arXiv subjects

Nicolas Bonod

Publications and source records attributed to Nicolas Bonod.

At least 19 recordsLinked to original sources

arXiv2026

Synthesis of hybrid gold-protein nanobioconjugates with strongly enhanced circular dichroism

Controlling chiral light-matter interactions with nanophotonics is a powerful strategy for amplifying molecular circular dichroism (CD) beyond its natural limits. Here, we present a plasmonic platform that uses superchiral near fields to amplify the optical activity of biological chromophores. Gold nanoparticles (3.6 nm in diameter) are electrostatically coupled with photosynthetic proteins which results in stable hybrid nanobioconjugates in aqueous solution. The Q-band absorption spectrum of the proteins overlaps with the localized surface plasmon resonance of the gold particles. In the visible region, a combination of numerical simulations and normalized CD measurements reveal an enhancement factor of 3 compared to free proteins. This observation reveals three effects. First, there is a local increase in electromagnetic chirality density in the protein environment. Second, there is an increase in the absorption of the proteins. Third, there is plasmon-induced circular dichroism. Our results quantitatively demonstrate that near-field super-chirality directly modulates biomolecular optical activity. These findings open new avenues for chiroptical nanodevices, biosensing platforms, and light-driven asymmetric photochemistry.

physics.optics↗

arXiv2024

Extracting Complete Resonance Characteristics From the Phase of Physical Signals

Resonances are common in wave physics and their full and rigorous characterization is crucial to correctly tailor the response of a system in both time and frequency domains. However, they have been conventionally described by the quality factor, a real-valued number quantifying the sharpness of a single peak in the amplitude spectrum, and associated with a singularity in the complex frequency plane. But the amplitude of a physical signal does not hold all the information on the resonance and it has not been established that even the knowledge of the full distribution of singularities carries this information. Here we derive a dimensionless quality function that fully characterizes resonances from the knowledge of the phase spectrum of the signal. This function is driven by the spectral derivative of the phase. It is equivalent to the imaginary part of the Wigner-Smith time delay but it has the advantage of being valid for arbitrary response functions, including the components of the S-matrix. The spectral derivative of the phase can be calculated numerically from simulations or experimental acquisitions of the phase spectrum. Alternatively, it can be retrieved from the distribution of poles and zeros in the complex frequency plane through an analytic expression, which demonstrates that singularities do not suffice to fully characterize the resonances and that both singularities and zeros must be taken into account to retrieve the quality function. This approach permits to extract all the characteristics of resonances from arbitrary spectral response functions without a priori knowledge on the physical system.

physics.optics↗

arXiv2024

Generalized Drude-Lorentz Model Complying with the Singularity Expansion Method

Deriving analytical expressions of dielectric permittivities is required for numerical and physical modeling of optical systems and the soar of non-hermitian photonics motivates their prolongation in the complex plane. Analytical models are based on the association of microscopic models to describe macroscopic effects. However, the question is to know whether the resulting Debye Drude Lorentz models are not too restrictive. Here we show that the permittivity must be treated as a meromorphic transfer function that complies with the requirements of complex analysis. This function can be naturally expanded on a set of complex singularities. This singularity expansion of the dielectric permittivity allows us to derive a generalized expression of the Debye Drude Lorentz model that complies with the requirements of complex analysis and the constraints of physical systems. We show that the complex singularities and other parameters of this generalized expression can be retrieved from experimental data acquired along the real frequency axis. The accuracy of this expression is assessed for a wide range of materials including metals, 2D materials and dielectrics, and we show how the distribution of the retrieved poles helps in characterizing the materials.

physics.optics↗

arXiv2023

True Circular Dichroism in Optically Active Achiral Metasurfaces and Its Relation to Chiral Near-Fields

Optically active achiral metasurfaces offer a promising way to detect chiral molecules based on chiroptic methods. The combination of plasmonic enhanced circular dichroism and reversible optical activity would boost the sensitivity and provide enantiomerselective surfaces while using a single sensing site. In this work, we use metasurfaces containing arrays of U-shaped resonators as a benchmark for analyzing the optical activity of achiral materials. Although the peculiar optical activity of these metasurfaces has 1 been quite well described, we present here an experimental and numerical quantitative determination of the different contributions to the measured optical activity. In particular, it is shown that linear birefringence and retardance contribute, but only marginally, to the apparent circular dichroism of the metasurface associated with the excitation of magnetoelectric modes. We then numerically demonstrate the peculiar near-field properties of the magneto-electric modes and explain how these properties could be reflected in the far-field polarimetric properties in the presence of chiral molecules. This work provides alternatives for the detection scheme of chiral molecules using plasmonic resonators.

physics.app-ph↗

arXiv2023

Multiple-Order Singularity Expansion Method

Physical systems and signals are often characterized by complex functions of frequency in the harmonic-domain. The extension of such functions to the complex frequency plane has been a topic of growing interest as it was shown that specific complex frequencies could be used to describe both ordinary and exceptional physical properties. In particular, expansions and factorized forms of the harmonic-domain functions in terms of their poles and zeros under multiple physical considerations have been used. In this work, we start from a general property of continuity and differentiability of the complex functions to derive the multiple-order singularity expansion method. We rigorously derive the common singularity and zero expansion and factorization expressions, and generalize them to the case of singularities of arbitrary order, whilst deducing the behaviour of these complex frequencies from the simple hypothesis that we are dealing with physically realistic signals.

physics.optics↗

arXiv2022

Crossing of the branch cut: the topological origin of a universal 2π-phase retardation in non-Hermitian metasurfaces

Full wavefront control by photonic components requires that the spatial phase modulation on an incoming optical beam ranges from 0 to 2π. Because of their radiative coupling to the environment, all optical components are intrinsically non-Hermitian systems, often described by reflection and transmission matrices with complex eigenfrequencies. Here, we show that Parity-Time symmetry breaking -- either explicit or spontaneous -- moves the position of Zero singularities of the reflection or transmission matrices from the real axis to the upper part of the complex frequency plane. A universal 0 to 2π-phase gradient of an output channel as a function of the real frequency excitation is thus realized whenever the discontinuity branch bridging a Zero and a Pole, i.e a pair of singularities, is crossing the real axis. This basic understanding is applied to engineer electromagnetic fields at interfaces, including, but not limited to, metasurfaces. Non-Hermitian topological features associated with exceptional degeneracies or branch cut crossing are shown to play a surprisingly pivotal role in the design of resonant photonic systems.

physics.optics↗

arXiv2021

Gaussian regularization for resonant states: open and dispersive optical systems

Resonant States (RS), also known as Quasi-Normal Modes (QNMs), are eigenstates that arise in spectral expansions of linear response functions of open systems. Manipulation of these spatially `divergent' oscillating functions requires a departure from the usual definitions of inner product, normalization and orthogonality typically encountered in the studies of closed systems. We show that once RS fields are expanded on a multipole basis, Gaussian regularization methods provide \emph{analytical} results for crucial RS inner product integrals \added{in the problematic region exterior to the scattering system}. Our demonstrations are carried out in the context of light scattering by spatially bounded objects composed of both electrically and magnetically dispersive media, with demonstrative analytic calculations being shown to \emph{completely} retrieve the results of exact Mie theory.

physics.optics↗

arXiv2020

Modal Expansion of the Impulse Response Function for Predicting the Time Dynamics of Open Optical Cavities

Light interaction with optical cavities is of fundamental interest to enhance the light-matter interaction and to shape the spectral features of the electromagnetic fields. Important efforts have been carried out to develop modal theories of open optical cavities relying on an expansion of the fields on the eigen-fields of the cavity. Here, we show how such an expansion predicts the temporal dynamics of optical resonators. We consider a Fabry-Perot cavity to derive the full analytical expressions of the internal and scattered field on the quasi-normal modes basis together with the complex eigen-frequencies. We evince the convergence and accuracy of this expansion before deriving the impulse response function (IRF) of the open cavity. We benefit from this modal expansion and IRF to demonstrate that the eigen-modes of the open cavity impact the signals only during the transient regimes and not in the permanent regime.

physics.optics↗

arXiv2019

Deep UV plasmonic enhancement of single protein autofluorescence in zero-mode waveguides

Single molecule detection provides detailed information about molecular structures and functions, but it generally requires the presence of a fluorescent marker which can interfere with the activity of the target molecule or complicate the sample production. Detecting a single protein with its natural UV autofluorescence is an attractive approach to avoid all the issues related to fluorescence labelling. However, the UV autofluorescence signal from a single protein is generally extremely weak. Here, we use aluminum plasmonics to enhance the tryptophan autofluorescence emission of single proteins in the UV range. Zero-mode waveguides nanoapertures enable observing the UV fluorescence of single label-free beta-galactosidase proteins with increased brightness, microsecond transit times and operation at micromolar concentrations. We demonstrate quantitative measurements of the local concentration, diffusion coefficient and hydrodynamic radius of the label-free protein over a broad range of zero-mode waveguide diameters. While the plasmonic fluorescence enhancement has generated a tremendous interest in the visible and near-infrared parts of the spectrum, this work pushes further the limits of plasmonic-enhanced single molecule detection into the UV range and constitutes a major step forward in our ability to interrogate single proteins in their native state at physiological concentrations.

physics.optics↗

arXiv2019

Evolutionary optimization of all-dielectric magnetic nanoantennas

Magnetic light and matter interactions are generally too weak to be detected, studied and applied technologically. However, if one can increase the magnetic power density of light by several orders of magnitude, the coupling between magnetic light and matter could become of the same order of magnitude as the coupling with its electric counterpart. For that purpose, photonic nanoantennas have been proposed, and in particular dielectric nanostructures, to engineer strong local magnetic field and therefore increase the probability of magnetic interactions. Unfortunately, dielectric designs suffer from physical limitations that confine the magnetic hot spot in the core of the material itself, preventing experimental and technological implementations. Here, we demonstrate that evolutionary algorithms can overcome such limitations by designing new dielectric photonic nanoantennas, able to increase and extract the optical magnetic field from high refractive index materials. We also demonstrate that the magnetic power density in an evolutionary optimized dielectric nanostructure can be increased by a factor 5 compared to state of the art dielectric nanoantennas. In addition, we show that the fine details of the nanostructure are not critical in reaching these aforementioned features, as long as the general shape of the motif is maintained. This advocates for the feasibility of nanofabricating the optimized antennas experimentally and their subsequent application. By designing all dielectric magnetic antennas that feature local magnetic hot-spots outside of high refractive index materials, this work highlights the potential of evolutionary methods to fill the gap between electric and magnetic light-matter interactions, opening up new possibilities in many research fields.

cond-mat.mes-hall↗

arXiv2018

Modal Expansion of the Scattered Field: Causality, Non-Divergence and Non-Resonant Contribution

Modal analysis based on the quasi-normal modes (QNM), also called resonant states, has emerged as a promising way for modeling the resonant interaction of light with open optical cavities. However, the fields associated with QNM in open photonic cavities diverge far away from the scatterer and the possibility of expanding the scattered field with resonant contributions only has not been established. Here, we address these two issues while restricting our study to the case of a dispersionless spherical scatterer. First, we derive the rigorous pole expansion of the $T$-matrix coefficients that link the scattered to the incident fields associated with an optical resonator. This expansion evinces the existence of a non-resonant term. Second, in the time domain, the causality principle allows us to solve the problem of divergence and to derive a modal expansion of the scattered field that does not diverge far from the scatterer.

physics.optics↗

arXiv2018

Free-Space Graphics with Electrically Driven Levitated Light Scatterers

Levitation of optical scatterers provides a new mean to develop free-space volumetric displays. The principle is to illuminate a levitating particle displaced at high velocity in three dimensions (3D) to create images based on persistence of vision (POV). Light scattered by the particle can be observed all around the volumetric display and therefore provides a true 3D image that does not rely on interference effects and remains insensitive to the angle of observation. The challenge is to control with a high accuracy and at high speed the trajectory of the particle in three dimensions. Systems that use light to generate free-space images either in plasma or with a bead are strictly dependent of the scanning method used. Mechanical systems are required to scan the particles in the volume which weakens the time dynamics. Here we use electrically driven planar Paul traps (PPTs) to control the trajectory of electrically charged particles. A single gold particle colloid is manipulated in three dimensions through AC and DC electrical voltages applied to a PPT. Electric voltages can be modulated at high frequencies (150 kHz) and allow for a high speed displacement of particles without moving any other system component. The optical scattering of the particle in levitation yields free-space images that are imaged with conventional optics. The trajectory of the particle is entirely encoded in the electric voltage and driven through stationary planar electrodes. We show in this paper, the proof-of-concept for the generation of 3D free space graphics with a single electrically scanned particle.

physics.app-ph↗

arXiv2018

Interplay between spontaneous decay rates and Lamb shifts in open photonic systems

In this letter, we describe the modified decay rate and photonic Lamb (frequency) shift of quantum emitters in terms of the resonant states of a neighboring photonic resonator. This description illustrates a fundamental distinction in the behaviors of closed (conservative) and open (dissipative) systems: the Lamb shift is bounded by the emission linewidth in closed systems while it overcomes this limit in open systems.

physics.optics↗

arXiv2018

Lamb shift multipolar analysis

It is now well established that radiative decay of quantum emitters can be strongly modified by their environment. In this paper we present an exact, within the weak-coupling approximation, multipole expression to compute the Lamb (frequency) shift induced by an arbitrary set of resonant scatterers on a nearby quantum emitter, using multi-scattering theory. We also adopt a Quasi-Normal Mode description to account for the line shape of the Lamb shift spectrum in the near-field of a plasmonic nanosphere. It is then shown that the Lamb shift resonance can be blue-shifted as the size of the nanoparticle increases, suggesting that nanoparticles may be used to tune this resonant interaction. Finally, a realistic calculation of the Lamb shift is made for a dimer configuration.

physics.optics↗

arXiv2017

Spectroscopy and Biosensing with Optically Resonant Dielectric Nanostructures

Resonant dielectric nanoparticles (RDNs) made of materials with large positive dielectric permittivity, such as Si, GaP, GaAs, have become a powerful platform for modern light science, enabling various fascinating applications in nanophotonics and quantum optics. In addition to light localization at the nanoscale, dielectric nanostructures provide electric and magnetic resonant responses throughout the visible and infrared spectrum, low dissipative losses and optical heating, low doping effect and absence of quenching, which are interesting for spectroscopy and biosensing applications. In this review, we present state-of-the-art applications of optically resonant high-index dielectric nanostructures as a multifunctional platform for light-matter interactions. Nanoscale control of quantum emitters and applications for enhanced spectroscopy including fluorescence spectroscopy, surface-enhanced Raman scattering (SERS), biosensing, and lab-on-a-chip technology are surveyed. We describe the theoretical background underlying these effects, overview realizations of specific resonant dielectric nanostructures and hybrid excitonic systems, and outlook the challenges in this field, which remain open to future research.

physics.optics↗

arXiv2017

Polarizability Expressions for Predicting Resonances in Plasmonic and Mie Scatterers

Polarizability expressions are commonly used in optics and photonics to model the light scattering by small particles. Models based on Taylor series of the scattering coefficients of the particles fail to predict the morphologic resonances hosted by dielectric particles. Here, we propose to use the factorization of the special functions appearing in the expression of the Mie scattering coefficients to derive point-like models. These models can be applied to reproduce both Mie resonances of dielectric particles and plasmonic resonances of metallic particles. They provide simple but robust tools to predict accurately the electric and magnetic Mie resonances in dielectric particles.

physics.optics↗

arXiv2017

All-Dielectric Metasurfaces Based on Cross-Shaped Resonators for Color Pixels with Extended Gamut

Printing technology based on plasmonic structures has many advantages over pigment based color printing such as high resolution, ultra-compact size and low power consumption. However, due to high losses and broad resonance behavior of metals in the visible spectrum, it becomes challenging to produce well-defined colors. Here, we investigate cross-shaped dielectric nanoresonators which enable high quality resonance in the visible spectral regime and, hence, high quality colors. We numerically predict and experimentally demonstrate that the proposed all-dielectric nanostructures exhibit high quality colors with selective wavelengths, in particular, due to lower losses as compared to metal based plasmonic filters. This results in fundamental colors (RGB) with high hue and saturation. We further show that a large gamut of colors can be achieved by selecting the appropriate length and width of individual $Si$ nanoantennas. Moreover, the proposed all-dielectric metasurface based color filters can be integrated with the well matured fabrication technology of electronic devices.

physics.optics↗

arXiv2016

Enhancement and Inhibition of Spontaneous Photon Emission by Resonant Silicon Nanoantennas

Substituting noble metals for high-index dielectrics has recently been proposed as an alternative strategy in nanophotonics to design broadband optical resonators and circumvent the ohmic losses of plasmonic materials. In this report, we demonstrate that subwavelength silicon nanoantennas can manipulate the photon emission dynamics of fluorescent molecules. In practice, it is showed that dielectric nanoantennas can both increase and decrease the local density of optical states (LDOS) at room temperature, a process that is inaccessible with noble metals at the nanoscale. Using scanning probe microscopy, we analyze quantitatively, in three dimensions, the near-field interaction between a 100 nm fluorescent nanosphere and silicon nanoantennas with diameters ranging between 170 nm and 250 nm. Associated to numerical simulations, these measurements indicate increased or decreased total spontaneous decay rates by up to 15 % and a gain in the collection efficiency of emitted photons by up to 85 %. Our study demonstrates the potential of silicon-based nanoantennas for the low-loss manipulation of solid-state emitters at the nanoscale and at room temperature.

physics.optics↗