THz Photonic Manipulation of Eccentric Core-shell Microparticles with Magneto-Plasmonic Shells

We propose core-shell microparticles that can be easily manipulated under THz illumination using electromagnetic forces. These particles consist of a dielectric core with an eccentric magneto-plasmonic shell that resonates at micrometer wavelengths. Such particles can be driven in any direction using an arbitrary static magnetic field, as a doped material can produce significant magneto-optical effects that are dependent on the field's intensity. By employing a simple illumination field of two counterpropagating, circularly polarised plane waves, the induced optical forces and angular accelerations are neutralised when the static field is off and the particle's eccentricity is zero. Moreover, with the system at magneto-plasmonic resonance and the static field present, the particle's radiation force vanishes for almost any eccentricity if oriented appropriately, which makes the method of particle manipulation robust to fabrication anomalies. Conversely, the particle spin depends on the offset between the core and shell when the magnetic field is applied, thus making the system useful for eccentricity detection. Tunable rotations can be produced according to the value chosen for the static field and the particle design. Angular accelerations in the order of 1000 kHz² and forces of 1 nN were observed for intensities around 4 mW/μm2 for the simulated sizes and materials at resonance. As the effect is scalable, similar results could be produced in the optical range by the appropriate choice of magneto-plasmonic shell size and material. Our work paves the way for developing small-scale viscosimeters, particle separation/sorting methods, smart drug delivery, and energy storage, among other applications.