Showing papers by "Paulo V. Santos published in 2020"

Large Nonreciprocal Propagation of Surface Acoustic Waves in Epitaxial Ferromagnetic/Semiconductor Hybrid Structures

Abstract: Nonreciprocal propagation of sound, that is, the different transmission of acoustic waves traveling in opposite directions, is a challenging requirement for the realization of devices such as acoustic isolators and circulators. Here, we demonstrate efficient nonreciprocal transmission of surface acoustic waves (SAWs) propagating in opposite directions in a $\mathrm{Ga}\mathrm{As}$ substrate coated with an epitaxial ${\mathrm{Fe}}_{3}\mathrm{Si}$ film. The nonreciprocity arises from the acoustic attenuation induced by the magnetoelastic (ME) interaction between the SAW strain field and spin waves in the ferromagnetic film, which depends on the SAW propagation direction and can be controlled via the amplitude and orientation of an external magnetic field. The acoustic-transmission nonreciprocity, defined as the difference between the transmitted acoustic powers for forward and backward propagation at the ME resonance, reaches values of up to 20%, which are, to our knowledge, the largest values of nonreciprocity reported for SAWs traveling in a semiconducting piezoelectric substrate covered by a ferromagnetic film. The experimental results are well accounted for by a model for the ME interaction, which also shows that the nonreciprocity can be further enhanced by optimization of the sample design. These results make ${\mathrm{Fe}}_{3}\mathrm{Si}/\mathrm{Ga}\mathrm{As}$ a promising platform for the realization of efficient nonreciprocal SAW devices.

Polariton-driven phonon laser.

Abstract: Efficient generation of phonons is an important ingredient for a prospective electrically-driven phonon laser. Hybrid quantum systems combining cavity quantum electrodynamics and optomechanics constitute a novel platform with potential for operation at the extremely high frequency range (30–300 GHz). We report on laser-like phonon emission in a hybrid system that optomechanically couples polariton Bose-Einstein condensates (BECs) with phonons in a semiconductor microcavity. The studied system comprises GaAs/AlAs quantum wells coupled to cavity-confined optical and vibrational modes. The non-resonant continuous wave laser excitation of a polariton BEC in an individual trap of a trap array, induces coherent mechanical self-oscillation, leading to the formation of spectral sidebands displaced by harmonics of the fundamental 20 GHz mode vibration frequency. This phonon “lasing” enhances the phonon occupation five orders of magnitude above the thermal value when tunable neighbor traps are red-shifted with respect to the pumped trap BEC emission at even harmonics of the vibration mode. These experiments, supported by a theoretical model, constitute the first demonstration of coherent cavity optomechanical phenomena with exciton polaritons, paving the way for new hybrid designs for quantum technologies, phonon lasers, and phonon-photon bidirectional translators. Efficient generation of phonons is an important ingredient for a prospective electrically-driven phonon laser for coherent control of quantum systems. Here, the authors report on laser-like phonon emission in a hybrid semiconductor microcavity that optomechanically couples BEC polaritons with phonons.

Anisotropic Spin-Acoustic Resonance in Silicon Carbide at Room Temperature

Abstract: We report on acoustically driven spin resonances in atomic-scale centers in silicon carbide at room temperature. Specifically, we use a surface acoustic wave cavity to selectively address spin transitions with magnetic quantum number differences of $\ifmmode\pm\else\textpm\fi{}1$ and $\ifmmode\pm\else\textpm\fi{}2$ in the absence of external microwave electromagnetic fields. These spin-acoustic resonances reveal a nontrivial dependence on the static magnetic field orientation, which is attributed to the intrinsic symmetry of the acoustic fields combined with the peculiar properties of a half-integer spin system. We develop a microscopic model of the spin-acoustic interaction, which describes our experimental data without fitting parameters. Furthermore, we predict that traveling surface waves lead to a chiral spin-acoustic resonance that changes upon magnetic field inversion. These results establish silicon carbide as a highly promising hybrid platform for on-chip spin-optomechanical quantum control enabling engineered interactions at room temperature.

Dynamic Local Strain in Graphene Generated by Surface Acoustic Waves.

Abstract: We experimentally demonstrate that the Raman-active optical phonon modes of single-layer graphene can be modulated by the dynamic local strain created by surface acoustic waves (SAWs). In particular, the dynamic strain field of the SAW is shown to induce a Raman scattering intensity variation as large as 15% and a phonon frequency shift of up to 10 cm-1 for the G band, for instance, for an effective hydrostatic strain of 0.24% generated in single-layer graphene atop a LiNbO3 piezoelectric substrate with a SAW resonator operating at a frequency of ∼400 MHz. Thus, we demonstrate that SAWs are powerful tools for modulating the optical and vibrational properties of supported graphene by means of the high-frequency localized deformations tailored by the acoustic transducers, which can also be extended to other 2D systems.

Cavity Optomechanics with Polariton Bose-Einstein Condensates

Abstract: We report the experimental study of a hybrid quantum solid state system comprising two-level artificial atoms coupled to cavity confined optical and vibrational modes. In this system combining cavity quantum electrodynamics and cavity optomechanics, excitons in quantum wells play the role of the two-level atoms and are strongly coupled to the optical field leading to mixed polariton states. The planar optical microcavities are laterally microstructured, so that polaritons can be confined in wires, 3D traps, and arrays of traps, providing an additional tuning degree of freedom for the polariton energies. Upon increasing the non-resonant laser excitation power, a Bose-Einstein condensation of the polaritons is observed. Optomechanical induced amplification type of experiments with an additional weak laser probe clearly identify the coupling of these Bose-Einstein condensates to 20~GHz breathing-like vibrations confined in the same cavities. With single continuous wave non-resonant laser excitation, and once the laser power overpasses the threshold for Bose-Einstein condensation in trap arrays, mechanical self-oscillation similar to phonon ``lasing'' is induced with the concomitant observation of Mollow-triplet type mechanical sidebands on the Bose-Einstein condensate emission. High-resolution spectroscopic photoluminescence experiments evidence that these vibrational side-band resolved lines are enhanced when neighboring traps are red-detuned with respect to the BEC emission at overtones of the fundamental 20 GHz breathing mode frequency. These results constitute the first demonstration of coherent cavity polariton optomechanics and pave the way towards a novel type of hybrid devices for quantum technologies, phonon lasers, and phonon-photon bidirectional translators.

Focusing Surface-Acoustic-Wave Microcavities on Ga As

Abstract: Focusing microcavities for surface acoustic waves (SAWs) produce highly localized strain and piezoelectric fields that can dynamically control excitations in nanostructures. Focusing transducers (FIDTs) that generate SAW beams that match nanostructure dimensions require pattern correction due to diffraction and wave-velocity anisotropy. The anisotropy correction is normally implemented by adding a quadratic term to the dependence of the wave velocity on the propagation angle. We show that a SAW focusing to a diffraction-limited size in $\mathrm{Ga}\mathrm{As}$ requires corrections that more closely follow the group-velocity wave front, which is not a quadratic function. Optical interferometric mapping of the resultant SAW displacement field reveals tightly focused SAW beams on $\mathrm{Ga}\mathrm{As}$ with a minimal beam waist. An additional set of Gouy-phase--corrected passive fingers creates an acoustic microcavity in the focal region with a small volume and a high quality factor. Our ${\ensuremath{\lambda}}_{\mathrm{SAW}}=5.6\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$ FIDTs are expected to scale well to the approximately 500-nm wavelength regime needed to study strong coupling between vibrations and electrons in electrostatic $\mathrm{Ga}\mathrm{As}$ quantum dots.

Dynamically Tuned Arrays of Polariton Parametric Oscillators

Abstract: Optical parametric oscillations (OPOs)—the nonlinear coherent coupling of an optically excited two-particle pump state to signal and idler states correlated in energy—is relevant for optical amplification and generation of correlated photons. OPOs require states with well-defined symmetries and energies; the fine-tuning of material properties and structural dimensions remains a challenge for the realization of scalable OPOs in semiconductor nanostructures. Here we demonstrate a pathway towards arrays of OPOs based on the dynamic control of microcavity exciton-polaritons confined in micrometer-sized intracavity traps by an acoustic wave. The spatially varying strain field of the wave induces state-dependent energy shifts of discrete polariton levels with the appropriate symmetry for OPO triggering. The robustness of the dynamic acoustic tuning is demonstrated by the synchronous excitation of an array of confined OPOs using a single wave, which thus opens the way for the realization of scalable nonlinear on-chip systems.

Attractive interactions, molecular complexes, and polarons in coupled dipolar exciton fluids

Abstract: Dipolar (or spatially indirect) excitons (IXs) in semiconductor double quantum well (DQW) subjected to an electric field are neutral species with a dipole moment oriented perpendicular to the DQW plane. Here, we theoretically study interactions between IXs in stacked DQW bilayers, where the dipolar coupling can be either attractive or repulsive depending on the relative positions of the particles. By using microscopic band structure calculations to determine the electronic states forming the excitons, we show that the attractive dipolar interaction between stacked IXs deforms their electronic wave function, thereby increasing the inter-DQW interaction energy and making the IX even more electrically polarizable. Many-particle interaction effects are addressed by considering the coupling between a single IX in one of the DQWs to a cloud of IXs in the other DQW, which is modeled either as a closed-packed lattice or as a continuum IX fluid. We find that the lattice model yields IX interlayer binding energies decreasing with increasing lattice density. This behavior is due to the dominating role of the intra-DQW dipolar repulsion, which prevents more than one exciton from entering the attractive region of the inter-DQW coupling. Finally, both models shows that the single IX distorts the distribution of IXs in the adjacent DQW, thus inducing the formation of an IX dipolar polaron (dipolaron). While the interlayer binding energy reduces with IX density for lattice dipolarons, the continuous polaron model predicts a nonmonotonous dependence on density in semiquantitative agreement with a recent experimental study [cf. Hubert et al., Phys. Rev. X 9, 021026 (2019)].

Remotely pumped GHz antibunched emission from single exciton centers in GaAs

Abstract: Quantum communication networks require on-chip transfer and manipulation of single particles as well as their interconversion to single photons for long-range information exchange. Flying excitons propelled by GHz surface acoustic waves (SAWs) are outstanding messengers to fulfill these requirements. Here, we demonstrate the acoustic manipulation of single exciton centers consisting of individual excitons bound to shallow impurities centers embedded in a semiconductor quantum well. Time-resolved photoluminescence studies show that the emission intensity and energy from these centers oscillate at the SAW frequency of 3.5 GHz. Furthermore, these centers can be remotely pumped via acoustic transport of flying excitons along a quantum well channel over several microns. Time correlation studies reveal that the centers emit anti-bunched light, thus acting as single-photon sources operating at GHz frequencies. Our results pave the way for the exciton-based on-demand manipulation and on-chip transfer of single excitons at microwave frequencies with a natural photonic interface.

Compact acousto-optic multimode interference device in (Al,Ga)As.

Abstract: Multimode interference (MMI) devices are key components in modern integrated photonic circuits. Here, we present acoustically tuned optical switches on an (Al,Ga)As platform that enable robust, compact and fast response systems improving on recently demonstrated technology. The device consists of a 2 × 2 MMI device fine-tuned in its center region by a focused surface acoustic wave (SAW) beam working in the low GHz range. In this way, we can tune the refractive index profile over a narrow modulation region and thus control the optical switching behaviour via the applied SAW intensity. Direct tuning of the MMI device avoids losses and phase errors inherent to arrayed waveguide based switches, while also reducing the dimensions of the photonic circuit.

Dynamically Tuned Arrays of Polariton Parametric Oscillators

Abstract: Optical parametric oscillations (OPOs) - a non-linear process involving the coherent coupling of an optically excited two particle pump state to a signal and an idler states with different energies - is a relevant mechanism for optical amplification as well as for the generation of correlated photons. OPOs require states with well-defined symmetries and energies: the fine-tuning of material properties and structural dimensions to create these states remains a challenge for the realization of scalable OPO-based functionalities in semiconductor nanostructures. Here, we demonstrate a pathway towards this goal based on the control of confined microcavity exciton-polaritons modulated by the spatially and time varying dynamical potentials produced by a surface acoustic waves (SAW). The exciton-polariton are confined in um-sized intra-cavity traps fabricated by structuring a planar semiconductor microcavity during the epitaxial growth process. OPOs in these structures benefit from the enhanced non-linearities of confined systems. We show that SAW fields induce state-dependent and time-varying energy shifts, which enable the energy alignment of the confined levels with the appropriate symmetry for OPO triggering. Furthermore, the dynamic acoustic tuning, which is fully described by a theoretical model for the modulation of the confined polaritons by the acoustic field, compensates for fluctuations in symmetry and dimensions of the confinement potential thus enabling a variety of dynamic OPO regimes. The robustness of the acoustic tuning is demonstrated by the synchronous excitation of an array of confined OPOs using a single acoustic beam, thus opening the way for the realization of scalable non-linear on-chip systems.

Acoustically Triggered Optical Memories

Abstract: Hysteretic behavior of the photon emission by microcavity exciton-polaritons (MPs), optical bistability, provides the foundation for photonic memories, switches, and optical logic. Here, we demonst...

Thermally Tunable Surface Acoustic Wave Cavities

Abstract: We experimentally demonstrate the dynamical tuning of the acoustic field in a surface acoustic wave (SAW) cavity defined by a periodic arrangement of metal stripes on LiNbO3 substrate. Applying a dc voltage to the ends of the metal grid results in a temperature rise due to resistive heating that changes the frequency response of the device up to 0.3%, which can be used to control the acoustic transmission through the structure. The timescale of the switching is demonstrated to be of about 200 ms. In addition, we have also performed finite-element simulations of the transmission spectrum of a model system, which exhibits a temperature dependence consistent with the experimental data. The advances shown here enable easy, continuous, dynamical control and could be applied for a variety of substrates.

Remotely pumped GHz anti-bunched emission from single exciton states

Abstract: Quantum communication networks require on-chip transfer and manipulation of single particles as well as their interconversion to single photons for long-range information exchange. Flying excitons propelled by GHz surface acoustic waves (SAWs) are outstanding messengers to fulfill these requirements. Here, we demonstrate the acoustic manipulation of two-level states consisting of individual excitons bound to shallow impurities centers embedded in a semiconductor quantum well. Time-resolved photoluminescence studies show that the emission intensity and energy from these centers oscillate at the SAW frequency of 3.5 GHz. Furthermore, these centers can be remotely pumped via acoustic transport along a quantum well channel over several micron. Time correlation studies reveal that the centers emit anti-bunched light, thus acting as single-photon sources operating at GHz frequencies. Our results pave the way for the exciton-based on-demand manipulation and on-chip transfer of single excitons at microwave frequencies with a natural photonic interface.

Electrically driven exciton-polariton optomechanics at super high frequencies

Abstract: Polaritons enable the resonant coupling of excitons and photons to vibrations in the application-relevant super high frequency (SHF, 3-30 GHz) domain. We introduce a novel platform for coherent optomechanics based on the coupling of exciton-polaritons and electrically driven SHF longitudinal acoustic phonons confined within the spacer region of a planar Bragg microcavity. An intrinsic property of the microcavity platform is the back-feeding of phonons via reflections at the sample boundaries, which enables frequency x quality factors products exceeding 10^14 Hz as well as huge modulation amplitudes of the optical transition energies (up to 8 meV). We show that the modulation is dominated by the phonon-induced energy shifts of the excitonic polariton component, thus leading to an oscillatory transition between the regimes of weak and strong light-matter coupling. These results open the way for polariton-based coherent optomechanics in the non-adiabatic, side-band-resolved regime of coherent control.