Peter R. Wiecha

Evolutionary multi-objective optimization of colour pixels based on dielectric nanoantennas

The rational design of photonic nanostructures consists of anticipating their optical response from systematic variations of simple models. This strategy, however, has limited success when multiple objectives are simultaneously targeted, because it requires demanding computational schemes. To this end, evolutionary algorithms can drive the morphology of a nano-object towards an optimum through several cycles of selection, mutation and cross-over, mimicking the process of natural selection. Here, we present a numerical technique that can allow the design of photonic nanostructures with optical properties optimized along several arbitrary objectives. In particular, we combine evolutionary multi-objective algorithms with frequency-domain electrodynamical simulations to optimize the design of colour pixels based on silicon nanostructures that resonate at two user-defined, polarization-dependent wavelengths. The scattering spectra of optimized pixels fabricated by electron-beam lithography show excellent agreement with the targeted objectives. The method is self-adaptive to arbitrary constraints and therefore particularly apt for the design of complex structures within predefined technological limits.

Origin of Second Harmonic Generation from individual Silicon Nanowires

We investigate Second Harmonic Generation from individual silicon nanowires and study the influence of resonant optical modes on the far-field nonlinear emission. We find that the polarization of the Second Harmonic has a size-dependent behavior and explain this phenomenon by a combination of different surface and bulk nonlinear susceptibility contributions. We show that the Second Harmonic Generation has an entirely different origin, depending on whether the incident illumination is polarized parallel or perpendicularly to the nanowire axis. The results open perspectives for further geometry-based studies on the origin of Second Harmonic Generation in nanostructures of high-index centrosymmetric semiconductors.

Enhanced nonlinear optical response from individual silicon nanowires

© 2015 American Physical Society. We report about the experimental observation and characterization of nonlinear optical properties of individual silicon nanowires of different dimensions. Our results show that the nonlinear light has different components, with one of them corresponding to the second-harmonic generation (SHG). The SHG strongly depends on the polarization of the optical excitation and nanowire diameter, and gives access to the local electromagnetic field intensity distribution. Furthermore, we show that the second harmonic, when observed, is enhanced compared to bulk silicon and is sensitive to optical resonances supported by the nanowires. This offers different perspectives on the definition of silicon-based nonlinear photonic devices.

Strongly Directional Scattering from Dielectric Nanowires

It has been experimentally demonstrated only recently that a simultaneous excitation of interfering electric and magnetic resonances can lead to unidirectional scattering of visible light in zero-dimensional dielectric nanoparticles. We show both theoretically and experimentally, that strongly anisotropic scattering also occurs in individual dielectric nanowires. The effect occurs even under either pure transverse electric or pure transverse magnetic polarized normal illumination. This allows for instance to toggle the scattering direction by a simple rotation of the incident polarization. Finally, we demonstrate that directional scattering is not limited to cylindrical cross sections but can be further tailored by varying the shape of the nanowires.

Polarization conversion in plasmonic nanoantennas for metasurfaces using structural asymmetry and mode hybridization

Polarization control using single plasmonic nanoantennas is of interest for subwavelength optical components in nano-optical circuits and metasurfaces. Here, we investigate the role of two mechanisms for polarization conversion by plasmonic antennas: Structural asymmetry and plasmon hybridization through strong coupling. As a model system we investigate L-shaped antennas consisting of two orthogonal nanorods which lengths and coupling strength can be independently controlled. An analytical model based on field susceptibilities is developed to extract key parameters and to address the influence of antenna morphology and excitation wavelength on polarization conversion efficiency and scattering intensities. Optical spectroscopy experiments performed on individual antennas, further supported by electrodynamical simulations based on the Green Dyadic Method, confirm the trends extracted from the analytical model. Mode hybridization and structural asymmetry allow address-ing different input polarizations and wavelengths, providing additional degrees of freedom for agile polarization conversion in nanophotonic devices.

Evolutionary multi-objective optimization for multi-resonant photonic nanostructures

The tailoring of optical properties of photonic nanostructures is usually based on a reference design. The target optical behavior is obtained by variations of an initial geometry. This approach however can be of limited versatility, in particular if complex optical properties are desired. In order to design double-resonant photonic nano-particles, we attack the problem in the inverse way: We mathematically define an optical response and optimize multiple of such objective functions concurrently, using an evolutionary multi-objective optimization algorithm coupled to full-field electro-dynamical simulations. We demonstrate that this approach is extremely versatile, that it allows the consideration of technological limitations and that it yields a correct prediction of the optical response of nano-structures fabricated by state-of-the-art electron beam lithography. We demonstrate the technique on multi-resonant photonic nanoparticles made from silicon, which belong to the emerging class of high refractive index dielectric nanostructures with applications such as field-enhanced spectroscopies.

Multi-resonant silicon nanoantennas by evolutionary multi-objective optimization

Photonic nanostructures have attracted a tremendous amount of attention in the recent past. Via their size, shape and material it is possible to engineer their optical response to user-defined needs. Tailoring of the optical response is usually based on a reference geometry for which subsequent variations to the initial design are applied. Such approach, however, might fail if optimum nanostructures for complex optical responses are searched. As example we can mention the case of complex structures with several simultaneous optical resonances. We propose an approach to tackle the problem in the inverse way: In a first step we define the desired optical response as function of the nanostructure geometry. This response is numerically evaluated using the Green Dyadic Method for fully retarded electro-dynamical simulations. Eventually, we optimize multiple of such objective functions concurrently, using an evolutionary multi-objective optimization algorithm, which is coupled to the electro-dynamical simulations code. A great advantage of this optimization technique is, that it allows the implicit and automatic consideration of technological limitations like the electron beam lithography resolution. Explicitly, we optimize silicon nanostructures such that they provide two user-defined resonance wavelengths, which can be individually addressed by crossed incident polarizations.

Local field enhancement and thermoplasmonics in multimodal aluminum structures

Aluminum nanostructures have recently been at the focus of numerous studies due to their properties including oxidation stability and surface plasmon resonances covering the ultraviolet and visible spectral windows. In this article, we reveal a facet of this metal relevant for both plasmonic purposes and photothermal conversion. The field distribution of high-order plasmonic resonances existing in two-dimensional Al structures is studied by nonlinear photoluminescence microscopy in a spectral region where electronic interband transitions occur. The polarization sensitivity of the field intensity maps shows that the electric field concentration can be addressed and controlled on demand. We use a numerical tool based on the Green dyadic method to analyze our results and to simulate the absorbed energy that is locally converted into heat. The polarization-dependent temperature increase of the Al structures is experimentally quantitatively measured, and is in an excellent agreement with theoretical predictions. Our work highlights Al as a promising candidate for designing thermal nanosources integrated in coplanar geometries for thermally assisted nanomanipulation or biophysical applications.

pyGDM—A python toolkit for full-field electro-dynamical simulations and evolutionary optimization of nanostructures

Abstract pyGDM is a python toolkit for electro-dynamical simulations in nano-optics based on the Green Dyadic Method (GDM). In contrast to most other coupled-dipole codes, pyGDM uses a generalized propagator, which allows to cost-efficiently solve large monochromatic problems such as polarization-resolved calculations or raster-scan simulations with a focused beam or a quantum-emitter probe. A further peculiarity of this software is the possibility to very easily solve 3D problems including a dielectric or metallic substrate. Furthermore, pyGDM includes tools to easily derive several physical quantities such as far-field patterns, extinction and scattering cross-section, the electric and magnetic near-field in the vicinity of the structure, the decay rate of quantum emitters and the LDOS or the heat deposited inside a nanoparticle. Finally, pyGDM provides a toolkit for efficient evolutionary optimization of nanoparticle geometries in order to maximize (or minimize) optical properties such as a scattering at selected resonance wavelengths. Program summary Program Title: pyGDM Program Files doi: http://dx.doi.org/10.17632/8wjcccv73j.1 Licensing provisions: GPLv3 Programming language: python, fortran Nature of problem: Full-field electrodynamical simulations of photonic nanostructures. This includes problems like optical scattering, the calculation of the near-field distribution or the interaction of quantum emitters with nanostructures. The program includes a module for automated evolutionary optimization of nanostructure geometries with respect to a specific optical response. Solution method: The optical response of photonic nanostructures is calculated using field susceptibilities (“Green Dyadic Method”) via a volume discretization. The approach is formally very similar to the coupled dipole approximation. Additional comments including restrictions and unusual features: Only 3D nanostructures. The volume discretization is limited to about 10000 meshpoints.

Enhanced nonlinear optical properties from individual silicon nanowires

We study second harmonic generation (SHG) from individual silicon nanowires. We show that second harmonic intensity is strongly dependent on the Mie resonances supported by the nanowire. The second harmonic polarization also has a size-dependent behavior, which is linked to different surface and bulk-like nonlinear susceptibility contributions. Silicon nanowires provide an interesting tool for nonlinear photonics applications.