Xavier Vidal

• PhD
• Head of the Quantum Research Labotatory at Tecnalia

Magnetometry with nitrogen-vacancy (NV) centers in diamond has so far been measured via emission of light from NV centers or via absorption at the singlet transition at 1042 nm. Here, we demonstrate a phenomenon of broadband optical absorption by the NV centers starting in the emission wavelength and reaching up to 1000 nm. The measurements are ena...

Diamond is a potential host material for laser applications due to its exceptional thermal properties, ultrawide bandgap, and color centers, which promise gain across the visible spectrum. More recently, coherent laser methods offer improved sensitivity for magnetometry. However, diamond fabrication is difficult in comparison to other crystalline m...

A challenge to this day in the development of diamond devices for quantum applications is the laterally defined and closely spaced positioning of nitrogen-vacancy centres with exceptional coherence properties. Here, we demonstrate a maskless, implantation-free method for the controlled in-plane positioning of NV centres using a combination of focus...

Widefield magnetometry based on nitrogen-vacancy centers enables high spatial resolution imaging of magnetic field distributions without a need for spatial scanning. In this work, we show nitrogen-vacancy center magnetic imaging of Fe3O4 nanoparticles within the gastrointestinal tract of Drosophila melanogaster larvae. Vector magnetic field imaging...

The use of diamond colour centres such as the nitrogen-vacancy (NV) centre is increasingly enabling quantum sensing and computing applications. Novel concepts like cavity coupling and readout, laser-threshold magnetometry and multi-pass geometries allow significantly improved sensitivity and performance via increased signals and strong light fields...

Diamond has long been identified as a potential host material for laser applications. This potential arises due to its exceptional thermal properties, ultra-wide bandgap, and color centers which promise gain across the visible spectrum. More recently, coherent laser methods offer new approaches to magnetometry. However, diamond fabrication is diffi...

Single substitutional nitrogen atoms [Formula: see text] are the prerequisite to create nitrogen-vacancy (NV) centers in diamonds. They not only serve as the electron donors to create the desired NV ⁻ center and provide charge stability against photo-ionisation but also are the main source of decoherence. Therefore, precise and quick determination...

Single substitutional nitrogen atoms N$_s^0$ are the prerequisite to create nitrogen-vacancy (NV) centers in diamonds. They serve as the electron donors to create the desired NV$^-$ center, provide charge stability against photo-ionisation, but also are the main source of decoherence. Therefore, precise and quick determination of N$_s^0$ concentrat...

Negatively charged nitrogen-vacancy (NV) centers in diamond are promising magnetic field quantum sensors. Laser threshold magnetometry theory predicts improved NV center ensemble sensitivity via increased signal strength and magnetic field contrast. Here, we experimentally demonstrate laser threshold magnetometry. We use a macroscopic high-finesse...

The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV ⁻ concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can b...

Negatively charged nitrogen-vacancy centres in diamond are promising quantum magnetic field sensors. Laser threshold magnetometry has been a theoretical approach for the improvement of NV-centre ensemble sensitivity via increased signal strength and magnetic field contrast. In this work we experimentally demonstrate laser threshold magnetometry. We...

Laser threshold magnetometry using the negatively charged nitrogen-vacancy (NV−) centre in diamond as a gain medium has been proposed as a technique to dramatically enhance the sensitivity of room-temperature magnetometry. We experimentally explore a diamond-loaded open tunable fibre-cavity system as a potential contender for the realisation of las...

We study the engineering of quantum magnetic sensor using laser generated from diamond nitrogen-vacancy (NV) centres in fibre cavity. The projected sensitivity of such a sensor is of the order of pT / Hz 1/2 .

Laser-threshold magetometry using the negatively charged nitrogen-vacancy (NV-) centre in diamond as a gain medium is a proposed technique to dramatically enhance the sensitivity of room-temperature magnetometry. We experimentally explore a diamond loaded open tunable fiber-cavity system as a potential contender for the realization of lasing with t...

Lens-axicon doublets have been used to produce Bessel-Gaussian beams, a narrow non-diffracting beam of relatively constant width. One problem of using Bessel-Gaussian beams is that there is a compromise between achieving a long effective focal length with a small central core radius and distributing the beam intensity between the central core and t...

Measuring photon autocorrelations via a Hanbury-Brown and Twiss (HBT) setup is a widely used experimental technique in quantum optics and is the base for quantum characterizations via Hong-Ou-Mandel interferometry, Bell inequality tests or quantum state tomography. The most common use for HBT measurements is the demonstration of single-photon emiss...

Over the past decade, exciton-polaritons in semiconductor microcavities have revealed themselves as one of the richest realizations of a light-based quantum fluid, subject to fascinating new physics and potential applications. For instance, in the regime of large two-body interactions, polaritons can be used to manipulate the quantum properties of...

The emergence of technologies operating at the nanometer scale for applications as varied as nano-fabrication and super-resolution microscopy has driven the need for ever more accurate spatial localization. In this context, nano-structures have been used as probes in order to provide a reference to track lateral drifts in the system. Yet nanometer...

Linear scattering processes are usually described as a function of the parameters of the incident beam. The wavelength, the intensity distribution, the polarization or the phase are among them. Here, we discuss and experimentally demonstrate how the angular momentum and the helicity of light influence the light scattering of spherical particles. We...

Linear scattering processes are usually described as a function of the parameters of the incident beam. The wavelength, the intensity distribution, the polarization or the phase are among them. Here, we discuss and experimentally demonstrate how the angular momentum and the helicity of light influence the light scattering of spherical particles. We...

Over the past decade, exciton-polaritons in semiconductor microcavities have attracted a great deal of interest as a driven-dissipative quantum fluid. These systems offer themselves as a versatile platform for performing Hamiltonian simulations with light, as well as for experimentally realizing nontrivial out-of-equilibrium phase transitions. In a...

Stimulated emission is the process fundamental to laser operation, thereby producing coherent photon output. Despite negatively charged nitrogen-vacancy (NV−) centres being discussed as a potential laser medium since the 1980s, there have been no definitive observations of stimulated emission from ensembles of NV− to date. Here we show both theoret...

Supplementary Note and Supplementary Figure

Lanthanide-doped glasses and crystals are attractive for laser applications because the metastable energy levels of the trivalent lanthanide ions facilitate the establishment of population inversion and amplified stimulated emission at relatively low pump power. At the nanometre scale, lanthanide-doped upconversion nanoparticles (UCNPs) can now be...

The phenomenon of molecular optical activity manifests itself as the rotation of the plane of linear polarization when light passes through chiral media. Measurements of optical activity and its wavelength dependence, optical rotatory dispersion, can reveal information about intricate properties of molecules, such as the 3D arrangement of atoms com...

The phenomenon of molecular optical activity manifests itself as the rotation of the plane of linear polarization when light passes through chiral media. Measurements of optical activity and its wavelength dependence, that is, optical rotatory dispersion, can reveal information about intricate properties of molecules, such as the three-dimensional...

We measure the far-field intensity of vortex beams going through nanoholes. The process is analyzed in terms of helicity and total angular momentum. It is seen that the total angular momentum is preserved in the process, and helicity is not. We compute the ratio between the two transmitted helicity components, $\gamma_{m,p}$. We observe that this r...

Measuring the optical activity from an ensemble of chiral molecules is a common tool to know their stereo-structure. These measurements are done in the same propagation direction of the probe beam of light, because that is the direction where most signal is emitted. We provide experimental and theoretical evidence that, even though other interestin...

We motivate metrology schemes based on topological singularities as a way to build robustness against deformations of the system. In particular, we relate reference settings of metrological systems to topological singularities in the measurement outputs. As examples we discuss optical nano-position sensing (i) using a balanced photodetector and a q...

The polarization of the light scattered by an optically dense, random solution of dielectric nanoparticles shows peculiar properties when the scatterers exhibit strong electric and magnetic polarizabilities. While the distribution of the scattering intensity in these systems shows the typical irregular speckle patterns, the helicity of the incident...

Circular dichroism, that is, the differential absorption of a system to left and right circularly polarized light, is one of the only techniques capable of providing morphological information of certain samples. In biology, for instance, circular dichroism spectroscopy is widely used to study the structure of proteins. More recently, it has also be...

An analysis of light–matter interactions based on symmetries can provide valuable insight, particularly because it reveals which quantities are conserved and which ones can be transformed within a physical system. In this context, helicity can be a useful addition to more commonly considered observables such as angular momentum. The question arises...

We propose a novel detection method based on the symmetry breaking induced by the bio-molecule to be detected. Briefly, by choosing a sensor presenting a particular symmetry, the revolution symmetry, the adsorption of an analyte will break this symmetry. By detecting this change in the symmetries of the system, the presence of bio-molecules can be...

We will show how to extract information from the Mie coefficients to properly design dual systems combined with chiral elements for having optically active structures. Such optically active elements will scatter the light omnidirectionally, where the amount of rotation of light is fixed in any given direction independently of the incident polarizat...

We present how the angular momentum of light can play an important role to induce a dual or anti-dual behaviour on a dielectric particle. Although the material the particle is made of is not dual, i.e. a dielectric does not interact with an electrical field in the same way as it does with a magnetic one, a spherical particle can behave as a dual sy...

This contribution reports the chiro-optic response of as-cast and photopatterned films of silver nanoparticles capped with photothermally-cleavable chiral ligands. We demonstrate broadband circular dichroism in these nanoparticle films, which is not present in dispersions of the nanoparticles capped with the chiral ligands. Long wavelength circular...

In this Letter, we show that the electromagnetic duality symmetry, broken in the microscopic Maxwell's equations by the presence of charges, can be restored for the macroscopic Maxwell's equations. The restoration of this symmetry is shown to be independent of the geometry of the problem. These results provide a tool for the study of light-matter i...

We present how the angular momentum of light can play an important role to induce a dual or anti-dual behaviour on a dielectric particle. Although the material the particle is made of is not dual, i.e. a dielectric does not interact with an electrical field in the same way as it does with a magnetic one, a spherical particle can behave as a dual sy...

We show the relation between optical activity and helicity for the scattering of chiral particles. The results show how to design chiral structures for an omnidirectional optically active response independent of the incident polarization.

We propose a novel detection method based on the symmetry breaking induced by the bio-molecule to be detected. This optical method provides substantial advantages over current approaches for the conception of biosensors.

Two conditions on symmetries are identified as necessary for a linear scattering system to be able to rotate the linear polarization of light: Lack of at least one mirror plane of symmetry and electromagnetic duality symmetry. Duality symmetry is equivalent to the conservation of the helicity of light in the same way that rotational symmetry is equ...

We unveil the relationship between two anomalous scattering processes known as Kerker conditions and the duality symmetry of Maxwell equations. We generalize these conditions and show that they can be applied to any particle with cylindrical symmetry, not only to spherical particles as the original Kerker conditions were derived for. We also explai...

We present an approach to produce plasmonic lithographic patterns with an extended circular dichroism response from the ultraviolet to the near infrared. We analyzed the optical chirality changes of the functionalized Ag nanospheres at every stage: from the chiral ligands to the patterned structures. The molecular chirality from the ligands is wave...

In this presentation, two results will be presented. First, the authors will generalize the Kerker conditions for systems with cylindrical symmetry. As it will be seen, the first Kerker condition is a particular case of a system with cylindrical and duality symmetry. In comparison, the second Kerker condition is a particular case of a system with c...

We present a novel method for the nano-positioning measurements of a sample. Based on symmetry considerations, the method transforms the problem of position sensing into identification of topologically robust features obtained from the scattered field. In particular, we exploit the cylindrical symmetry of a control defect appended to the sample. Ex...

We present a femtosecond-pulsed laser-based method for writing subwavelength metallic nanostructures in a polymer. The pulsed laser induces two-photon initiated in situ reduction of a metal salt and simultaneous polymerization of a photoresist.

We present a new method to address multipolar resonances and to control the scattered field of a spherical scatterer. This method is based on the engineering of the multipolar content of the incident beam. We propose experimentally feasible techniques to generate light beams which contain only a few multipolar modes. The technique uses incident bea...

We present a new method to address multipolar resonances and to control the scattered field of a spherical scatterer. This method is based on the engineering of the multipolar content of the incident beam. We propose experimentally feasible techniques to generate light beams which contain only a few multipolar modes. The techniques uses incident be...

The scattering of circularly polarised Gaussian beams impinging on a cylindrical nanohole in a metallic thin film contains phase singularities. These experimental observations are due to changes in the helicity of the scattered photons.

We propose a novel metamaterial-based waveguide coupler for use as a de-multiplexer at telecommunication wavelengths. This device consists of coupled dielectric microchannel waveguides, one of which has a dispersive core with an index of refraction that is negative within a narrow frequency band at telecommunication wavelengths. We study the near f...

We present a novel laser-based method for directly writing metallic nanostructures in a polymer matrix with sub-wavelength resolution [1]. The method involves two-photon lithography (TPL) in which a femtosecond-pulsed laser induces two-photon initiated in situ reduction of a metal salt and simultaneous polymerization of a negative photoresist. Inte...

We present an approach to produce micropatterns of metallic nanoparticles (NPs) that preserve key optical properties of the individual NPs. The technique uses a photothermal reaction induced by two-photon direct laser writing. The studied NP property is plasmon chirality, which was obtained via chemical conjugation of Ag NPs with chiral ligands. Th...

We report an alternative approach to produce micropatterns of metallic nanoparticles using photothermal-reaction-assisted two-photon direct laser writing. The patterns are achieved using a facile surface treatment of silver nanoparticles (Ag NPs) functionalized with thermally cleavable ligands; N -(tert-butoxycarbonyl)-L-cysteine methyl ester. The...

This article presents a new method for fabricating highly conductive gold nanostructures within a polymeric matrix with subwavelength resolution. The nanostructures are directly written in a gold precursor-doped photoresist using a femtosecond pulsed laser. The laser energy is absorbed by a two-photon dye, which induces simultaneous reduction of go...

The second-harmonic generation from transparent strontium-barium niobate crystals is experimentally studied and explained from the emission patterns of randomly scattered nonlinear domains and the far field interference of the light generated from such domains.

A novel method for fabricating metallic nanostructures with sub-wavelength resolution in a polymer using simultaneous two-photon initiated reduction of a metal salt and polymerization of a negative photoresist is demonstrated. Gold nanoparticle-doped lines as narrow as 200 nm were fabricated. The dependence of line-width on laser intensity and scan...

We show that the weak second harmonic light generated from a random distribution of nonlinear domains of transparent Strontium Barium Niobate crystals can display a particularly intense generation in the forward direction. By using a theoretical model able to analyze the optical response of arbitrary distributions of three-dimensional nonlinear vol...

We detected the second-order nonlinear response from single isolated spheres comprised from a centrosymmetric material but covered by a layer of a material with strong second-order nonlinear properties and isolated from an ensemble by the optical trapping technique. We show that when large size parameter spheres are used, the measured second-harmon...

An efficient solid-phase based synthesis was presented to covalently bind a large number of molecules with high nonlinear optical (NLO) performance to the surface of colloidal crystalline arrays (CCA) nanoparticles. The nanospheres maintained a large surface charge density due to unaltered sulfate groups. The latexes are chosen with similar electro...

Many nonlinear photonic materials exhibit large extinction coefficients as a result of the active molecule absorption band. The authors show that it is possible to redshift the absorption resonance of such material without a significant alteration of the nonlinear response. A drastic reduction in the absorption coefficient at the wavelength of inte...

This paper reviews the progress in implementing several new configurations to obtain a more flexible nonlinear optical interaction. In particular, this work reviews the experimental work on weak localization in nonlinear random media, second harmonic generation in the neighborhood of metallic nano particles, second harmonic from the high bands in o...

We show that to partially phase match second harmonic generation is possible in structures of randomly oriented domains. In certain conditions making the dispersion in domain size large has no negative effect for such generation.

Two trains of light pulses at periods that are equally shifted from the harmonics of a missing fundamental are combined in a nonlinear crystal. As a result of a noncollinear phase-matched second-order nonlinear generation, a new train of pulses is obtained. When the temporal width of the input pulses is large, the frequency of the resulting pulse t...

We show that the second order nonlinear generation of light, a process that it is assumed to require highly ordered materials, is also possible in structures of randomly oriented nonlinear domains. We explain theoretically why in such disordered structures the efficiency of the nonlinear generation of light grows linearly with the number of domains...

Two trains of light pulses at periods that are equally shifted from the harmonics of a missing fundamental, are combined in a nonlinear crystal. As a result of a noncollinear phase matched second order nonlinear generation, a new train of pulses is obtained. When the temporal width of the input pulses is large, the frequency of the resulting pulse...

It is widely accepted that quadratic nonlinear processes, such as parametric generation or amplification, require the use of materials with a high degree of ordering. In some occasions, such ordering is found at a nanoscale, and in other cases, the order is at a micron scale. When such ordering is not intrinsic to the material, one may introduce a...

In dielectric structures such as photonic crystals that combine two types of materials it is likely that one may find a large degree of disorder. This is also true for nonlinear photonic crystals that combine two different orientations of the same material or a nonlinear with a linear material. Such a disorder may not always be detrimental for the...

The second order NLO response of a new family of acridine-based chromophores is greatly enhanced due to the presence of a tautomeric minor form with high charge-transfer capabilities.

In this paper, a new 2-D frequency converter based on second harmonic generation (SHG) in GaAs photonic crystal waveguides is proposed. The input waveguide, where the second order nonlinear process takes place, is coupled to a secondary waveguide that is designed to allow only SH propagation. A row of photonic crystal microcavity resonators is then...

In this paper, we present experimental evidence of phase matched third harmonic generation at 355 nm from a beam at 1064 nm incident on a photonic crystal made of 120 nm in diameter polystyrene spheres ordered in an fee lattice. Numerical calculations show that at the lower angle edge of the Bragg reflected band, the periodical structure induced ef...

We show that the second order parametric nonlinear interaction from waves that are counter propagating may be perfectly phase matched in the framework of a photonic crystal. We observe that the large momentum mismatch of such parametric process is compensated because of the interfaces present in the nonlinear photonic crystal considered, which sepa...