46 research outputs found
Near-Field Directionality Beyond the Dipole Approximation: Electric Quadrupole and Higher-Order Multipole Angular Spectra
Within the context of spin-related optical phenomena, the near-field
directionality is generally understood from the quantum spin Hall effect of
light, according to which the transverse spin of surface or guided modes is
locked to the propagation direction. So far, most previous works have been
focused on the spin properties of circularly polarized dipolar sources.
However, in near-field optics, higher-order multipole sources (e.g.,
quadrupole, octupole, and so on) might become relevant, so a more in-depth
formulation would be highly valuable. Building on the angular spectrum
representation, we provide a general, analytical, and ready-to-use treatment in
order to address the near-field directionality of any multipole field,
particularizing to the electric quadrupole case. Besides underpinning and
upgrading the current framework on spin-dependent directionality, our results
may open up new perspectives for engineering light-matter coupling at the
nanoscale.Comment: 7 pages, 2 figures. Supplemental Material (19 pages). Supplemental
tools (calculator of angular spectra and animation) available at
https://doi.org/10.5281/zenodo.267790
Interferometric evanescent wave excitation of nano-antenna for ultra-sensitive displacement and phase metrology
We propose a method for ultra-sensitive displacement and phase metrology
based on the interferometric evanescent wave excitation of nano-antennas. We
show that with a proper choice of nano-antenna, tiny displacements or relative
phase variations can be converted into sensitive scattering direction changes
in the Fourier -space. These changes stem from the strong position
dependence of the imaginary Poynting vector orientation within interfering
evanescent waves. Using strongly-evanescent standing waves, high sensitivity is
achieved in the nano-antenna's zero scattering direction, which varies linearly
with displacement over a long range. With weakly-evanescent wave interference,
even higher sensitivity to tiny displacement or phase changes can be reached
around chosen location. The high sensitivity of the proposed method can form
the basis for many applications
Complex refraction metasurfaces for locally enhanced propagation through opaque media
Metasurfaces with linear phase gradients can redirect light beams. We propose
controlling both phase and amplitude of a metasurface to extend Snell's law to
the realm of complex angles, enabling a non-decaying transmission through
opaque media with complex refractive indices. This leads to the discovery of
non-diffracting and non-decaying solutions to the wave equation in opaque
media, in the form of generalised cosine and Bessel-beams with a complex
argument. While these solutions present nonphysical exponentially growing side
tails, we address this via a windowing process, removing the side tails of the
field profile while preserving significant transmission enhancement through an
opaque slab on a small localized region. Such refined beam profiles may be
synthesized by passive metasurfaces with phase and amplitude control at the
opaque material's interface. Our findings, derived from rigorous solutions of
the wave equation, promise new insights and enhanced control of light
propagation in opaque media.Comment: 9 pages, 3 figure
Avoiding metallic walls: Use of modal superposition in plasmonic waveguides to reduce propagation loss
We theoretically explore the possibility of reducing the propagation loss in
a metal-insulator-metal (MIM) waveguide, using mode combinations to achieve
wall-avoiding field distributions along a certain propagation length. We
present analytical results for several waveguides showing notable loss
reduction, and we discuss the tradeoffs between low loss and high confinement
present in this technique