Revealing the spectral response of a plasmonic structure using tunnel electrons

Plasmonic micro- and nanostructures may be used to locally convert electron current into light beams or surface plasmons and are thus expected to play a key role in integrating nanophotonics into electronic devices. Such... [ view full abstract ]

Plasmonic micro- and nanostructures may be used to locally convert electron current into light beams or surface plasmons and are thus expected to play a key role in integrating nanophotonics into electronic devices. Such structures are also used in all kinds of optical microcomponents and plasmonic circuits. Some of these structures, such as optical nanoantennas, plasmonic crystals or plasmonic lenses, have already been the subject of numerous studies in which they are excited by a laser beam or by near-field coupling to a fluorescent molecule. However, their excitation by electrical current, in particular by inelastic tunneling, has been rarely studied. A characteristic of inelastic tunneling is that it is extremely local and broadband in frequency, which requires adapting the design of the excited plasmonic structures that are often designed to operate at a given frequency. Thus, it is essential to be able to accurately measure the spectral response of these plasmonic structures under local electrical excitation. To this end, we have developed an experimental setup that combines a scanning tunneling microscope (STM), an optical microscope and an optical imaging spectrometer. The tunneling current is used to electrically excite surface plasmons on a gold microstructure under the STM tip. The scattering and leakage radiation of the excited surface plasmons are collected by the optical microscope and an image of this light emission in the Fourier space is projected on the entrance slit of the spectrometer which disperses light onto a cooled CCD camera. The spectral response of the plasmonic structure and the energy-wavevector dispersion of its modes are thus measured. We show that this technique may be used to optimize the design of plasmonic lenses for integrated electric microsources of radially polarized light beams.