Quantum Dot Transfer from the Organic Phase to Acrylic Monomers for the Controlled Integration of Single-Photon Sources by Photopolymerization.

This paper reports on a new strategy for obtaining homogeneous dispersion of grafted quantum dots (QDs) in a photopolymer matrix and their use for the integration of single-photon sources by two-photon polymerization (TPP) with nanoscale precision. The method is based on phase transfer of QDs from organic solvents to an acrylic matrix. The detailed protocol is described, and the corresponding mechanism is investigated and revealed. The phase transfer is done by ligand exchange through the introduction of mono-2-(methacryloyloxy) ethyl succinate (MES) that replaces oleic acid (OA). Infrared (IR) measurements show the replacement of OA on the QD surface by MES after ligand exchange. This allows QDs to move from the hexane phase to the pentaerythritol triacrylate (PETA) phase. The QDs that are homogeneously dispersed in the photopolymer without any clusterization do not show any significant broadening in their photoluminescence spectra even after more than 3 years. The ability of the hybrid photopolymer to create micro- and nanostructures by two-photon polymerization is demonstrated. The homogeneity of emission from 2D and 3D microstructures is confirmed by confocal photoluminescence microscopy. The fabrication and integration of a single-photon source in a spatially controlled manner by TPP is achieved and confirmed by auto-correlation measurements.

Strategy for Patterning Single Colloidal Gold Nanoparticles on Two Photon Polymerized and Functionalized Ultrathin Nanostructures.

Local and deterministic trapping of single nanoparticles (NPs) has always been a challenging topic due to the difficulties faced at such a small particle size. These difficulties are concerned with the stability, simplicity, robustness, and efficiency of the used trapping technique. Here, we used two-photon polymerization (TPP) of a prefunctionalized photopolymer to obtain a nanometric polymer layer and selectively attract single colloidal gold NPs (AuNPs). Thanks to a deep photochemical study of the threshold energy, we identified a photopolymerization regime allowing one to tune the polymer size and immobilize single gold nanoparticles. This method is promising for the fabrication of single photon sources.

Phase-resolved all-fiber reflection-based s-NSOM for on-chip characterization.

We report on a phase-resolved, reflection-based, scattering-type near-field scanning optical microscope technique with a convenient all-fiber configuration. Exploiting the flexible positioning of the near-field probe, our technique renders a heterodyne detection for phase measurement and point-to-point frequency-domain reflectometry for group index and loss measurement of waveguides on a chip. The important issue of mitigating the measurement errors due to environmental fluctuations along fiber-optic links has been addressed. We perform systematic measurements on different types of silicon waveguides which demonstrate the accuracy and precision of the technique. With a phase compensation approach on the basis of a common-path interferometer, the phase drift error is suppressed to ∼ 0.013°/s. In addition, characterizations of group index, group velocity dispersion, propagation loss, insertion loss, and return loss of component waveguides on a chip are all demonstrated. The measurement accuracy of the propagation loss of a ∼ 0.2 cm long nano-waveguide reaches ±1 dB/cm. Our convenient and versatile near-field characterization technique paves the way for in-detail study of complex photonic circuits on a chip.

Waveguide efficient directional coupling and decoupling via an integrated plasmonic nanoantenna.

The development of integrated photonic devices has led to important advancements in the field of light-matter interaction at the nanoscale. One of the main focal points is the coupling between single photon emitters and optical waveguides aiming to achieve efficient optical confinement and propagation. In this work, we focus on the characterization of a hybrid dielectric/plasmonic waveguide consisting of a gold triangular nanoantenna placed on top of a TiO2 waveguide. The strong directionality of the device is experimentally demonstrated by comparing the intensity scattered by the nanotriangle to the one scattered by a SNOM tip for different illumination geometries. The ability of the plasmonic antenna to generate powerful coupling between a single emitter and the waveguide will also be highlighted through numerical simulations.

Hybrid plasmonic nano-emitters with controlled single quantum emitter positioning on the local excitation field.

Hybrid plasmonic nano-emitters based on the combination of quantum dot emitters (QD) and plasmonic nanoantennas open up new perspectives in the control of light. However, precise positioning of any active medium at the nanoscale constitutes a challenge. Here, we report on the optimal overlap of antenna's near-field and active medium whose spatial distribution is controlled via a plasmon-triggered 2-photon polymerization of a photosensitive formulation containing QDs. Au nanoparticles of various geometries are considered. The response of these hybrid nano-emitters is shown to be highly sensitive to the light polarization. Different light emission states are evidenced by photoluminescence measurements. These states correspond to polarization-sensitive nanoscale overlap between the exciting local field and the active medium distribution. The decrease of the QD concentration within the monomer formulation allows trapping of a single quantum dot in the vicinity of the Au particle. The latter objects show polarization-dependent switching in the single-photon regime.

Aperiodic differential method associated with FFF: an efficient electromagnetic computational tool for integrated optical waveguides modelization.

A reformulation of the differential theory associated with fast Fourier factorization used for periodic diffractive structures is presented. The incorporation of a complex coordinate transformation in the propagation equations allows the modeling of semi-infinite open problems through an artificially periodized space. Hence, the outgoing wave conditions of an open structure must be satisfied. On the other hand, the excitation technique must be adjusted to adapt with guided structures. These modifications turn the differential theory into an aperiodic tool used with guided optical structure. Our method is verified through numerical results and comparisons with the aperiodic Fourier modal method showing enhanced convergence and accuracy, especially when complex-shaped photonic guided devices are considered.

Excitation of surface plasmon polaritons in a gold nanoslab on ion-exchanged waveguide technology.

Integrated metaphotonic devices has opened new horizons to control light-guiding properties at nanoscale; particularly interesting is the application of plasmonic nanostructures coupled to dielectric waveguides to reduce the inherent light propagation losses in metallic metamaterials. In this contribution, we show the feasibility of using ion-exchanged glass waveguides (IExWg) as a platform for the efficient excitation of surface plasmon polaritons (SPP). These IExWg provide high coupling efficiency and low butt-coupling with conventional dielectric optical waveguides and fibers, overcoming the hard fabrication tunability of commonly used CMOS-guiding platforms. We present a near-field scanning optical microscopy characterization of the propagation characteristics of SPP supported in a gold nanoslab fabricated on top of an IExWg. We found that the SPP can be only be excited with the fundamental TM photonic mode of the waveguide. Thanks to the low propagation loss, low birefringence, and compatibility with optical fibers, glass waveguide technology is a promising platform for the development of integrated plasmonic devices operating at visible and near infrared wavelengths with potential applications in single molecule emission routing or biosensing devices.

Meta-model-based multi-objective optimization for robust color reproduction using hybrid diffraction gratings.

We propose an efficient and versatile optimization scheme, based on the combination of multi-objective genetic algorithms and neural-networks, to reproduce specific colors through the optimization of the geometrical parameters of metal-dielectric diffraction gratings. To illustrate and assess the performance of this approach, we tailor the chromatic response of a structure composed of three adjacent hybrid V-groove diffraction gratings. To be close to the experimental situation, we include the feasibility constraints imposed by the fabrication process. The strength of our approach lies in the possibility to simultaneously optimize different contradictory objectives, avoiding time-consuming electromagnetic calculations.

In-plane electric field confinement engineering in graphene-based hybrid plasmonic waveguides.

Surface plasmon polaritons (SPPs) are surface modes confined to metal-dielectric interfaces. This confinement enhances the electromagnetic field and therefore, SPPs are sensitive to surface conditions. The properties of two dimensional materials such as graphene thus can be enhanced and used to engineer nanoscale components for optical communications. However, SPPs are transverse magnetic modes with electric fields out-of-plane that limit flexibility. In this contribution, we numerically analyze the confinement and in-plane enhancement in graphene-based hybrid plasmonic waveguides. We find that plasmonic modes supported by metal nanoparticle chain waveguides provide higher in-plane enhancement compared to those supported by nano-strip and slot hybrid plasmonic waveguides. Our results contribute to the performance improvement of graphene light absorption devices, including electro-optic modulators and photodetectors.

Nanoscale plasmonic TM-pass polarizer integrated on silicon photonics.

Transverse electric (TE)-pass polarizers integrated in silicon-on-insulator photonic circuits, based on hybrid plasmonic absorption, have been widely discussed as a key component for applications in information and communication technologies. Nevertheless, the complementary transverse magnetic (TM)-pass polarizer has not been developed due to the TM nature of plasmonic modes. Here, we experimentally demonstrate a nano-scale TM-pass polarizer based on TE-polarized plasmonic absorption using a periodic metal nanoparticle chain integrated on a silicon waveguide. Currently, the measured extinction ratio is 23 dB µm-1 and the insertion loss is 2.4 dB µm-1 at the central wavelength of 1.59 µm, in a device with a footprint of 630 nm (0.4λ). This polarization-selective absorption is analyzed using dispersion curves and transmission near-field scanning optical microscopy. The nanophotonic device completes an integrated polarizer based on plasmonic absorption and provides the necessary footprint for high density integration in photonic integrated circuits.

Metamodeling of high-contrast-index gratings for color reproduction.

We propose a metamodel-based optimization technique to tailor the chromatic response of high-contrast-index gratings. The algorithm, which couples a population-based metaheuristic with a neural network, is used to retrieve the optimal geometrical parameters of a grating to reproduce a prescribed color. By means of some examples, we assess the possibilities and limitations of our optimization scheme. The numerical evidence found shows that the metamodel approach offers an alternative to traditional metaheuristic techniques that not only provides the best solution for a given geometry and a material but also significantly improves the computing time required for the optimization process.

Reciprocity and Babinet's principles applied to the enhancement of the electric and magnetic local density of states in integrated plasmonics on silicon photonics.

Here, reciprocity and Babinet's principles were applied to the design of integrated plasmonic structures on silicon photonic waveguides. Numerical analyses and near-field optical microscopy observations show that one of the hybrid photonic-plasmonic structures exhibits high confinement and enhancement of the electric field, and, through Babinet's principle, the magnetic field of its complementary structure is confined and enhanced as well. Reciprocally, due to the modification of the electric and magnetic local density of states, enhanced emission of electric and magnetic dipoles by Purcell effect were obtained into specific silicon photonic modes. Such structures can be advantageously implemented for on-chip integrated single-photon sources.

Imaging of guided waves using an all-fiber reflection-based NSOM with self-compensation of a phase drift.

A phase-resolved reflection-based near-field scanning optical microscopy (NSOM) technique with an original all-fiber configuration is presented. Our system consists of an intrinsically phase-stable common-path interferometer. The reflection from the waveguide input facet or from an integrated fiber Bragg grating is used as the reference beam. This arrangement effectively suppresses the phase drift caused by environmental fluctuations. By raster scanning a silicon atomic force microscope probe, we measure the complex near fields of the propagating and stationary waves in silicon nanowaveguides. Our robust, align-free, cost-effective, and shot-noise-limited near-field imaging technique paves the way for versatile optical characterizations of nanophotonic structures on a chip.

Optimization of all-dielectric structures for color generation.

In this work, we propose an inversion scheme to tailor the chromatic response of an all-dielectric structure. To this end, we couple, through a previously defined objective functional involving the concept of color difference, a forward solver with an optimization algorithm. The former is based on the differential method, whereas the latter is based on particle swarm optimization. The optimal geometrical parameters of the structure that generates a specific color are obtained through the solution of an approximation problem. We illustrate the performance of our inversion scheme through examples and discuss its limitations and potential applications.

Full control of far-field radiation via photonic integrated circuits decorated with plasmonic nanoantennas.

We theoretically develop a hybrid architecture consisting of photonic integrated circuit and plasmonic nanoantennas to fully control optical far-field radiation with unprecedented flexibility. By exploiting asymmetric and lateral excitation from silicon waveguides, single gold nanorod and cascaded nanorod pair can function as component radiation pixels, featured by full 2π phase coverage and nanoscale footprint. These radiation pixels allow us to design scalable on-chip devices in a wavefront engineering fashion. We numerically demonstrate beam collimation with 30° out of the incident plane and nearly diffraction limited divergence angle. We also present high-numerical-aperture (NA) beam focusing with NA ≈0.65 and vector beam generation (the radially-polarized mode) with the mode similarity greater than 44%. This concept and approach constitutes a designable optical platform, which might be a future bridge between integrated photonics and metasurface functionalities.

Local density of electromagnetic states in plasmonic nanotapers: spatial resolution limits with nitrogen-vacancy centers in diamond nanospheres.

One of the most explored single quantum emitters for the development of nanoscale fluorescence lifetime imaging is the nitrogen-vacancy (NV) color center in diamond. An NV center does not experience fluorescence bleaching or blinking at room temperature. Furthermore, its optical properties are preserved when embedded into nanodiamond hosts. This paper focuses on the modeling of the local density of states (LDOS) in a plasmonic nanofocusing structure with an NV center acting as local illumination sources. Numerical calculations of the LDOS near such a nanostructure were done with a classical electric dipole radiation placed inside a diamond sphere as well as near-field optical fluorescence lifetime imaging of the structure. We found that Purcell factors higher than ten can be reached with diamond nanospheres of radius less than 5 nm and at a distance of less than 20 nm from the surface of the structure. Although the spatial resolution of the experiment is limited by the size of the nanodiamond, our work supports the analysis and interpretation of a single NV color center in a nanodiamond as a probe for scanning near-field optical microscopy.

Direct measurement of the effective infrared dielectric response of a highly doped semiconductor metamaterial.

We have investigated the effective dielectric response of a subwavelength grating made of highly doped semiconductors (HDS) excited in reflection, using numerical simulations and spectroscopic measurement. The studied system can exhibit strong localized surface resonances and has, therefore, a great potential for surface-enhanced infrared absorption (SEIRA) spectroscopy application. It consists of a highly doped InAsSb grating deposited on lattice-matched GaSb. The numerical analysis demonstrated that the resonance frequencies can be inferred from the dielectric function of an equivalent homogeneous slab by accounting for the complex reflectivity of the composite layer. Fourier transform infrared reflectivity (FTIR) measurements, analyzed with the Kramers-Kronig conversion technique, were used to deduce the effective response in reflection of the investigated system. From the knowledge of this phenomenological dielectric function, transversal and longitudinal energy-loss functions were extracted and attributed to transverse and longitudinal resonance modes frequencies.

Hybrid integrated optical waveguides in glass for enhanced visible photoluminescence of nanoemitters.

Integrated optical devices able to control light-matter interactions on the nanoscale have attracted the attention of the scientific community in recent years. However, most of these devices are based on silicon waveguides, limiting their use for telecommunication wavelengths. In this contribution, we propose an integrated device that operates with light in the visible spectrum. The proposed device is a hybrid structure consisting of a high-refractive-index layer placed on top of an ion-exchanged glass waveguide. We demonstrate that this hybrid structure serves as an efficient light coupler for the excitation of nanoemitters. The numerical and experimental results show that the device can enhance the electromagnetic field confinement up to 11 times, allowing a higher photoluminescence signal from nanocrystals placed on its surface. The designed device opens new perspectives in the generation of new optical devices suitable for quantum information or for optical sensing.

Compact interferometer transducer based on surface plasmon phase resonance.

We propose a new monolithic interferometric configuration and implement a novel method for spectroscopic phase shift detection of surface plasmon resonance (SPR) sensors. The interference pattern is obtained using a nonpolarizing beam splitter cube with two attached right angle prisms in such a way that each interference field undergoes two total internal reflections (TIR) at prisms/air interface and one attenuated total reflection (ATR) through surface plasmon interaction. The evanescent part of the interferogram around the Zero optical path difference (ZOPD) is sampled and detected in the far field, thanks to a bidimensional array of scattering optical near-field probes deposited on the corresponding prism surface. A Fourier transform of the sampled interferogram is performed to measure the input light wavelength, while a direct comparison of the interferogram in TM and TE polarization modes allows us to determine the differential phase shift induced by the SPR layer. The phase shift measurement is made possible thanks to a remarkable time stability of the interferogram in the glass bulk. By tuning the input laser wavelength around the resonance, we show a good agreement between experimental and theoretical calculations for both amplitude and phase spectral responses.

On-chip hybrid photonic-plasmonic light concentrator for nanofocusing in an integrated silicon photonics platform.

The enhancement and confinement of electromagnetic radiation to nanometer scale have improved the performances and decreased the dimensions of optical sources and detectors for several applications including spectroscopy, medical applications, and quantum information. Realization of on-chip nanofocusing devices compatible with silicon photonics platform adds a key functionality and provides opportunities for sensing, trapping, on-chip signal processing, and communications. Here, we discuss the design, fabrication, and experimental demonstration of light nanofocusing in a hybrid plasmonic-photonic nanotaper structure. We discuss the physical mechanisms behind the operation of this device, the coupling mechanisms, and how to engineer the energy transfer from a propagating guided mode to a trapped plasmonic mode at the apex of the plasmonic nanotaper with minimal radiation loss. Optical near-field measurements and Fourier modal analysis carried out using a near-field scanning optical microscope (NSOM) show a tight nanofocusing of light in this structure to an extremely small spot of 0.00563(λ/(2n(rmax)))(3) confined in 3D and an exquisite power input conversion of 92%. Our experiments also verify the mode selectivity of the device (low transmission of a TM-like input mode and high transmission of a TE-like input mode). A large field concentration factor (FCF) of about 4.9 is estimated from our NSOM measurement with a radius of curvature of about 20 nm at the apex of the nanotaper. The agreement between our theory and experimental results reveals helpful insights about the operation mechanism of the device, the interplay of the modes, and the gradual power transfer to the nanotaper apex.