arXiv subjects

Fabio Pezzoli

Publications and source records attributed to Fabio Pezzoli.

12 recordsLinked to original sources

arXiv2026

Bright Telecom Spin-Photon Interface in Silicon Photonics

Silicon is an attractive host for scalable quantum photonics, but the absence of bright telecom-band emitters with optically addressable spin states has limited its use for spin-photon interfaces. Here we demonstrate the Al1-center, an aluminum--carbon defect in silicon, as a bright waveguide-integrated single-photon emitter with a ground-state spin. Using isotopically purified silicon-on-insulator nanophotonic devices, we isolate individual Al1-centers and observe high-purity single-photon emission with $g^{(2)}(0)=0.04$ without background subtraction. Time-resolved photoluminescence spectroscopy reveals a fast excited-state lifetime of 135 ns, nearly an order of magnitude shorter than the benchmark provided by the well-studied T-center. Resonant photoluminescence excitation measurements further resolve the zero-phonon transition and reveal a narrow homogeneous linewidth reaching 47 MHz, threefold narrower than the T-center under comparable temperature. Through magneto-optical spectroscopy, we resolve the spin-dependent transitions of the bound-exciton manifold and achieve spin-selective optical pumping, fulfilling the prerequisite for quantum state initialization and readout. These results establish the Al1-center as a bright telecom-band spin-photon interface in silicon photonics and introduce a promising platform for integrated quantum networks.

quant-ph↗

arXiv2026

Growth and characterization of planar hexagonal Ge on CdS

Hexagonal group-IV semiconductors have attracted increasing interest owing to their unconventional electronic and optical properties compared to the cubic diamond phase. However, the stabilization of these metastable allotropes in planar heterostructures remains a major challenge. In this work, we demonstrate the epitaxial growth of planar hexagonal germanium on non-basal $m$-plane CdS substrates by low-energy plasma-enhanced chemical vapor deposition. The role of growth temperature in the formation and stabilization of the hexagonal phase is investigated. X-ray diffraction, scanning transmission electron microscopy, and polarization-resolved Raman spectroscopy reveal the formation of epitaxial hexagonal germanium with the expected crystal symmetry. In particular, the Raman response exhibits the characteristic polarization selection rules of the E$_\text{2g}$ phonon mode of hexagonal Ge. Conversely, photoluminescence spectroscopy does not reveal any Ge-related emission feature. Combined transmission electron microscopy observations and atomistic modeling show that strain relaxation is governed by a limited set of dislocation mechanisms, which efficiently relieve most of the mismatch strain within a few nanometers from the interface and can involve localized cubic stacking insertions. At greater distances from the interface, the progressive loss of hexagonal order is increasingly dominated by stacking-fault disorder, particularly I3-type defects. These results establish CdS as a promising template for the planar stabilization of hexagonal Ge and provide insight into the defect mechanisms governing strain relaxation in metastable group-IV heterostructures.

cond-mat.mtrl-sci↗

arXiv2025

Formation of C-centers in Si-based systems by light ion irradiation

Atomic-scale crystal defects in Si are quantum-light sources offering tantalizing integration with existing photonic technologies. Yet, the controlled creation of near-infrared color centers for long- haul quantum communication and information still remains a challenge. In this work, we utilize light ions, such as H+ and He+, to gently generate quantum emitters in a crystalline Si matrix. Temperature-dependent photoluminescence measurements demonstrate the presence of optically-active defects, whose fluorescence matches the primary telecom window around 1550 nm. In addition, time-resolved investigations unveil long-lived excitonic states in the μs regime, thus confirming the formation of interstitial oxygen-carbon complexes, termed C-centers. Finally, we explored controlled ion irradiation strategies to seamlessly generate C-centers also in Ge-on-Si heterostructures, which offer an advanced technological platform for the future realization of integrated quantum photonics. This analysis, informed by practical color center synthesis and proof-of-principle experiments in epitaxial architectures, indicates intriguing prospects and profitable strategies to advance the burgeoning field of light-based quantum technologies.

cond-mat.mtrl-sci↗

arXiv2025

Optical spin pumping in silicon

The generation of an out-of-equilibrium population of spin-polarized carriers is a keystone process for quantum technologies and spintronics alike. It can be achieved through the so-called optical spin orientation by exciting the material with circularly polarized light. Although this is an established technique for studying direct band-gap semiconductors, it has been proven limited in materials like Si that possess weak oscillator strengths for the optical transitions. In this study, we address the problem by presenting an all-optical analog of the spin pumping method. This involves the optical creation of a non-equilibrium spin population within an absorber, which subsequently transfers spin-polarized carriers to a nearby indirect gap semiconductor, resulting in polarized emission from the latter. By applying this concept to a Ge-on-Si heterostructure we observe luminescence from Si with an unrivaled polarization degree as high as 9%. The progressive etching of the absorbing layer, assisted by magneto-optic experiments, allows us to ascertain that the polarized emission is determined by effective spin injection aided by the carrier lifetime shortening due to extended defects. These findings can facilitate the use of highly promising spin-dependent phenomena of Si, whose optical exploitation has thus far been hampered by fundamental limitations associated with its peculiar electronic structure.

cond-mat.mes-hall↗

arXiv2025

Optical control of the spin-Hall effect in a two-dimensional hole gas

Relativistic effects influence the motion of charged particles in solids by intertwining spin and momentum. The resulting phenomena exhibit rich and intriguing properties that can unveil radically new quantum devices. In this context, the two-dimensional hole gas formed in group IV heterostructures is a particularly promising platform, owning to a notable spin-orbit coupling. However, the exploitation of spin-momentum locking and precise manipulation of spin currents has remained elusive thus far. Here we use the modulation-doping technique to break inversion symmetry at novel Ge1-xSnx/Ge interfaces and explore spin-orbit phenomena in the emergent Rashba-coupled hole gases. Magneto-optical investigations demonstrate the unusual establishment of a staggered band alignment with carrier lifetime in the ns range. Optical spin orientation is then leveraged to directly inject spin-polarized currents in the Rashba-split 2D gas. Spin-to-charge conversion is shown to genuinely occur at the staggered gap through the inverse spin-Hall effect. This provides unprecedented access to low-order contributions of the spin-orbit Hamiltonian. Moreover, it leads to the startling demonstration that the spin Hall angle can be optically controlled by modifying the Rashba coupling through the photoexcitation density. Ge1-xSnx quantum wells thus offer innovative solutions and functionalities stemming from their unique spin-dependent properties and intriguing quantum phenomena at the crossroad between transport and photonic realms.

cond-mat.mes-hall↗

arXiv2024

THz Higher-Order Topological Photonics in Ge-on-Si Heterostructures

We design germanium-based higher-order topological cavities for terahertz applications by breaking the symmetry of a two-dimensional photonic crystal following the Su-Schrieffer-Heeger model. Calculations demonstrate the parity inversion of the electric field in differently deformed unit cells. The interface between domains of opposite topology presents edge and corner modes. The former are chiral, locking light propagation to its helicity. The latter prove that Ge-based structures can be used as high-order topological photonic crystals. These findings can accelerate the development of Si-photonic components working in a spectral range of high technological interest.

physics.app-ph↗

arXiv2020

Magneto-optical determination of the carrier lifetime in coherent Ge(1-x)Sn(x)/Ge heterostructures

We present a magneto-optical study of the carrier dynamics in compressively strained Ge(1-x)Sn(x) films having Sn compositions up to 10% epitaxially grown on blanket Ge on Si (001) virtual substrates. We leverage the Hanle effect under steady-state excitation to study the spin-dependent optical transitions in presence of an external magnetic field. This allowed us to obtain direct access to the dynamics of the optically-induced carrier population. Our approach singled out that at cryogenic temperatures the effective lifetime of the photogenerated carriers in coherent Ge(1-x)Sn(x) occurs in the sub-ns time scale. Supported by a model estimate of the radiative lifetime, our measurements indicate that carrier recombination is dominated by non-radiative processes. Our results thus provide central information to advance the fundamental understanding of carrier kinetics in this novel direct-gap group-IV material system. Such knowledge can be a stepping stone in the quest for the implementation of Ge(1-x)Sn(x)-based functional devices.

physics.optics↗

arXiv2017

Carrier and Spin Coherent Dynamics in Strained Germanium-Tin Semiconductor on Silicon

Germanium-Tin is emerging as a material exhibiting excellent photonic properties. Here we demonstrate optical initialization and readout of spins in this intriguing group IV semiconductor alloy and report on spin quantum beats between Zeeman-split levels under an external magnetic field. Our optical experiments reveal robust spin orientation in a wide temperature range and a persistent spin lifetime that approaches the ns regime at room temperature. Besides important insights into nonradiative recombination pathways, our findings disclose a rich spin physics in novel epitaxial structures directly grown on a conventional Si substrate. This introduces a viable route towards the synergic enrichment of the group IV semiconductor toolbox with advanced spintronics and photonic capabilities.

cond-mat.mtrl-sci↗

arXiv2017

Optically reconfigurable polarized emission in Germanium

Light polarization can conveniently encode information. Yet, the ability to steer polarized optical fields is notably demanding but crucial to develop practical methods for data encryption and to gather fundamental insights into light-matter interactions. Here we demonstrate the dynamic manipulation of the chirality of light in the telecom wavelength regime. This unique possibility is enrooted in the multivalley nature of the conduction band of a conventional semiconductor, namely Ge. In particular, we demonstrate that optical pumping suffices to govern the kinetics of spin-polarized carriers and eventually the chirality of the radiative recombination. We found that the polarization of the optical transitions can be remarkably swept through orthogonal eigenstates without magnetic field control or phase shifter coupling. Our results anticipate spin-enhanced optoelectronic functionalities in materials backed by the mainstream microelectronics industry and provide guiding information for the selection and design of functionalized emitters based on novel group IV compounds.

cond-mat.mtrl-sci↗

arXiv2016

Temperature-dependent photoluminescence characteristics of GeSn epitaxial layers

GeSn epitaxial heterostructures are emerging as prominent candidates for the monolithic integration of light sources on Si substrates. Here we propose a judicious explanation for their temperature-dependent photoluminescence (PL) that is based upon the so far disregarded optical activity of dislocations. By working at the onset of plastic relaxation, which occurs whenever the epilayer releases the strain accumulated during growth on the lattice mismatched substrate, we demonstrate that dislocation nucleation can be explicitly seen in the PL data. Notably, our findings point out that a monotonous thermal PL quenching can be observed in coherent films, in spite of the indirect nature of the GeSn bandgap. Our investigation, therefore, contributes to a deeper understanding of the recombination dynamics in this intriguing group IV alloy and offers insights into crucial phenomena shaping the light emission efficiency.

cond-mat.mtrl-sci↗

arXiv2016

Giant g factor tuning of long-lived electron spins in Ge

Control of electron spin coherence via external fields is fundamental in spintronics. Its implementation demands a host material that accommodates the highly desirable but contrasting requirements of spin robustness to relaxation mechanisms and sizeable coupling between spin and orbital motion of charge carriers. Here we focus on Ge, which, by matching those criteria, is rapidly emerging as a prominent candidate for shuttling spin quantum bits in the mature framework of Si electronics. So far, however, the intrinsic spin-dependent phenomena of free electrons in conventional Ge/Si heterojunctions have proved to be elusive because of epitaxy constraints and an unfavourable band alignment. We overcome such fundamental limitations by investigating a two dimensional electron gas (2DEG) confined in quantum wells of pure Ge grown on SiGe-buffered Si substrates. These epitaxial systems demonstrate exceptionally long spin relaxation and coherence times, eventually unveiling the potential of Ge in bridging the gap between spintronic concepts and semiconductor device physics. In particular, by tuning spin-orbit interaction via quantum confinement we demonstrate that the electron Landé g factor and its anisotropy can be engineered in our scalable and CMOS-compatible architectures over a range previously inaccessible for Si spintronics.

cond-mat.mtrl-sci↗

arXiv2015

Optical orientation of electron spins and valence band spectroscopy in germanium

We have investigated optical orientation in the vicinity of the direct gap of bulk germanium. The electron spin polarization is studied via polarization-resolved photoluminescence excitation spectroscopy unfolding the interplay between doping and ultrafast electron transfer from the center of the Brillouin zone towards its edge. As a result, the direct-gap photoluminescence circular polarisation can vary from 30% to -60% when the excitation laser energy increases. This study provides also simultaneous access to the resonant electronic Raman scattering due to inter-valence band excitations of spin-polarized holes, yielding a fast and versatile spectroscopic approach for the determination of the energy spectrum of holes in semiconducting materials.

cond-mat.mes-hall↗