Plasmons can be excited by inelastic tunneling of electrons [1,2]. This provides a broadband source of plasmons that can be integrated into plasmonic nanocircuitry. The emission wavelength and bandwidth can be controlled via plasmonic resonances which enhance the inelastic tunneling process.
Here we present an electrically-driven multi-element Yagi-Uda antenna that emits light into one specific direction . Furthermore, we discuss mode-specific electric excitation of plasmons in a two-wire transmission line and its application towards polarization-controlled nano-light sources.
Nanoscale optical integration is nowadays a strategic technological challenge and the ability of generating and manipulating nonlinear optical processes in sub-wavelength volumes is pivotal to realize efficient sensing probes and photonic sources for the next-generation communication technologies. Yet, confining nonlinear processes below the diffraction limit remains a challenging task because phase-matching is not a viable approach at the nanoscale. The localized fields associated to the resonant modes of plasmonic and dielectric nanoantennas offer a route to enhance and control nonlinear processes in highly confined volumes. In my talk I will discuss two nonlinear platforms based on plasmonic and dielectric nanostructures. The first relies on a broken symmetry antenna design, which brings about an efficient second harmonic generation (SHG). We recently applied this concept to an extended array of non-centrosymmetric nanoantennas for sensing applications. I will also show the evidence of a cascaded second-order process in Third Harmonic Generation (THG) in these nanoantennas.
Recently, dielectric nanoantennas emerged as an alternative to plasmonic nanostructures for nanophotonics applications, thanks to their sharp magnetic and electric Mie resonances along with the low ohmic losses in the visible/near-infrared region of the spectrum. I will present our most recent studies on the nonlinear properties of AlGaAs dielectric nanopillars. The strong localized modes along with the broken symmetry in the crystal structure of AlGaAs allow obtaining more than two orders of magnitude higher SHG efficiency with respect to plasmonic nanoantennas with similar spatial footprint and using the same pump power. I will also discuss a few key strategies we recently adopted to optically switch the SHG in these antennas even on the ultrafast time scale. Finally, I will show how to effectively engineer the sum frequency generation via the Mie resonances in these nanoantennas. These results draw a viable blueprint towards room-temperature all optical logic operation at the nanoscale.
The optimization of nonlinear optical processes at the nanoscale is a key challenge in nanoscience. In this framework, plasmon-enhanced nonlinear effects together with the development of innovative nanoantenna designs and hybrid nanostructures are receiving a lot of attention [1-2]. We recently devised a plasmonic nanoantenna working in the near-infrared region of the electromagnetic spectrum, which allows boosting the SHG efficiency. This is achieved by optimizing the nanoantenna geometry to feature (i) a double resonant response at both the excitation and emission wavelengths, (ii) a spatial overlap between the modes involved in the process and (iii) a broken symmetry, to enable dipole-allowed SHG. We found that this nanoantenna behaves like a strongly coherent nanoscale light source, featuring a marked THG along with an intense SHG [3-4].
We find evidence of a SHG-mediated cascaded effect in THG . We have identified a THG polarization behavior that strongly deviates from that of a bulk (3)-mediated effect and unveils a significant contribution coming from the cascaded coherent sum of a SHG photon and a pump photon.
 Kauranen, M.; Zayats, A. V., Nature Photon. 2012, 6, 737-748.
 Butet, J.; Brevet, P. F.; Martin, O. J. F., ACS Nano 2015, 9, 10545–10562.
 Celebrano, M.; et al., Nat. Nanotechnol. 2015, 10, 412−417.
 Sartorello, G.; et al., ACS Photon. 2016, 3, 1517−1522.
 Mu, X.; et al., Y., Opt. Lett. 2000, 25, 117-119.
Antennas play a key role in today’s wireless communication networks and it would be hugely beneficial to extent their use into the optical regime. However, classical signal generators do not work at those frequencies and therefore new concepts are needed. Here, we demonstrate how to electrically drive an optical nanoantenna using an atomic-scale feed gap provided by a gold-particle pushed into a precisely tailored interstice between two antenna arms. Upon applying a voltage, inelastic electron tunneling leads to current fluctuations in the optical regime and, hence, light emission. We show how the antennas spectrally shape the emission, how the exact particle position influences these properties and how to increase the directivity via Yagi-Uda arrangements or plasmonic waveguides structures in order to make electricallydriven optical nanoantennas more suitable for on-chip data communication.
We introduce the possibility of performing two-pulse correlation measurements in order to probe the dynamics of twophoton
photoluminescence in Au nanostructures. Our preliminary results obtained from single-crystal Au nanorods are
consistent with the two-step model for the photoluminescence process.
We explore the possibility to control the polarization state of light confined into sub-diffraction volumes by means of
plasmonic optical antennas. To this aim, we describe a resonant cross antenna, constituted of two perpendicular two-wire
antennas sharing the same gap, which is able to maintain the polarization state in the plane of the antenna. We also
discuss how, by proper tuning of the arm length in a slightly off-resonance cross antenna, it is possible to effectively
realize a nanoscale quarter-waveplate antenna. We present experimental results for the preparation of individual cross
antennas by means of focused ion beam milling starting from single-crystalline Au microflakes, and finally show
preliminary characterization results based on two-photon photoluminescence confocal imaging with linearly-polarized
Light emitted from the aperture of a near-field optical probe in the close vicinity of a dielectric object propagates in classically `forbidden' as well as `allowed' directions; the two zones are separated by the critical angle for total internal reflection. The new `tunnel' near-field optical microscopy (TNOM) technique makes use of forbidden and allowed radiation, in contrast to standard scanning near-field optical microscopy (SNOM or NSOM), which records only the allowed light. Scan images obtained with allowed and forbidden light are complementary to some extent; the latter, however, provide high contrast and resolution even in situations in which standard SNOM/NSOM shows little or no contrast. The influence of topography on image formation is analyzed and discussed.
The concepts of near-field optical microscopy and experimental and theoretical work carried out in Switzerland over the last 10 years are reviewed. After a description of the pioneering experiments of the mid-1980s, we focus on the recent efforts of the three Swiss laboratories currently working in the field in close collaboration. This newly refreshed initiative in near-field optics is supported by the Swiss Priority Program Optique.