Yogesh Jeyaram

Plasmons reveal the direction of magnetization in nickel nanostructures.

We have applied the surface-sensitive nonlinear optical technique of magnetization-induced second harmonic generation (MSHG) to plasmonic, magnetic nanostructures made of Ni. We show that surface plasmon contributions to the MSHG signal can reveal the direction of the magnetization. Both the plasmonic and the magnetic nonlinear optical responses can be tuned; our results indicate novel ways to combine nanophotonics, nanoelectronics, and nanomagnetics and suggest the possibility for large magneto-chiral effects in metamaterials.

Plasmon-Enhanced Sub-Wavelength Laser Ablation: Plasmonic Nanojets

In response to the incident lights electric field, the electron density oscillates in the plasmonic hotspots producing an electric current. Associated Ohmic losses raise the temperature of the material within the plasmonic hotspot above the melting point. A nanojet and nanosphere ejection can then be observed precisely from the plasmonic hotspots.

U-Shaped Switches for Optical Information Processing at the Nanoscale

4–6 ] In such devices, light waves would be used instead of electrons. The possibility arises from the fact that light waves can couple to collective excitations of electrons at the surfaces of metallic nanostructures, a prop-erty referred to as surface plasmon resonance. Because these optically induced resonances occur at the surfaces and interfaces of the nanostructures, they can readily be investigated with a surface- and interface-specifi c optical technique, such as second-harmonic generation (SHG). SHG is a nonlinear optical technique that, within the dipole approximation, is forbidden in materials with a center of symmetry. Consequently, SHG is highly sensitive to regions with broken symmetry, such as surfaces (or interfaces), and it has been successfully applied to the study of plasmonic nano-materials with different geometries.

Geometrical guidance and trapping transition of human sperm cells

The guidance of human sperm cells under confinement in quasi-2D microchambers is investigated using a purely physical method to control their distribution. Transport property measurements and simulations are performed with diluted sperm populations, for which effects of geometrical guidance and concentration are studied in detail. In particular, a trapping transition at convex angular wall features is identified and analyzed. We also show that highly efficient microratchets can be fabricated by using curved asymmetric obstacles to take advantage of the spermatozoa specific swimming strategy.

Influence of swimming strategy on microorganism separation by asymmetric obstacles

It has been shown that a nanoliter chamber separated by a wall of asymmetric obstacles can lead to an inhomogeneous distribution of self-propelled microorganisms. Although it is well established that this rectification effect arises from the interaction between the swimmers and the noncentrosymmetric pillars, here we demonstrate numerically that its efficiency is strongly dependent on the detailed dynamics of the individual microorganism. In particular, for the case of run-and-tumble dynamics, the distribution of run lengths, the rotational diffusion, and the partial preservation of run orientation memory through a tumble are important factors when computing the rectification efficiency. In addition, we optimize the geometrical dimensions of the asymmetric pillars in order to maximize the swimmer concentration and we illustrate how it can be used for sorting by swimming strategy in a long array of parallel obstacles.

Volumetric Method of Moments and Conceptual Multilevel Building Blocks for Nanotopologies

On the basis of the relationship between charge dimensionality and singular field behavior, it is proven that in a volumetric description of a volume current carrying topology, half rooftops of different binary hierarchical level are allowed without introducing numerical difficulties. This opens the possibility to use a very efficient multilevel hierarchical meshing scheme in a volumetric method-of-moments (V-MoM) algorithm. The new meshing scheme is validated by numerical calculations and experiments. It paves the way toward a much more efficient use of MoM in the description of arbitrarily shaped nanostructures at infrared and optical frequencies.

Distributing the Optical Near‐Field for Efficient Field‐Enhancements in Nanostructures

We are grateful to Saloomeh Shariati from the crypto group in the Universite Catholique de Louvain, for helpful discussion on the measures of the uniformity in images. We acknowledge financial support from the fund for scientific research Flanders (FWO-V), the K. U. Leuven (CREA, GOA), Methusalem Funding by the Flemish government and the Belgian Inter-University Attraction Poles IAP Programmes. V. K. V. and S. V. are grateful for the support from the FWO-Vlaanderen. B. DC. is thankful to the IWT.

Rendering dark modes bright by using asymmetric split ring resonators

We have studied both theoretically and experimentally symmetric and asymmetric planar metallic Split Ring Resonators. We demonstrate that introducing structural asymmetry makes it possible to excite several higher order modes of both even (l = 2) and odd (l = 3, 5) order, which are otherwise inaccessible for a normally incident plane wave in symmetric structures. Experimentally we observe that the even mode resonances of asymmetric resonators have a quality factor 5.8 times higher than the higher order odd resonances.

Disrupting the wall accumulation of human sperm cells by artificial corrugation

Many self-propelled microorganisms are attracted to surfaces. This makes their dynamics in restricted geometries very different from that observed in the bulk. Swimming along walls is beneficial for directing and sorting cells, but may be detrimental if homogeneous populations are desired, such as in counting microchambers. In this work, we characterize the motion of human sperm cells ∼60 μm long, strongly confined to ∼25 μm shallow chambers. We investigate the nature of the cell trajectories between the confining surfaces and their accumulation near the borders. Observed cell trajectories are composed of a succession of quasi-circular and quasi-linear segments. This suggests that the cells follow a path of intermittent trappings near the top and bottom surfaces separated by stretches of quasi-free motion in between the two surfaces, as confirmed by depth resolved confocal microscopy studies. We show that the introduction of artificial petal-shaped corrugation in the lateral boundaries removes the tendency of cells to accumulate near the borders, an effect which we hypothesize may be valuable for microfluidic applications in biomedicine.

Light-matter interactions mediated by nanoscale confinement in plasmonic resonators

Plasmonic resonators are nanosized metallic antennas that convert electromagnetic waves at optical frequencies into localized fields, providing an effective route to couple photons in and out of nanoscale volumes. This unique ability makes these nanostructures excellent tools to study and manipulate light-matter interaction at the nanoscale. The strong coupling of a plasmonic resonator to light, resulting in optical cross-sections of more than 10 times the particles physical size, is driven by collective oscillations of the conduction electrons in the metal - the so-called surface plasmon resonances.