Artur Movsesyan

Plasmonic systems, which support collective oscillations of electrons that couple strongly with light, have provided a rich source of physical insights in the study of light-matter interaction and are tightly embedded in a variety of fields and applications, from biosensing to photonic computing. Advances in nanofabrication techniques have played an important role in the increased relevance of plasmonics, offering tools to create nanostructures with different features and properties. Among the various nanofabrication techniques, colloidal chemistry has proven powerful and versatile as a scalable bottom-up fabrication framework. The shape and size of the metal nanoparticles can be controlled by changing different properties of the seed and of the environmental conditions for their growth, including the use of light. Light-driven or light-modulated nanoparticle growth and reshaping is being demonstrated as a useful tool to increase the range of available structures for nanofabrication. In particular, the polarization state of light induces a directional symmetry breaking in the local physicochemical response of the nanostructure, which shows strong potential for controlling the evolution of its shape under various fabrication conditions. In this article, we overview examples and opportunities of using the polarization of light in photochemical processes using plasmonic nanostructures to extend our catalogue of available geometries and superstructures.

Hot carrier and heat generation in plasmonic nanoparticles are critically important in plasmon-assisted chemistry and catalysis, sensing, and optoelectronic applications. The selective control of these effects plays a key role in determining the efficiency of chemical processes and the performance of associated devices. A major challenge lies in distinguishing and precisely controlling these effects under experimental conditions, particularly since most applications rely on continuous-wave (CW) irradiation, where both phenomena occur simultaneously. In this work, we investigate the polymerization of diazonium salts on gold nanocubes under CW and femtosecond (fs) irradiation as a probe for evaluating the efficiency of hot-electron generation and heat production. Our findings reveal that continuous excitation of the quadrupole plasmonic mode promotes a hot-electron-driven process, owing to the absorptive character of the quadrupole resonance. Conversely, excitation of the dipole mode under CW irradiation results in chaotic polymer growth, driven by lattice heat generation. However, fs excitation of the dipole mode induces polarization-dependent polymerization precisely at plasmonic hot spots. We attribute this phenomenon to the synergistic effect of vibrational heating of adsorbate molecules occurring on the picosecond scale due to energy transfer from hot electrons, which leads to the formation of the first polymer layers on the nanocube corners, followed by a lattice-heating-driven reaction that proceeds more rapidly at the created anchoring sites. These results offer insights into the ultrafast processes occurring in gold nanoparticles upon irradiation and pave the way for designing energetically efficient chemical reactions and optoelectronic devices.

Chiral molecules in nature usually show optical activity only in the deep ultraviolet, whereas artificial chiral plasmonic nanostructures can generate much stronger responses at visible and near-infrared wavelengths. An important challenge is whether the abundant biomolecular chirality in nature can be directly transferred to achiral plasmonic systems without elaborate three-dimensional nanofabrication. Here we show that the mechanical stretching of protein molecules anchored within achiral gold nanoparticle assemblies strongly enhances and reversibly modulates plasmon-coupled circular dichroism. Stretching amplifies the chiroptical response to an ellipticity of 1.18° and a dissymmetry factor of 0.2, far exceeding conventional hotspot-based strategies. Repeated stretching and relaxation further enable reversible switching over more than 100 cycles. Simulations and in situ spectroscopy indicate that the deformation of protein changes its conformation and dipole alignment, thereby strengthening the plasmonic chiral response. These findings establish a route to achieve dynamically controllable chiroptical activity in achiral plasmonic assemblies, revealing how small biomolecular deformations can strongly influence plasmonic responses of much larger nanostructures.

Propriétés plasmoniques des nanoparticules métalliques : au-delà de la résonance dipolaire: Ces dernières années, l’intérêt pour les nanoparticules métalliques a grandement augmenté du fait de la spécificité de leurs propriétés optiques. Ces propriétés se réfèrent aux résonances de plasmon de surface localisées pour les nanoparticules métalliques dont la taille est plus petite que la longueur d’onde d’illumination. Ces résonances sont directement liées à l’oscillation des électrons de conduction du métal et mènent à un champ proche électromagnétique fort localisé à la surface de la nanoparticule. Les nanoparticules métalliques sont utilisées dans des applications variées, telles que la détection biochimique, les lasers, la génération de couleurs, la spectroscopie exaltée de surface, la thérapie photo thermique du cancer, etc. Le plus souvent, les applications sont basées sur des facteurs externes. Dans cette thèse, nous étudions en profondeur les facteurs qui peuvent affecter la position spectrale de ces modes. De plus, nous parlerons de l’influence du montage expérimental sur les spectres optiques enregistrés. Outre le mode dipolaire, une nanostructure plasmonique peut montrer plusieurs autres modes qui peuvent être potentiellement utilisés pour des applications. Nous étudierons l’excitation et les mécanismes révélant les modes plasmoniques « cachés ». Dans la dernière étape de cette étude, nous utiliserons les propriétés multi-résonantes d’un nanocylindre d’or pour démontrer l’exaltation de fluorescence de boîtes quantiques par un effet plasmonique résonant double. Cette approche possède un grand potentiel pour être utilisée dans des applications d’énergie solaire.

Exploring the mechanisms and potential of hot-electron (HE) generation is a crucial facet of contemporary nanooptics and nanoelectronics research. In this work, we examine the limits of localization and enhancement in HE generation, particularly in the context of photochemical processes such as hydrocarbon fuel synthesis. For this study, we developed a nonlinear, quantum, self-consistent formalism incorporating multipole Kreibig parameters. We employed a plasmonic trimer consisting of two broadband antenna nanoparticles (NPs) and a small "reactor" nanorod (NR) with a narrow resonance. In this Fano scheme, the total absorption of the NP-NR-NP trimer exhibits a pronounced Fano effect─specifically, a Fano dip. Notably, we observe a significant enhancement in HE generation when computing surface maps within the NR. We refer to this configuration as an HE Super-Generator. From a fundamental perspective, the proposed nanooptical regime─combining Fano interference with the antenna effect─represents a scenario approaching the practical upper limit of HE-based quantum effects achievable in plasmonics. Our findings point to a promising strategy for future optoelectronic and photochemical applications at the classical-quantum interface in plasmonic systems.

Shape symmetry breaking in the formation of inorganic nanostructures is of significant current interest. It was typically achieved through the growth of colloidal nanoparticles with adsorbed chiral molecules. Photochemical processes induced through asymmetric plasmon excitation by circularly polarized light in surface immobilized nanostructures also led to symmetry breaking. Here, we show that chiral symmetry breaking can be achieved by randomly rotating gold@silver core-shell nanobars in colloidal solution using circularly polarized illumination, where orientational averaging does not eliminate the symmetry breaking of an asymmetric plasmon-induced galvanic replacement reaction. Different morphological effects that are produced by circularly vs linearly polarized light illumination demonstrate the intricate effect of light polarization on the localized plasmonic-induced photochemical response. The essential features of this symmetry breaking, such as illumination wavelength dependence, were reproduced by simulations of circularly polarized light-excited-plasmon-induced hot-electron generation as the source for asymmetric metal deposition. The symmetry breaking becomes smaller in more symmetric geometrical shapes, such as triangular nanoprisms and nanocubes, and down to zero in spherical ones. The degree of symmetry breaking rises when the nanobars are immobilized on a substrate and illuminated from a single direction.

Chiral molecules, a cornerstone of chemical sciences with applications ranging from pharmaceuticals to molecular electronics, come in mirror-image pairs called enantiomers. However, their synthesis often requires complex control of their molecular geometry. We propose a strategy called "electromagnetic enantiomers" for inducing chirality in molecules located within engineered nanocavities using light, eliminating the need for intricate molecular design. This approach works by exploiting the strong coupling between a nonchiral molecule and a chiral mode within a nanocavity. We provide evidence for this strong coupling through angular emission patterns verified by numerical simulations and with complementary evidence provided by luminescence lifetime measurements. In simpler terms, our hypothesis suggests that chiral properties can be conveyed on to a molecule with a suitable chromophore by placing it within a specially designed chiral nanocavity that is significantly larger (hundreds of nanometers) than the molecule itself. To demonstrate this concept, we showcase an application in display technology, achieving efficient emission of circularly polarized light from a nonchiral molecule. The electromagnetic enantiomer concept offers a simpler approach to chiral control, potentially opening doors for asymmetric synthesis.

Shape symmetry breaking in the formation of inorganic nanostructures is of large current interest. It was typically achieved through growth of colloidal nanoparticles with adsorbed chiral molecules. Photochemical processes induced through asymmetric plasmon excitation by circularly polarized light in surface immobilized nanostructures also led to symmetry breaking. Here we show that chiral symmetry breaking can be achieved on randomly rotating gold@silver core-shell nanobars in colloidal solution, by means of circularly polarized illumination, where orientational averaging does not eliminate the symmetry breaking of an asymmetric plasmon-induced galvanic replacement reaction. Different morphological effects that are produced by circularly vs. linearly polarized light illumination demonstrate the intricate effect of light polarization on the localized plasmonic-induced photochemical response. The symmetry breaking becomes smaller in more symmetric geometrical shapes such as triangular nanoprisms and nanocubes, down to zero in spherical ones. The symmetry breaking rises when the nanobars are immobilized on a substrate and illuminated from a single direction.

The collective light-matter interaction of chiral supramolecular aggregates or molecular ensembles with confined light fields remains a mystery beyond the current theoretical description. Here, we programmably and accurately build models of chiral plasmonic complexes, aiming to uncover the entangled effects of excitonic correlations, intra- and intermolecular charge transfer, and localized surface plasmon resonances. The intricate interplay of multiple chirality origins has proven to be strongly dependent on the site-specificity of chiral molecules on plasmonic nanoparticle surfaces spanning the nanometer to sub-nanometer scale. This dependence is manifested as a distinct circular dichroism response that varies in spectral asymmetry/splitting, signal intensity, and internal ratio of intensity. The inhomogeneity of the surface-localized plasmonic field is revealed to affect excitonic and charge-transfer mixed intermolecular couplings, which are inherent to chirality generation and amplification. Our findings contribute to the development of hybrid classical-quantum theoretical frameworks and the harnessing of spin-charge transport for emergent applications.

Due to their broken symmetry, chiral plasmonic nanostructures have unique optical properties and numerous applications. However, there is still a lack of comprehension regarding how chirality transfer occurs between circularly polarized light (CPL) and these structures. Here, we thoroughly investigate the plasmon‐assisted growth of chiral nanoparticles from achiral Au nanocubes (AuNCs) via CPL without the involvement of any chiral molecule stimulators. We identify the structural chirality of our synthesized chiral plasmonic nanostructures using circular differential scattering (CDS) spectroscopy, which is correlated with scanning electron microscopy imaging at both the single‐particle and ensemble levels. Theoretical simulations, including hot‐electron surface maps, reveal that the plasmon‐induced chirality transfer is mediated by the asymmetric distribution of hot electrons on achiral AuNCs under CPL excitation. Furthermore, we shed light on how this plasmon‐induced chirality transfer can also be utilized for chiral growth in bimetallic systems, such as Ag or Pd on AuNCs. The results presented here uncover fundamental aspects of chiral light‐matter interaction and have implications for the future design and optimization of chiral sensors and chiral catalysis, among others.