafer.

Quantum confinement of exciton-polaritons in a structured (Al,Ga)As microcavity

Abstract: The realization of quantum functionalities with polaritons in an all-semiconductor platform requires the control of the energy and spatial overlap of the wave functions of single polaritons trapped in potentials with precisely controlled shape and size. In this study we reach the confinement of microcavity polaritons in traps with an effective potential width down to 1 \textmu{}m, produced by patterning the active region of the (Al,Ga)As microcavity between two molecular beam epitaxy growth runs. We correlate spectroscopic and structural data to show that the smooth surface relief of the patterned traps translates into a graded confinement potential characterized by lateral interfaces with a finite lateral width. We show that the structuring method is suitable for the fabrication of arrays of proximal traps, supporting hybridization between adjacent lattice sites.

Optical waveguide devices modulated by surface acoustic waves

Abstract: This paper reviews the application of coherent acoustic phonons in the form of piezoelectrically-generated surface acoustic waves to control the response of integrated photonic acousto-optic waveguide structures. We first address the fundamental properties of the acoustic fields in piezoelectric materials as well as the piezoelectric generation of surface acoustic waves using interdigital transducers. The mechanism responsible for the interaction between light and the acoustic modes in the photonic waveguide structures and the modulation of the response of the devices is carefully reviewed. Next, we discuss the most important developments in the field of integrated acousto-optical device applications, with focus on devices built upon three-dimensional optical waveguides. Finally, prospects for acousto-optic device applications by use of surface acoustic waves are summarized.

Acoustically regulated optical emission dynamics from quantum dot-like emission centers in GaN/InGaN nanowire heterostructures

Abstract: We report on experimental studies of the effects induced by surface acoustic waves on the optical emission dynamics of GaN/InGaN nanowire quantum dots. We employ stroboscopic optical excitation with either time-integrated or time-resolved photoluminescence detection. In the absence of the acoustic wave, the emission spectra reveal signatures originated from the recombination of neutral exciton and biexciton confined in the probed nanowire quantum dot. When the nanowire is perturbed by the propagating acoustic wave, the embedded quantum dot is periodically strained and its excitonic transitions are modulated by the acousto-mechanical coupling. Depending on the recombination lifetime of the involved optical transitions, we can resolve acoustically driven radiative processes over time scales defined by the acoustic cycle. At high acoustic amplitudes, we also observe distortions in the transmitted acoustic waveform, which are reflected in the time-dependent spectral response of our sensor quantum dot. In addition, the correlated intensity oscillations observed during temporal decay of the exciton and biexciton emission suggest an effect of the acoustic piezoelectric fields on the quantum dot charge population. The present results are relevant for the dynamic spectral and temporal control of photon emission in III-nitride semiconductor heterostructures.

Asymmetric g Tensor in Low-Symmetry Two-Dimensional Hole Systems

Abstract: New experiments and calculations determine, for the first time, the full ``hole $g$ tensor,'' a parameter that determines the spin behavior of valence-band holes and a key ingredient to developing novel spin-based technologies such as quantum information protocols.

Tunneling blockade and single-photon emission in GaAs double quantum wells

Abstract: We report on the selective excitation of single impurity-bound exciton states in a GaAs double quantum well (DQW). The structure consists of two quantum wells (QWs) coupled by a thin tunnel barrier. The DQW is subject to a transverse electric field to create spatially indirect inter-QW excitons with electrons and holes located in different QWs. We show that the presence of intra-QW charged excitons (trions) blocks carrier tunneling across the barrier to form indirect excitons, thus opening a gap in their emission spectrum. This behavior is attributed to the low binding energy of the trions. Within the tunneling blockade regime, emission becomes dominated by processes involving excitons bound to single shallow impurities, which behave as two-level centers activated by resonant tunneling. The quantum nature of the emission is confirmed by the antibunched photon emission statistics. The narrow distribution of emission energies ($\ensuremath{\sim}10$ meV) and the electrical connection to the QWs make these single-exciton centers interesting candidates for applications in single-photon sources.

Attractive dipolar coupling between stacked exciton fluids

Abstract: The interaction between aligned dipoles is long-ranged and highly anisotropic: it changes from repulsive to attractive depending on the relative positions of the dipoles. We report on the observation of the attractive component of the dipolar coupling between excitonic dipoles in stacked semiconductor bilayers. We show that the presence of a dipolar exciton fluid in one bilayer modifies the spatial distribution and increases the binding energy of excitonic dipoles in a vertically remote layer. The binding energy changes are explained by a many-body polaron model describing the deformation of the exciton cloud due to its interaction with a remote dipolar exciton. The results open the way for the observation of theoretically predicted new and exotic collective phases, the realization of interacting dipolar lattices in semiconductor systems as well as for engineering and sensing their collective excitations.

Acoustic Field for the Control of Electronic Excitations in Semiconductor Nanostructures

Abstract: We investigate the confinement of surface acoustic waves (SAWs)in two-dimensional (2D)cavities on GaAs defined by focusing interdigital transducers (IDTs). These IDTs have curved-shape fingers with shapes designed to improve beam focusing by taking into account the anisotropy of the elastic and piezoelectric properties of the GaAs surface. We calculate the field distribution within the cavities using a finite element model, which also determines the electrical response of the structures. The calculated profiles are compared with the field distribution measured using scanning interferometry. The results demonstrate the feasibility of generation of μm-sized SAW fields for the modulation of semiconductor nanostructures.

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

Abstract: 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).