Martin Neugebauer

Polarization Tailored Light Driven Directional Optical Nanobeacon

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.

The photonic wheel - demonstration of a state of light with purely transverse angular momentum

In classical mechanics, a system may possess angular momentum which can be either transverse (e.g. in a spinning wheel) or longitudinal(e.g. for a spiraling seed falling from a tree) with respect to the direction of motion. However, for light, a typical massless wave system,the situation is less versatile. Photons are well-known to exhibit intrinsic angular momentum which is longitudinal only: the spin angularmomentum defining the polarization and the orbital angular momentum associated with a spiraling phase front. Here we show that itis possible to generate a novel state of the light field that contains purely transverse angular momentum, the analogue of a spinningmechanical wheel. We realize this state by tight focusing of a polarization tailored light beam and measure it using an optical nano-probingtechnique. Such a novel state of the light field can find applications in optical tweezers and spanners where it allows for additionalrotational degree of freedom not achievable in single-beam configurations so far.

Polarization-controlled directional scattering for nanoscopic position sensing

Controlling the propagation and coupling of light to sub-wavelength antennas is a crucial prerequisite for many nanoscale optical devices. Recently, the main focus of attention has been directed towards high-refractive-index materials such as silicon as an integral part of the antenna design. This development is motivated by the rich spectral properties of individual high-refractive-index nanoparticles. Here we take advantage of the interference of their magnetic and electric resonances to achieve strong lateral directionality. For controlled excitation of a spherical silicon nanoantenna, we use tightly focused radially polarized light. The resultant directional emission depends on the antennas position relative to the focus. This approach finds application as a novel position sensing technique, which might be implemented in modern nanometrology and super-resolution microscopy set-ups. We demonstrate in a proof-of-concept experiment that a lateral resolution in the Ångström regime can be achieved.

Geometric spin Hall effect of light in tightly focused polarization-tailored light beams

,100401(2009)].TheunderlyingeffectisphenomenologicallysimilartothespinHalleffectoflightbutdoesnotdependonthespecificlight-matter interaction and can be interpreted as a purely geometric effect. Thus, it was named the geometricspin Hall effect of light. Here, we experimentally investigate the appearance of this effect in tightly focusedvector beams. We use an experimental nanoprobing technique in combination with a reconstruction algorithm toverify the relative shifts of the components of the electric energy density and the shift of the intensity in the focalplane. By that, we experimentally demonstrate the geometric spin Hall effect of light in a highly nonparaxialbeam.DOI: 10.1103/PhysRevA.89.013840 PACS number(s): 42

Lateral spin transport in paraxial beams of light

We investigate the lateral transport of (longitudinal) spin angular momentum in a special polarization tailored light beam composed of a superposition of a y-polarized zero-order and an x-polarized first-order Hermite-Gaussian mode. This phenomenon is linked to the relative Gouy phase shift between the individual modes upon propagation, but can also be interpreted as a geometric phase effect. Experimentally, we demonstrate the implementation of such a mode and measure the spin density upon propagation.

Exploiting cellophane birefringence to generate radially and azimuthally polarised vector beams

We exploit the birefringence of cellophane to convert a linearly polarised Gaussian beam into either a radially or azimuthally polarised beam. For that, we fabricated a low-cost polarisation mask consisting of four segments of cellophane. The fast axis of each segment is oriented appropriately in order to rotate the polarisation of the incident linearly polarised beam as desired. To ensure the correct operation of the polarisation mask, we tested the polarisation state of the generated beam by measuring the spatial distribution of the Stokes parameters. Such a device is very cost efficient and allows for the generation of cylindrical vector beams of high quality.

Mirror-Symmetric Heterogeneous Resonant Nanostructures: Extrinsic Chirality and Spin-Polarized Scattering

We investigate a geometrically symmetric gold-silicon sphere heterodimer and reveal its extrinsic chiroptical response caused by the interaction with a substrate. The chiroptical response is obtained for oblique incidence owing to the coalescence of extrinsic chirality, heterogeneity and substrate induced break of symmetry. To quantify the chiral response we utilize k-space polarimetry. We elucidate the physics of the involved phenomena by considering scattering properties of the heterodimer in free space and find that incident linearly polarized light is scattered in a spin-split fashion. We corroborate our finding with a coupled dipole model and find that the spin-split behavior originates from the heterogeneity of the structure. This spin-split scattering, combined with the substrate-induced break of symmetry, leads to an extrinsic chiroptical response. Our work sheds new light on the potential and optical properties of heterogeneous nanostructures and paves the way for designing spectrally tunable polarization controlled heterogeneous optical elements.

Exciting a chiral dipole moment in an achiral nanostructure

Controlling the electric and magnetic dipole moments of optical nanostructures is a fundamental prerequisite for light routing and polarization multiplexing at the nanoscale. A versatile approach for inducing tailored dipole moments is structured illumination. Here, we discuss the excitation of a chiral dipole moment in an achiral silicon nanoparticle. In particular, we make use of the electric and magnetic polarizabilities of the silicon nanoparticle to coherently excite a superposition of parallel electric and magnetic dipole moments phase-shifted by ±π/2, which resembles the fundamental mode of a three-dimensional chiral nanostructure. We demonstrate the wavelength dependence of the excitation scheme and measure the spin and orbital angular momenta in the emission of the induced chiral dipole moments. Our results highlight the capabilities of such tunable chiral dipole emitters—not limited by structural properties—as flexible sources of spin-polarized light for nanoscopic devices.

Chirality of Symmetric Resonant Heterostructures

We investigate a geometrically symmetric gold-silicon sphere heterodimer and reveal its extrinsic chiroptical response caused by the interaction with a substrate. The chiroptical response is obtained for oblique incidence owing to the coalescence of extrinsic chirality, heterogeneity and substrate induced break of symmetry. To quantify the chiral response we utilize k-space polarimetry. We elucidate the physics of the involved phenomena by considering scattering properties of the heterodimer in free space and find that incident linearly polarized light is scattered in a spin-split fashion. We corroborate our finding with a coupled dipole model and find that the spin-split behavior originates from the heterogeneity of the structure. This spin-split scattering, combined with the substrate-induced break of symmetry, leads to an extrinsic chiroptical response. Our work sheds new light on the potential and optical properties of heterogeneous nanostructures and paves the way for designing spectrally tunable polarization controlled heterogeneous optical elements.

Chirality of Symmetric Resonant Heterostructures: Chirality of Symmetric Resonant Heterostructures

We investigate a geometrically symmetric gold-silicon sphere heterodimer and reveal its extrinsic chiroptical response caused by the interaction with a substrate. The chiroptical response is obtained for oblique incidence owing to the coalescence of extrinsic chirality, heterogeneity and substrate induced break of symmetry. To quantify the chiral response we utilize k-space polarimetry. We elucidate the physics of the involved phenomena by considering scattering properties of the heterodimer in free space and find that incident linearly polarized light is scattered in a spin-split fashion. We corroborate our finding with a coupled dipole model and find that the spin-split behavior originates from the heterogeneity of the structure. This spin-split scattering, combined with the substrate-induced break of symmetry, leads to an extrinsic chiroptical response. Our work sheds new light on the potential and optical properties of heterogeneous nanostructures and paves the way for designing spectrally tunable polarization controlled heterogeneous optical elements.