arXiv subjects

Xavier Marie

Publications and source records attributed to Xavier Marie.

At least 19 recordsLinked to original sources

arXiv2026

Alloy engineering of excitonic properties in TMD monolayers

We investigate monolayer MoS$_{2x}$Se$_{2(1-x)}$ alloys across the full composition range using optical spectroscopy. We demonstrate continuous tuning of the optical gap over $\sim$0.35 eV, accompanied by a systematic reduction of the B--A exciton splitting, in agreement with density functional theory calculations. Temperature-dependent measurements reveal a progressive increase of the average phonon energy from Se-rich to S-rich alloys that follows a simple reduced-mass scaling model. Polarization-resolved spectroscopy further shows a monotonic increase of the circular polarization from nearly zero in MoSe$_2$ to $\sim$15\% in MoS$_2$ at 78 K. The observed evolution of the polarization is attributed to alloy-induced modifications of the electronic structure that modify bright--dark exciton mixing and the associated valley depolarization. These findings establish alloy engineering as an effective strategy for controlling excitonic properties in TMD monolayers.

cond-mat.mtrl-sci↗

arXiv2026

Entangled Telecom Photon Generation using Twisted Van der Waals Crystals

Nanoscale quantum light sources are essential building blocks for integrated quantum photonic systems. Here, we report a wavelength-scale entangled-photon source based on van der Waals-engineered NbOBr$_2$, and benchmark its performance for telecom-wavelength quantum light generation. By exploiting the material's second-order nonlinearity, we generate quantum-correlated photon pairs via spontaneous parametric down-conversion. We then use a 90$^{\circ}$ twisted stacking to induce quantum interference in photon-pair generation, yielding polarization-entangled photons. This approach enables tunability of the quantum optical state via control of the excitation laser polarization. We experimentally obtain entanglement fidelities exceeding 95% for Bell states, along with a high coincidence-to-accidental ratio of $\sim$335, and a brightness approximately one order of magnitude higher than recently reported telecom sources based on transition metal dichalcogenide (TMD) 2D materials. These results establish twisted van der Waals engineering as a powerful platform for highly tunable, high-brightness quantum light sources at telecom wavelengths.

physics.optics↗

arXiv2026

Sub-nm range momentum-dependent exciton transfer from a 2D semiconductor to graphene

Heterostructures made from atomically thin semiconductors (here MoSe$_2$) and graphene are uniquely poised to investigate photoinduced charge and energy transfer in the 2D limit. Here, using picosecond time-resolved photoluminescence spectroscopy at cryogenic temperatures on two types of MoSe$_2$/graphene heterostrutures, we unveil key features of the underlying mechanisms. First, the shortening of the MoSe$_2$ bright exciton lifetime is marginally affected by the number of graphene layers to which MoSe$_2$ is coupled. Second, exciton transfer vanishes when a sub-nm thick spacer of hexagonal boron nitride decouples MoSe$_2$ from graphene. These results indicate that charge tunneling govern bright exciton relaxation in MoSe$_2$/graphene and that longer-range, F\"orster-type energy transfer (FRET) does not affect bright excitons. However, sub-ps FRET to graphene accelerates the relaxation of ``hot'' excitons formed upon optical excitation, leading to photoluminescence quenching factors that exceed expectations based on the shortening of the bright exciton lifetime. Our work has direct implications for energy harvesting and funneling using van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2026

Gate-tuneable single-photon emitters in WSe2 monolayer created via AFM nanoindentation on rigid SiO2/Si substrates

Single-photon emitters (SPEs) hosted by two-dimensional (2D) semiconducting materials are envisioned for next-generation quantum applications. However, SPE creation in 2D semiconductors on rigid substrates like SiO2/Si via nanoindentation is a technological gap, critical for interfacing SPEs with photonic circuits and cavities. Here, we report a protocol for deterministically creating SPEs in monolayer WSe2 on SiO2/Si substrates using a sharp diamond AFM (atomic force microscope) tip. A displacement-controlled indentation process is developed, allowing indent depths > 150 nm necessary for creating SPEs. Sharp defect peaks (~200 {\mu}eV) are observed in cryogenic (4K) photoluminescence (PL) spectrum at nanoindented sites and are stable upto ~ 120K. 76% of sites exhibit sharp defect-bound peaks confirmed by power-dependent, temperature-dependent, and time-resolved PL (TRPL). AFM and PL mapping link these peaks to indent periphery. The peaks show sub-linewidth spectral jitter, no blinking, and single-photon nature in second-order autocorrelation measurements. SPEs can be switched on/off, and background emissions suppressed using electrical gating. Gate-voltage dependent TRPL indicate that SPE dynamics can be tuned, depending on nature of SPE, pointing the way to higher-purity SPEs. Our work is directly applicable to other 2D materials and photonic circuit/cavity compatible rigid substrates and is a significant step for scalable SPE technologies.

cond-mat.mes-hall↗

arXiv2026

Emission Dynamics of Rydberg Excitons in $\mathbf{\mathrm{Cu_2O}}$: Distinguishing Second Harmonic Generation from Secondary Emission

Rydberg excitons in $\mathrm{Cu_2O}$ simultaneously give rise to two very different optical responses under resonant two-photon excitation: a coherent second-harmonic signal mediated by the excitonic second order susceptibility tensor $\chi^{(2)}$, and a secondary emission originating from the radiative decay of real exciton populations. Distinguishing these two channels is essential for interpreting nonlinear and quantum-optical experiments based on high-$n$ states, yet their temporal, spectral, and power-dependent signatures often overlap. Here we use time-resolved resonant two-photon excitation to cleanly separate SHG and SE and to map how each depends on $n$, temperature, excitation power, and crystal quality. This approach reveals the markedly different sensitivities of the two processes to phonons, defects, and many-body effects, and establishes practical criteria for identifying SE and SHG in a wide range of experimental conditions. Our results provide a unified framework for interpreting emission from Rydberg excitons and offer guidelines for future studies aiming to exploit their nonlinear response and long-range interactions.

cond-mat.quant-gas↗

arXiv2025

Non-radiative energy transfer between boron vacancies in hexagonal boron nitride and other 2D materials

Boron vacancies ($V_B^-$) in hexagonal boron nitride (hBN) have emerged as a promising platform for two-dimensional quantum sensors capable of operating at atomic-scale proximity. However, the mechanisms responsible for photoluminescence quenching in thin hBN sensing layers when placed in contact with absorptive materials remain largely unexplored. In this Letter, we investigate non-radiative F\"orster resonance energy transfer (FRET) between $V_B^-$ centers and either monolayer graphene or 2D semiconductors. Strikingly, we find that the FRET rate is negligible for hBN sensing layers thicker than 3 nm, highlighting the potential of $V_B^-$ centers for integration into ultra-thin quantum sensors within van der Waals heterostructures. Furthermore, we experimentally extract the intrinsic radiative decay rate of $V_B^-$ defects.

cond-mat.mtrl-sci↗

arXiv2025

High luminescence efficiency of multi-valley excitonic complexes in heavily doped WSe2 monolayer

Monolayers of group-VI transition-metal dichalcogenides (TMDs) are two-dimensional semiconductors that exhibit exceptionally strong light-matter coupling yet typically suffer from low emission quantum yields. In this letter, we investigate the heavily n-doped regime of a WSe$_2$ monolayer and show that multi-particle excitonic complexes produce photoluminescence signals up to two orders of magnitude stronger than in the neutral state. Time-resolved photoluminescence and differential reflectivity measurements reveal that the quantum yield rises with carrier density and exceeds 50% for electron concentrations above 10$^{13}$ cm$^{-2}$. These findings establish TMD monolayers as a platform for exploring excitonic complexes in high-density electron gases and point toward new opportunities for efficient, atomically thin light emitters.

cond-mat.mes-hall↗

arXiv2025

Thermal assisted transport of biexcitons in monolayer WSe2

Studies of excitonic transport in transition metal dichalcogenide monolayers have attracted increasing interest in recent years in order to develop nano-optoelectronic devices made with 2D materials. These studies began with low to moderate optical excitation regimes, and more recently have focused on high injection regimes where nonlinear effects appear. This article is focused on the transport of biexcitons by spatially and temporally resolved photoluminescence spectroscopy at high excitation flux. The study is carried out on a high-quality WSe$_2$ monolayer encapsulated in hexagonal boron nitride. The results show that a Seebeck current affects transport in connection with the presence of hot biexcitons. In particular, we observe the formation of spatial rings, also called halos, which have been observed in other excitonic gases. These results tend to generalize the importance of high-energy populations in excitonic transport in TMD, even for complex and heavy excitonic particles.

cond-mat.mtrl-sci↗

arXiv2025

Sub 10 nm Nanochannels Enable Directional Quasi Ballistic Exciton Transport over 5 {\mu}m at Room Temperature

Nanoscale potential wells provide a powerful means to engineer energy landscapes in low dimensional materials, enabling control over quantum states, carrier dynamics, and optoelectronic responses. Such confinement governs phenomena including charge localization, transport anisotropy, band structure modulation, and light matter interaction strength. However, realizing clean and well defined nanostructures remains technically challenging, as fabrication techniques such as focused ion beam (FIB) milling and electron beam lithography frequently introduce structural disorder, residual contamination, or detrimental interactions with the underlying substrate. Here, we develop a femtosecond laser direct writing technique to create sub 10 nm wide dielectric nanochannels with smooth, continuous boundaries on hexagonal boron nitride (hBN) substrates, without using resists or chemical etchants. As a demonstration, these nanochannels are employed to define programmable dielectric landscapes in monolayer molybdenum diselenide (MoSe2), forming excitonic energy funnels that suppress scattering and significantly extend the exciton transport distance. Transport is reshaped from isotropic diffusion with submicron range to directional super diffusion exhibiting quasi ballistic transport exceeding 5 um, more than 20 times longer than in unpatterned systems. The smooth dielectric boundaries further enable precise control over exciton trajectories, allowing for programmable transport pathways. This dry, scalable, and substrate compatible approach offers a robust platform for exciton engineering and integrated quantum photonic devices.

physics.optics↗

arXiv2025

Silicon nanoantennas for tailoring the optical properties of MoS2 monolayers

Silicon-based dielectric nanoantennas provide an effective platform for engineering light-matter interactions in van der Waals semiconductors. Here, we demonstrate near-field coupling between monolayer MoS2 and silicon nanoantennas arranged in hexagonal lattices with tunable geometric parameters, leading to a three-fold enhancement in photoluminescence and an excitation-wavelength-dependent emission that aligns with Mie-resonant modes. Raman spectroscopy reveals an up to 8-fold enhancement in the vibrational modes of MoS2, while second-harmonic generation exhibits a 20 to 30-fold increase in efficiency, closely correlating with the presence of the underlying nanoantennas. Our experiments and simulations quantify the tunable benefits of the near-field interactions, taking into account thin-film interference and strain-induced effects. Our findings present dielectric nanoantennas as a promising platform for tailoring linear and nonlinear optical properties in 2D materials, with potential applications in nanophotonic devices and integrated photonics.

physics.optics↗

arXiv2025

Charge state tuning of spin defects in hexagonal boron nitride

Boron vacancies in hexagonal boron nitride (hBN) are among the most extensively studied optically active spin defects in van der Waals crystals, due to their promising potential to develop two-dimensional (2D) quantum sensors. In this letter, we demonstrate the tunability of the charge state of boron vacancies in ultrathin hBN layers, revealing a transition from the optically active singly negatively charged state to the optically inactive doubly negatively charged state when sandwiched between graphene electrodes. Notably, there is a photoluminescence quenching of a few percent upon the application of a bias voltage between the electrodes. Our findings emphasize the critical importance of considering the charge state of optically active defects in 2D materials, while also showing that the negatively charged boron vacancy remains robust against external perpendicular electric fields. This stability makes it a promising candidate for integration into various van der Waals heterostructures.

cond-mat.mtrl-sci↗

arXiv2025

Laser patterning of the room temperature van der Waals ferromagnet 1$T$-CrTe$_2$

Lamellar crystalline materials, whose layers are bond by van der Waals forces, can be stacked to form ultrathin artificial heterostructures, and in particular vertical magnetic junctions when some of the stacked materials are (ferro)magnetic. Here, using the room temperature van der Waals ferromagnet 1$T$-CrTe$_2$, we report a method for patterning lateral magnetic junctions. Exploiting the heat-induced phase transformation of the material into Cr$_x$Te$_y$ compounds ($x/y>1/2$), we use local laser heating to imprint patterns at the micron-scale. Optimizing laser heat dissipation, we further demonstrate the crucial role of the substrate to control the phase transformation. If plain, unstructured poorly heat-conducting substrates allow for direct writing of magnetic patterns, structured $h$-BN layers can serve as heat stencils to draw potentially thinner patterns. Besides, $h$-BN encapsulation turns out to be heat-protective (in addition from protecting against oxidation as it is generally used for), allowing the demonstration of room temperature ferromagnetism in $<$7~nm-thick 1$T$-CrTe$_2$.

cond-mat.mtrl-sci↗

arXiv2024

Energy shifts and broadening of excitonic resonances in electrostatically-doped semiconductors

Tuning the density of resident electrons or holes in semiconductors provides crucial insight into the composition of excitonic complexes that are observed as absorption or photoluminescence resonances in optical studies. Moreover, we can change the way these resonances shift and broaden in energy by controlling the quantum numbers of the resident carriers with magnetic fields and doping levels, and by selecting the quantum numbers of the photoexcited or recombining electron-hole (e-h) pair through optical polarization. We discuss the roles of distinguishability and optimality of excitonic complexes, showing them to be key ingredients that determine the energy shifts and broadening of optical resonances in charge-tunable semiconductors. A distinguishable e-h pair means that the electron and hole undergoing photoexcitation or recombination have quantum numbers that are not shared by any of the resident carriers. An optimal excitonic complex refers to a complex whose particles come with all available quantum numbers of the resident carriers. All optical resonances may be classified as either distinct or indistinct depending on the distinguishability of the e-h pair, and the underlying excitonic complex can be classified as either optimal or suboptimal. The universality of these classifications, inherited from the fundamental Pauli exclusion principle, allows us to understand how optical resonances shift in energy and whether they should broaden as doping is increased. This understanding is supported by conclusive evidence that the decay of optical resonances cannot be simply attributed to enhanced screening when resident carriers are added to a semiconductor. Finally, applying the classification scheme in either monolayer or moire heterobilayer systems, we relate the energy shift and amplitude of the neutral exciton resonance to the compressibility of the resident carrier gas.

cond-mat.mtrl-sci↗

arXiv2024

Direct measurement of the lifetime and coherence time of Cu2O Rydberg excitons

Rydberg states of excitons are promising quantum objects to engineer giant nonlinearities in a solid-state system. For this purpose, a deeper understanding of the dynamics of Rydberg excitons and of their potential for coherent manipulation becomes important. We report experimental results where two-photon absorption is resonant with various Rydberg states of excitons in copper oxide and we detect their emission dynamics on a streak camera with sub-picosecond resolution. This technique enables the direct measurement of the Rydberg states lifetimes, which are in good agreement with the expected Rydberg scaling law. Moreover, we observe several intriguing dynamics including the presence of long-lived coherent oscillations. Finally, we independently measure the coherence time of the Rydberg states using a modified Michelson interferometer and find a good agreement with the coherent oscillations detected in the exciton emission dynamics. The lifetimes also reveal the absence of inhomogeneous broadening in the current high-precision spectroscopic data for the S series, which together with the presence of significant coherence time confirms the suitability of the system for coherent engineering.

quant-ph↗

arXiv2024

Quantum light generation with ultra-high spatial resolution in 2D semiconductors via ultra-low energy electron irradiation

Single photon emitters (SPEs) are building blocks of quantum technologies. Defect engineering of 2D materials is ideal to fabricate SPEs, wherein spatially deterministic and quality-preserving fabrication methods are critical for integration into quantum devices and cavities. Existing methods use combination of strain and electron irradiation, or ion irradiation, which make fabrication complex, and limited by surrounding lattice damage. Here, we utilise only ultra-low energy electron beam irradiation (5 keV) to create dilute defect density in hBN-encapsulated monolayer MoS2, with ultra-high spatial resolution (< 50 nm, extendable to 10 nm). Cryogenic photoluminescence spectra exhibit sharp defect peaks, following power-law for finite density of single defects, and characteristic Zeeman splitting for MoS2 defect complexes. The sharp peaks have low spectral jitter (< 200 {\mu}eV), and are tuneable with gate-voltage and electron beam energy. Use of low-momentum electron irradiation, ease of processing, and high spatial resolution, will disrupt deterministic creation of high-quality SPEs.

cond-mat.mes-hall↗

arXiv2024

Using Light to Polarize and Detect Electron Spins in Silicon

Despite decades of research, demonstration of all-optical detection and control of free electron spins in silicon remains elusive. Here, we directly probe the electron spin properties in bulk silicon by measuring the polarization of luminescence following circularly polarized light excitation. The experiments performed for both direct and indirect gap excitation allow not only an experimental determination of the optical selection rules in silicon for the different phononassisted transitions but they also lead to the measurement of the spin relaxation of electrons in conditions which are not accessible using transport techniques. We also measure the spin properties of free excitons in bulk silicon, a very little explored field.

cond-mat.mtrl-sci↗

arXiv2024

Exciton self-trapping in twisted hexagonal boron nitride homostructures

One of the main interests of 2D materials is their ability to be assembled with many degrees of freedom for tuning and manipulating excitonic properties. There is a need to understand how the structure of the interfaces between atomic layers influences exciton properties. Here we use cathodoluminescence and time-resolved cathodoluminescence experiments to study how excitons interact with the interface between two twisted hexagonal boron nitride (hBN) crystals with various angles. An efficient capture of free excitons by the interface is demonstrated, which leads to a population of long-lived and interface-localized (2D) excitons. Temperature dependent experiments indicate that for high twist angles, these excitons localized at the interface further undergo a selftrapping. It consists in a distortion of the lattice around the exciton on which the exciton traps itself. Our results suggest that this exciton-interface interaction causes the broad 4-eV optical emission of highly twisted hBN-hBN structures. Exciton self-trapping is finally discussed as a common feature of sp2 hybridized boron nitride polytypes and nanostructures due to the ionic nature of the B-N bond and the small size of their excitons.

cond-mat.mtrl-sci↗

arXiv2024

Large-scale characterization of Cu2O monocrystals via Rydberg excitons

Rydberg states of excitons can reach microns in size and require extremely pure crystals. We introduce an experimental method for the rapid and spatially-resolved characterization of Rydberg excitons in copper oxide (Cu2O) with sub-micron resolution over large zones. Our approach involves illuminating and imaging the entire sample on a camera to realize a spatially-resolved version of resonant absorption spectroscopy, without any mobile part. This yields spatial maps of Rydberg exciton properties, including their energy, linewidth and peak absorption, providing a comprehensive quality assessment of the entire sample in a single shot. Furthermore, by imaging the sample photoluminescence over the same zone, we establish a strong relationship between the spectral quality map and the photoluminescence map of charged oxygen vacancies. This results in an independent, luminescence-based quality map that closely matches the results obtained through resonant spectroscopy. Our findings reveal that Rydberg excitons in natural Cu2O crystals are predominantly influenced by optically-active charged oxygen vacancies, which can be easily mapped. Together, these two complementary methods provide valuable insights into Cu2O crystal properties.

cond-mat.quant-gas↗