OP: Ultrafast and Optomechanical Properties of Individual Plasmonic Antennas

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Title: Understanding the energy relaxation pathways in optical antennas made from assemblies of metal nanoparticlesNon-Technical DescriptionMetal nanoparticles support the collective motion of their conduction band electrons in response to incident light, an effect known as a surface plasmon resonance. When those nanoparticles approach each other to within distances of less than their diameters, the surface plasmons start to couple just like connected harmonic oscillators, making it possible to engineer the overall optical response and design antennas that operate in the visible frequency range in complete analogy to radio frequency receivers and transmitters. The difference to radio antennas is that the dimensions are reduced to tens of nanometers (10-9 meter). While this concept of plasmon coupling and its use to receive and transmit radiation is fairly well understood, our understanding of heating losses (i.e. Ohmic resistance), which always occur in metals and are even more important in nanoparticles interacting with visible light, is limited for assemblies of nanoparticles with specifically designed plasmon resonances. This project aims to address this question and to provide detailed insight into how the overall geometry of the nanoparticle antenna affects the energy relaxation of absorbed photons that eventually produce heat by directly following the fate of the excitation energy with very short laser pulses. The knowledge gained through this work will make it possible to potentially minimize heating losses, but also and more importantly exploit the dependence of the energy relaxation dynamics on the antenna geometry to design fast opto-electronic switches and modulators.Technical DescriptionThe goal of this proposal is to determine the effect of the geometry of antennas made from different arrangements of metal nanoparticles that have various sizes and shapes on the electron-phonon coupling and acoustic vibrations. Specifically, the proposed project will address the following two objectives: (1) Establish the dependence of electronic energy relaxation on the nanoparticle antenna geometry and the type of the excited surface plasmon mode; and, (2) Investigate the mechanism for the excitation and damping of acoustic vibrations of strongly coupled nanoparticle antennas. To accomplish these goals, electron microscopy will be combined with single-particle transient extinction spectroscopy employing wavelength tunable pulses to investigate the same individual nanoparticle antennas. Single-particle spectroscopy techniques are necessary to correlate the optical and structural properties of individual nanoparticles antennas having different geometries because especially the damping of the acoustic vibrations is otherwise determined by extrinsic factors, i.e. nanoparticle size polydispersity. It is expected that the outcomes of the proposed studies will yield a detailed insight into how the structural parameters of a nanoparticle antenna, including the surrounding medium, can be engineered to optimize desired electron-phonon relaxation times and how impulsively launched acoustic vibrations can be exploited to modulate the optical signal from the antenna itself as well as quantum emitters located in the antenna gaps. These research efforts will lead to important contributions toward understanding the relationship between the ultrafast energy relaxation dynamics and the structure dependent collective plasmon modes in nanoparticle antennas.