Aso Rahimzadegan

All-dielectric reciprocal bianisotropic nanoparticles

The study of high-index dielectric nanoparticles currently attracts a lot of attention. They do not suffer from absorption but promise to provide control on the properties of light comparable to plasmonic nanoparticles. To further advance the field, it is important to identify versatile dielectric nanoparticles with unconventional properties. Here, we show that breaking the symmetry of an all-dielectric nanoparticle leads to a geometrically tunable magneto-electric coupling, i.e. an omega-type bianisotropy. The suggested nanoparticle exhibits different backscatterings and, as an interesting consequence, different optical scattering forces for opposite illumination directions. An array of such nanoparticles provides different reflection phases when illuminated from opposite directions. With a proper geometrical tuning, this bianisotropic nanoparticle is capable of providing a

Fundamental limits of optical force and torque

2\pi

Optical force and torque on dipolar dual chiral particles

phase change in the reflection spectrum while possessing a rather large and constant amplitude. This allows creating reflectarrays with near-perfect transmission out of the resonance band due to the absence of an usually employed metallic screen.

Optical alignment of oval graphene flakes

Optical force and torque provide unprecedented control on the spatial motion of small particles. A valid scientific question, that has many practical implications, concerns the existence of fundamental upper bounds for the achievable force and torque exerted by a plane wave illumination with a given intensity. Here, while studying isotropic particles, we show that different light-matter interaction channels contribute to the exerted force and torque; and analytically derive upper bounds for each of the contributions. Specific examples for particles that achieve those upper bounds are provided. We study how and to which extent different contributions can add up to result in the maximum optical force and torque. Our insights are important for applications ranging from molecular sorting, particle manipulation, nanorobotics up to ambitious projects such as laser-propelled spaceships.

Disordered photonic metasurfaces for complex light field control (Conference Presentation)

On the one hand, electromagnetic dual particles preserve the helicity of light upon interaction. On the other hand, chiral particles respond differently to light of opposite helicity. These two properties on their own constitute a source of fascination. Their combined action, however, is less explored. Here, we study on analytical grounds the force and torque as well as the optical cross sections of dual chiral particles in the dipolar approximation exerted by a particular wave of well-defined helicity: A circularly polarized plane wave. We put emphasis on particles that possess a maximally electromagnetic chiral and hence dual response. Besides the analytical insights, we also investigate the exerted optical force and torque on a real particle using the example of a metallic helix that is designed to approach the maximal electromagnetic chirality condition. Various applications in the context of optical sorting but also nanorobotics can be foreseen considering the particles studied in this contribution.

Improved Plasmonic Filter, Ultra-Compact Demultiplexer, and Splitter

Patterned graphene, as an atomically thin layer, supports localized surface plasmon polaritons at mid-infrared or far-infrared frequencies. This provides a pronounced optical force/torque in addition to large optical cross sections and will make it an ideal candidate for optical manipulation. Here, we study the optical force and torque exerted by a linearly polarized plane wave on circular and oval graphene flakes (single layers of graphene). While the torque vanishes for circular flakes, the finite torque allows rotating and orienting oval flakes relative to the electric field polarization. Depending on the wavelength, the alignment is either parallel or perpendicular to the electric field vector. In our contribution, we rely on a full-wave numerical simulation and also on an analytical model that treats the graphene flakes in a dipole approximation. The presented results reveal a good level of control on the spatial alignment of graphene flakes subjected to far-infrared illumination.

Theory of optical forces on small particles by multiple plane waves

Optical metasurface can provide control over wavefront, polarization and spectrum of light fields while having just nanoscale thickness, making them promising candidates for flat optical components. Most metasurfaces studied so far consist of two-dimensional subwavelength arrays of designed metallic or dielectric scatterers. Deviations from a periodic, ordered arrangement are usually associated with a deterioration of the optical properties. However, the introduction of controlled disorder also provides interesting opportunities to engineer the optical response of metasurfaces. For example, the introduction of disorder can decrease unwanted anisotropy in the optical response [1], it suppresses scattering into discrete diffraction orders, and it can enhance the metasurfaces’ channel capacity [2]. Here we investigate different types of disordered metasurfaces. We demonstrate that the introduction of rotational disorder at the unit-cell level enables the realization of chiral plasmonic metasurfaces supporting pure circular dichroism and circular birefringence. We show experimentally that the polarization eigenstates of these metasurfaces, which coincide with the fundamental right- and left-handed circular polarizations, do not depend on the wavelength over the spectral range of the metasurface resonances. Thereby, our metasurfaces mimic the behaviour of natural chiral media, while providing a stronger chiral response. Furthermore, we systematically investigate how the introduction of different types of positional disorder influences the complex transmittance spectra of Mie-resonant silicon metasurfaces, showing that disorder provides an independent degree of freedom for engineering their spatial and spectral dispersion. [1] S. S. Kruk et al., Phys. Rev. B 88, 201404(R) (2013). [2] D. Veksler et al., ACS Photonics 2, 661 (2015).