Enantiomorphing Chiral Plasmonic Nanostructures:A Counterintuitive Sign Reversal of the Nonlinear Circular Dichroism

Plasmonic nanostructures have demonstrated a remarkable ability to control light in ways never observed in nature, as the optical response is closely linked to their flexible geometric design. Due to lack of mirror symmetry, chiral nanostructures allow twisted electric field “hotspots” to form at the material surface. These hotspots depend strongly on the optical wavelength and nanostructure geometry. Understanding the properties of these chiral hotspots is crucial for their applications; for instance, in enhancing the optical interactions with chiral molecules. Here, the results of an elegant experiment are presented: by designing 35 intermediate geometries, the structure is “enantiomorphed” from one handedness to the other, passing through an achiral geometry. Nonlinear multiphoton microscopy is used to demonstrate a new kind of double‐bisignate circular dichroism due to enantiomorphing, rather than wavelength change. From group theory, a fundamental origin of this plasmonic chiroptical response is proposed. The analysis allows the optimization of plasmonic chiroptical materials

Second harmonic generation from chiral gold nanoparticles

The synthesis, characterization and application of noble metal nanoparticles is the subject of intense research, due to the promising characteristics of these nanoparticles for applications in optics, catalysis, electronics and metamaterials. Noble metal nanoparticles display interesting nonlinear optical properties. These properties are caused by their high density of free electrons and the associated surface plasmons. Second harmonic generation (SHG) has been used to characterize, amongst others, gold nanoparticles. Separately, SHG has been used to characterize the chirality of organic materials as well as metamaterials. However, no report has been made of the nonlinear optical characterization of chiral noble metal nanoparticles. We report the SHG characterization of a nanocomposite of poly(methyl methacrylate) and chiral gold nanoparticles. Chirality is observed in the second harmonic generated light under correct polarization of incoming and detected light. These results demonstrate the ability of SHG to probe the chirality of noble metal nanoparticles. Further nonlinear optical characterization can improve the understanding of the origin of chirality in these nanoparticles.status: publishe

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.status: publishe

Optical second harmonic generation in a low-bandgap polymer

Recently, much research has been performed on developing low-bandgap polymers for e.g. harvesting solar energy. In the quest to improve these properties, little attention has been paid to their nonlinear optical properties, despite their interesting linear optical spectra and structural similarities to certain nonlinear optically active compounds. We characterized the optical second harmonic generation of corona poled films of poly(cyclopenta[2,1-b;3,4-b′] dithiophen-4-ylidenedioctylmalonate). The unexpectedly large nonlinear optical susceptibilities and the thermal and temporal stability of the material compare favorably to other novel nonlinear optical materials despite the lack of a donor-acceptor dye. Additionally, the polymer displays a very low absorption in the relevant wavelength region. These results demonstrate the promise of these materials for nonlinear optical devices.publisher: Elsevier articletitle: Optical second harmonic generation in a low-bandgap polymer journaltitle: Materials Chemistry and Physics articlelink: http://dx.doi.org/10.1016/j.matchemphys.2014.06.003 content_type: article copyright: Copyright © 2014 Elsevier B.V. All rights reserved.status: publishe

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.status: publishe

Tunneling of holes is observed by second-harmonic generation

Al2O3 passivated n-doped Si substrates are investigated by second-harmonic generation (SHG) upon applying an external electric field by a corona wire. The observed change in the SHG response upon applying an external electric field is attributed to charge transfer in the semiconductor. Capacitance-voltage measurements are performed to affirm this conclusion. Upon applying a large negative electric field over the structure, a clear alteration in SHG signal is observed, which corresponds to tunneling of holes from the n-doped silicon into the aluminum oxide layer. © 2013 American Institute of Physics.status: publishe

Aperture-SNOM reveals plasmonic magnetic near-fields (presentation video)

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Anisotropy versus circular dichroism in second harmonic generation from fourfold symmetric arrays of G-shaped nanostructures

Circular dichroism in optical second harmonic generation (CD-SHG) is studied in planar symmetrical arrays of G-shaped and mirror-G-shaped nanostructures. Anisotropic CD-SHG measurements demonstrate a strong dependence of the value and the sign of the CD effect on the angle of incidence of the fundamental radiation. We show that both dipole and higher order multipole components of the second order susceptibility are responsible for the CD response from G-shaped nanostructures.status: publishe

Volumetric method of moments and conceptual multilevel building blocks for nanotopologies

Based on 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 multi-level hierarchical meshing scheme in a Volumetric Method of Moments (MoM) algorithm. The new meshing scheme is validated by numerical calculations and experiments. It paves the way towards a much more efficient use of MoM in the description of arbitrarily shaped nano-structures at IR and optical frequencies

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 particle-s physical size, is driven by collective oscillations of the conduction electrons in the metal - the so-called surface plasmon resonances.2 page(s