Appropriate combinations of laser beams can be used to trap and manipulate small particles with optical tweezers as well as to induce significant optical binding forces between particles. These interaction forces are usually strongly anisotropic depe
nding on the interference landscape of the external fields. This is in contrast with the familiar isotropic, translationally invariant, van der Waals and, in general, Casimir-Lifshitz interactions between neutral bodies arising from random electromagnetic waves generated by equilibrium quantum and thermal fluctuations. Here we show, both theoretically and experimentally, that dispersion forces between small colloidal particles can also be induced and controlled using artificially created fluctuating light fields. Using optical tweezers as gauge, we present experimental evidence for the predicted isotropic attractive interactions between dielectric microspheres induced by laser-generated, random light fields. These light induced interactions open a path towards the control of translationally invariant interactions with tuneable strength and range in colloidal systems.
We address the scattering cross sections, and their consequences, for submicrometer Germanium spheres. It is shown that there is a wide window in the near infrared where light scattering by these particles is fully described by their induced electric
and magnetic dipoles. In this way, we observe remarkable anisotropic scattering angular distributions, as well as zero forward or backward scattered intensities, which until recently was theoretically demonstrated only for hypothetically postulated magnetodielectric spheres. Also, interesting new effects of the optical forces exerted on these objects are now obtained.
High-permittivity dielectric particles with resonant magnetic properties are being explored as constitutive elements of new metamaterials and devices in the microwave regime. Magnetic properties of low-loss dielectric nanoparticles in the visible or
infrared are not expected due to intrinsic low refractive index of optical materials in these regimes. Here we analyze the dipolar electric and magnetic response of loss-less dielectric spheres made of moderate permittivity materials. For low material refractive index there are no sharp resonances due to strong overlapping between different multipole contributions. However, we find that Silicon particles with refractive index 3.5 and radius approx. 200nm present a dipolar and strong magnetic resonant response in telecom and near-infrared frequencies, (i.e. at wavelengths approx. 1.2-2 micrometer). Moreover, the light scattered by these Si particles can be perfectly described by dipolar electric and magnetic fields, quadrupolar and higher order contributions being negligible.
