52 research outputs found
Effective electric and magnetic properties of metasurfaces in transition from crystalline to amorphous state
In this paper we theoretically study electromagnetic reflection,
transmission, and scattering properties of periodic and random arrays of
particles which exhibit both electric-mode and magnetic-mode resonances. We
compare the properties of regular and random grids and explain recently
observed dramatic differences in resonance broadening in the electric and
magnetic modes of random arrays. We show that randomness in the particle
positioning influences equally on the scattering loss from both electric and
magnetic dipoles, however, the observed resonance broadening can be very
different depending on the absorption level in different modes as well as on
the average electrical distance between the particles. The theory is
illustrated by an example of a planar metasurface composed of cut-wire pairs.
We show that in this particular case at the magnetic resonance the array
response is almost not affected by positioning randomness due to lower
frequency and higher absorption losses in that mode. The developed model allows
predictions of behavior of random grids based on the knowledge of
polarizabilities of single inclusions.Comment: 13 pages, 5 figures, and submitted to PR
Shadow-free multimers as extreme-performance meta-atoms
We generalize the concept of parity-time symmetric structures with the goal
to create meta-atoms exhibiting extraordinary abilities to overcome the
presumed limitations in the scattering of overall lossless particles, such as
non-zero forward scattering and the equality of scattering and extinction
powers for all lossless particles. Although the forward scattering amplitude
and the extinction cross section of our proposed meta-atoms vanish, they
scatter incident energy into other directions, with controllable
directionality. These meta-atoms possess extreme electromagnetic properties not
achievable for passive scatterers. As an example, we study meta-atoms
consisting of two or three small dipole scatters. We consider possible
microwave realizations in the form of short dipole antennas loaded by lumped
elements. The proposed meta-atom empowers extraordinary response of a
shadow-free scatterer and theoretically enables most unusual material
properties when used as a building block of an artificial medium.Comment: 14 pages, 9 Figure
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Unscrambling Structured Chirality with Structured Light at the Nanoscale Using Photoinduced Force
We show that the gradient force generated by the near field of a chiral nanoparticle carries information about its chirality. On the basis of this physical phenomenon we propose a new microscopy technique that enables the prediction of spatial features of chirality of nanoscale samples by exploiting the photoinduced optical force exerted on an achiral tip in the vicinity of the test specimen. The tip-sample interactive system is illuminated by structured light to probe both the transverse and longitudinal (with respect to the beam propagation direction) components of the sample's magnetoelectric polarizability as the manifestation of its sense of handedness, i.e., chirality. We specifically prove that although circularly polarized waves are adequate to detect the transverse polarizability components of the sample, they are unable to probe the longitudinal component. To overcome this inadequacy and probe the longitudinal chirality, we propose a judiciously engineered combination of radially and azimuthally polarized beams as optical vortices possessing pure longitudinal electric and magnetic field components along their vortex axis, respectively. The proposed technique may benefit branches of science such as stereochemistry, biomedicine, physical and material science, and pharmaceutics
Enantiospecific Detection of Chiral Nanosamples Using Photoinduced Force
We propose a high-resolution microscopy technique for enantiospecific detection of chiral samples down to sub-100-nm size based on force measurement. We delve into the differential photoinduced optical force ΔF exerted on an achiral probe in the vicinity of a chiral sample when left and right circularly polarized beams separately excite the sample-probe interactive system. We analytically prove that ΔF is entangled with the enantiomer type of the sample enabling enantiospecific detection of chiral inclusions. Moreover, we demonstrate that ΔF is linearly dependent on both the chiral response of the sample and the electric response of the tip and is inversely related to the quartic power of probe-sample distance. We provide physical insight into the transfer of optical activity from the chiral sample to the achiral tip based on a rigorous analytical approach. We support our theoretical achievements by several numerical examples highlighting the potential application of the derived analytic properties. Lastly, we demonstrate the sensitivity of our method to enantiospecify nanoscale chiral samples with chirality parameter on the order of 0.01 and discuss how the sensitivity of our proposed technique can be further improved
Huge local field enhancement in perfect plasmonic absorbers
In this Letter we theoretically study the possibility of total power
absorption of light in a planar grid modelled as an effective sheet with zero
optical thickness. The key prerequisite of this effect is the simultaneous
presence of both resonant electric and magnetic modes in the structure. We show
that the needed level of the magnetic mode is achievable using the effect of
substrate-induced bianisotropy which also allows the huge local field
enhancement at the same wavelength where the maximal absorption holds.Comment: 4 pages, 4 figure
Classification of bianisotropic metasurfaces from reflectance and transmittance measurements
Upon using fundamental electromagnetic properties of metasurfaces we build a
platform to classify reciprocal bianisotropic metasurfaces from typical
experimental measurements and determine isotropic, anisotropic, bi-isotropic
(chiral), and bianisotropic (so-called omega) properties. We provide
experimental guidelines to identify each class by measuring macroscopic
scattering parameters, i.e., reflection and transmission coefficients upon
plane wave illumination with linear and/or circular polarization. We explicitly
provide a recipe of what metasurface properties can and cannot be inferred by
means of chosen polarization, reflection, and transmission properties. We also
clarify common confusions in the classification of anisotropic versus chiral
metasurfaces based on circular dichroism measurements presented in the recent
literature.Comment: 16 pages, 8 figures, 2 table
Parametric Mie resonances and directional amplification in time-modulated scatterers
We provide a theoretical description of light scattering by a spherical
particle whose permittivity is modulated in time at twice the frequency of the
incident light. Such a particle acts as a finite-sized photonic time crystal
and, despite its sub-wavelength spatial extent, can host optical parametric
amplification. Conditions of parametric Mie resonances in the sphere are
derived. We show that time-modulated materials provide a route to tailor
directional light amplification, qualitatively different from that in
scatterers made from a gain media. We design two characteristic time-modulated
spheres that simultaneously exhibit light amplification and desired radiation
patterns, including those with zero backward and/or vanishing forward
scattering. The latter sphere provides an opportunity for creating shadow-free
detectors of incident light.Comment: 8 pages, 4 figure
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Helicity maximization below the diffraction limit
Optimally chiral electromagnetic fields with maximized helicity density, recently introduced by Hanifeh et al. [M. Hanifeh, M. Albooyeh, and F. Capolino, ACS Photonics 7, 2682 (2020)10.1021/acsphotonics.0c00304], enable chirality characterization of optically small nanoparticles. Here we demonstrate a technique to obtain optimally chiral near fields that leads to the maximization of helicity density under the constraint of constant energy density, beyond the diffraction limit. We show how optimally chiral illumination induces balanced electric and magnetic dipole moments in an achiral dielectric nanoantenna, which leads to generating optimally chiral scattered and total near fields. In particular, we explore helicity and energy densities in the near field of a spherical dielectric nanoantenna illuminated by an optimally chiral combination of azimuthally and radially polarized beams. This beam combination generates parallel induced electric and magnetic dipole moments in the nanoantenna that in turn generate an optimally chiral scattered field with the same helicity sign of the incident field. The application of helicity maximization to near fields results in helicity enhancement at the nanoscale, which is of great advantage in the detection of nanoscale chiral samples, microscopy, and optical manipulation of chiral nanoparticles
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