Stefaan Vandendriessche

Nonlinear superchiral meta-surfaces: Tuning chirality and disentangling non-reciprocity at the nanoscale

Circularly polarized light is incident on a nanostructured chiral meta-surface. In the nanostructured unit cells whose chirality matches that of light, superchiral light is forming and strong optical second harmonic generation can be observed.

Adsorption Kinetics of Ultrathin Polymer Films in the Melt Probed by Dielectric Spectroscopy and Second-Harmonic Generation

We studied the adsorption kinetics of supported ultrathin films of dye-labeled polystyrene (l-PS) by combining dielectric spectroscopy (DS) and the interface-specific nonlinear optical second harmonic generation (SHG) technique. While DS is sensitive to the fraction of mobile dye moieties (chromophores), the SHG signal probes their anisotropic orientation. Time-resolved measurements were performed above the glass transition temperature on two different sample geometries. In one configuration, the l-PS layer is placed in contact with the aluminum surface, while in the other one, the deposition is done on a strongly adsorbed layer of neat PS. From the time dependence of the dielectric strength and SHG signal of the l-PS layer in contact with the metal, we detected two different kinetics regimes. We interpret these regimes in terms of the interplay between adsorption and orientation of the adsorbing labeling moieties. At early times, dye moieties get adsorbed adopting an orientation parallel to the surface. When adsorption proceeds to completeness, the kinetics slows down and the dye moieties progressively orient normal to the surface. Conversely, when the layer of l-PS layer is deposited on the strongly adsorbed layer of neat PS, both the dielectric strength and the SHG signal do not show any variation with time. This means that no adsorption takes place.

The role of chiral local field enhancements below the resolution limit of Second Harmonic Generation microscopy

While it has been demonstrated that, above its resolution limit, Second Harmonic Generation (SHG) microscopy can map chiral local field enhancements, below that limit, structural defects were found to play a major role. Here we show that, even below the resolution limit, the contributions from chiral local field enhancements to the SHG signal can dominate over those by structural defects. We report highly homogeneous SHG micrographs of star-shaped gold nanostructures, where the SHG circular dichroism effect is clearly visible from virtually every single nanostructure. Most likely, size and geometry determine the dominant contributions to the SHG signal in nanostructured systems.

Synthesis and Characterization of Holmium-Doped Iron Oxide Nanoparticles

Rare earth atoms exhibit several interesting properties, for example, large magnetic moments and luminescence. Introducing these atoms into a different matrix can lead to a material that shows multiple interesting effects. Holmium atoms were incorporated into an iron oxide nanoparticle and the concentration of the dopant atom was changed in order to determine its influence on the host crystal. Its magnetic and magneto-optical properties were investigated by vibrating sample magnetometry and Faraday rotation measurements. The luminescent characteristics of the material, in solution and incorporated in a polymer thin film, were probed by fluorescence experiments.

Characterization of magnetization-induced second harmonic generation in iron oxide polymer nanocomposites.

We have measured the magnetization-induced second harmonic generation (MSHG) of a nanocomposite consisting of iron oxide nanoparticles in a polymer film. The existing theoretical framework is extended to include DC magnetic fields in order to characterize the MSHG signal and analyze the measurements. Additionally, magnetic hysteresis loops are measured for four principal polarizer-analyzer configurations, revealing the P(IN)-P(OUT) and S(IN)-P(OUT) polarizer-analyzer configurations to be sensitive to the transverse magnetic field. These results demonstrate the use of MSHG and the applied formalism as a tool to study magnetic nanoparticles and their magnetic properties.

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.

Faraday rotation and its dispersion in the visible region for saturated organic liquids.

Faraday rotation and its dispersion have been measured and calculated in the 400-800 nm wavelength range for a set of saturated organic liquids. The resulting Verdet constants are fitted and trends are analyzed. Comparisons are made to both the polarizability and diamagnetic susceptibility. The data are applied to a connectivity index model, allowing prediction of Verdet constants of aliphatic organic liquids from 400 to 800 nm. The observed correlations and connectivity model improve the understanding of Faraday rotation in diamagnetic materials, allowing for future optimization.

Sandwich Approach toward Inverse Opals with Linear and Nonlinear Optical Functionalities

Three-dimensionally ordered macroporous materials have unique structural and optical properties, making them useful for numerous applications in catalysis, membrane science, and optics. Accessible and economic fabrication of these materials is essential to fully explore the many possibilities that these materials present. A new templating method to fabricate three-dimensionally ordered macroporous materials without overlayers is presented. The resulting structures are freestanding inverse opals with large-area uniformity. The versatility and power of our fabrication method is demonstrated by synthesizing inverse opals displaying fluorescence, chirality, upconversion, second harmonic generation, and third harmonic generation. This economical and versatile fabrication method will facilitate research on inverse opals in general and on linear and nonlinear optical effects in 3D photonic crystals specifically. The relative ease of synthesis and wide variety of resulting materials will help the characterization and improvement of existing anomalous dispersion effects in these structures, while providing a platform for the discovery and demonstration of novel effects.

Magneto-optical harmonic susceptometry of superparamagnetic materials

We describe a technique to optically characterize superparamagnetism. Faraday rotation measurements are performed on a superparamagnetic nanocomposite using small alternating current magnetic fields. The superparamagnetism of the iron oxide nanoparticles causes signals at the uneven harmonics of the magnetic field frequency. These signals provide information on the magnetic moment of the superparamagnetic nanoparticles. Dia- and paramagnetism do not cause signals at higher harmonics, resulting in a high sensitivity to superparamagnetism, even in samples with large dia- or paramagnetic contributions. This technique provides a rapid, economical method to characterize superparamagnetism in composite samples not easily accessible by other techniques.

Photoelastic modulator non-idealities in magneto-optical polarization measurements

Modifying and detecting the polarization of light is increasingly important in many contexts such as Faraday isolators and electro-optical modulators. In order to control the polarization of light, it is necessary to know the polarization characteristics of the materials used in the applications. To be able to (magneto-)optically characterize novel materials, we designed a setup using a single photoelastic modulator (PEM) to simultaneously detect natural and magnetic circular dichroism and circular birefringence over a large spectral range. We then theoretically analyzed and experimentally characterized the effect of non-idealities in the PEM on the setup and the resulting data. Our results demonstrate an influence of PEM non-idealities on the measured signals, resulting in non-negligible mixing of circular birefringence and circular dichroism signals. Our measurements of the wavelength dependence of these non-idealities reveal larger non-idealities towards shorter wavelengths. These results illustrate the necessity to take PEM non-idealities into account when working with PEMs, especially at shorter wavelengths or when dealing with signals spanning different orders of magnitude. PEM non-idealities, while frequently neglected in experimental setup design and theoretical derivations, are expected to be more complicated and possibly exert a larger influence on obtained results for experimental setups with multiple PEMs.