Electron emission by a material surface under intense ultrashort pulsed laser light is actively studied for its application in various fields, from biochemistry to aerospace. Different strategies are proposed to push back against the electronic breakdown happening at high laser irradiance. Here, we investigate the effect of adding a thin dielectric top layer of copper oxide (CuO) on a noble plasmonic metal, copper (Cu), on the electron emission processes. Using a low energy and photoemission electron microscope (LEEM - PEEM), we measured, at a subwavelength spatial scale, electron emission spatial distribution and total electron energy distribution (TEED) of both pristine and thermally oxidized copper. We report that (i) the preferred electron emission sites on the surface correspond to step edge bunches and surface defects of the Cu lattice, (ii) the growth of a thin CuO layer improves the electron emission properties of Cu surface. The thin dielectric CuO coating is a source of near-field plasmon-polariton waveguiding and confinement in the immediate vicinity of the metal surface. The electron emission regime changes from a photoelectric weak field regime to a strong field tunnelling regime beyond a light irradiance threshold of 0.7 GW/cm² (0.073 V/nm). This irradiance threshold value is one order of magnitude lower than that required before thermal oxidation. Under equivalent illumination conditions (irradiance, exposure time) above a threshold irradiance, the total charge extracted from Cu with a CuO surface layer is 100 times greater than that of a clean metal.

We have studied the surface structure of a single crystal (beta-Ga2O3(010) using quantitative Low Energy Electron Diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). The XPS measurements show spectra typical of stoichiometricGa(2)O(3) with a clean surface. LEED consistently shows a p(1x1) pattern, free of surface reconstruction. Quantitative LEED I(V) curves are acquired for 41 distinct diffraction spots. The experimental I(V) curves are compared to simulations over the first five layers. The best fits to the experimental LEED I(V) curves acquired at all diffraction spots are then used to calculate the interplanar relaxation and atomic rumpling. Significant atomic rumpling and interplanar relaxation are found over the first 5 atomic layers. As a result of rumpling a polarization of similar to 2 mu C/cm(2) develops in the topmost surface layer. The structural results are in good agreement with previous density functional theory calculations and experimental X-ray photoelectron diffraction.

In this work, an investigation on plasmonic hotspots of Au tilted nanocolumns and their application to the uracil detection by surface-enhanced Raman scattering (SERS) is addressed. These Au tilted nanocolumns are fabricated by employing a low-cost and easy-to-implement fabrication technique, which is the glancing angle deposition with magnetron sputtering. Moreover, these Au nanocolumns can generate a great density of highly confined electric field zones, called hotspots, located in the nanogaps between the nearest nanocolumns. The photoemission electron microscopy is employed to obtain the statistics on the distribution of these plasmonic hotspots with a high accuracy (spatial resolution of 10-20 nm) in the spectral range from 680 nm to 1080 nm. Thus, the SERS intensity can be correlated with the density and the intensity distribution of these plasmonic hotspots. Finally, the plasmonic hotspots of the Au nanocolumns are used to assess the limit of detection (LOD) of the uracil molecule by SERS. This LOD is determined at 6 nM demonstrating the good sensitivity of our Au nanocolumns compared to the existing literature.

The analysis of the local distribution of the electric field, induced by localized surface plasmon resonance (LSPR), is crucial for selecting the morphology of gold nanoparticles (AuNPs) for specific applications. The reported study is based on an LSPR-induced near-field two-photon photopolymerization (NF2P) reaction. The initiation of NF2P is triggered by LSPR-enhanced near-field light, while in the far-field, oxygen inhibits this reaction. The spatial extent of the NF2P reaction is compared to the local electric field distribution depending on AuNP morphologies, established by numerical simulations. Overall, our results demonstrate that the photopolymerization is not only driven by the local near-field enhancement but also strongly depends on the topologies of nano-object, and the photopolymerization extension was more confined in anisotropic and sharp structures compared to isotropic ones. Additionally, we showcased its capability to confine polymerization reactions within nanoscale volumes with the possibility of controlling the localization of polymer lobes at a single triangle apex, for instance, via light polarization.

Over the past 20 years, hybrid plasmonics for nanoemitters of light or for nanoabsorbers, based on weak or strong coupling between metallic nanocavities and active media (emissive or absorbing entities), have given rise to important research efforts. One of the main current challenges is the control of the nanoscale spatial distribution and associated symmetry of the active medium in the vicinity of the metallic nanoparticles. In this review, we first recall the main principles of weak and strong coupling by stressing the importance of controlling the spatial distribution of the active medium and present the main approaches developed for achieving this control. Nine different approaches are identified. We then focus our attention on one of them based on plasmonic photopolymerization and discuss the flexibility of this approach in terms of control of the spatial symmetry of the hybrid nanosystem metal-polymer nanoemitters and the resulting polarization dependence of the light emission. The different approaches are analyzed and compared with each other, and some future perspectives and challenges are finally discussed.

Progress in single molecule fluorescence experiments have enabled an in-depth characterization of fluorophores, ranging from their photophysical rates to the orientation of their emission dipole moments in three dimensions. However, one crucial spatial information remains elusive: the molecule orientation relative to its emission dipole moment. One can retrieve the latter only by the use of another non-colinear transition dipole moment. We experimentally demonstrate the optical retrieval of this information for single terrylene (Tr) molecules in a 30 nm thin para-terphenyl matrix. We show, through second-order correlation measurements at varying excitation power and polarization, that Tr molecules experience an optically induced deshelving of their triplet states, mediated by two orthogonal intra-molecular triplet-triplet absorption dipole moments. We take advantage of these two transition dipole moments to retrieve the full orientation of the Tr molecule, employing a 3-level scheme for the molecule photophysics and analytical calculations for the exciting electric field distribution. This modelling approach enables us to accurately describe both varying power and polarization measurements, giving access to the molecule's photophysical rates and to its complete orientation in three dimensions. This includes the orientation of the singlet emission dipole moment in the laboratory frame, and the orientation of the molecule plane with respect to the singlet emission dipole moment. Using fluorescence and triplet-triplet absorption, we retrieve the three-dimensional orientation of a single fluorescent molecule: the orientation of its singlet absorption dipole moment and the orientation of the molecular plane around the latter.

Since the description of diffraction from the seventeenth century and the development of optical microscopy that has followed, many approaches have been developed for breaking the diffraction limit. Following the first proposition of near-field optical microscopy made in the 1920s, the first experimental demonstrations started in the early 1980s through scanning near-field optical microcopy, within the context of the swift development of scanning probe microscopies. Since, different alternative approaches and concepts have emerged for probing the optical near-field with a sub-wavelength resolution. The chapter is divided into four main sections. In Sect. 4.2, important theoretical principles will be reminded. They will allow the reader to acquire a general background in near-field optics. Section 4.3 describes how it is possible to probe the near-field with physical optical nano-antenna. In particular, different approaches of scanning near-field optical microscopy will be discussed. Section 4.4 is dedicated to the way free electrons can be used for probing the near-field. Section 4.5 deals with the use of nanoscale photochemistry for probing the optical near-field. These three approaches present respective features and assets that will be illustrated by examples of achievements from the literature.

Strong-field quantum electronics is emerging as a potential candidate in information processing but still coherence vs decoherence is a primary concern of the concept. Strong-field coherent processes in band gap materials have led during the last decade to the emergence of high harmonic generation in semiconductors, petahertz electronics, or strong-field quantum states. However, the coherent behavior of the sub-optical cycle-driven electrons has never been directly observed. We report here on the experimental evidence of coherent ultrashort emission of hot electrons from a nanostructured semiconductor. Our method uses sub-wavelength electric field enhancement to localize the electron emission within a nanometer-scale spot. We found similarities with the electron emission from metallic nanotips in the strong-field regime, a topic that has opened a vast domain of applications during the last decade. The electron spectra display both odd and even harmonic orders of the driving femtosecond laser frequency, a signature of the coherent nature of the electron emission and their attosecond timing. Our findings complete our knowledge of phenomena governing coherent strong-field processes in semiconductors and open perspectives for the generation of future quantum devices operating in the strong-field regime.

Surface enhanced Raman spectroscopy (SERS) is a powerful non-invasive technique to detect and identify molecule traces. The accurate identification of molecules is based on the detection of distinctive vibrational modes characteristic of a molecule adsorbed onto the surface. We investigated the detection performances of three commercial SERS substrates: nanostructured Au supports from Hamamatsu, Premium Ag–Au supports from SERSitive, and RAM–SERS–Au from Ocean Insight, which were tested with solutions of thiophenol (C 6 H 6 S) at 10 –6 M and 10 –8 M concentration. SERS measurements were performed systematically with 633 and 785 nm excitation wavelengths and Raman mappings were recorded randomly on the surfaces. The spectral quality (baseline intensity and signal-to-noise ratio), the thermal stability under laser illumination, and the Raman intensity distribution of the Hamamatsu substrate and our own fabricated gold substrate were discussed for the detection of 10 –8 M thiophenol molecules. The detection of crystal violet (CV), a toxic dye, is demonstrated at 5.10 –9 M.

Surface-enhancedRaman scattering (SERS) is a well-establishedsurface-sensitive technique for detecting trace amounts of molecularanalytes. While the impact of surface singularities on plasmonic materialshas been widely studied, the fabrication of cost-effective, efficientSERS substrates remains a challenge. In this paper, we present thestudy of large-area nanorough Au SERS-active substrates, elaborated by thermal evaporation deposition by photoemission electron microscopy(PEEM), a high-resolution near-field mapping technique, to accessthe statistical properties of the hot-spot distribution. We experimentallydemonstrate that the near-field PEEM and far-field Raman statisticalsignatures of nanorough Au surfaces are quantitatively correlatedwhen used for molecular sensing. The maximum of the SERS signal ofthiophenol (TP) molecules diluted to 10(-6) M is observednear the film percolation threshold for which the hot-spot densityis maximum. Finally, SERS measurements from solutions of TP, crystalviolet, and rhodamine B molecules at 10(-8) M demonstratedthe sensitivity of our substrates for molecular sensing.

We present a new method to pattern assemblies of gold nanorods (GNRs) on substrates and a study of the near-field coupling induced by the aggregation of the nanoparticles. The combination of deep-UV lithography and controlled deposition of functionalized GNRs generates complex GNR assemblies. Near-field coupling is investigated by photoemission electron microscopy (PEEM) on single, dimer, and elongated aggregates of GNRs. This comparative study exhibits different kinds of near-field coupling efficiency that occur depending of the incident light polarization, interparticle gaps and angles between the nanorods. Hot spots of the near-field are associated with the interparticle gap regions.

We developed a model based on Boundary-Element-Method to describe the tip-assisted electric field enhancement under laser illumination. The nanometric distribution of enhanced field is at the basis for an interpretation of near-field laser ablation applications.

Localized surface plasmon-induced photopolymerization of free-radical acrylate monomers is an efficient, smart, and versatile method for preparing metal/polymer hybrid nanoparticles (NPs) with accurate control of the thickness and spatial distribution of the polymer on the NP surface. Despite a growing number of practical demonstrations, the mechanism leading to polymerization of the acrylate monomer by localized surface plasmon resonance (LSPR) is still controversial. It could be related to either a photochemical mechanism enhanced by electromagnetic hot spots (enhanced near field) or thermoplasmonic (photothermal heating) or electrochemical (via hot-carrier injection) mechanisms, as proposed in different studies. After developing a high-resolution characterization method based on transmission electron microscopy and by tuning the photopolymer composition and the irradiation conditions, the LSPR-induced physicochemical mechanism is revealed. We demonstrate that the photochemical pathway is the main mechanism under the mild irradiation conditions chosen for this process. In a more general way, photopolymerization proves to be a powerful tool to investigate the coupling between metal nanostructures and organic moieties.

The enhancement and control of non-linear phenomena at a nanometer scale has a wide range of applications in science and in industry. Among these phenomena, high-harmonic generation in solids is a recent focus of research to realize next generation petahertz optoelectronic devices or compact all solid state EUV sources. Here, we report on the realization of the first nanoscale high harmonic source. The strong field regime is reached by confining the electric field from a few nanojoules femtosecond laser in a single 3D semiconductor waveguide. We reveal a strong competition between enhancement of coherent harmonics and incoherent fluorescence favored by excitonic processes. However, far from the band edge, clear enhancement of the harmonic emission is reported with a robust sustainability offering a compact nanosource for applications. We illustrate the potential of our harmonic nano-device by performing a coherent diffractive imaging experiment. Ultra-compact UV/X-ray nanoprobes are foreseen to have other applications such as petahertz electronics, nano-tomography or nano-medicine.

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.

Surface plasmon resonance can be used to manipulate light at the nanoscale. It was used here to trigger photopolymerization of an atom transfer radical polymerization (ATRP) molecular system, leading to a thin polymer shell at the surface of the metal nanostructure. The polymerization can be reactivated from the first polymer shell to covalently graft a second monomer layer with precise control over the thickness at the nanometric scale, depending on the photonic parameters. This route can be applied to different nanoobjects and allows an anisotropic surface modification in agreement with the spatial localization of the enhanced electromagnetic field near the nanostructure. This new route opens the door towards the preparation of multifunctional hybrid metal/polymer nanostructures.

A simple hand calculation method based on group theory is proposed to predict the near field maps of finite metallic nanoparticles (MNP) of canonical geometries: prism, cube, hexagon, disk, sphere, etc. corresponding to low order localized surface plasmon resonance excitations. In this article, we report the principles of the group theory approach and demonstrate, through several examples, the general character of the group theory method which can be applied to describe the plasmonic response of particles of finite or infinite symmetry point groups. Experimental validation is achieved by collection of high-resolution subwavelength near-field maps by photoemission electron microscopy (PEEM) on a representative set of Au colloidal particles exhibiting either finite (hexagon) or infinite (disk, sphere) symmetry point groups.

La microscopie de photoémission d’électrons constitue une technique non intrusive d’investigation du champ proche optique. Son domaine d’application actuel autorise une approche physique multidimensionnelle des phénomènes d’optique de champ proche aux échelles spatiale (nm), temporelle (fs) et spectrale (meV).

The two-photon luminescence (TPL) of gold nanoparticles (NP) was shown to result from the excitation of hot carriers, the plasmonic NP resonances playing an important role both for plasmon enhanced absorption and plasmon enhanced emission. However, the exact parameters enabling to control or optimize the NP nonlinear luminescence still need to be understood in detail. In this paper, we report the two-photon excited photoluminescence of single gold nanorods exhibiting identical aspect ratio (close to 4) and thus identical plasmonic resonances, but increasing volumes V (707 <V< 160 103 nm3 i.e. rod diameters varying between 6 and 40 nm). The two-photon luminescence intensity of a high number of colloidal nanorods was investigated at the single object level, combining polarization resolved TPL and simultaneously acquired topography. Non-monotonic TPL variations are evidenced, nanorods with an intermediate size (diameter around 10 nanometers) exhibiting the highest TPL signal intensity. A model is proposed considering both the local field enhancement effects at the NP and the size-dependent electron thermalization processes. BEM (Boundary Elements Method) simulations are used to compute the fields at both the transverse and longitudinal plasmon resonance. A good fitting of the experimental data is obtained considering integration of the fields over the whole the NP volume.

Although bulk gold is known to present very low luminescence quantum yield, gold nanoparticles can present a huge nonlinear luminescence with an apparent high influence of the particle shape. The two-photon luminescence (TPL) of Au nanorods (NR) was investigated at the single-object level, combining polarization-resolved TPL and simultaneously acquired topography using atomic force microscopy (AFM). We evidence that the huge TPL observed in these rods is the result of an interplay between their two different localized surface plasmon resonances. Excitation of gold NR at its longitudinal plasmon frequency first lead to increased absorption and thus increased production of electron-hole pairs that happen to recombine radiatively through the transverse plasmon band i.e. with an enhanced emission at the NR transverse plasmon band. Above fundamental aspects, we show that these specific properties make TPL a highly sensitive technique for the characterization of nanoobjects from their topology to their organization.