Polarization Tailored Light Driven Directional Optical Nanobeacon Martin Neugebauer, Thomas Bauer, Peter Banzer,* and Gerd Leuchs Max Planck Institute for the Science of Light, Guenther-Scharowsky-Straße 1/Bldg. 24, 91058 Erlangen, Germany Institute of Optics, Information and Photonics, University Erlangen−Nuremberg, Staudtstraße 7/B2, 91058 Erlangen, Germany ABSTRACT: We experimentally demonstrate all-optical control of the emission directivity of a dipole-like nanoparticle with spinning dipole moment sitting on the interface to an optical denser medium. The particle itself is excited by a tightly focused polarization tailored light beam under normal incidence. The position dependent local polarization of the focal field allows for tuning the dipole moment via careful positioning of the particle relative to the beam axis. As an application of this scheme, we investigate the polarization dependent coupling to a planar two- dimensional dielectric waveguide. KEYWORDS: Nanoantenna, tight focusing, transversally spinning dipole, dielectric interface, waveguide I n the past decade, directional emission and coupling of nanophotonic devices have gained increasing attention. Using optical nanoantennas to couple light selectively to plasmonic or dielectric waveguides on the nanoscale is one of the main ingredients for on-chip or interchip integrated photonic circuits. 1−5 To enhance this coupling, a huge variety of different complex antenna designs have been proposed and experimentally examined. Examples include the well-known concepts of Yagi-Uda 6−8 or graded antennas 9 at the nanoscale and other designs to achieve directional emission. 10−15 Recently, sensitive optical control over the emission direction- ality has been reported for a polarization dependent near-field effect. 16,17 By employing an elliptically polarized spinning dipole emitter, it was shown, that the evanescent components of the dipole’ s near- field result in angular dependent constructive and destructive interference. This leads to directional emission and coupling into a waveguide when the spinning axis of the dipole is oriented parallel to the waveguide interface. This coupling was demonstrated experimentally by shining circularly polarized light under grazing incidence onto a plasmonic slit, resulting in the excitation and directional propagation of surface plasmon polaritons. 16 Here, we show that a full two-dimensional optically controllable directionality (nanobeacon) based on this effect can be achieved. We address and tune the dipole moment of a spherical subwavelength nanoparticle (radius ≪ λ) sitting on a dielectric interface by excitation with a tightly focused vector beam under normal incidence. Because of its small size, the particle senses most dominantly the local electric field, with its induced dipole moment being proportional to the latter, p ∝ E(x, y, z). 18 For generating a transversally spinning electric dipole with maximum directivity, we therefore require an excitation beam with suitable polarization properties, that is, a local electric field vector with ± π/2 phase difference between the longitudinal and the transverse component. 19 When focusing spatially coherent light to a tight spot, this required phase difference is directly given in many beam configurations (e.g., in a tightly focused, linearly polarized Gaussian beam). 18 In this work, we choose a tightly focused radially polarized beam, which exhibits a strong longitudinal electric field on axis, surrounded by transverse field components. 20 Due to this strong position dependence of the local polarization of the focal field, the particle’s dipole moment can be easily and sensitively tuned via careful positioning of the subwavelength particle relative to the optical axis of the tightly focused beam. The additional feature of cylindrical symmetry of the chosen beam enables full control over the angle of the directive emission in the full azimuthal range of 2π. The focal field distribution of the deployed radially polarized beam focused tightly onto the dielectric interface is plotted in Figure 1. For the calculation, we use vectorial diffraction theory 21,22 with the same parameters as in the experiment discussed later. As expected, the amplitudes and relative phases of the components of the electric field E x , E y , and E z vary strongly with respect to the lateral position in the focal plane. 20 The strongest component of the electric energy density for the chosen input beam configuration is |E z | 2 (Figure 1d). In addition, the z component of the electric field is ± π/2 out of phase compared to the transverse components E x and E y (Figure 1b,c). As mentioned above, this ± π/2 phase difference between longitudinal and transverse electric field components of the incoming beam is required to excite a spinning electric dipole in the nanoparticle (with its axis parallel to the interface) and achieve maximum directivity of the scattered light. 16 The ellipticity of the induced dipole then depends on the radial distance of the nanoparticle relative to the beam center. If the particle is sitting in the center of the focal spot, meaning on the optical axis, it is excited by the longitudinal electric field component only. For this position, a symmetric far-field pattern Received: January 28, 2014 Revised: April 7, 2014 Published: April 11, 2014 Letter pubs.acs.org/NanoLett © 2014 American Chemical Society 2546 dx.doi.org/10.1021/nl5003526 | Nano Lett. 2014, 14, 2546−2551