Due to their unique properties, magnetic nanoparticles, often made of iron oxides, have received significant attention in chemistry, solid state physics, and the life sciences. Although a magnetic field is the most obvious mean by which one can manipulate magnetic nanoparticles, we here demonstrate that magnetic nanoparticles can be individually controlled by optical manipulation. We quantify the interaction of optically trapped individual magnetic nanoparticles with the electrical field by determining the spring constant. Also, by finite element modeling we determine the extinction, scattering and absorption cross sections of magnetic nanoparticles as well as the real and imaginary parts of their complex polarizability. In comparison to magnetic manipulation, optical manipulation has the advantage, due to the tight focusing of the laser beam, that it allows for manipulation of a single particle at a time. Also, one can imagine applications where it is advantageous to employ both magnetic and optical manipulations simultaneously.
In typical colloidal suspensions, the corresponding optical polarizability is positive, and thus enhanced scattering takes place as optical beams tend to catastrophically collapse during propagation. Recently, light penetration deep inside scattering suspensions has been realized by engineering dielectric or plasmonic nanoparticle polarizibilities. In particular, we have previously demonstrated two types of soft-matter systems with tunable optical nonlinearities - the dielectric and metallic colloidal suspensions, in which the effects of diffraction and scattering were overcome, hence achieving deep penetration of a light needle through the suspension.
In this work, we show that waveguides can be established in soft matter systems such as metallic nanosuspensions through the formation of plasmonic resonant solitons. First, we show that, due to plasmonic resonance, a 1064nm laser beam (probe) would not experience appreciable nonlinear self-action while propagating through 4cm cuvette containing the metallic nanosuspension of gold spheres (40nm), whereas a 532nm laser beam (pump) can readily form a spatial soliton due to nonlinear self-trapping. Second, we demonstrate effective guidance of the probe beam, which would otherwise diffract significantly through the nanosuspensions, due to the soliton-induced waveguide from the pump beam. Such guidance was observed when the power of the probe beam was varied from 20mW to 500mW at constant pump beam power, with more pronounced guidance realized from lower to higher probe beam power. Interestingly, due to the presence of the probe beam, the pump beam undergoes self-trapping at an even lower power. These results may bring about the possibility of engineering plasmonic soliton-based waveguides for many applications.