Optical response of finite-thickness ultrathin plasmonic films
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Research Letter
Optical response of finite-thickness ultrathin plasmonic films Igor V. Bondarev and Hamze Mousavi, Math & Physics Department, North Carolina Central University, Durham, NC 27707, USA Vladimir M. Shalaev, School of Electrical & Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA Address all correspondence to Igor V. Bondarev at [email protected] (Received 30 May 2018; accepted 26 July 2018)
Abstract We show that the optical response of ultrathin metallic films of finite lateral size and thickness can feature peculiar magneto-optical effects resulting from the spatial confinement of the electron motion. In particular, the frequency dependence of the magnetic permeability of the film exhibits a sharp resonance structure shifting to the red as the film aspect ratio increases. The films can also be negatively refractive in the IR frequency range under proper tuning. We show that these magneto-optical properties can be controlled by adjusting the film chemical composition, plasmonic material quality, the aspect ratio, and the surroundings of the film.
Introduction Contemporary techniques of material fabrication allow one to produce stoichiometrically perfect ultrathin metallic films of controlled thickness down to a few monolayers.[1–4] Such films are the major components used to create ultrathin metasurfaces for advanced applications in modern optoelectronics and quantum optics including the ultrafast information processing, microscopy, imaging, sensing, and probing the light–matter interactions at the nanoscale.[5–15] As opposed to bulk metals whose electron plasma spectrum and associated optical response are controlled by the material band structure, the plasma properties and optical response of metallic films can also be controlled by adjusting their thickness, chemical composition, and by using properly chosen substrate and superstrate materials.[16] This unique tunability makes ultrathin metallic (or plasmonic) films an attractive platform for the design of advanced multifunctional metasurfaces with increased operational bandwidths and reduced losses, metasurfaces that are capable of utilizing the true quantum nature of light to achieve new improved functionalities.[17] With the decrease of the thickness, however, the vertical electron confinement becomes stronger, which can lead to new finite-size and dimensionality-related effects.[11,16] These effects require a deeper theoretical insight to understand how important they are and how they can be used to control the optical properties of ultrathin plasmonic films. Two of us have recently shown theoretically the plasma frequency of ultrathin metallic films to acquire the spatial dispersion typical of two-dimensional (2D) materials such as graphene, gradually shifting to the red as the film thickness decreases, while being constant for relatively thick films.[16] This leads to the spatial dispersion and the non-locality connected therewith of the dielectric response function of the
film. The dielectric response non-lo
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