Cg Biris

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.

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.

The origin of second harmonic generation hotspots in chiral optical metamaterials [Invited]

Recently, a great amount of research has been triggered by the prediction, formulated by Pendry and co-authors, that novel and enhanced nonlinear optical phenomena could be observed in metamaterials.[1] This prediction is based on the fact that, in metamaterials, local field enhancements can have a dramatic influence over the optical properties of the material. One of the largest contributions to such local field enhancements is attributable to surface plasmon resonances. Plasmons are collective oscillations of the electrons under the influence of lights electromagnetic field. Plasmons occur naturally on the surfaces of homogeneous metal films, where they usually dissipate quickly and cancel each others influence. However, upon patterning the metal surface at the nanoscale, plasmons can be manipulated in a manner similar to classical waveguiding, whereby propagation or standing wave patterns can be achieved. In other words, artificial structuring allows for nanoengineering the position and intensity of the local fields. Incidentally, nonlinear optical effects, such as second-, third-, or forth-harmonic generation, scale as the second, third or fourth power of the electromagnetic intensity, respectively. Therefore, it is perfectly reasonable to assume that the large electromagnetic local field enhancements in metamaterials should yield large or previously unobserved nonlinear optical effects.

Fano resonance resulting from a tunable interaction between molecular vibrational modes and a double continuum of a plasmonic metamolecule.

Coupling between tunable broadband modes of an array of plasmonic metamolecules and a vibrational mode of carbonyl bond of poly(methyl methacrylate) is shown experimentally to produce a Fano resonance, which can be tuned in situ by varying the polarization of incident light. The interaction between the plasmon modes and the molecular resonance is investigated using both rigorous electromagnetic calculations and a quantum mechanical model describing the quantum interference between a discrete state and two continua. The predictions of the quantum mechanical model are in good agreement with the experimental data and provide an intuitive interpretation, at the quantum level, of the plasmon-molecule coupling.

Excitation of dark plasmonic cavity modes via nonlinearly induced dipoles: applications to near-infrared plasmonic sensing

We demonstrate that dark plasmon modes of cavity-shaped plasmonic structures made of metallic nanowires can be excited by local dipoles induced via second-harmonic generation. The optical properties of these plasmonic cavity modes are thoroughly characterized by using a numerical method that provides a complete description of the optical field at both the fundamental frequency and the second harmonic. In particular, we show that the optical properties of these plasmonic cavity modes are strongly dependent on the geometry of the plasmonic cavity and the material parameters of its constituents. This enhanced sensitivity of dark plasmonic cavity modes to the surrounding dielectric environment can find applications in plasmonic sensing. Specifically, this novel approach to sensing reveals that detection limits of 10(-5) refractive index units can readily be achieved by using wavelength-sized plasmonic devices.

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.

Nonlinear pulsed excitation of high- Q optical modes of plasmonic nanocavities

We present a comprehensive theoretical and numerical analysis of the physical mechanisms pertaining to the nonlinear pulsed excitation of optical modes in plasmonic cavities made of metallic nanowires. Our analysis is based on extensive numerical simulations carried out both in the frequency and time domains. The numerical algorithm used in our study is based on the multiple scattering method and allows us to include in our analysis the effects of both the surface and bulk nonlinear polarizations generated at the second harmonic (SH). In particular, we investigate the physical properties of plasmonic modes excited at the SH as the result of the interaction of femtosecond optical pulses with plasmonic nanocavities. We show that such cavities have two distinct types of modes, namely, plasmonic cavity modes and multipole plasmon modes generated via the hybridization of modes of single nanowires. Our analysis reveals that the properties of the latter modes depend only weakly on the cavity geometry, whereas the lifetime and quality factor of plasmonic cavity modes vary considerably with the system parameters.

Polarization-induced tunability of localized surface plasmon resonances in arrays of sub-wavelength cruciform apertures

We demonstrate experimentally that by engineering the structural asymmetry of the primary unit cell of a symmetrically nanopatterned metallic film the optical transmission becomes strongly dependent on the polarization of the incident wave. By considering a specific plasmonic structure consisting of square arrays of nanoscale asymmetric cruciform apertures we show that the enhanced optical anisotropy is induced by the excitation inside the apertures of localized surface plasmon resonances. The measured transmission spectra of these plasmonic arrays show a transmission maximum whose spectral location can be tuned by almost 50% by simply varying the in-plane polarization of the incident photons. Comprehensive numerical simulations further prove that the maximum of the transmission spectra corresponds to polarization-dependent surface plasmon resonances tightly confined in the two arms of the cruciform aperture. Despite this, there are isosbestic points where the transmission, reflection, and absorption spectra are polarization-independent, regardless of the degree of asymmetry of the apertures.

Nonlinear surface-plasmon whispering-gallery modes in metallic nanowire cavities

We demonstrate that the surface second-harmonic generation can lead to the formation of nonlinear plasmonic whispering-gallery modes (WGMs) in microcavities made of metallic nanowires. Since these WGMs are excited by induced surface nonlinear dipoles, they can be generated even when they are not coupled to the radiation continuum. Consequently, the quality factor of these nonlinear modes can be as large as the theoretical limit imposed by the optical losses in the metal. Remarkably, our theoretical analysis shows that nonlinear plasmonic WGMs are characterized by fractional azimuthal modal numbers. This suggests that the plasmonic cavities investigated here can be used to generate multicolor optical fields with fractional angular momentum. Applications to plasmonic sensors are also discussed.

Resonant mixing of optical orbital and spin angular momentum by using chiral silicon nanosphere clusters

We present an in-depth analysis of the resonant intermixing between optical orbital and spin angular momentum of Laguerre-Gaussian (LG) beams, mediated by chiral clusters made of silicon nanospheres. In particular, we establish a relationship between the spin and orbital quantum numbers characterizing the LG beam and the order q of the rotation symmetry group 𝒞q of the cluster of nanospheres for which resonantly enhanced coupling between the two components of the optical angular momentum is observed. Thus, similar to the case of diffraction grating-mediated transfer of linear momentum between optical beams, we demonstrate that clusters of nanospheres that are invariant to specific rotation transformations can efficiently transfer optical angular momentum between LG beams with different quantum numbers. We also discuss the conditions in which the resonant interaction between LG beams and a chiral cluster of nanospheres leads to the generation of superchiral light.