CDDA: extension and analysis of the discrete dipole approximation for chiral systems.

Discrete dipole approximation (DDA) is a computational method broadly used to solve light scattering problems. In this work, we propose an extension of DDA that we call Chiral-DDA (CDDA), to study light-chiral matter interactions with the capability of describing the underlying physics behind. Here, CDDA is used to solve and analyze the interaction of a nanoantenna (either metallic or dielectric) with a chiral molecule located in its near field at different positions. Our method allowed to relate near field interactions with far field spectral response of the system, elucidating the role that the nanoantenna electric and magnetic polarizabilities play in the coupling with a chiral molecule. In general, this is not straightforward with other methods. We believe that CDDA has the potential to help researchers revealing some of the still unclear mechanisms responsible for the chiral signal enhancements induced by nanoantennas.

Hybrid magnetite-gold nanoparticles as bifunctional magnetic-plasmonic systems: three representative cases.

Hybrid systems based on magnetite and gold nanoparticles have been extensively used as bifunctional materials for bio- and nano-technology. The properties of these composites are assumed to be closely related to the magnetite to gold mass ratio and to the geometry of the resulting hetero-structures. To illustrate this, we compare and analyze the optical and magnetic properties of core-shell, dumbbell-like dimers and chemical cross-linked pairs of magnetite and gold nanoparticles in detail. We explore how the combination of gold with magnetite can lead to an improvement of the optical properties of these systems, such as tunability, light scattering enhancement or an increase of the local electric field at the interface between magnetic and plasmonic constituents. We also show that although the presence of gold might affect the magnetic response of these hybrid systems, they still show good performance for magnetic applications; indeed the resulting magnetic properties are more dependent on the NP size dispersion. Finally, we identify technological constraints and discuss prospective routes for the development of further magnetic-plasmonic materials.

Analysis of the spectral behavior of localized plasmon resonances in the near- and far-field regimes.

The angular spectrum representation of electromagnetic fields scattered by metallic particles much smaller than the incident wavelength is used to interpret and analyze the spectral response of localized surface plasmon resonances (LSPs) both in the near-field and far-zone regimes. The previously observed spectral redshift and broadening of the LSP peak, as one moves from the far-zone to the near-field region of the scatterer, is analyzed on studying the role and contribution of the evanescent and propagating plane wave components of the emitted field. For such dipolar particles, it is found that the evanescent waves are responsible for those broadenings and shifts. Further, we prove that the shift is a universal phenomenon, and hence, it constitutes a general law, its value increasing as the imaginary part of the nanostructure permittivity grows. Our results should be of use for the prediction and interpretation of the spectral behavior in applications where the excitation of LSPs yield field enhancements like those assisting surface-enhanced Raman spectroscopy or equivalent processes.

Experimental verification of the spectral shift between near- and far-field peak intensities of plasmonic infrared nanoantennas.

Theory predicts a distinct spectral shift between the near- and far-field optical response of plasmonic antennas. Here we combine near-field optical microscopy and far-field spectroscopy of individual infrared-resonant nanoantennas to verify experimentally this spectral shift. Numerical calculations corroborate our experimental results. We furthermore discuss the implications of this effect in surface-enhanced infrared spectroscopy.

Magnetic and electric coherence in forward- and back-scattered electromagnetic waves by a single dielectric subwavelength sphere.

Magnetodielectric small spheres present unusual electromagnetic scattering features, theoretically predicted a few decades ago. However, achieving such behaviour has remained elusive, due to the non-magnetic character of natural optical materials or the difficulty in obtaining low-loss highly permeable magnetic materials in the gigahertz regime. Here we present unambiguous experimental evidence that a single low-loss dielectric subwavelength sphere of moderate refractive index (n=4 like some semiconductors at near-infrared) radiates fields identical to those from equal amplitude crossed electric and magnetic dipoles, and indistinguishable from those of ideal magnetodielectric spheres. The measured scattering radiation patterns and degree of linear polarization (3-9 GHz/33-100 mm range) show that, by appropriately tuning the a/λ ratio, zero-backward ('Huygens' source) or almost zero-forward ('Huygens' reflector) radiated power can be obtained. These Kerker scattering conditions only depend on a/λ. Our results open new technological challenges from nano- and micro-photonics to science and engineering of antennas, metamaterials and electromagnetic devices.

Resolving the electromagnetic mechanism of surface-enhanced light scattering at single hot spots.

Light scattering at nanoparticles and molecules can be dramatically enhanced in the 'hot spots' of optical antennas, where the incident light is highly concentrated. Although this effect is widely applied in surface-enhanced optical sensing, spectroscopy and microscopy, the underlying electromagnetic mechanism of the signal enhancement is challenging to trace experimentally. Here we study elastically scattered light from an individual object located in the well-defined hot spot of single antennas, as a new approach to resolve the role of the antenna in the scattering process. We provide experimental evidence that the intensity elastically scattered off the object scales with the fourth power of the local field enhancement provided by the antenna, and that the underlying electromagnetic mechanism is identical to the one commonly accepted in surface-enhanced Raman scattering. We also measure the phase shift of the scattered light, which provides a novel and unambiguous fingerprint of surface-enhanced light scattering.

Extended discrete dipole approximation and its application to bianisotropic media.

In this research we introduce the formalism of the extension of the discrete dipole approximation to a more general range of tensorial relative permittivity and permeability. Its performance is tested in the domain of applicability of other methods for the case of composite materials (nanoshells). Then, some early results on bianisotropic nanoparticles are presented, to show the potential of the Extended Discrete Dipole Approximation (E-DDA) as a new tool for calculating the interaction of light with bianisotropic scatterers.

Nanoscopic surface inspection by analyzing the linear polarization degree of the scattered light.

We present an optical method for the nanoscopic inspection of surfaces. The method is based on the spectral and polarization analysis of the light scattered by a probe nanoparticle close to the inspected surface. We explore the sensitivity to changes either in the probe-surface distance or in the refractive index of the surface.

Surface inspection by monitoring spectral shifts of localized plasmon resonances.

We present a numerical study of the spectral variations of localized surface plasmon resonances (LSPR) in a 3D-probe metallic nanoparticle scanned over an inhomoegeneous dielectric surface. The possibilities for both, index monitoring and lateral resolution at nanoscale level are explored, with special attention paid to the shape of the probe and the profile of the near field underneath.

Backscattering of metallic microstructures with small defects located on flat substrates.

Micron-sized structures on flat substrates supporting submicron defects are analyzed by means of a parameter based on integrated backscattering calculations. This analysis is performed for different particle and defect sizes, optical properties and for two different configurations (defect on the microstructure or on the substrate). Calculations in the far field are complemented by some near field results. It is shown that information about the defect presence, size and optical properties) can be obtained from the proposed backscattering parameter.

Monitoring small defects on surface microstructures through backscattering measurements.

The detection and characterization of small defects on particles or structures of a size comparable with the incident wavelength and located on flat substrates is numerically analyzed. For metallic scattering systems, calculations based on the extinction theorem of physical optics show that geometrical information about a small Gaussian defect can be obtained by partially integrating the backscattering patterns.