Yannick DE WILDE

Résumé: High refractive index dielectric nanoantennas strongly modify the decay rate via the Purcell effect through the design of radiative channels. Due to their dielectric nature, the field is mainly confined inside the nanostructure and in the gap, which is hard to probe with scanning probe techniques. Here we use single-molecule fluorescence lifetime imaging microscopy (smFLIM) to map the decay rate enhancement in dielectric GaP nanoantenna dimers with a median localization precision of 14 nm. We measure, in the gap of the nanoantenna, decay rates that are almost 30 times larger than on a glass substrate. By comparing experimental results with numerical simulations we show that this large enhancement is essentially radiative, contrary to the case of plasmonic nanoantennas, and therefore has great potential for applications such as quantum optics and biosensing.

Résumé: Spherical dielectric resonators are highly attractive for light manipulation, thanks to their intrinsic electric and magnetic resonances. Here, we present measurements of the mid-infrared far-field thermal radiation of single subwavelength dielectric spheres deposited on a gold substrate, of radii ranging from 1 to 2.5 μm, which agree quantitatively with simulated absorption cross sections. For SiO2microspheres, we evidence the excitation of both surface phonon-polariton (SPhP) modes and geometrical electric and magnetic Mie modes. The transition from a phonon-mode-dominated to a Mie-mode-dominated emission spectrum is observed, with a threshold radius of ∼1.5 μm. We also show that the presence of the metallic substrate augments the computed spheres absorption cross-section due to increased local field enhancement, arising from the near-field interaction of the spheres oscillating charges with their image in the metallic mirror. In contrast, measurements of single subwavelength SPhP-inactive PTFE spheres reveal that the mid-infrared response of such lossy spheres is dominated by their bulk absorption. Our results demonstrate how engineering the geometrical and dielectric properties of subwavelength scatterers can enable the control of thermal emission near room temperature, with exciting perspectives for applications such as radiative cooling.

Résumé: Light emission by fluorophores can be computed from the knowledge of the absorption spectrum. However, at long wavelengths, the calculated emission may diverge if the decay of the imaginary part of the permittivity is not modelled with precision. We report a technique to obtain the permittivity of fluorophores such as dye molecules from fluorescence measurements. We find that the Brendel-Bormann model enables to fit the emission spectra accurately.

Résumé: The electromagnetic modes of a sub-wavelength sized antenna can be excited by thermal fluctuations. This thermal radiation is extremely weak, requiring in general the fabrication of a huge number of identical antennas to be detectable. Here, we will first demonstrate experimentally emission spectrum measurement and superresolved mapping of a single nanoantenna, sub-λ in the infrared spectral domain, based on the detection of its thermal radiation despite an overwhelming background thermal radiation [1]. To achieve a background free detection of the thermal radiation from single or a few sub-λ sized resonators, we have developed an infrared spatial modulation spectroscopy (IR-SMS) technique using a lateral modulation of the sample heated at ~150 °C combined with lock-in detection of the infrared signal detected through a Fourier transform infrared spectrometer. We have applied it in combination with thermal radiation scanning tunnelling microscopy [2] to study the thermal radiation from single plasmonic metal-insulator-metal (MIM) antennas both in the near field and in the far field. Our studies performed on single MIMs have revealed the surprising result that when silica is used as insulating material, its strong dispersion in the mid-infrared domain is such that the fundamental spatial mode of the antenna can be thermally excited at various wavelengths. This causes multiple resonances in the thermal radiation spectrum to which correspond the same spatial distribution of near-field thermal radiation [1].

Résumé: The study of hybrid modes in a single dimer of neighboring antennas is an essential step to optimize the far-field electromagnetic (EM) response of large-scale metasurfaces or any complex antenna structure made up of subwavelength building blocks. Here we present far-field infrared spatial modulation spectroscopy (IR-SMS) measurements of a single thermally excited asymmetric dimer of square metal-insulator-metal (MIM) antennas separated by a nanometric gap. Through thermal fluctuations, all the EM modes of the antennas are excited, and hybrid bonding and anti-bonding modes can be observed simultaneously. We study the latter within a plasmon hybridization model, and analyze their effect on the far-field response.

Résumé: Copyright © 2020 American Chemical Society. Single-molecule localization microscopy is a powerful technique with vast potential to study light-matter interactions at the nanoscale. Nanostructured environments can modify the fluorescence emission of single molecules, and the induced decay-rate modification can be retrieved to map the local density of optical states (LDOS). However, the modification of the emitter's point spread function (PSF) can lead to its mislocalization, setting a major limitation to the reliability of this approach. In this paper, we address this by simultaneously mapping the position and decay rate of single molecules and by sorting events by their decay rate and PSF size. With the help of numerical simulations, we are able to infer the dipole orientation and to retrieve the real position of mislocalized emitters. We have applied our approach of single-molecule fluorescence lifetime imaging microscopy (smFLIM) to study the LDOS modification of a silver nanowire over a field of view of ∼ 10 μm2 with a single-molecule localization precision of ∼ 15 nm. This is possible thanks to the combined use of an EMCCD camera and an array of single-photon avalanche diodes, enabling multiplexed and super-resolved fluorescence lifetime imaging.

Résumé: © 2019 The Author(s) 2019 OSA. We propose a modulation method to record background-free far field FTIR spectra of single sub -lambda sized objects which we combine with near-field TRSTM measurements to characterize the thermal emission of plasmonic antennas and silica rods. The fundamental mode of MIM nanoantennas is excited at multiple wavelengths.

Résumé: © 2018 American Physical Society. The far-field spectral and near-field spatial responses of an individual metal-insulator-metal nanoantenna are reported, using thermal fluctuations as an internal source of the electromagnetic field. The far-field spectra, obtained by combining Fourier transform infrared spectroscopy with spatial modulation based on a light falloff effect in a confocal geometry, have revealed two distinct emission peaks attributed to the excitation of the fundamental mode of the nanoantenna at two distinct wavelengths. Superresolved near-field images of the thermally excited mode have been obtained by thermal radiation scanning tunneling microscopy. Experimental results are supported by numerical simulations showing that it is possible to excite the same mode at different wavelengths near a resonance of the insulating dielectric material forming the antenna.

Résumé: © 2018 Electromagnetics Academy. All rights reserved. Nanoantennas have the ability to spatially and spectrally manipulate light at the nanoscale. Arranged in arrays, they create metasurfaces with homogeneous optical properties but periodicity and coupling between nanoantennas can be detrimental to the study of their intrinsic optical response. In order to investigate the electromagnetic properties of a single nanoantenna, we use a set-up based on thermal radiation scanning tunneling microscopy (TRSTM) to characterize an isolated nanoantenna from the near field to the far field by achieving sub-wavelength imaging in the mid-infrared.

Résumé: © 2017 American Physical Society.We observe time-correlated emission between a single CdSe/CdS/ZnS quantum dot exhibiting single-photon statistics and a fluorescent nanobead located micrometers apart. This is accomplished by coupling both emitters to a silver nanowire. Single plasmons are created on the latter from the quantum dot, and transfer energy to excite in turn the fluorescent nanobead. We demonstrate that the molecules inside the bead show the same blinking behavior as the quantum dot.

Résumé: © 2015 IEEE. Infrared near-field scanning optical microscopy (NSOM) has been used to investigate surface plasmon polaritons (SPPs) which are generated on an integrated device by means of a semi-conductor laser. Devices which realize a slit-doublet experiment with SPPs have been investigated. Using NSOM imaging, we demonstrate in situ generation of SPPs with quantum cascade lasers at λ=7.5 μm and with laser diodes at λ=1.3 μm. Using thermal radiation scanning optical microscopy (TRSTM), we measure the infrared thermal emission of a sample, which is scattered in the near-field by means of a tip. We have recorded images and Fourier transform infrared (FTIR) spectra of the near-field thermal emission of materials which support surface polaritons. We show that the TRSTM probes both the spatial and frequency dependence of the electromagnetic local density of states (EM-LDOS). The modes of SPPs confined on metallic patterns are observed with TRSTM imaging. At the surface of materials supporting surface phonon polaritons, the measured near-field thermal emission spectra are quasi-monochromatic, in striking contrast with blackbody-like far-field thermal emission. TRSTM combined with FTIR spectroscopy is a promising method for infrared spectroscopy with nanoscale resolution, i.e. well beyond the diffraction limit.

Résumé: © 2015 American Chemical Society. We present a novel method to extract the various contributions to the photonic local density of states from near-field fluorescence maps. The approach is based on the simultaneous mapping of the fluorescence intensity and decay rate and on the rigorous application of the reciprocity theorem. It allows us to separate the contributions of the radiative and the apparent nonradiative local density of states to the change in the decay rate. The apparent nonradiative contribution accounts for losses due to radiation out of the detection solid angle and to absorption in the environment. Data analysis relies on a new analytical calculation, and does not require the use of numerical simulations. One of the most relevant applications of the method is the characterization of nanostructures aimed at maximizing the number of photons emitted in the detection solid angle, which is a crucial issue in modern nanophotonics.

Résumé: We image in near-field the transverse modes of semiconductor distributed feedback (DFB) lasers operating at λ ≈ 1.3 μm and employing metallic gratings. The active region is based on tensile-strained InGaAlAs quantum wells emitting transverse magnetic polarized light and is coupled via an extremely thin cladding to a nano-patterned gold grating integrated on the device surface. Single mode emission is achieved, which tunes with the grating periodicity. The near-field measurements confirm laser operation on the fundamental transverse mode. Furthermore - together with a laser threshold reduction observed in the DFB lasers - it suggests that the patterning of the top metal contact can be a strategy to reduce the high plasmonic losses in this kind of systems. © 2013 Optical Society of America.

Résumé: We present a semiconductor-based approach to compensate plasmonic losses. The core idea is to employ an electrically pumped laser diode and to overlap its active region with the evanescent field of a surface plasmon wave. In order to keep the losses at a manageable level, we rely on hybrid waveguide modes stemming from the coupling of a dielectric and a plasmonic mode. The laser device we demonstrate operates-at telecom wavelengths-on such a hybrid plasmonic mode. The device operates by electrical injection at room temperature. The near-field imaging of the laser facet provides evidence of the stimulated emission into the hybrid mode and confirms the prediction of the numerical simulations. © 2013 American Institute of Physics.

Résumé: We demonstrate a semiconductor laser-based approach which enables plasmonic active devices in the telecom wavelength range. We show that optimized laser structures based on tensile-strained InGaAlAs quantum wells-coupled to integrated metallic patternings-enable surface plasmon generation in an electrically driven compact device. Experimental evidence of surface plasmon generation is obtained with the slit-doublet experiment in the near-field, using near-field scanning optical microscopy measurements. © 2012 American Chemical Society.

Résumé: While freely propagating photons cannot be focused below their diffraction limit, surface-plasmon polaritons follow the metallic surface to which they are bound, and can lead to extremely sub-wavelength energy volumes. These properties are lost at long mid-infrared and THz wavelengths where metals behave as quasi-perfect conductors, but can in principle be recovered by artificially tailoring the surface-plasmon dispersion. We demonstrate - in the important mid-infrared range of the electromagnetic spectrum - the generation onto a semiconductor chip of plasmonic excitations which can travel along long distances, on bent paths, to be finally focused into a sub-wavelength volume. The demonstration of these advanced functionalities is supported by full near-field characterizations of the electromagnetic field distribution on the surface of the active plasmonic device. © 2012 Optical Society of America.

Résumé: Surface-plasmon polaritons (SPPs) are electromagnetic waves which are bound at a metal/dielectric interface. SPPs dispersion relation allows bent propagation and can lead to sub-wavelength energy concentration. These properties, well known in the visible and near-infrared, are lost at mid-infrared and THz wavelengths. Here we demonstrate an integrated device which is able to recover and exploit the confinement properties of SPPs. It operates in the mid-infrared wavelengths by electrical injection, It generates plasmonic excitations whose dispersion is artificially tailored via proper patterning of a purely metallic surface. We illustrate the power of this approach by demonstrating bending, focusing and sub-wavelength energy concentration. We demonstrate a compact (<0.1 mm2) device, which is electrically driven and is able to generate, couple, propagate on a chip over macroscopic distances, and focus mid infrared radiation into a subwavelength region. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

Résumé: Recently a surface plasmon polariton (SPP) source based on an electrically operated semiconductor laser has been demonstrated. Here we present a numerical investigation of the light-SPP coupling process involved in the device. The problem consists in the coupling via a diffraction grating between a dielectric waveguide mode-the laser mode-and a SPP mode. The issue of the coupling efficiency is discussed, and the dependence on various geometrical parameters of both the grating and the dielectric waveguide is studied in detail. A maximum coupling efficiency of ≈24% is obtained at telecom wavelengths, which could lead to a high-power integrated SPP source when combined to a laser medium. © 2011 Optical Society of America.

Résumé: Disordered semi-continuous metallic films are a particularly striking example of complex photonic systems. They exhibit peculiar optical properties that cannot be explained from the behavior of bulk metals or ensembles of isolated nanoparticles [1]. The interplay between surface-plasmon excitations and scattering by multiscale (fractal) metallic clusters leads to spatial localization of the electromagnetic field in subwavelength areas (hot spots). A feature of these hot-spots modes is the expected coexistence of both localized and delocalized modes at the same frequency [2,3], a situation referred to as inhomogeneous localization. © 2011 IEEE.

Résumé: A surface plasmon polariton (SPP) mode is directly excited on a metal/air interface using an integrated mid-infrared (MIR) quantum cascade laser. We demonstrate the SPP generation and propagation via far-field and near-field MIR imaging. We also demonstrate bending and focusing of MIR surface-plasmons using a sub-wavelength metal patterning. © 2011 OSA.

Résumé: We present a technique based on full-field heterodyne holography for the mapping of the scattered field of plasmonic gold nanoantennas in all three dimensions. A spectroscopic study allowed us to determine resonant and non-resonant wavelengths at which we conducted a full characterization of the scattered field for different polarizations and for a wide range of antennas. These results are compared to numerical simulations. © 2011 OSA.

Résumé: Nanoantennas have the unique ability to affect the emission pattern of a dipole in free space. We present a technique based on full-field heterodyne holography for the mapping of the scattered field of plasmonic gold nanodisk chains in all three dimensions. A spectroscopic study allowed us to determine the resonant and nonresonant wavelengths at which we conducted a full characterization of the scattered field on a chosen nanodisk chain. © 2011 Optical Society of America.

Résumé: We measure the statistical distribution of the local density of optical states (LDOS) on disordered semicontinuous metal films. We show that LDOS fluctuations exhibit a maximum in a regime where fractal clusters dominate the film surface. These large fluctuations are a signature of surface-plasmon localization on the nanometer scale. © 2010 The American Physical Society.

Résumé: Optical nanoantennas are studied by full field heterodyne numerical holography in total internal reflection. After a spectroscopic characterization, the 3D scattering pattern of gold nanodisc chains was measured for wavelengths in and out of resonance. © 2010 Optical Society of America.

Résumé: Surface-plasmon polaritons (SPPs) are propagating electromagnetic modes bound at a metal-dielectric interface. We report on electrical generation of SPPs by reproducing the analogue in the near field of the slit-doublet experiment, in a device which includes all the building blocks required for a fully integrated plasmonic active source: an electrical generator of SPPs, a coupler, and a passive metallic waveguide. SPPs are generated upon injection of electrical current, and they are then launched at the edges of a passive metallic strip. The interference fringes arising from the plasmonic standing wave on the surface of the metallic strip are unambiguously detected with apertureless near-field scanning optical microscopy. © 2010 The American Physical Society.

Résumé: We report a fast imaging method based on full field heterodyne interferometry for the purpose of frequency resolved temperature imaging. An integrated circuit is supplied with a modulated current resulting into a temperature modulation. The frequency content for this modulation is detected using an object beam and a reference beam, frequency-shifted to create a beating of the interference pattern. We obtain frequency domain spectra of the temperature with excellent precision. © 2010 American Institute of Physics.

Résumé: We describe the use of Raman spectroscopy to detect and quantify, for the first time, the presence of the imprinting template in single molecularly imprinted polymer microspheres. The polymers were imprinted with the β-blocking drugs propranolol and atenolol, and precipitation polymerization was used to obtain spherical particles of diameters of 200 nm and 1.5 μm. The size of the Raman laser spot being between 1 μm and a few μm, the nanoparticles were used for bulk detection whereas with micrometer-sized particles, quantitative measurements on single particles were possible. The laser power, and consequently the acquisition times, needed to be adapted as a function of the polymer and template used in order to avoid burning. Analyte quantification from Raman spectra is straightforward by determining the peak height of a typical Raman band of the analyte, and by using a typical polymer peak for normalization. Relatively low detection limits down to 1 μM have been reached for the detection of S-propranolol through bulk measurements on MIP nanoparticles. © 2009 Elsevier B.V. All rights reserved.

Résumé: Linear and non-linear thermo-optical dynamical regimes were investigated in a photonic crystal cavity. First, we have measured the thermal relaxation time in an InP-based nano-cavity with quantum dots in the presence of optical pumping. The experimental method presented here allows one to obtain the dynamics of temperature in a nanocavity based on reflectivity measurements of a cw probe beam coupled through an adiabatically tapered fiber. Characteristic times of 1.0±0.2μs and 0.9±0.2 μs for the heating and the cooling processes were obtained. Finally, thermal dynamics were also investigated in a thermo-optical bistable regime. Switch-on/off times of 2 μs and 4 μs respectively were measured, which could be explained in terms of a simple non-linear dynamical representation. ©2008 Optical Society of America.

Résumé: We demonstrate pulsed, room-temperature operation of single-mode surface-plasmon distributed-feedback quantum cascade lasers operating at λ≈7.5 μm. The grating is implemented via the sole patterning of the top metallic contact. The presence of the grating yields an important loss reduction with respect to unpatterned surface-plasmon devices. ©2008 IEEE.

Résumé: We report the observation by scanning near-field optical microscopy of the evanescent electric field on the surface of a quantum cascade laser. The devices have been designed in order to let a consistent portion of its optical mode to leak out of the top surface. The laser wavelength in vacuum is ≈ 7.7 μm. ©OSA.

Résumé: Near-field scanning optical microscopes (NSOMs) enable one to perform subwavelength optical imaging by scanning a nanosized probe in the near field at the surface of a specimen. NSOMs generally use a subwavelength aperture, a scattering tip, or a fluorescent nanoobject as local probes of the near-field. We review the basic principles of the different types of NSOMs. Illustrative examples are given to show how these probes can be used to perform optical mapping and characterization of materials with nanoscopic resolution. © 2007 American Institute of Physics.

Résumé: In standard near-field scanning optical microscopy (NSOM), a subwavelength probe acts as an optical 'stethoscope' to map the near field produced at the sample surface by external illumination. This technique has been applied using visible, infrared, terahertz and gigahertz radiation to illuminate the sample, providing a resolution well beyond the diffraction limit. NSOM is well suited to study surface waves such as surface plasmons or surface-phonon polaritons. Using an aperture NSOM with visible laser illumination, a near-field interference pattern around a corral structure has been observed, whose features were similar to the scanning tunnelling microscope image of the electronic waves in a quantum corral. Here we describe an infrared NSOM that operates without any external illumination: it is a near-field analogue of a night-vision camera, making use of the thermal infrared evanescent fields emitted by the surface, and behaves as an optical scanning tunnelling microscope. We therefore term this instrument a 'thermal radiation scanning tunnelling microscope' (TRSTM). We show the first TRSTM images of thermally excited surface plasmons, and demonstrate spatial coherence effects in near-field thermal emission. ©2006 Nature Publishing Group.