Broadband scattering by tapered nanoantennas

1 Introduction Plasmonic nanoantennas have become a subject of considerable interest [1–3]. Numerous intriguing applications of nanoantennas in areas as diverse as optical and quantum communication, nonlinear optics, sensing, and photovoltaics have been discussed [3]. Ar-rayed nanoantennas like Yagi–Uda architectures down-scaled to nanometer dimensions are particularly suited for such applications because they offer high directivity [4] and strong emission enhancement at the same time [5]. However, classical Yagi–Uda nanoantennas are intrinsi-cally narrowband, creating the need for new antenna concepts able to provide broadband functionality. Indeed, broadband nanoantennas open up novel opportunities for high-bandwidth wireless on-chip communication [6], spec-troscopy [7], surface-enhanced Raman scattering [8], and light harvesting devices [9]. Furthermore, tapered nanoan-tennas in particular have been predicted to provide a high directivity over a wide spectral band [6, 10, 11] and, in addition , to offer completely new functionalities like mul-tichannel sensing [12] and cascaded four-wave mixing [13], that require only simple excitation from the far-field [13]. In this Letter, we demonstrate the experimental realization of this new design of tapered Yagi–Uda optical nano-antenna, and confirm its large spectral bandwidth by far