arXiv subjects

Peter Banzer

Publications and source records attributed to Peter Banzer.

At least 37 records · Page 2Linked to original sources

arXiv2019

Orbital-to-Spin Angular Momentum Conversion Employing Local Helicity

Spin-orbit interactions in optics traditionally describe an influence of the polarization degree of freedom of light on its spatial properties. The most prominent example is the generation of a spin-dependent optical vortex upon focusing or scattering of a circularly polarized plane-wave by a nanoparticle, converting spin to orbital angular momentum of light. Here, we present a mechanism of conversion of orbital-to-spin angular momentum of light upon scattering of a linearly polarized vortex beam by a spherical silicon nanoparticle. We show that focused linearly polarized Laguerre-Gaussian beams of first order ($\ell = \pm 1$) exhibit an $\ell$-dependent spatial distribution of helicity density in the focal volume. By using a dipolar scatterer the helicity density can be manipulated locally, while influencing globally the spin and orbital angular momentum of the beam. Specifically, the scattered light can be purely circularly polarized with the handedness depending on the orbital angular momentum of the incident beam. We corroborate our findings with theoretical calculations and an experimental demonstration. Our work sheds new light on the global and local properties of helicity conservation laws in electromagnetism.

physics.optics↗

arXiv2019

Interaction of light carrying orbital angular momentum with a chiral dipolar scatterer

The capability to distinguish the handedness of circularly polarized light is a well-known intrinsic property of a chiral nanostructure. It is a long-standing controversial debate, however, whether a chiral object can also sense the vorticity, or the orbital angular momentum (OAM), of a light field. Since OAM is a non-local property, it seems rather counter-intuitive that a point-like chiral object could be able to distinguish the sense of the wave-front of light carrying OAM. Here, we show that a dipolar chiral nanostructure is indeed capable of distinguishing the sign of the phase vortex of the incoming light beam. To this end, we take advantage of the conversion of the sign of OAM, carried by a linearly polarized Laguerre-Gaussian beam, into the sign of optical chirality upon tight focusing. Our study provides for a deeper insight into the discussion of chiral light-matter interactions and the respective role of OAM.

physics.optics↗

arXiv2019

Huygens' Dipole for Polarization-Controlled Nanoscale Light Routing

Structured illumination allows for satisfying the first Kerker condition of in-phase perpendicular electric and magnetic dipole moments in any isotropic scatterer that supports electric and magnetic dipolar resonances. The induced Huygens' dipole may be utilized for unidirectional coupling to waveguide modes that propagate transverse to the excitation beam. We study two configurations of a Huygens' dipole -- longitudinal electric and transverse magnetic dipole moments or vice versa. We experimentally show that only the radially polarized emission of the first and azimuthally polarized emission of the second configuration are directional in the far-field. This polarization selectivity implies that directional excitation of either TM or TE waveguide modes is possible. Applying this concept to a single nanoantenna excited with structured light, we are able to experimentally achieve scattering directivities of around 23 dB and 18 dB in TM and TE modes, respectively. This strong directivity paves the way for tunable polarization-controlled nanoscale light routing and applications in optical metrology, localization microscopy and on-chip optical devices.

physics.optics↗

arXiv2019

Multi-twist polarization ribbon topologies in highly-confined optical fields

Electromagnetic plane waves, solutions to Maxwell's equations, are said to be `transverse' in vacuum. Namely, the waves' oscillatory electric and magnetic fields are confined within a plane transverse to the waves' propagation direction. Under tight-focusing conditions however, the field can exhibit longitudinal electric or magnetic components, transverse spin angular momentum, or non-trivial topologies such as Möbius strips. Here, we show that when a suitably spatially structured beam is tightly focused, a 3-dimensional polarization topology in the form of a ribbon with two full twists appears in the focal volume. We study experimentally the stability and dynamics of the observed polarization ribbon by exploring its topological structure for various radii upon focusing and for different propagation planes.

physics.optics↗

arXiv2019

Experimental demonstration of linear and spinning Janus dipoles for polarisation and wavelength selective near-field coupling

The electromagnetic field scattered by nano-objects contains a broad range of wave vectors and can be efficiently coupled to waveguided modes. The dominant contribution to scattering from subwavelength dielectric and plasmonic nanoparticles is determined by electric and magnetic dipolar responses. Here, we experimentally demonstrate spectral and phase selective excitation of Janus dipoles, sources with electric and magnetic dipoles oscillating out of phase, in order to control near-field interference and directional coupling to waveguides. We show that by controlling the polarisation state of the dipolar excitations and the excitation wavelength to adjust their relative contributions, directionality and coupling strength can be fully tuned. Furthermore, we introduce a novel spinning Janus dipole featuring cylindrical symmetry in the near and far field, which results in either omnidirectional coupling or noncoupling. Controlling the propagation of guided light waves via fast and robust near-field interference between polarisation components of a source is required in many applications in nanophotonics and quantum optics.

physics.optics↗

arXiv2019

Vectorial vortex generation and phase singularities upon Brewster reflection

We experimentally demonstrate the emergence of a purely azimuthally polarized vectorial vortex beam with a phase singularity upon Brewster reflection of focused circularly polarized light from a dielectric substrate. The effect originates from the polarizing properties of the Fresnel reflection coefficients described in Brewster's law. An astonishing consequence of this effect is that the reflected field's Cartesian components acquire local phase singularities at Brewster's angle. Our observations are crucial for polarization microscopy and open new avenues for the generation of exotic states of light based on spin-to-orbit coupling, without the need for sophisticated optical elements.

physics.optics↗

arXiv2018

Transverse Kerker Scattering for Angstrom Localization of Nanoparticles

Angstrom precision localization of a single nanoantenna is a crucial step towards advanced nanometrology, medicine and biophysics. Here, we show that single nanoantenna displacements down to few Angstroms can be resolved with sub-Angstrom precision using an all-optical method. We utilize the tranverse Kerker scattering scheme where a carefully structured light beam excites a combination of multipolar modes inside a dielectric nanoantenna, which then upon interference, scatters directionally into the far-field. We spectrally tune our scheme such that it is most sensitive to the change in directional scattering per nanoantenna displacement. Finally, we experimentally show that antenna displacement down to 3 Angstrom is resolvable with a localization precision of 0.6 Angstrom.

physics.optics↗

arXiv2018

Constructing a chiral dipolar mode in an achiral nanostructure

We discuss the excitation of a chiral dipolar mode in an achiral silicon nanoparticle. In particular, we make use of the electric and magnetic polarizabilities of the silicon nanoparticle to construct this chiral electromagnetic mode which is conceptually similar to the fundamental modes of 3D chiral nanostructures or molecules. We describe the chosen tailored excitation with a beam carrying neither spin nor orbital angular momentum and investigate the emission characteristics of the chiral dipolar mode in the helicity basis, consisting of parallel electric and magnetic dipole moments, phase shifted by $\pm π/2$. We demonstrate the wavelength dependence and measure the spin and orbital angular momentum in the emission of the excited chiral mode.

physics.optics↗

arXiv2018

Chiroptical response of a single plasmonic nanohelix

We investigate the chiroptical response of a single plasmonic nanohelix interacting with a weakly-focused circularly-polarized Gaussian beam. The optical scattering at the fundamental resonance is characterized experimentally, and the chiral behavior of the nanohelix is explained based on a multipole analysis. The angularly resolved emission of the excited nanohelix is verified experimentally and it validates the theoretical results. Further, we study the first higher-order resonance and explain the formation of chiral dipoles in both cases.

physics.optics↗

arXiv2018

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.

physics.optics↗

arXiv2018

Weak measurement of elliptical dipole moments by C point splitting

We investigate points of circular polarization in the far field of elliptically polarized dipoles and establish a relation between the angular position and helicity of these C points and the dipole moment. In the case of highly eccentric dipoles, the C points of opposite handedness exhibit only a small angular separation and occur in the low intensity region of the emission pattern. In this regard, we introduce an optical weak measurement approach that utilizes the transverse electric (azimuthal) and transverse magnetic (radial) far-field polarization basis. Projecting the far field onto a spatially varying post-selected polarization state reveals the angular separation and the helicity of the C points. We demonstrate the applicability of this approach and determine the elliptical dipole moment of a particle sitting on an interface by measuring the C points in its far field.

physics.optics↗

arXiv2018

Tailoring multipolar Mie scattering with helicity and angular momentum

Linear scattering processes are usually described as a function of the parameters of the incident beam. The wavelength, the intensity distribution, the polarization or the phase are among them. Here, we discuss and experimentally demonstrate how the angular momentum and the helicity of light influence the light scattering of spherical particles. We measure the backscattering of a $4μm$ TiO$_2$ single particle deposited on a glass substrate. The particle is probed at different wavelengths by different beams with total angular momenta ranging from $-8$ to $+8$ units. It is observed that the spectral behavior of the particle is highly dependent on the angular momentum and helicity of the incoming beam. While some of the properties of the scattered field can be described with a simple resonator model, the scattering of high angular momentum beams requires a deeper understanding of the multipolar modes induced in the sphere. We observe that tailoring these induced multipolar modes can cause a shift and a spectral narrowing of the peaks of the scattering spectrum. Furthermore, specific combinations of helicity and angular momentum for the excitation lead to differences in the conservation of helicity by the system, which has clear consequences on the scattering pattern.

physics.optics↗

arXiv2017

The magnetic and electric transverse spin density of spatially confined light

When a beam of light is laterally confined, its field distribution can exhibit points where the local magnetic and electric field vectors spin in a plane containing the propagation direction of the electromagnetic wave. The phenomenon indicates the presence of a non-zero transverse spin density. Here, we experimentally investigate this transverse spin density of both magnetic and electric fields, occurring in highly-confined structured fields of light. Our scheme relies on the utilization of a high-refractive-index nano-particle as local field probe, exhibiting magnetic and electric dipole resonances in the visible spectral range. Because of the directional emission of dipole moments which spin around an axis parallel to a nearby dielectric interface, such a probe particle is capable of locally sensing the magnetic and electric transverse spin density of a tightly focused beam impinging under normal incidence with respect to said interface. We exploit the achieved experimental results to emphasize the difference between magnetic and electric transverse spin densities.

physics.optics↗

arXiv2016

Exotic Looped Trajectories of Photons in Three-Slit Interference

The validity of the superposition principle and of Born's rule are well-accepted tenants of quantum mechanics. Surprisingly, it has recently been predicted that the intensity pattern formed in a three-slit experiment is seemingly in contradiction with the predictions of the most conventional form of the superposition principle when exotic looped trajectories are taken into account. However, the probability of observing such paths is typically very small and thus rendering them extremely difficult to measure. In this work, we confirm the validity of Born's rule and present the first experimental observation of these exotic trajectories as additional paths for the light by directly measuring their contribution to the formation of optical interference fringes. We accomplish this by enhancing the electromagnetic near-fields in the vicinity of the slits through the excitation of surface plasmons. This process effectively increases the probability of occurrence of these exotic trajectories, demonstrating that they are related to the near-field component of the photon's wavefunction.

quant-ph↗

arXiv2016

The ubiquitous photonic wheel

A circularly polarized electromagnetic plane wave carries an electric field that rotates clockwise or counterclockwise around the propagation direction of the wave. According to the handedness of this rotation, its \emph{longitudinal} spin angular momentum density is either parallel or antiparallel to the propagation of light. However, there are also light waves that are not simply plane and carry an electric field that rotates around an axis perpendicular to the propagation direction, thus yielding \emph{transverse} spin angular momentum density. Electric field configurations of this kind have been suggestively dubbed "photonic wheels". It has been recently shown that photonic wheels are commonplace in optics as they occur in electromagnetic fields confined by waveguides, in strongly focused beams, in plasmonic and evanescent waves. In this work we establish a general theory of electromagnetic waves {propagating along a well defined direction, which carry} transverse spin angular momentum density. We show that depending on the shape of {these waves, the} spin density may be either perpendicular to the \emph{mean} linear momentum (globally transverse spin) or to the linear momentum \emph{density} (locally transverse spin). We find that the latter case generically occurs only for non-diffracting beams, such as the Bessel beams. Moreover, we introduce the concept of \emph{meridional} Stokes parameters to operationally quantify the transverse spin density. To illustrate our theory, we apply it to the exemplary cases of Bessel beams and evanescent waves. These results open a new and accessible route to the understanding, generation and manipulation of optical beams with transverse spin angular momentum density.

physics.optics↗

arXiv2016

Tighter spots of light with superposed orbital angular momentum beams

The possibility of focusing light to an ever tighter spot has important implications for many applications and fields of optics research, such as nano-optics and plasmonics, laser-scanning microscopy, optical data storage and many more. The size of lateral features of the field at the focus depends on several parameters, including the numerical aperture of the focusing system, but also the wavelength and polarization, phase and intensity distribution of the input beam. Here, we study the smallest achievable focal feature sizes of coherent superpositions of two co-propagating beams carrying opposite orbital angular momentum. We investigate the feature sizes for this class of beams not only in the scalar limit, but also use a fully vectorial treatment to discuss the case of tight focusing. Both our numerical simulations and our experimental results confirm that lateral feature sizes considerably smaller than those of a tightly focused Gaussian light beam can be observed. These findings may pave the way for improving the resolution of imaging systems or may find applications in nano-optics experiments.

physics.optics↗

arXiv2016

Experimental generation of amplitude squeezed vector beams

We present an experimental method for the generation of amplitude squeezed high-order vector beams. The light is modified twice by a spatial light modulator such that the vector beam is created by means of a collinear interferometric technique. A major advantage of this approach is that it avoids systematic losses, which are detrimental as they cause decoherence in continuous-variable quantum systems. The utilisation of a spatial light modulator (SLM) gives the flexibility to switch between arbitrary mode orders. The conversion efficiency with our setup is only limited by the efficiency of the SLM. We show the experimental generation of Laguerre-Gauss (LG) modes with radial indices up to 1 and azimuthal indices up to 3 with complex polarization structures and a quantum noise reduction up to -0.9dB$\pm$0.1dB. The corresponding polarization structures are studied in detail by measuring the spatial distribution of the Stokes parameters.

quant-ph↗

arXiv2016

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 remarkably 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 antenna's 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 setups. We demonstrate in a proof-of-concept experiment, that a lateral resolution in the Angstrom regime can be achieved.

physics.optics↗