Abstract
Optical patch nano-antennas possess unique absorption, field enhancement and concentration capabilities – but their crosssection, as well as their response outside of normal incidence are not well understood. Here we explain the large cross-section by considering that each patch nanoantenna is a cavity excited from both sides. Such a simple physical picture allows to fully understand the influence of the angle of incidence – that odd resonances have a very high absorption cross-section which decreases when the incidence angle increases, while even resonances cannot be excited in normal incidence. A direct application would be to use these structures as an optical nanometric set-square.
Highlights
Gap-plasmon resonators are a new kind of optical metallic resonator[1], that has emerged as promising[2,3,4,5,6], from a fundamental point of view[7,8,9] as well as for all the applications that could be based on these structures given their extraordinary efficiency at concentrating light[2, 10]
The light confinement associated to their resonances is so high that an unprecedentedly high Purcell effect can be obtained using this kind of optical patch antennas[17,18,19,20]
We show that the influence of the incidence angle can be accurately taken into account if a single gap-plasmon resonator is modeled as a cavity excited from both ends, making the structure a clear example of interferometrically controlled absorption[26], or of a coherent absorber[27]
Summary
Gap-plasmon resonators are a new kind of optical metallic resonator[1], that has emerged as promising[2,3,4,5,6], from a fundamental point of view[7,8,9] as well as for all the applications that could be based on these structures given their extraordinary efficiency at concentrating light[2, 10]. The light confinement associated to their resonances is so high that an unprecedentedly high Purcell effect can be obtained using this kind of optical patch antennas[17,18,19,20] These resonators are literally cavities for a peculiar plasmonic mode called a gap-plasmon[1] which presents a very large effective index. We give analytic formula for the losses induced by the resonances inside the cavity and link it to the incidence angle Such a description allows to understand why the absorption decreases when the incidence angle increases[25] and explains the extraordinary absorption cross-section of the patch antenna compared to other kinds of gap-plasmon resonators[9, 28]. Such a mechanism is not present in disordered structures, where the surface plasmons are excited[20] and absorb a non-negligible part of the energy but play a www.nature.com/scientificreports/
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