arXiv subjects

P. V. Santos

Publications and source records attributed to P. V. Santos.

At least 19 recordsLinked to original sources

arXiv2026

Synchronization in coherently and dissipatively coupled spinor polariton time crystals

The spinor degree of freedom associated to exciton-polariton condensates can spontaneously self-oscillate breaking time translation symmetry, thus showing a continuous time-crystal (CTC) behavior. An open question in such driven-dissipative and non-linear quantum open systems is what happens when CTCs are brought together to interact. Here we experimentally study polariton condensates in coupled traps, evidencing mutual induction and synchronization of the pseudospin temporal GHz dynamics in the CTC phase. The individual and relative orientation of the (limit cycle) precessing pseudospins can be tuned by the optical excitation power, displaying both ferro and anti-ferro dynamical configurations. We theoretically show that the exciton reservoir, and both the coherent and long-range dissipative inter-trap coupling, play important roles in the CTC dynamics. The investigation of time-broken symmetry is thus extended here to more complex non-hermitian systems opening the path to study self-sustained collective dynamics in lattices of non-linear quantum condensates.

cond-mat.other↗

arXiv2026

Coherent Control of a Polariton Continuous Time Crystal

Spontaneous breaking of time-translation symmetry and the emergence of self-sustained oscillations in quantum driven-dissipative systems is the hallmark of continuous time crystals (CTCs). An outstanding challenge is to achieve precise, coherent control of such dynamical phases, whose complex nonlinear dynamics makes them inherently difficult to manipulate. Here, we experimentally demonstrate coherent control of a solid-state CTC realized in a non-resonantly excited spin-polarized exciton-polariton condensate in a Ga(Al)As microcavity. We exploit two complementary control channels: an additional weak control laser and the optomechanical interaction with confined GHz phonons. By tuning the control laser energy and power, we stabilize distinct dynamical regimes, including frequency pushing, injection locking with continuous tuning of the limit-cycle frequency, and full suppression of the autonomous dynamics via phase locking. Under appropriate detuning conditions, the control excitation generates coherent mechanical self-oscillation through the polariton--phonon deformation-potential interaction, evidenced by spectral sidebands. The resulting dynamical back-action provides a phonon-mediated locking channel that fixes the frequency of the CTC. Together, the two channels dramatically enhance the temporal coherence of the GHz limit-cycle dynamics, as established through linewidth narrowing and time-resolved first-order correlation measurements $\gone(τ)$. Our work establishes a way to harness the unique aspects of CTCs for practical applications in the GHz-range.

physics.optics↗

arXiv2026

Strong Coupling beyond the High-Q Limit and Linewidth Narrowing in a Multi-Exciton Planar Microcavity

We systematically study the linewidths of multilevel exciton-polariton modes as a function of the detuning in a planar hybrid microcavity (MC) with low quality factor (Q~300) operating in the linear optical response regime. Using optical reflectivity, we observe that, counterintuitively, the linewidths of the polariton modes undergo a pronounced spectral narrowing as detuning is reduced. Using optical reflectivity, we observe that, counterintuitively, the linewidths of the polariton modes undergo a pronounced spectral narrowing as detuning is reduced. By benchmarking the experimental results against three commonly used constant-loss theoretical descriptions, we find that this behavior is only partially reproduced, highlighting limitations of conventional strong-coupling models when applied to non-standard MC architectures. Our results suggest that frequency-dependent self-energy effects or correlated dissipation mechanisms, typically neglected in simplified treatments, may play an important role.

physics.optics↗

arXiv2025

Polariton cascade phonon laser

Phonon lasers, as their photon counterparts, rely on the physics of stimulated emission. Arguably, because light does not require a material substrate to propagate, while sound does, the impact of the two technologies has however been highly contrasting, with "sasers" (for sound amplification by stimulated emission of radiation) mostly remaining as an academic curiosity. This might be changing due to the possibility to use coherent sound generation for on-chip processing of information at ultra-high frequencies, and in the quantum realm, in integrated photonic and optomechanical devices. Inspired by the concept of unipolar lasers based on the quantum engineering of states in semiconductor heterostructures, we propose and implement a quantum cascade phonon laser (QCPL). A condensate of exciton-photon quasiparticles (polaritons) is optically induced in a microstructured semiconductor device to jump down a ladder of engineered levels. This down-cascade is accompanied by the efficient stimulated emission of phonons of $\sim 20$, $\sim 60$, and $\sim 100$~GHz, which are designed to strongly interact with the polaritons on the same chip. The proposed concept opens the path for the design of integrated high-frequency optomechanical devices, as for example for non-reciprocal photon transport and multi-wavelength Brillouin lasers.

physics.optics↗

arXiv2025

Shapiro resonances in ac-self-modulated exciton-polariton Josephson junctions

We experimentally investigate the dynamics of exciton polariton Josephson junctions when the coupling between condensates is periodically modulated through self-induced mechanical oscillations. The condensates energy detuning, the analog of the bias voltage in superconducting junctions, displays a plateau behavior akin to the Shapiro steps. At each step massive tunneling of particles occurs featuring Shapiro-like spikes. These characteristic changes are observed when the condensates Josephson frequency $ω_\textrm{J}$ is an integer multiple of the modulation frequency $ω_\mathrm{M}$.

cond-mat.other↗

arXiv2025

GHz helical acoustic drum modes on a chip

On-chip laterally confined GHz acoustic modes with tunable helicity open the way for advanced optomechanical functionalities. Here, we demonstrate a novel concept for the implementation of these functionalites through the electrical excitation of GHz membrane-like drum modes. Our concept relies on the strong dependence of the frequency spectrum of Lamb acoustic modes on the thickness of the propagating medium. Lamb modes generated by a piezoelectric resonator in a thicker (or thinner) substrate region remain confined in this region via reflections at the lateral boundaries with thinner (thicker) substrate regions. If the generation region is disk-shaped, the lateral reflections form drum-like modes, which we experimentally confirm by radio-frequency spectroscopy as well as by maps of the surface displacements. Furthermore, we show that an array of sector shaped piezoelectric transducers powered with appropriate radio-frequency phases creates helical modes with tunable helicity, which can then be transferred to an optical beam. Our analytical and finite-element models yield useful insights into the acoustic coupling of bulk and surface modes that can guide adaptation to other material systems. The acoustic drum modes thus provide a flexible platform for acousto-optical chiral functionalities in the GHz frequency range.

physics.optics↗

arXiv2024

Dynamical Reorientation of Spin Multipoles in Silicon Carbide by Transverse Magnetic Fields

The long-lived and optically addressable high-spin state of the negatively charged silicon vacancy ($\mathrm{V_{Si}}$) in silicon carbide makes it a promising system for applications in quantum technologies. Most studies of its spin dynamics have been performed in external magnetic fields applied along the symmetry axis. Here, we find that the application of weak magnetic fields perpendicular to the symmetry axis leads to nontrivial behavior caused by dynamical reorientation of the $\mathrm{V_{Si}}$ spin multipole under optical excitation. Particularly, we observe the inversion of the quadrupole spin polarization in the excited state and appearance of the dipole spin polarization in the ground state. The latter is much higher than thermal polarization and cannot be induced solely by optical excitation. Our theoretical calculations reproduce well all sharp features in the spin resonance spectra, and shine light on the complex dynamics of spin multipoles in these kinds of solid-state systems.

quant-ph↗

arXiv2024

Solid-state continuous time crystal with a built-in clock

Time crystals (TCs) are many-body systems displaying spontaneous breaking of time translation symmetry. Here, we demonstrate a TC using driven-dissipative condensates of microcavity exciton-polaritons, spontaneously formed from an incoherent particle bath. In contrast to other realizations, the TC phases can be controlled by the power of continuous-wave non-resonant optical drive exciting the condensate and optomechanical interactions with phonons. Those phases are for increasing power: (i) Larmor precession of pseudo-spins - a signature of continuous TC, (ii) locking of the frequency of precession to self-sustained coherent phonons - stabilized TC, (iii) doubling of TC frequency by phonons - a discrete TC with continuous excitation. These results establish microcavity polaritons as a platform for the investigation of time-broken symmetry in non-hermitian systems.

cond-mat.other↗

arXiv2023

Identification of acoustically induced spin resonances of Si vacancy centers in 4H-SiC

The long-lived and optically addressable spin states of silicon vacancies ($\mathrm{V}_\mathrm{Si}$) in 4H-SiC make them promising qubits for quantum communication and sensing. These color centers can be created in both the hexagonal (V1) and in the cubic (V2) local crystallographic environments of the 4H-SiC host. While the spin of the V2 center can be efficiently manipulated by optically detected magnetic resonance at room temperature, spin control of the V1 centers above cryogenic temperatures has so far remained elusive. Here, we show that the dynamic strain of surface acoustic waves can overcome this limitation and efficiently excite magnetic resonances of V1 centers up to room temperature. Based on the width and temperature dependence of the acoustically induced spin resonances of the V1 centers, we attribute them to transitions between spin sublevels in the excited state. The acoustic spin control of both kinds of $\mathrm{V}_\mathrm{Si}$ centers in their excited states opens new ways for applications in quantum technologies based on spin-optomechanics.

quant-ph↗

arXiv2022

Giant optomechanical coupling and dephasing protection with cavity exciton-polaritons

Electronic resonances can significantly enhance the photon-phonon coupling in cavity optomechanics, but are normally avoided due to absorption losses and dephasing by inhomogeneous broadening. We experimentally demonstrate that exciton-polaritons in semiconductor microcavities enable GHz optomechanics with single-particle resonant couplings reaching record values in the 10s of MHz range. Moreover, this resonant enhancement is protected from inhomogeneous broadening by the Rabi gap. Single-polariton non-linearities and the optomechanical strong-coupling regime become accessible in this platform.

cond-mat.mes-hall↗

arXiv2022

Microcavity phonoritons -- a coherent optical-to-microwave interface

Optomechanical systems provide a pathway for the bidirectional optical-to-microwave interconversion in (quantum) networks. We demonstrate the implementation of this functionality and non-adiabatic optomechanical control in a single, $μ$m-sized potential trap for phonons and exciton-polariton condensates in a structured semiconductor microcavity. The exciton-enhanced optomechanical coupling leads to self-oscillations (phonon lasing) -- thus proving reversible photon-to-phonon conversion. We show that these oscillations are a signature of the optomechanical strong coupling signalizing the emergence of elusive phonon-exciton-photon quasiparticles -- the phonoritons. We then demonstrate full control of the phonoriton spectrum as well as coherent microwave-to-photon interconversion using electrically generated GHz-vibrations and a resonant optical laser beam. These findings establish the zero-dimensional polariton condensates as a scalable coherent interface between microwave and optical domains with enhanced microwave-to-mechanical and mechanical-to-optical coupling rates.

physics.optics↗

arXiv2022

Scanning X-ray diffraction microscopy of a 6 GHz surface acoustic wave

Surface acoustic waves at frequencies beyond a few GHz are promising components for quantum technology applications. Applying scanning X-ray diffraction microcopy we directly map the locally resolved components of the three-dimensional strain field generated by a standing surface acoustic wave on GaAs with wavelength $λ\simeq500\,$nm corresponding to frequencies near 6 GHz. We find that the lattice distortions perpendicular to the surface are phase-shifted compared to those in propagation direction. Model calculations based on Rayleigh waves confirm our measurements. Our results represent a break through in providing a full characterization of a radio frequency surface acoustic wave beyond plain imaging.

cond-mat.mes-hall↗

arXiv2022

Photoluminescence of double quantum wells: asymmetry and excitation laser wavelength effects

Circularly polarized photoluminescence (PL) spectroscopy measured at 19 K on GaAs/AlGaAs symmetric and asymmetric double quantum wells (DQW) is reported. The PL is obtained by exciting the sample with a circularly polarized (left or right) laser in order to create an initial unbalanced distribution of electron spins in the conduction band and, in this way, obtain the electron spin lifetime $τ_s$. The effects of the excitation laser wavelength were estimated by exciting with laser wavelengths of 701.0 nm, 787.0 nm, 801.5 nm and 806.5 nm. The increase of $τ_s$ with the excitation wavelength is attributed to the lower initial quasi-momentum $\bf{k}$ of the excited carriers, which also reduces spin-orbit relaxation processes. $τ_s$ was found to be higher in asymmetric DQWs: this is attributed to the wider QWs in these samples, which reduces spin relaxation due to the Dresselhaus mechanism. In addition, we also detected a smaller contribution from the Rashba mechanism by comparing samples with built-in electric fields of different orientations defined by doped barrier layers.

cond-mat.mes-hall↗

arXiv2021

Optomechanical parametric oscillation of a quantum light-fluid lattice

Two-photon coherent states are one of the main building pillars of non-linear and quantum optics. It is the basis for the generation of minimum-uncertainty quantum states and entangled photon pairs, applications not obtainable from standard coherent states or one-photon lasers. Here we describe a fully-resonant optomechanical parametric amplifier involving a polariton condensate in a trap lattice quadratically coupled to mechanical modes. The quadratic coupling derives from non-resonant virtual transitions to extended discrete excited states induced by the optomechanical coupling. Non-resonant continuous wave (cw) laser excitation leads to striking experimental consequences, including the emergence of optomechanically induced inter-site parametric oscillations and inter-site tunneling of polaritons at discrete inter-trap detunings corresponding to sums of energies of the two involved mechanical oscillations (20 and 60 GHz confined vibrations). We show that the coherent mechanical oscillations correspond to parametric resonances with threshold condition different to that of standard linear optomechanical self-oscillation. The associated Arnold tongues display a complex scenario of states within the instability region. The observed new phenomena can have applications for the generation of entangled phonon pairs, squeezed mechanical states relevant in sensing and quantum computation, and for the bidirectional frequency conversion of signals in a technologically relevant range.

cond-mat.mes-hall↗

arXiv2021

Optical anisotropies of asymmetric double GaAs (001) Quantum Wells

In the present work, we were able to identify and characterize a new source of in-plane optical anisotropies (IOAs) occurring in asymmetric DQWs; namely a reduction of the symmetry from $D_{2d}$ to $C_{2v}$ as imposed by asymmetry along the growth direction. We report on reflectance anisotropy spectroscopy (RAS) of double GaAs quantum wells (DQWs) structures coupled by a thin ($<2$ nm) tunneling barrier. Two groups of DQWs systems were studied: one where both QWs have the same thickness (symmetric DQW) and another one where they have different thicknesses (asymmetric DQW). RAS measures the IOAs arising from the intermixing of the heavy- and light- holes in the valence band when the symmetry of the DQW system is lowered from $D_{2d}$ to $C_{2v}$. If the DQW is symmetric, residual IOAs stem from the asymmetry of the QW interfaces; for instance, associated to Ga segregation into the AlGaAs layer during the epitaxial growth process. In the case of an asymmetric DQW with QWs with different thicknesses, the AlGaAs layers (that are sources of anisotropies) are not distributed symmetrically at both sides of the tunneling barrier. Thus, the system losses its inversion symmetry yielding an increase of the RAS strength. The RAS line shapes were compared with reflectance spectra in order to assess the heavy- and light- hole mixing induced by the symmetry breakdown. The energies of the optical transitions were calculated by numerically solving the one-dimensional Schrödinger equation using a finite-differences method. Our results are useful for interpretation of the transitions occurring in both, symmetric and asymmetric DQWs.

cond-mat.mes-hall↗

arXiv2021

Acoustically induced coherent spin trapping

Hybrid spin-optomechanical quantum systems offer high flexibility, integrability and applicability for quantum science and technology. Particularly, on-chip surface acoustic waves (SAWs) can efficiently drive spin transitions in the ground states (GSs) of atomic-scale, color centre qubits, which are forbidden in case of the more frequently used electromagnetic fields. Here, we demonstrate that strain-induced spin interactions within their optically excited state (ES) can exceed by two orders of magnitude the ones within the GS. This gives rise to novel physical phenomena, such as the acoustically induced coherent spin trapping (CST) unvealed here. The CST manifests itself as the spin preservation along one particular direction under the coherent drive of the GS and ES by the same acoustic field. Our findings provide new opportunities for the coherent control of spin qubits with dynamically generated strain fields that can lead towards the realization of future spin-acoustic quantum devices.

quant-ph↗

arXiv2021

Remotely pumped GHz antibunched emission from single exciton centers in GaAs

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.

cond-mat.mes-hall↗

arXiv2020

Anisotropic Spin-Acoustic Resonance in Silicon Carbide at Room Temperature

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 $\pm$1 and $\pm$2 in the absence of external microwave electromagnetic fields. These spin-acoustic resonances reveal a non-trivial 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, which 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.

cond-mat.mes-hall↗