arXiv subjects

Nicolas Stenger

Publications and source records attributed to Nicolas Stenger.

At least 19 recordsLinked to original sources

arXiv2026

Resonant Excitation Induced Vibronic Mollow Triplets

The Mollow triplet is the definitive spectral signature of an optically dressed quantum emitter. We predict that for emitters coupled to localized phonons, this signature is not confined to the zero-phonon line. Under a strong resonant drive, we show that Mollow triplets are strikingly replicated on the associated phonon sidebands -a surprising result, given that phonon sidebands are typically viewed as incoherent, inelastic scattering pathways. These vibronic Mollow triplets are a direct fingerprint of dynamically generated dressed states that hybridize the emitter's electronic, photonic, and vibrational degrees of freedom. We develop a scalable analytical formalism to model this effect in complex, multi-mode molecular systems, such as dibenzoterrylene. Our work provides the precise driving conditions for observing these novel spectral features, establishing a new signature of coherence in vibronically coupled systems.

quant-ph↗

arXiv2025

Integrated on-chip quantum light sources on a van der Waals platform

Scalable photonic quantum information technologies require a platform combining quantum light sources, waveguides, and detectors on a single chip. Here, we introduce a van der Waals platform comprising strain-engineered bilayer WSe$_2$ quantum emitters, integrated on multimode WS$_2$ waveguides with optimized grating couplers, enabling efficient on-chip quantum light sources. The emitters exhibit bright, highly polarized emission that couples efficiently into WS$_2$ waveguides. Under resonant p-shell excitation, we observe high-purity, waveguide-coupled single-photon emission, measured using both an off-chip Hanbury Brown-Twiss configuration ($g^{(2)}(0) = 0.003^{+0.030}_{-0.003}$) and an on-chip configuration ($g^{(2)}(0) = 0.076\pm0.023$). For a single output, the out-coupled single-photon count rate at the first lens reaches approximately 320 kHz under continuous-wave p-shell excitation, corresponding to an estimated waveguide-coupled rate of 1.7 MHz. These results demonstrate an efficient, integrated single-photon source and establish a pathway toward scalable photonic quantum information processing centered around nanoengineered van der Waals materials.

physics.optics↗

arXiv2025

Systematic investigation of the generation of luminescent emitters in hBN via irradiation engineering

Hexagonal boron nitride (hBN), a two-dimensional (2D) material, garners interest for hosting bright quantum emitters at room temperature. A great variety of fabrication processes have been proposed with various yields of quantum emitters. In this work, we study the influence of several parameters, such as irradiation energy, annealing environment, and the type of hBN, on the emitter density in hBN. Our results show (i) high emitter density with oxygen irradiation at 204 eV, (ii) post-annealing in carbon-rich atmospheres significantly increases emitter density, reinforcing carbon's potential role, (iii) no significant effect of oxygen pre-annealing, and (iv) a slightly increased emitter density from hBN crystals with lower structural quality. Although the precise origin of the emitters remains unclear, our study shows that oxygen irradiation and subsequent inert annealing in a carbon-rich environment play a crucial role in emitter generation, while the other processing parameters have a smaller influence. As such, our systematic study and findings show relevant advances towards the reproducible formation of visible-frequency quantum emitters in hBN.

cond-mat.mtrl-sci↗

arXiv2025

Creation and Microscopic Origins of Single-Photon Emitters in Transition Metal Dichalcogenides and Hexagonal Boron Nitride

We highlight recent advances in the controlled creation of single-photon emitters in van der Waals materials and in the understanding of their atomistic origin. We focus on quantum emitters created in monolayer transition-metal dichalcogenide semiconductors, which provide spectrally sharp single-photon emission at cryogenic temperatures, and the ones in insulating hBN, which provide bright and stable single-photon emission up to room temperature. After introducing the different classes of quantum emitters in terms of band-structure properties, we review the defect creation methods based on electron and ion irradiation as well as local strain engineering and plasma treatments. A main focus of the review is put on discussing the microscopic origin of the quantum emitters as revealed by various experimental platforms, including optical and scanning probe methods.

cond-mat.mes-hall↗

arXiv2025

Ultrafast control of coherent acoustic lattice dynamics in the transition metal dichalcogenide alloy WSSe

Coherent acoustic phonons (CAPs)$-$propagating strain waves that can dynamically modify the structure and symmetry of a crystal$-$offer unique opportunities for controlling material properties. We investigate CAP generation in the Janus-like layered alloy tungsten sulfide selenide (WS$_x$Se$_{1-x}$, hereafter WSSe). Employing high-fluence photoexcitation at 400 nm combined with ultrafast transient reflection spectroscopy, we capture the carrier-lattice dynamics governed by a cascade of processes including rapid exciton formation, phonon recycling, and thermoelastic deformation. These phenomena precede the emergence of a robust CAP mode at 27 GHz. Notably, the CAP amplitude in WSSe substantially exceeds that observed in the symmetric parent crystals WS$_2$ and WSe$_2$, which we attribute to an enhanced coupling mediated by a built-in out-of-plane electric field arising from the inversion asymmetry of the WSSe alloy. Furthermore, the implementation of a tailored two-pulse excitation sequence enables optical control of the CAP, underscoring the potential of WSSe and related Janus-like layered alloys as versatile building blocks in optomechanical and nanoacoustic device applications.

cond-mat.mtrl-sci↗

arXiv2025

Fourier-Tailored Light-Matter Coupling in van der Waals Heterostructures

Dielectric structures can support low-absorption optical modes, which are attractive for engineering light-matter interactions with excitonic resonances in two-dimensional (2D) materials. However, the coupling strength is often limited by the electromagnetic field being confined inside the dielectric, reducing spatial overlap with the active excitonic material. Here, we demonstrate a scheme for enhanced light-matter coupling by embedding excitonic tungsten disulfide (WS$_2$) within dielectric hexagonal boron nitride (hBN), forming a van der Waals (vdW) heterostructure that optimizes the field overlap and alignment between excitons and optical waveguide modes. To tailor diffractive coupling between free-space light and the waveguide modes in the vdW heterostructure, we fabricate Fourier surfaces in the top hBN layer using thermal scanning-probe lithography and etching, producing sinusoidal topographic landscapes with nanometer precision. We observe the formation of exciton-polaritons with a Rabi splitting indicating that the system is at the onset of strong coupling. These results demonstrate the potential of Fourier-tailored vdW heterostructures for exploring advanced optoelectronic and quantum devices.

physics.optics↗

arXiv2025

Quantifying the creation of negatively charged boron vacancies in He-ion irradiated hexagonal boron nitride

Hexagonal boron nitride (hBN) hosts luminescent defects possessing spin qualities compatible with quantum sensing protocols at room temperature. Vacancies, in particular, are readily obtained via exposure to high-energy ion beams. While the defect creation mechanism via such irradiation is well understood, the occurrence rate of optically active negatively charged vacancies ($V_B^-$) is an open question. In this work, we exploit focused helium ions to systematically generate optically active vacancy defects in hBN flakes at varying density. By comparing the density-dependent spin splitting measured by magnetic resonance to calculations based on a microscopic charge model, in which we introduce a correction term due to a constant background charge, we are able to quantify the number of $V_B^-$ defects generated by the ion irradiation. We find a lower bound for the fraction (0.2%) of all vacancies in the optically active, negatively charged state. Our results provide a protocol for measuring the generation efficiency of $V_B^-$, which is necessary for understanding and optimizing luminescent centers in hBN.

cond-mat.mes-hall↗

arXiv2024

Collective single-photon emission and energy transfer in thin-layer dielectric and plasmonic systems

We study the collective photon decay of multiple quantum emitters embedded in a thin high-index dielectric layer such as hexagonal boron nitride (hBN), with and without a metal substrate. We first explore the significant role that guided modes including surface plasmon modes play in the collective decay of identical singlephoton emitters (super- and subradiance). Surprisingly, on distances relevant for collective emission, the guided or surface-plasmon modes do not always enhance the collective emission. We identify configurations with inhibition, and others with enhancement of the dipole interaction due to the guided modes. We interpret our results in terms of local and cross densities of optical states. In the same structure, we show a remarkably favorable configuration for enhanced F\"orster resonance energy transfer between a donor and acceptor in the dielectric layer on a metallic substrate. We compare our results to theoretical limits for energy transfer efficiency.

physics.optics↗

arXiv2024

Confocal polarization tomography of dielectric nanocavities

We employ polarization tomography to characterize the modal properties of a dielectric nanocavity with sub-wavelength mode confinement. Our analysis of reflection spectra shows that the Fano-lineshape depends strongly on the polarization in a confocal configuration, and that the lineshape can be transformed into a Lorentzian-like peak for a certain polarization. For this polarization setting, the background is almost fully suppressed in a finite range of frequencies. This enables us to identify another resonance that has not yet been experimentally reported for these nanocavities. Lastly, we use symmetry-forbidden polarizations and show that, surprisingly, the modal resonance features of the system remain visible.

physics.optics↗

arXiv2024

Two-dimensional materials as ideal substrates for molecular quantum emitters

The generation and manipulation of non-classical states of light is central to quantum technologies. Color centers in insulators have been extensively studied for single-photon generation, but organic molecules immobilized on substrates have gained attention due to their superior scalability, large oscillator strengths, and tunable emission frequency. Here, we use first-principles calculations to investigate the photoemission from organic molecules adsorbed on 2D materials. Focusing on terrylene on hexagonal boron nitride (hBN), we obtain zero phonon line (ZPL) energies and emission lineshapes in excellent agreement with experiments. Notably, antisite defects in hBN can immobilize the molecule without influencing its key emission features. We further show that the main effect of the 2D substrate is to introduce sharp sidebands near the ZPL as a fingerprint of hindered rotational, translational, or bending modes of the molecule. Our findings provide insight into how substrate interactions shape the optical properties of molecular systems for quantum applications.

cond-mat.mtrl-sci↗

arXiv2024

Encapsulated void resonators in lossy dielectric van der Waals heterostructures

Dielectric optical resonators traditionally rely on materials with the combination of high refractive indices and low optical losses. Such materials are scarce for operation in visible spectrum and shorter wavelengths. This limitation can be circumvented by relaxing the requirement of low losses. We demonstrate that highly lossy dielectric materials can be structured to support optical resonances that confine light in air voids. We theoretically design void resonances in the visible spectrum and identify resonant modes supported by void arrays. Experimentally, we fabricate void arrays in tungsten diselenide and characterize the confined resonances using far-field reflectance measurements and scanning near-field optical microscopy. Using van der Waals heterostructure assembly, we encapsulate the voids with hexagonal boron nitride which reduces the void volume causing a large spectral blue shift of the void resonance exceeding 150 nm. Our work demonstrates a versatile optical platform for lossy materials, expanding the range of suitable materials and the spectral range of photonic devices.

physics.optics↗

arXiv2024

On-demand heralded MIR single-photon source using a cascaded quantum system

We propose a novel mechanism for generating single photons in the mid-Infrared (MIR) using a solid-state or molecular quantum emitter. The scheme utilises cavity QED effects to selectively enhance a Frank-Condon transition, deterministically preparing a single Fock state of a polar phonon mode. By coupling the phonon mode to an antenna, the resulting excitation is then radiated to the far field as a single photon with a frequency matching the phonon mode. By combining macroscopic QED calculations with methods from open quantum system theory, we show that optimal parameters to generate these MIR photons occur for modest light-matter coupling strengths, which are achievable with state-of-the-art technologies. Combined, the cascaded system we propose provides a new quasi-deterministic source of heralded single photons in a regime of the electromagnetic spectrum where this previously was not possible.

physics.optics↗

arXiv2024

Full quantitative near-field characterization of strongly coupled exciton-plasmon polaritons in thin-layered WSe2 on a monocrystalline gold platelet

Exciton-plasmon polaritons (EPPs) are attractive both for the exploration of fundamental phenomena and applications in nanophotonics. Previous studies of EPPs mainly relied on far-field characterization. Here, using near-field optical microscopy, we quantitatively characterize the dispersion of EPPs existing in 13-nm-thick tungsten diselenide (WSe$_2$) deposited on a monocrystalline gold platelet. We extract from our experimental data a Rabi splitting of 81 meV, and an experimental effective polariton loss of 55 meV, demonstrating that our system is in the strong-coupling regime. Furthermore, we measure for the first time at visible wavelengths the propagation length of these EPPs for each excitation energy of the dispersion relation. To demonstrate the quantitative nature of our near-field method to obtain the full complex-valued wavevector of EPPs, we use our near-field measurements to predict, via the transfer matrix method, the far-field reflectivities across the exciton resonance. These predictions are in excellent agreement with our experimental far-field measurements. Our findings open the door towards the full near-field study of light-manipulating devices at the nanoscale.

physics.optics↗

arXiv2023

A room-temperature moir\'e interlayer exciton laser

Moir\'e superlattices in van der Waals heterostructures offer highly tunable quantum systems with emergent electronic and excitonic properties such as superconductivity, topological edge states, and moir\'e-trapped excitons. Theoretical calculations predicted the existence of the moir\'e potential at elevated temperatures; however, its impact on the optical properties of interlayer excitons (IXs) at room temperature is lacking, and the benefits of the moir\'e effects for lasing applications remain unexplored. We report that the moir\'e potential in a molybdenum disulfide/tungsten diselenide (MoS2/WSe2) heterobilayer system can significantly enhance light emission, elongate the IX lifetime, and modulate the IX emission energy at room temperature. By integrating a moir\'e superlattice with a silicon topological nanocavity, we achieve ultra-low-threshold lasing at the technologically important telecommunication O-band thanks to the significant moir\'e modulation. Moreover, the high-quality topological nanocavities facilitate the highest spectral coherence of < 0.1 nm linewidth among all reported two-dimensional material-based laser systems. Our findings not only open a new avenue for studying correlated states at elevated temperatures, but also enable novel architectures for integrated on-chip photonics and optoelectronics.

physics.optics↗

arXiv2022

Combining experiments on luminescent centres in hexagonal boron nitride with the polaron model and ab initio methods towards the identification of their microscopic origin

The two-dimensional material hexagonal boron nitride (hBN) hosts luminescent centres with emission energies of 2 eV which exhibit pronounced phonon sidebands. We investigate the microscopic origin of these luminescent centres by combining ab initio calculations with non-perturbative open quantum system theory to study the emission and absorption properties of 26 defect transitions. Comparing the calculated line shapes with experiments we narrow down the microscopic origin to three carbon-based defects: $\mathrm{C_2C_B}$, $\mathrm{C_2C_N}$, and $\mathrm{V_NC_B}$. The theoretical method developed enables us to calculate so-called photoluminescence excitation (PLE) maps, which show excellent agreement with our experiments. The latter resolves higher-order phonon transitions, thereby confirming both the vibronic structure of the optical transition and the phonon-assisted excitation mechanism with a phonon energy 170 meV. We believe that the presented experiments and polaron-based method accurately describe luminescent centres in hBN and will help to identify their microscopic origin.

cond-mat.mtrl-sci↗

arXiv2022

Signatures of Non-Markovianity in Cavity-QED with Color Centers in 2D Materials

Light-matter interactions of defects in two dimensional materials are expected to be profoundly impacted by strong coupling to phonons. In this work, we combine ab initio calculations of a defect in hBN, with a fully quantum mechanical and numerically exact description of a cavity-defect system to elucidate this impact. We show that even at weak light-matter coupling, the dynamical evolution of the cavity-defect system has clear signatures of non-markovian phonon effects, and that linear absorption spectra show the emergence of hybridised light-matter-phonon states in regimes of strong light-matter coupling. We emphasise that our methodology is general, and can be applied to a wide variety of material/defect systems.

quant-ph↗

arXiv2022

Nanometer-scale photon confinement in topology-optimized dielectric cavities

Nanotechnology enables in principle a precise mapping from design to device but relied so far on human intuition and simple optimizations. In nanophotonics, a central question is how to make devices in which the light-matter interaction strength is limited only by materials and nanofabrication. Here, we integrate measured fabrication constraints into topology optimization, aiming for the strongest possible light-matter interaction in a compact silicon membrane, demonstrating an unprecedented photonic nanocavity with a mode volume of $V\sim3\times10^{-4}\,λ^3$, quality factor $Q\sim1100$, and footprint $4\,λ^2$ for telecom photons with a $λ\sim 1550$ nm wavelength. We fabricate the cavity, which confines photons inside 8 nm silicon bridges and use near-field optical measurements to perform the first experimental demonstration of photon confinement to a single hotspot well below the diffraction limit in dielectrics.

physics.optics↗

arXiv2022

Quantitative near-field characterization of surface plasmon polaritons on monocrystalline gold platelets

The subwavelength confinement of surface plasmon polaritons (SPPs) makes them attractive for various applications such as sensing, light generation and solar energy conversion. Near-field microscopy associated with interferometric detection allows to visualize both the amplitude and phase of SPPs. However, their full quantitative characterization in a reflection configuration is challenging due to complex wave patterns arising from the interference between several excitation channels. Here, we present near-field measurements of SPPs on large monocrystalline gold platelets in the visible spectral range. We study systematically the influence of the incident angle of the exciting light on the SPPs launched by an atomic force microscope tip. We find that the amplitude and phase signals of these SPPs are best disentangled from other signals at grazing incident angle relative to the edge of the gold platelet. Furthermore, we introduce a simple model to explain the phase shift observed between the SPP amplitude and phase profiles. Using this model, the wavelength and propagation length of the tip-launched plasmons are retrieved by isolating and fitting their signals far from the platelets edges. Our experimental results are in excellent agreement with theoretical models using gold refractive index values. The presented method to fully characterize the SPP complex wavevector could enable the quantitative analysis of polaritons occurring in different materials at visible wavelengths.

physics.optics↗