"The interaction of light with structures much smaller than its wavelength, i.e. far below the diffraction limit, is enhanced by the effect that electromagnetic fields cause in the charge of the object. The excitation of plasmons enhances and focuses the light in the proximity of the nanostructure, mediating the energy exchange between photons and electrons. As the size of metal nanostructures and optoelectronic nanodevices approaches atomic scale dimensions, quantization effects in their electronic and plasmon structure gain increasing relevance in light scattering. Understanding the coupling of photons with electrons in the presence of quantum effects is crucial for improving the functionality of optoelectronic nanodevices like light emitting diodes or for the performance of nanoparticles in fields like medicine, or catalysis. In this proposal we will study the quantum limits of light emission and scattering by metallic and molecular nanowires of nanometer sizes. We will identify their plasmon resonances and correlate them with their quantized electronic structure. The goal is to prove that nanowires of atomic sizes behave as optical antennas due to the quantization of their plasmon structure. This would mean that excitation of plasmon resonances can enhance the coupling between photons and electronic transitions in the nanowire. Since this research project bridges the fields of atomic-scale spectroscopy and nanooptics, a novel experimental approach is proposed. We will use low temperature scanning tunnelling and force microscopies, coupled to a light excitation and detection set-up, to resolve at the atomic scale both electronic structure and light scattering/emission by the atomic-sized antennas in response to optical/electron excitations. To enhance the field focusing at the quantum object we will use nanofabricated optical antennas as tips. An in-vacuum Fourier Transform detection scheme will be developed to extend the spectral detection to the mid-infrared."
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