Probing Broadband Light Absorbing Metallic Nanogrooves with Electron Energy-Loss Spectroscopy (EELS)

Nicolas Stenger Department of Photonics Engineering, Technical University of Denmark, Kongens Lyngby, Denmark Center for Nanostructured Graphene (CNG), Technical University of Denmark, Kongens Lyngby, Denmark Søren Raza Department of Photonics Engineering, Technical University of Denmark, Kongens Lyngby, Denmark Center for Nanostructured Graphene (CNG), Technical University of Denmark, Kongens Lyngby, Denmark

We report an experimental study of GSPs in ultra-sharp gold convex nanogrooves using EELS [1]. The geometry of these nanogrooves is characterized by gradual and relatively slow variations in the gap width when moving deeper inside a groove. This means that the groove GSP modes can be considered as being formed by local metal-insulator-metal (MIM) GSP modes (that is, by GSP modes supported by constant-gap MIM configurations) that are weighted accordingly, a representation which is widely used in integrated optics and plasmonics for effective-index approximation. In EELS experiments, the strongly confined electrical fields of moving electrons excite thereby local MIM GSP modes, corresponding to the position of the electron beam inside the groove. Overall, the considered groove geometry is ideal for studying MIM GSP modes, as the width of the insulating layer (gap size) decreases as the position of the electron probe is moved down the nanogroove, thus allowing us to map the evolution of MIM GSP modes for varying gap size in a single groove. We intentionally propagate the electron beam along the axis of the groove within the mirror-symmetry plane in order to allow for probing of modes near the groove bottom and to study the optically dark modes. We verify experimentally the existence of the MIM asymmetric GSP mode in the crevice of the groove, i.e., at extremely narrow gaps of only 5 nm.

Finally, we discuss the implication of these lossy asymmetric GSP modes on the broadband light absorption properties of metallic nanogrooves in general [2].

References:

[1] S. Raza, N. Stenger, A. Pors, T. Holmgaard, S. Kadkhodazadeh, J. B. Wagner, K. Pedersen, M. Wubs, S. I. Bozhevolnyi, and N. A. Mortensen, Nat. Commun. 5, 4125 (2014).

[2] T. Søndergaard, S. M. Novikov, T. Holmgaard, R. L. Eriksen, J. Beermann, Z. Han, K. Pedersen, and S. I. Bozhevolnyi, Nat. Commun. 3, 1–6 (2012).

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