Valley coherence refers to a phase-coherent superposition of inequivalent momentum valleys, in which quantum information can be encoded in the relative valley phase. Chiral nematic liquid crystals, by imposing a flip of the spin angular momentum upon light reflection, provide an effective photonic environment for optically coupling excitons of the K and K' valleys in monolayer semiconducting transition-metal dichalcogenides. We experimentally demonstrate that using such liquid crystal as a substrate, it is possible through nearfield interaction to engineer a room temperature mechanism for inducing the intervalley coupling. Our results show that this approach provides a simple and scalable route toward valleytronic functionalities based on controlled coherent emission from valleys with opposite Berry curvature.

We present "torchGDM", a numerical framework for nano-optical simulations based on the Green's Dyadic Method (GDM). This toolkit combines a hybrid approach, allowing for both fully discretized nano-structures and structures approximated by sets of effective electric and magnetic dipoles. It supports simulations in three dimensions and for infinitely long, two-dimensional structures. This capability is particularly suited for multi-scale modeling, enabling accurate near-field calculations within or around a discretized structure embedded in a complex environment of scatterers represented by effective models. Importantly, torchGDM is entirely implemented in PyTorch, a well-optimized and GPU-enabled automatic differentiation framework. This allows for the efficient calculation of exact derivatives of any simulated observable with respect to various inputs, including positions, wavelengths or permittivity, but also intermediate parameters like Green's tensors. We anticipate that this toolkit will be valuable for applications merging nano-photonics and machine learning, as well as for solving nano-photonic optimization and inverse problems, such as the global design and characterization of metasurfaces, where optical interactions between structures are critical.

Controlling the motion of neutral excitons in optically active media is a mandatory development to enable the conception of advanced circuits and devices for applications in excitronics, quantum photonics, and optoelectronics. Recently, proof of unidirectional exciton transport from high- to low-bandgap material is evidenced using a high-quality lateral heterostructure separating transition metal dichalcogenide monolayers (TMD-MLs). In this paper, by combining room-temperature micro-photoluminescence far-field imaging with a statistical description of exciton transport, the underlying excitonic local distribution and fluxes taking place near lateral heterojunctions are unveiled. The complex 2D excitonic transport properties found near a linear interface separating WSe2 from MoSe2 TMD-MLs are studied and reveal two distinct diffusion regimes profoundly affecting the effective diffusion length. Then, it is shown that combining two and three of these interfaces, allows advanced in-plane control of the excitonic distribution and flux over large distances. Exciton focalization and trapping, allowing an increase in the local exciton density up to three orders of magnitude are demonstrated. Finally, flux collimation is achieved with the formation of parallel current lines extending a few micrometers away from the source. We believe that the deterministic shaping and positioning of the exciton distribution and flux shown here will be key toward the conception of realistic excitronic devices.

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.

Improving the brightness and efficiency of single photon sources by the mean of optically resonant nanostructures is a major stake for the development of efficient nanodevices for quantum communications. For a couple of decades, these resonant nanostructures have mainly been made of noble metal that sustain strong localized resonances (LSPR) that can be used to manipulate, concentrate or redirect visible light. Such properties have led to numerous actual or potential applications in integrated optics, sensors, nonlinear optics, field-enhanced spectroscopies, or photovoltaics. Recently, an alternative emerged with high refractive index dielectric nanostructures, which offer the same range of applications as plasmonics by manipulating Mie optical resonances instead of LSPR [1]. These resonances can be efficiently tuned by modifying the size, shape, and material of those nanostructures (e.g. silicon, n ~ 4) [2]. Furthermore, high index dielectric nanostructures offer several key advantages when compared to their metallic counterparts: absorption losses are far weaker for wavelengths longer than the direct band gap, access to semiconductor (CMOS) technology for nanostructure fabrication, and presence of intrinsic strong magnetic and electric resonances, providing an unique opportunity to spatially separate and redistribute the energy of the magnetic and electric parts of the electromagnetic field in the near field, otherwise inextricably connected in the far field [3]. We discuss here the effect of simple high-index dielectric nanoantennas on the spontaneous emission of model quantum emitters. First, we accurately positioned arrays of nanodiamonds hosting NV colored centers in the gap of silicon dimer nanoantennas, using atomic force microscopy (AFM – [4]) nanoxerography [5]. The NV center is an ideal model system exhibiting single photon emission properties at room temperature, which is well adapted for proof of concept experiments in quantum nano-optics. Since the local density of photonic states (LDOS) is modified by the nanostructure, we show by the mean of time-resolved photoluminescence acquisitions that the photodynamics of these quantum emitters can be enhanced by the coupling to the nanoantenna, down to the single photon emission regime [5]. Our experimental results are in good agreement with multipolar analysis and numerical simulations based on the Green Dyadic Method - GDM. Second, in order to go a step further, we explore the control of the directive emission from quantum dipolar sources by the mean of complex silicon antennas. The latter, made of a given number of building (nano)blocks, are optimized by an Evolutionary Algorithm (EA) coupled to full-field electrodynamical GDM simulations. The geometries obtained allow to tailor the emission direction of a single dipolar source, maximizing the intensity of the emitted light within a given solid angle. Our numerical and preliminary experimental results demonstrate the efficiency of such compact EA-optimized antennas for the control of quantum nanosources. Acknowledgements : This work was supported by Programme Investissements d’Avenir through the grants ANR-10-LABX-0037-NEXT (MILO) and NanoX n° ANR-17-EURE- 0009 (Q-META), by the HiLight ANR project ANR-19-CE24-0026, by LAAS-CNRS micro and nanotechnologies platform member of the French Renatech network, by the CNRS and INSA (PhD grant), by the Région Midi-Pyrénées via the Institute for Quantum technologies in Occitanie (IQO), and by the computing facility center CALMIP of the University of Toulouse under grants P12167 and P1107. References : Kallel et al, Tunable enhancement of light absorption and scattering in Si1xGex nanowires, Phys. Rev. B 12: 085318, 2012. P.R. Wiecha et al., Strongly directional scattering from dielectric nanowires, ACS Photonics 4: pp 2036–2046, 2017. Montagnac et al., Control of light emission of quantum emitters coupled to silicon nanoantenna using cylindrical vector beams, Light : Science and Applications 12 (1), 239, 2023. Humbert et al., Versatile, rapid and robust nanopositioning of single-photon emitters by AFM-nanoxerography, Nanotechnology 33, 215301, 2022. Humbert et al., Large-scale controlled coupling of single-photon emitters to high index dielectric nanoantennas using AFM nanoxerography, Nanoscale 15, 599-608, 2023. Figure 1

Achieving high-resolution and cost-effective microdisplays for augmented and virtual reality applications now requires shaping the color conversion quantum-dot-based layers into structures corresponding to micropixels. We propose herein an alternative and versatile approach for addressing this challenge, nanoxerography, an electrostatic-based technique to selectively direct the assembly of colloidal nano-objects on precisely chosen areas of electrets. Coassembly of red- and green-emitting core-shell Cd(Se,S)/(Cd,Zn)S quantum nanoplatelets was performed on passive (PMMA/ITO/glass) and active (PMMA/ITO/GaN mu LEDS) substrates. Topographical, optical, and electro-optical characterizations were further realized. Lateral resolution for subpixels of 800 nm (using charge injection by atomic force microscopy) and 3 mu m (using charge injection by electrical microcontact printing) over surfaces larger than 1 cm(2) in the later case was demonstrated. Nanoxerography was also proven to not alter the optical properties of the assembled quantum nanoplatelets. Without even the need of optical black barriers, no crosstalk between quantum nanoplatelet-based subpixels was observed for interspace greater than 1 mu m. Red emitting quantum nanoplatelets assembled as 7.5 mu m squared subpixels on active GaN blue-lit active substrates give an external quantum efficiency and internal quantum efficiency of 7.5 and 17%, respectively.

We optimize silicon nano-antennas to enhance and steer the emission of local quantum sources. We combine global evolutionary optimization (EO) with frequency domain electrodynamical simulations, and compare design strategies based on resonant and non-resonant building blocks. Specifically, we investigate the performance of models with different degrees of freedom but comparable amount of available material. We find that simpler geometric models allow significantly faster convergence of the optimizer, which, expectedly, comes at the cost of a reduced optical performance. We finally analyze the physical mechanisms underlying the directional emission that also comes with an emission rate enhancement, and find a surprising robustness against perturbations of the source emitter location. This makes the structures highly interesting for actual nano-fabrication. We believe that optimized, all-dielectric silicon nano-antennas have high potential for genuine breakthroughs in a multitude of applications in nanophotonics and quantum technologies.

Controlling the motion of neutral excitons in optically active media is a mandatory development to enable the conception of advanced circuits and devices for applications in excitronics, quantum photonics, and optoelectronics. Recently, a proof of unidirectional exciton transport from high- to low-band-gap material has been evidenced using a high-quality lateral heterostructure separating transition metal dichalcogenide monolayers (TMD-MLs). In this paper, by combining room-temperature micro-photoluminescence far-field imaging with a statistical description of exciton transport, we unveil the underlying excitonic local distribution and fluxes taking place near lateral heterojunctions. We study the complex 2D excitonic transport properties found near a linear interface separating WSe$_2$ from MoSe$_2$ TMD-MLs and reveal two distinct diffusion regimes profoundly affecting the effective diffusion length. Then, we show that combining two and three of these interfaces, allows advanced in-plane control of the excitonic distribution and flux over large distances. We demonstrate exciton focalization and trapping, allowing an increase in the local exciton density up to three orders of magnitude. Finally, we achieve flux collimation with the formation of parallel current lines extending a few micrometers away from the source. We believe that the deterministic shaping and positioning of the exciton distribution and flux here will be a key towards the conception of realistic excitronic devices.

Light emission of europium (Eu 3+ ) ions placed in the vicinity of optically resonant nanoantennas is usually controlled by tailoring the local density of photon states (LDOS). We show that the polarization and shape of the excitation beam can also be used to manipulate light emission, as azimuthally or radially polarized cylindrical vector beam offers to spatially shape the electric and magnetic fields, in addition to the effect of silicon nanorings (Si-NRs) used as nanoantennas. The photoluminescence (PL) mappings of the Eu 3+ transitions and the Si phonon mappings are strongly dependent of both the excitation beam and the Si-NR dimensions. The experimental results of Raman scattering and photoluminescence are confirmed by numerical simulations of the near-field intensity in the Si nanoantenna and in the Eu 3+ -doped film, respectively. The branching ratios obtained from the experimental PL maps also reveal a redistribution of the electric and magnetic emission channels. Our results show that it could be possible to spatially control both electric and magnetic dipolar emission of Eu 3+ ions by switching the laser beam polarization, hence the near field at the excitation wavelength, and the electric and magnetic LDOS at the emission wavelength. This paves the way for optimized geometries taking advantage of both excitation and emission processes.

Optical Mie resonators based on silicon nanostructures allow tuning of light-matter-interaction with advanced design concepts based on complementary metal–oxide–semiconductor (CMOS) compatible nanofabrication. Optically active materials such as transition-metal dichalcogenide (TMD) monolayers can be placed in the near-field region of such Mie resonators. Here, we experimentally demonstrate and verify by numerical simulations coupling between a MoSe 2 monolayer and the near-field of dielectric nanoresonators. Through a comparison of dark-field (DF) scattering spectroscopy and photoluminescence excitation experiments (PLE), we show that the MoSe 2 absorption can be enhanced via the near-field of a nanoresonator. We demonstrate spectral tuning of the absorption via the geometry of individual Mie resonators. We show that we indeed access the optical near-field of the nanoresonators, by measuring a spectral shift between the typical near-field resonances in PLE compared to the far-field resonances in DF scattering. Our results prove that using MoSe 2 as an active probe allows accessing the optical near-field above photonic nanostructures, providing complementary information to sophisticated near-field microscopy equipment.

The hierarchically directed assembly of multiple types of colloidal nano-objects on surfaces is of interest for developing disruptive applications combining their original properties. We propose herein a versatile, electrostatically driven strategy to arrange various kinds of colloids vertically in the shape of 3D micropatterns by nanoxerography. We made the proof of concept of this vertical combinatorial nano-object patterning using two types of photoluminescent CdSe(S)/CdZnS core/shell nanoplatelets emitting in the red and green wavelengths as model colloidal nanoparticles. The key experimental parameters were investigated to tune the thickness of each independent level of nanoplatelets within the vertical stack. We finally applied such a concept to make dual-colored nanoplatelet patterns. Interestingly, we proved numerically that the relatively high index of the nanoplatelet level is responsible for the partially directed emissions observed in photoluminescence experiments.

Improving the brightness of single-photon sources by means of optically resonant nanoantennas is a major stake for the development of efficient nanodevices for quantum communications. We demonstrate that nanoxerography by atomic force microscopy makes possible the fast, robust and repeatable positioning of model quantum nanoemitters (nitrogen-vacancy NV centers in nanodiamonds) on a large-scale in the gap of silicon nanoantennas with a dimer geometry. By tuning the parameters of the nanoxerography process, we can statistically control the number of deposited nanodiamonds, yielding configurations down to a unique single photon emitter coupled to these high index dielectric nanoantennas, with high selectivity and enhanced brightness induced by a near-field Purcell effect. Numerical simulations are in very good quantitative agreement with time-resolved photoluminescence experiments. A multipolar analysis reveals in particular all the aspects of the coupling between the dipolar single emitter and the Mie resonances hosted by these simple nanoantennas. This proof of principle opens a path to a genuine and large-scale spatial control of the coupling of punctual quantum nanoemitters to arrays of optimized optically resonant nanoantennas. It paves the way for future fundamental studies in quantum nano-optics and toward integrated photonics applications for quantum technologies.

We design planar silicon antennas for controlling the emission rate of magnetic or electric dipolar emitters. Evolutionary algorithms coupled to the Green Dyadic Method lead to different optimized geometries which depend on the nature and orientation of the dipoles. We discuss the physical origin of the obtained configurations thanks to modal analysis but also emphasize the role of nanoscale design of the LDOS. We complete our study using finite element method and demonstrate an enhancement up to 2 × 103 of the magnetic Purcell factor in europium ions. Our work brings together random optimizations to explore geometric parameters without constraint, a first order deterministic approach to understand the optimized designs and a modal analysis which clarifies the physical origin of the exaltation of the magnetic Purcell effect.

Quantum nano-optics aims at transposing the concepts of quantum optics at the nanoscale. In this context, nitrogen-vacancy color centers in nanodiamonds are particularly interesting model sources as they emit single photons one by one with a broad spectral distribution between 600 and 800 nm. We deposit such quantum emitters at the extremities of silicon nanowires and analyze the effects of this interaction via spectrally resolved confocal microscopy. We demonstrate that the single-photon emission can be guided over several microns under conservation of their quantum statistics, while the silicon nanowires act as spectral filters for the quantum emission. Numerical simulations are in very good agreement with experiments, and indicate that the modal landscape of these nanowaveguides and the according coupling efficiencies with quantum emitters can be designed such that only light from emitters of specific orientations is efficiently guided through the nanowire, enabling a unique quantum state selectivity at a micrometer length scale. Our work opens the door to a genuine modal control of single-photon transfer in subwavelength waveguides, paving the way to future fundamental studies and toward integrated and multiwavelength photonics applications in quantum nano-optics.

The scattering properties of metallic optical antennas are typically examined through the lens of their plasmonic resonances. However, non-plasmonic transition metals also sustain surface waves in the visible. We experimentally investigate in this work the far-field diffraction properties of apertured optical antennas milled on non-plasmonic W films and compare the results with plasmonic references in Ag and Au. The polarization-dependent diffraction patterns and the leakage signal emerging from apertured antennas in both kinds of metals are recorded and analyzed. This thorough comparison with surface plasmon waves reveals that surface waves are launched on W and that they have the common abilities to confine the visible light at metal-dielectric interfaces offering the possibility to tailor the far-field emission. The results have been analyzed through theoretical models accounting for the propagation of a long range surface mode launched by subwavelength apertures, that is scattered in free space by the antenna. This surface mode on W can be qualitatively described as an analogy in the visible of the Zenneck wave in the radio regime. The nature of the new surface waves have been elucidated from a careful analysis of the asymptotic expansion of the electromagnetic propagators, which provides a convenient representation for explaining the Zenneck-like character of the excited waves and opens new ways to fundamental studies of surface waves at the nanoscale beyond plasmonics.

Processing information with conventional integrated circuits remains beset by the interconnect bottleneck: circuits made of smaller active devices need longer and narrower interconnects, which have become the prime source of power dissipation and clock rate saturation. Optical interchip communication provides a fast and energy-saving option that still misses a generic on-chip optical information processing by interconnect-free and reconfigurable Boolean arithmetic logic units (ALU). Considering metal plasmons as a platform with dual optical and electronic compatibilities, we forge interconnect-free, ultracompact plasmonic Boolean logic gates and reconfigure them, at will, into computing ALU without any redesign nor cascaded circuitry. We tailor the plasmon mode landscape of a single 2.6 μm2 planar gold cavity and demonstrate the operation and facile reconfiguration of all 2-input logic gates. The potential for higher complexity of the same logic unit is shown by a multi-input excitation and a phase control to realize an arithmetic 2-bit adder.

Dataset supports the paper: Clement Majorel, Christian Girard, Aurelien Cuche, Arnaud Arbouet, and Peter R. Wiecha Quantum Theory of Near-field Optical Imaging with Rare-earth Atomic Clusters. JOSA B 37(5), 1474-1484 (2020)

Monolayers (MLs) of transition metal dichalcogenides (TMDs) such as WSe2 and MoSe2 can be placed by dry stamping directly on broadband dielectric resonators, which have the ability to enhance the spontaneous emission rate and brightness of solid-state emitters at room temperature. We show strongly enhanced emission and directivity modifications in room-temperature photoluminescence mapping experiments. By varying TMD material (WSe2 vs MoSe2) transferred on silicon nanoresonators with various designs (planarized vs nonplanarized), we experimentally separate the different physical mechanisms that govern the global light emission enhancement. For WSe2 and MoSe2, we address the effects of Mie resonances and strain in the monolayer. For WSe2, an important additional contribution comes from out-of-plane exciton dipoles. This paves the way for more targeted designs of TMD-Si nanoresonator structures for room-temperature applications.

Scanning near-field optical imaging (SNOM) using local active probes provides in general images of the electric part of the photonic local density of states. However, certain atomic clusters can supply more information by simultaneously revealing both the magnetic (m-LDOS) and the electric (e-LDOS) local density of states in the optical range. For example, nanoparticles doped with rare-earth elements like europium or terbium provide both electric dipolar (ED) and magnetic dipolar (MD) transitions. In this theoretical article, we develop a quantum description of active systems (rare earth ions) coupled to a photonic nanostructure, by solving the optical Bloch equations together with Maxwell's equations. This allows us to access the population of the emitting energy levels for all atoms excited by the incident light, degenerated at the extremity of the tip of a near-field optical microscope. We show that it is possible to describe the collected light intensity due to ED and MD transitions in a scanning configuration. By carrying out simulations on different experimentally interesting systems, we demonstrate that our formalism can be of great value for the interpretation of experimental configurations including various external parameters (laser intensity, polarization and wavelength, the SNOM probe size, the nature of the sample ...).

In the context of the emerging field of quantum plasmonics, we demonstrate in this manuscript the wavelength-dependent propagation and sorting of single plasmons launched in a two-dimensional crystalline gold flake by a broadband quantum nanoemitter. The stream of single plasmons in the visible is produced by a nanodiamond hosting a single nitrogen-vacancy color center positioned in the near field of the mesoscopic metallic microplatelet. Spatially and spectrally resolved images of the single plasmon propagation in the pristine hexagonal flake, and then in the same structure after insertion of a Bragg mirror, are obtained by filtered image-plane acquisitions on a leakage-radiation microscope. Our work on two-dimensional crystalline structures paves the way to future fundamental studies and applications in quantum plasmonics.