Digital holography is an interference-based imaging technique capable of recording both the amplitude and phase of an electromagnetic field. It can be obtained at the laser illumination wavelength, but also with second-harmonic generation, since the latter is produced in a coherent process. Here we describe the development of a harmonic holographic microscope for 3D single-shot mapping of second-harmonic emitters. The knowledge of the scattered field, in amplitude and phase, in a given plane, that of the camera, allows its reconstruction in any other plane using the angular spectrum representation of the optical fields, a process called 3D numerical back-propagation. In order to probe the polarization dependence of the sample nonlinear response, we implement polarization multiplexing, in which a Wollaston prism creates two off-axis reference beams with orthogonal polarizations and non-parallel propagation directions. Each reference only interferes with the corresponding polarization component in the sample SHG emission, thus providing two independent sets of interference fringes which are easily separated in the angular spectrum representation. From a single measurement, two second-harmonic fields corresponding to orthogonal polarizations can be back-propagated. In the particular case of collagen, the second-harmonic polarization state can reveal the orientation or disorder of molecules and fibers. We demonstrate the feasibility of the method by reconstructing the spatial distribution of the second-harmonic field generated by collagen fibers in a rat-tail tendon sample and show that polarization-multiplexed holography can provide single-shot 3D mapping of biophysical parameters such as the helical pitch angle of collagen molecules.

Digital holography is an imaging technique that enables a 3-dimensional reconstruction of both the amplitude and phase of an electromagnetic field after its interaction with an object. Second harmonic generation being a coherent process, it can also be used to generate interferences and holograms. Since collagen molecules exhibit a significant second order response, we apply harmonic holography to porcine cornea samples to obtain single-shot measurements of the 3D spatial distribution of the collagen fibers. In addition, we propose polarization multiplexing to study the polarization dependence of the sample response.

Digital holography is an imaging technique that enables a 3-dimensional reconstruction of the electromagnetic field scattered by an object in both amplitude and phase. We demonstrated its use in microscopy in linear regime for the full 3-D mapping of the field scattered by single nanostructures such as nano-antennas [1] and near-field probes [2]. Holography is a technique based on interferences, which can be obtained at the laser illumination wavelength, but also with Second Harmonic Generation (SHG), since the latter is produced in a coherent process [3, 4]. Here, we describe the development of a harmonic holographic microscope for single-shot mapping of the second harmonic 3D radiation pattern near samples with nonzero second harmonic susceptibilities. The knowledge of the scattered field (amplitude and phase) in a given plane (that of the camera) allows its reconstruction in any other plane using e.g. the angular spectrum representation of the optical fields [5], and assuming propagation in homogeneous media, a process called 3D numerical back-propagation [6]. In addition to providing 3D reconstruction, thus enhancing the imaging capabilities beyond those of back focal-plane imaging, the harmonic holography microscope also benefits from an amplification effect since the signal from the sample is multiplied by an intense reference in the interference term [7], making the method particularly well suited to measure the weak SHG signals [8]. After a first validation on dielectric samples made of nonlinear micro-crystals and cornea collagen, we are implementing the technique to obtain SHG fields radiated by plasmonic nano-antennas [9].

Plasmonic resonators featuring nanoscale can exhibit strongly enhanced optical near-fields that have been extensively used in surface enhanced spectroscopy (Raman and Fluorescence) and in biosensing. However, deterministic nanostructures do not provide numerous degrees of freedom to control optically these local field enhancements. By comparison, wavefront shaping techniques in disordered scattering media provide numerous degrees of freedom to control light focusing in space and time. To associate local field enhancements and far-field wavefront control, we use disordered plasmonic metasurfaces close to the percolation threshold that feature both hotspots and delocalized plasmonic modes that can be controlled using a spatial light modulator. By controlling the phase of an incoming femtosecond pulsed laser on a disordered gold metasurface, we optimize the two-photon induced luminescence (TPL) at a chosen position with a typical two-order of magnitude enhancement, indicating a far-field optimization of the optical near-field. The optimization process is performed using a random genetic algorithm and provides the highest enhancement when the metasurface features the highest degree of morphological complexity, close to the percolation threshold (Nano Lett. 20, 3291 (2020)). Far-field wavefront shaping also provides new degrees of freedom to provide statistical imaging schemes that offset the structural complexity of disordered nanophotonic systems. For instance, using nonlinear luminescence images measured with randomly wavefront-shaped femtosecond excitations, we can independently map the localized and delocalized plasmonic modes on a disordered gold metasurface (ACS Photonics 8, 1973 (2021)).

Using a statistical analysis of nonlinear luminescence images measured with randomly wavefront-shaped femtosecond excitations, we provide direct insight on both the localized and delocalized plasmonic modes featured by disordered gold metasurfaces. We can independently image areas where far-field wavefront shaping can control the optical properties and areas with strong subwavelength optical hotspots. In practice, the fraction of the disordered plasmonic surface on which wavefront control is feasible depends strongly on the nanoscale morphology of the sample. Close to the percolation threshold, the entire surface is sensitive to wavefront shaping, and we observe the largest densities of delocalized modes as well as the strongest optical hotspots. These results demonstrate how statistical imaging schemes can offset the complexity of disordered nanophotonic systems in order to characterize their optical properties.

We demonstrate the local optimization of nonlinear luminescence from disordered gold metasurfaces by shaping the phase of femtosecond excitation. This process is enabled by the far-field wavefront control of plasmonic modes delocalized over the sample surface, leading to a coherent enhancement of subwavelength electric fields. In practice, the increase in nonlinear luminescence is strongly sensitive to both the nanometer-scale morphology and the level of structural complexity of the gold metasurface. We typically observe a 2 orders of magnitude enhancement of the luminescence signal for an optimized excitation wavefront compared to a random one. These results demonstrate how disordered metasurfaces made of randomly coupled plasmonic resonators, together with wavefront shaping, provide numerous degrees of freedom to program locally optimized nonlinear responses and optical hotspots.

Plasmonic nanoantennas featuring nanoscale gaps can exhibit strongly enhanced optical near-fields that have been extensively used in surface enhanced spectroscopy (Raman and Fluorescence) and in biosensing. However, deterministic nanostructures do not provide enough degrees of freedom to control optically these local field enhancements. By comparison, wavefront shaping techniques in disordered scattering media provide numerous degrees of freedom to control light focusing in space and time [1]. To associate local field enhancements and far-field wavefront control, we use disordered plasmonic surfaces close to the percolation threshold (see Fig. 1-a) that feature both hotspots [2] and delocalized plasmon modes. Disordered plasmonic surfaces can be controlled using a spatial light modulator [3].

Engineering the wavefront of light in random media allows the control of wave propagation in space and time by exploiting the spatial and spectral degrees of freedom introduced by multiple scattering (M. Mounaix et al, Phys. Rev. Lett. 116, 253901 (2016)). To apply this far-field control strategy and focus electromagnetic energy at the nanoscale, it is necessary to introduce scatterers that feature strongly enhanced and confined optical fields such as plasmonic nanoantennas. In particular, semi-continuous gold films close to the percolation threshold feature high local field enhancements (S. Gresillon et al, Phys. Rev. Lett. 82, 4520 (1999)) but also propagating surface plasmon waves that can be controlled using a spatial light modulator (P. Bondareff et al, ACS Photonics 2, 1658 (2015)). In this presentation, we demonstrate how controlling the phase of an incoming pulsed laser on a chosen 10 µm x 10 µm area of a random plasmonic metasurface allows us to optimize the two-photon luminescence (TPL) of gold at a given position of the sample. The optimized TPL intensities, that are associated with strong local field enhancements, are increased by a factor of 50 for semi-continuous films that are close to percolation compared to samples far from it, demonstrating that the morphology and randomness of the plasmonic film play an essential role in the control of nonlinear luminescence. Furthermore, we show that TPL intensities can be enhanced at any position of a percolated film, opening exciting perspectives for the wavefront engineering of local field enhancements in random plasmonic metasurfaces.

In this chapter, we will present the general concepts of surface-enhanced fluorescence (SEF) before discussing in detail the effect of a simple continuous metallic thin film to improve detection and imaging at the interface. The understanding of the interplay between fluorophores, propagating photons, and the surface plasmon (SP) in most basic geometry captures the essence of SEF. We show that this configuration provides significant improvements when applied to bioimaging. This chapter is divided into further three sections. The first section introduces the principles of SEF and the second to SP coupling and its combined effect on fluorophores. The third section is devoted to analyzing the various cases of fluorescence enhancement near metallic surface for imaging applications.

In this work, we present methods to control at will the spatio-temporal profile of a transmitted ultrashort pulse at the output of a thick scattering medium. By measuring either the Multispectral [3] or the Time-Resolved Transmission Matrix [4], we can fully describe the propagation of the broadband pulse either in the spectral or in the temporal domain. With a single phase-only SLM, one can manipulate the spatial degrees of freedom to adjust the delay between different optical paths. Therefore, spatial and spectral/temporal distortions of the output light can both be compensated at the same time.

We report the broadband characterization of the propagation of light through a multiple scattering medium by means of its multispectral transmission matrix. Using a single spatial light modulator, our approach enables the full control of both the spatial and spectral properties of an ultrashort pulse transmitted through the medium. We demonstrate spatiotemporal focusing of the pulse at any arbitrary position and time with any desired spectral shape. Our approach opens new perspectives for fundamental studies of light-matter interaction in disordered media, and has potential applications in sensing, coherent control, and imaging.

We report broadband characterization of the propagation of light through a multiply scattering medium by means of its Multi-Spectral Transmission Matrix. Using a single spatial light modulator, our approach enables the full control of both spatial and spectral properties of an ultrashort pulse transmitted through the medium. We demonstrate spatiotemporal focusing of the pulse at any arbitrary position and time with any desired spectral shape. Our approach opens new perspectives for fundamental studies of light-matter interaction in disordered media, and has potential applications in coherent control and imaging.

We experimentally study the optical field distribution on disordered plasmonic networks by far-field wavefront shaping. We observe nonlocal fluctuations of the field intensity mediated by plasmonic modes up to a distance of 10 mu m from the excitation area. In particular we quantify the spatial extent of these fluctuations as a function of the metal filling fraction in the plasmonic network, and we identify a clear increase around percolation due to the existence of extended plasmonic modes. This paves the way toward far-field coherent control of plasmonic modes on similar disordered plasmonic networks. We expect these results to be relevant for quantum networks, coherent control, and light matter interactions in such disordered films where long-range interactions are critical.

We present a method to measure the spectrally-resolved transmission matrix of a multiply scattering medium, thus allowing for the deterministic spatiospectral control of a broadband light source by means of wavefront shaping. As a demonstration, we show how the medium can be used to selectively focus one or many spectral components of a femtosecond pulse and how it can be turned into a controllable dispersive optical element to spatially separate different spectral components to arbitrary positions.

Les sondes locales permettent de mesurer ou modifier les propriétés de surface de la matière avec une résolution de quelques atomes. Le principe des sondes locales est similaire à celui du stéthoscope du médecin, et consiste à mettre une petite sonde le plus proche possible de la surface pour une zone d’observation qui n’est plus limitée par la propagation et la longueur d’onde mais seulement par la distance sonde-surface.

The measurement of the polychromatic transmission matrix of a multiply scattering medium is reported, thus allowing control the propagation of an ultrashort pulse through the medium. We also report on our effort towards fast transmission matrix measurement.

We present a simple scheme to determine the diffusion properties of a thin slab of strongly scattering material by measuring the speckle contrast resulting from the transmission of a femtosecond pulse with controlled bandwidth. In contrast with previous methods, our scheme does not require time measurements nor interferometry. It is well adapted to the characterization of samples for pulse shaping, nonlinear excitation through scattering media, and biological imaging.

Single biomolecule behaviour can reveal crucial information about processes not accessible by ensemble measurements. It thus represents a real biotechnological challenge. Common optical microscopy approaches require pico- to nano-molar concentrations in order to isolate an individual molecule in the observation volume. However, biologically relevant conditions often involve micromolar concentrations, which impose a drastic reduction of the conventional observation volume by at least three orders of magnitude. This confinement is also crucial for mapping sub-wavelength heterogeneities in cells, which play an important role in many biological processes. We propose an original approach, which couples Fluorescence Correlation Spectroscopy (FCS), a powerful tool to retrieve essential information on single molecular behaviour, and nano-fakir substrates with strong field enhancements and confinements at their surface. These electromagnetic singularities at nanometer scale, called "hotspots", are the result of the unique optical properties of surface plasmons. They provide an elegant means for studying single-molecule dynamics at high concentrations by reducing dramatically the excitation volume and enhancing the fluorophore signal by several orders of magnitude. The nano-fakir substrates used are obtained from etching optical fiber bundles followed by sputtering of a gold thin-film. It allows one to design reproducible arrays of nanotips.

Active plasmonic devices are much promising for optical devices and circuits at the nanoscale. We show that single nanoparticles coupled to metallic surfaces are good candidates for integrated components with nanometric dimensions. The localized plasmon of the nanoparticle launches propagating surface plasmons in the metallic thin film. Direct particle observation using leaky wave microscope geometry permits easy detection through the interference of the direct transmitted excitation light and the surface plasmon leaky mode. Investigations of the optical response of a nanoparticle deposited on metallic thin metal films reveals unexpectedly high transmission of light associated to contrast inversion in the images.