Dries Vercruysse

Unidirectional Side Scattering of Light by a Single-Element Nanoantenna

Unidirectional side scattering of light by a single-element plasmonic nanoantenna is demonstrated using full-field simulations and back focal plane measurements. We show that the phase and amplitude matching that occurs at the Fano interference between two localized surface plasmon modes in a V-shaped nanoparticle lies at the origin of this effect. A detailed analysis of the V-antenna modeled as a system of two coherent point-dipole sources elucidates the mechanisms that give rise to a tunable experimental directivity as large as 15 dB. The understanding of Fano-based directional scattering opens a way to develop new directional optical antennas for subwavelength color routing and self-referenced directional sensing. In addition, the directionality of these nanoantennas can increase the detection efficiency of fluorescence and surface enhanced Raman scattering.

Dark and bright localized surface plasmons in nanocrosses

A metallic nanocross geometry sustaining broad dipole and sharp higher order localized surface plasmon resonances is investigated. Spectral tunability is achieved by changing the cross arm length and the angle between the arms. The degree of rotational symmetry of the nanocross is varied by adding extra arms, changing the arm angle and shifting the arm intersection point. The particles symmetry is shown to have a crucial influence on the plasmon coupling to incident radiation. Pronounced dipole, quadrupole, octupole and Fano resonances are observed in individual cross structures. Furthermore, the nanocross geometry proves to be a useful building block for coherently coupled plasmonic dimers and trimers where the reduced symmetry results in hybridized subradiant and superradiant modes and multiple Fano interferences. Finite difference time domain calculations of absorption and scattering cross-sections as well as charge density profiles are used to reveal the nature of the different plasmon modes. Experimental spectra for the discussed geometries support the calculations.

Mode parity-controlled fano- and lorentz-like line shapes arising in plasmonic nanorods

We present the experimental observation of spectral lines of distinctly different shapes in the optical extinction cross-section of metallic nanorod antennas under near-normal plane wave illumination. Surface plasmon resonances of odd mode parity present Fano interference in the scattering cross-section, resulting in asymmetric spectral lines. Contrarily, modes with even parity appear as symmetric Lorentzian lines. Finite element simulations are used to verify the experimental results. The emergence of either constructive or destructive mode interference is explained with a semianalytical 1D line current model. This simple model directly explains the mode-parity dependence of the Fano-like interference. Plasmonic nanorods are widely used as half-wave optical dipole antennas. Our findings offer a perspective and theoretical framework for operating these antennas at higher-order modes.

All-Dielectric Antenna Wavelength Router with Bidirectional Scattering of Visible Light

An optical antenna forms the subwavelength bridge between free space optical radiation and localized electromagnetic energy. Its localized electromagnetic modes strongly depend on its geometry and material composition. Here, we present the design and experimental realization of a novel V-shaped all-dielectric antenna based on high-index amorphous silicon with a strong magnetic dipole resonance in the visible range. As a result, it exhibits extraordinary bidirectional scattering into diametrically opposite directions. The scattering direction is effectively controlled by the incident wavelength, rendering the antenna a passive bidirectional wavelength router. A detailed multipole decomposition analysis reveals that the excitation and abrupt phase change of an out-of-plane polarized magnetic dipole and an in-plane electric quadrupole are essential for the directivity switching. Previously, noble metals have been extensively exploited for plasmonic directional nanoantenna design. However, these inevitably suffer from high intrinsic ohmic losses and a relatively weak magnetic response to the incident light. Compared to a similar gold plasmonic nanoantenna design, we show that the silicon-based antennas demonstrate stronger magnetic scattering with minimal absorption losses. Our results indicate that all-dielectric antennas will open exciting possibilities for efficient manipulation of light-matter interactions.

On the Use of Group Theory in Understanding the Optical Response of a Nanoantenna

Symmetry holds a prominent position in defining the optical response of a nanoantenna. In this work, we harness a mathematical tool, group representation theory, combine it with the eigenmode analysis for a nanoantenna, and illustrate how the symmetry allows or forbids the energetic coupling (i.e., interference) between a nanoantennas eigenmodes. We do this especially using a nanobar structure and a symmetric cross structure. Further, the consequence of symmetry-breaking is illustrated by an asymmetric cross-shaped nanostructure, i.e., a strong asymmetric Fano-type resonance line shape due to mode interference is observed with the physics behind elaborated. Both numerical and experimental evidences are provided.

Nanoparticle Scattering for Multijunction Solar Cells: The Tradeoff Between Absorption Enhancement and Transmission Loss

This paper contains a combined experimental and simulation study of the effect of Al and AlInP nanoparticles on the performance of multijunction (MJ) solar cells. In particular, we investigate oblique photon scattering by the nanoparticle arrays as a means of improving thinned subcells or those with low diffusion lengths, either inherently or due to radiation damage. Experimental results show the feasibility of integrating nanoparticle arrays into the antireflection coatings of commercial InGaP/InGaAs/Ge solar cells, and computational results show that nanoparticle arrays can improve the internal quantum efficiency via optical path length enhancement. However, a design that improves the external quantum efficiency of a state-of-the-art cell has not been found, despite the large parameter space studied. We show a clear tradeoff between oblique scattering and transmission loss and present design principles and insights into how improvements can be made.

Accurate label-free 3-part leukocyte recognition with single cell lens-free imaging flow cytometry

Three-part white blood cell differentials which are key to routine blood workups are typically performed in centralized laboratories on conventional hematology analyzers operated by highly trained staff. With the trend of developing miniaturized blood analysis tool for point-of-need in order to accelerate turnaround times and move routine blood testing away from centralized facilities on the rise, our group has developed a highly miniaturized holographic imaging system for generating lens-free images of white blood cells in suspension. Analysis and classification of its output data, constitutes the final crucial step ensuring appropriate accuracy of the system. In this work, we implement reference holographic images of single white blood cells in suspension, in order to establish an accurate ground truth to increase classification accuracy. We also automate the entire workflow for analyzing the output and demonstrate clear improvement in the accuracy of the 3-part classification. High-dimensional optical and morphological features are extracted from reconstructed digital holograms of single cells using the ground-truth images and advanced machine learning algorithms are investigated and implemented to obtain 99% classification accuracy. Representative features of the three white blood cell subtypes are selected and give comparable results, with a focus on rapid cell recognition and decreased computational cost.

High throughput cell sorter based on lensfree imaging of cells

Introduction: The trend towards digitization of healthcare promises a future of improved, insight driven, more economical and patient-centered care models. Besides digitization of health records, it has spurred massive research in nano- and bioMEMS devices that sense health related parameters in a compact format. A myriad of devices including innovative CMOS FET structures, nanophotonic and nanomechanical structures, nanowires and nanopores[1,2,3] are being explored as suitable sensors for bio-molecules in ultralow concentrations. Large scale integration offered by silicon technology becomes indispensable when analyzing large amounts of different bio-entities. The most compelling example is the sequencing of DNA with CMOS based platforms. Millions of sensing sites are required to cover the informative genetic aberrations of the human genome, that in total contains 3 billion base pairs. Silicon CMOS technology has been the enabling technology[4,5] to reduce the cost of analysis to a few 100 USD per analysis. Equal emphasis should be put on scaling the fluidic networks required to prepare samples towards those molecular detection devices in order to speed up the total analysis time starting directly from patient material. Figure 1 shows such a silicon microfluidic chip[6], to detect single nucleotide polymorphisms (SNPs), those SNP aberrations are important e.g. for analyzing cancer subtypes, drug response prediction.

Nanoparticle scattering for radiation-hard multi-junction space solar cells

We investigate how an array of nanoparticles embedded in the anti-reflection coating can improve the radiation hardness of multi-junction space solar cells. In space, high-energy electron and proton radiation reduces solar cell efficiency. Most notably, the InGaAs-middle-cell diffusion lengths are degraded, reducing photocurrent. Metal nanoparticles can scatter incident photons obliquely into the semiconductor, reducing their penetration depths and hence causing charge carriers to be photogenerated closer to the junctions. We postulate that this can improve radiation hardness by improving carrier collection at end of life. In this work, GaInP/InGaAs/Ge solar cells with optimised double-layer AlOx/TiOx ARCs were fabricated with regular arrays of Al nanoparticles deposited on top. An electro-optical simulation tool was also developed, and validated by comparison to the measured quantum efficiency and reflectance spectra, with good agreement. Using the validated simulation tool, we predict the photocurrent before and after high energy electron irradiation. The fraction of the initial photocurrent remaining after irradiation is predicted to improve for certain nanoparticle arrays. However, the overall photocurrent both before and after irradiation is reduced by the presence of the particles. Hence a net benefit is not predicted for the studied array dimensions.

Unidirectional scattering and emission of light mediated by a single-element nanoantenna

A V-shaped single-element metallic nanoantenna is found to enable unidirectional scattering of light. We experimentally and by means of FDTD simulations show unidirectional scattering of a plane wave and unidirectional emission from photoluminescent molecules coupled to the antenna. An eigenmode analysis reveals the physical origin of this effect and elegantly explains how the direction of the scattered light is reversed depending the excitation type.