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In this dissertation, I have verified that the striking purple color of the intermetallic compound AuAl₂, also known as purple gold, originates from surface plasmons (SPs). This contrasts to a previous assumption that this color is due to an interband absorption transition. The existence of SPs was demonstrated by launching them in thin AuAl2 films in the Kretschmann configuration, which enables us to measure the SP dispersion relation. I observed that the SP energy in thin films of purple gold is around 2.1 eV, comparable to previous work on the dielectric function of this material. Furthermore, SP sensing using AuAl₂ also shows the ability to measure the change in the refractive index of standard sucrose solution. AuAl₂ in nanoparticle form is also discussed in terms of plasmonic applications, where Mie scattering theory predicts that the particle bears nearly uniform absorption over the entire visible spectrum with an order magnitude higher than a lightabsorbing carbonaceous particle. The second topic of this dissertation focuses on plasmon enhanced fluorescence in gold nanoparticles (Au NPs). Here, I investigated the distance-dependent fluorescence emission of rhodamine green 110 fluorophores from Au NPs with tunable spacers. These spacers consist of polyelectrolyte multilayers (PEMs) consisting of poly(allylamine hydrochloride) and poly(styrene sulfonate) assembled at pH 8.4. The distance between Au NPs and fluorophores was varied by changing the ambient pH from 3 to 10 and back, which causes the swelling and deswelling of PEM spacer. Maximum fluorescence intensity with 4.0-fold enhancement was observed with 7-layer coated Au NPs at ambient pH 10 referenced to pH 3. The last topic of this dissertation examines a novel approach to assemble nanoparticles, in particular, dimers of gold nanospheres (NSs). 16 nm and 60 nm diameter NSs were connected using photocleavable molecules as linkers. I showed that the orientation of the dimers can be controlled with the polarization of UV illumination that cleaves the linkers, making dipolar patches. This type of assembly provides a simple method with potential applications in multiple contexts, such as biomedicine and nanorobotics. / PHD / This dissertation covers three related topics. The first is an investigation of the optical properties of the unusually colored purple gold, which is a blend of gold and aluminum with the chemical formula is AuAl₂. This compound is interesting in that the origin of this color is different from most other metals. In the case of gold, for example, the metal gold is yellow color by absorbing the blue component from white light, leaving behind yellow color reflected light. The blue light is absorbed by electrons that change their state from a lower energy to a higher one. In purple gold, the color results from a different phenomenon known as “surface plasmons.” Surface plasmons are waves consisting of many electrons that move back and forth near an interface between a metal and an electrical insulator. The energy of surface plasmons in purple gold is low and corresponds to the purple color in this compound. Recently, published theoretical work supports the possibility of surface plasmons in purple gold. In this dissertation, I experimentally verify the presence of surface plasmons in purple gold. To launch surface plasmons, light was reflected off of a purple gold film deposited on the hypotenuse of a prism with varying angles of incidence. Surface plasmons can be observed by the sudden dimming of reflected light. From this, I was able to extract the surface plasmon dispersion relation, which is the relation between the inverse of the wavelength and the energy of the surface plasmons. In addition, I computed the light absorption properties of purple v gold when it is used in a nanoparticle form. The computational result showed that small purple gold nanoparticles absorb light very well, which may be useful in photothermal cancer therapy and solar steam generation. The second dissertation topic comprises a study of fluorescent molecules. These are compounds that reemit light with a different and redder color than the color of the light that illuminates them. In this experiment, green fluorescent molecules were placed near the surface of gold nanoparticles to observe how the brightness of the light emission is affected by the distance between the molecule and the metal. The underlying mechanism is based on localized surface plasmon resonances in gold nanoparticles. Localized surface plasmon resonances are waves consisting of many electrons that oscillate inside the particle, and they only occur when light at certain frequency illuminate the particle. On the resonance, the particle also exhibits the brighter light around the particle’s surface but the dimmer light away from the particle’s surface. The light enhancement from the particle can change the light emission of the fluorescent molecules. If the fluorescent molecules were placed in the range of localized surface plasmon resonances, the light emission is increased owing to the brighter light from the particle. However, if the fluorescent molecules were placed further away from the range of localized surface plasmon resonances, the light emission is decreased owing to the dimmer light from the particle. The distance between the surface of gold nanoparticle and the fluorescent molecules was varied by wrapping the gold particles with ultra-thin films of different plastic polymers before attaching fluorescent molecules to the surface of the films. These polymer films have the property that they swell and shrink when the acidity and basicity of the solution of gold particles changes, which allows me to vary the distance between the gold particles and fluorescent molecules. The results showed that the observed light gets dimmer when the solution is more acidic. On the other hand, the brighter light is noticed when the solution is more basic, and this observation is repeatable many times. Moreover, my work differs from other published works vi in that the particles with the polymer films are more robust and stable than the other particles. This allows more design flexibility and suggests applications in biomedical or environmental research where the particles can be used to locally measure properties, such as acidity in confined spacers such as living cells. It may be possible to use this technique for tumor cells in our body or toxic pollutants in the air or water. The last dissertation topic involves assembling nanoparticles to build them into larger structures. In this experiment, I fabricated particle dimers that consisted of two gold nanospheres of different sizes. They were attached together by using small molecules that are sensitive to ultraviolet (UV) light, where these molecules allow small gold nanospheres to be attached to large gold nanospheres only in those locations on the large nanospheres that have been illuminated with a sufficient amount of UV light. To achieve this alignment, UV light with a linear polarization (a specific electric field direction) was used to select the area on the large nanospheres where the UV light was particularly intense and therefore able to break the molecules, leaving positively charged surface patches on the spheres. This results in the electrostatic attraction between the positive patches on the large gold nanospheres and the negatively charged small gold nanospheres. With this method, I was able to make dimers of nanospheres in a preferred alignment by changing the polarization of UV light. The experimental results showed a good yield of dipolar patches, which allows multifunctional nanostructures with applications in nanomedicine, optical sensing, nanoelectronics, etc.

La génération de seconde harmonique magnétique (mSHG pour magnetic Second Harmonic Generation) est un phénomène physique très sensible apparaissant grâce aux brisures de symétrie aux niveaux des surfaces et interfaces des structures métalliques magnétiques. Elle constitue donc un outil puissant pour explorer ce type d'interfaces et des nanostructures. Dans ce travail, nous nous intéressons aux couplages et interactions entre la mSHG et les ondes électromagnétiques pouvant se propager en surface des matériaux. Un intérêt spécifique est porté sur l’ excitation de (i) plasmon polaritons de surfaces (SPP) dans des films métalliques en structures multicouches, (ii) d'anomalies de diffraction (dîtes de Wood) dans des nanostructures métalliques périodiques. Pour étudier l'influence de l'excitation linéaire et non-linéaire des SPP sur la mSHG, l'intensité du signal réfléchi par génération de seconde harmonique (SH) et le contraste magnétique lié à ce signal ont été mesurés par la technique de l'effet Kerr magnéto-optique transverse (MOKE) en fonction de l'angle d'incidence. Via l'utilisation de sources lasers femtosecondes émettant dans le proche infrarouge, domaine spectral où les variations de la dispersion des SPP et du coefficient d'amortissement sont significatives, nous avons pu distinguer les différentes contributions linéaires et non-linéaires aux processus d'excitation. Ce travail de thèse a ainsi permis de montrer que l’accord de phase entre la mSHG et les ondes électromagnétiques de surface peuvent contribuer très efficacement à l'augmentation des signaux SH et de contraste magnétique associé. / Owing to surface and interface sensitivity, the magnetic Second Harmonic Generation (mSHG) represents a useful tool to probe magnetic interfaces and nanostructures. This work investigates the coupling and interaction of the mSHG with electromagnetic waves propagating along the surface. Two types of surface waves have been studied: (i) surface plasmon polaritons (SPP) at surfaces of metallic thin films and multilayers, and (ii) the diffraction anomaly at the surface of periodically arranged metallic nanostructures. To study influence of linear and nonlinear excitation of surface waves on the mSHG, the reflected second harmonic (SH) intensity and the magnetic SH contrast in the transverse magneto-optical geometry were measured as a function of the angle of incidence. The use of different femtosecond light sources in the near-infrared optical range, where the SPP dispersion and damping exhibit significant variations, made it possible to disentangle linear and nonlinear contributions to the excitation of surface waves. In this thesis, it is proven that phase-matching of the mSHG and surface electromagnetic waves can lead to the enhancement of both the SH yield and the nonlinear magneto-optical signal. These results are important for controlling of the nonlinear magneto-optical response and could impact the development of magnetic storage devices, label-free biosensors and nonlinear magneto-optical switches.