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.

Strong magnetic response of submicron silicon particles in the infrared.

High-permittivity dielectric particles with resonant magnetic properties are being explored as constitutive elements of new metamaterials and devices. Magnetic properties of low-loss dielectric nanoparticles in the visible or infrared are not expected due to intrinsic low refractive index of optical media in these regimes. Here we analyze the dipolar electric and magnetic response of lossless dielectric spheres made of moderate permittivity materials. For low material refractive index (<∼3) there are no sharp resonances due to strong overlapping between different multipole contributions. However, we find that Silicon particles with index of refraction∼3.5 and radius∼200 nm present strong electric and magnetic dipolar resonances in telecom and near-infrared frequencies, (i.e. at wavelengths≈1.2-2 mm) without spectral overlap with quadrupolar and higher order resonances. The light scattered by these Si particles can then be perfectly described by dipolar electric and magnetic fields.

Angle-suppressed scattering and optical forces on submicrometer dielectric particles.

We show that submicrometer silicon spheres, whose polarizabilities are completely given by their two first Mie coefficients, are an excellent laboratory to test effects of both angle-suppressed and resonant differential scattering cross sections. Specifically, outstanding scattering angular distributions, with zero forward- or backward-scattered intensity, (i.e., the so-called Kerker conditions), previously discussed for hypothetical magnetodielectric particles, are now observed for those Si objects in the near infrared. Interesting new consequences for the corresponding optical forces are derived from the interplay, both in and out of resonance, between the electric- and magnetic-induced dipoles.

Optical forces on small magnetodielectric particles.

We present a study of the optical force on a small particle with both electric and magnetic response, immersed in an arbitrary non-absorbing medium, due to a generic incident electromagnetic field. Expressions for the gradient force, radiation pressure and curl components are obtained for the force due to both the electric and magnetic dipoles excited in the particle. In particular, for the magnetic force we tentatively introduce the concept of curl of the spin angular momentum density of the magnetic field, also expressed in terms of 3D generalizations of the Stokes parameters. From the formal analogy between the conservation of momentum and the optical theorem, we discuss the origin and significance of the self-interaction force between both dipoles; this is done in connection with that of the angular distribution of scattered light and of the extinction cross section.

Radiative corrections to the polarizability tensor of an electrically small anisotropic dielectric particle.

Radiative corrections to the polarizability tensor of isotropic particles are fundamental to understand the energy balance between absorption and scattering processes. Equivalent radiative corrections for anisotropic particles are not well known. Assuming that the polarization within the particle is uniform, we derived a closed-form expression for the polarizability tensor which includes radiative corrections. In the absence of absorption, this expression of the polarizability tensor is consistent with the optical theorem. An analogous result for infinitely long cylinders was also derived. Magneto optical Kerr effects in non-absorbing nanoparticles with magneto-optical activity arise as a consequence of radiative corrections to the electrostatic polarizability tensor.

Full transmission through perfect-conductor subwavelength hole arrays.

Light transmission through 2D subwavelength hole arrays in perfect-conductor films is shown to be complete (100%) at some resonant wavelengths even for arbitrarily narrow holes. Conversely, the reflection on a 2D planar array of nonabsorbing scatterers is shown to be complete at some wavelengths regardless how small the scatterers are. These results are proven analytically and corroborated by rigorous numerical solution of Maxwell's equations. This work supports the central role played by dynamical diffraction during light transmission through subwavelength hole arrays and it provides a systematics to analyze more complex geometries and many of the features observed in connection with transmission through hole arrays.

Resonant radiation pressure on neutral particles in a waveguide.

A theoretical analysis of electromagnetic forces on neutral particles in a hollow waveguide is presented. We show that the effective scattering cross section of a very small (Rayleigh) particle can be strongly modified inside a waveguide. The coupling of the scattered dipolar field with the waveguide modes induces a resonant enhanced backscattering state of the scatterer-guide system close to the onset of new modes. The particle effective cross section can then be as large as the wavelength even far from any transition resonance. As we will show, a small particle can be strongly accelerated along the guide axis while being highly confined in a narrow zone of the cross section of the guide.