Ultra-compact Archimedes spiral plasmonic lens with a circular groove for low power optical trapping in the far-field region.

Abstract

Particle levitation is crucial in optical trapping considering contamination and alteration of the character of the particle due to physical contact with the structure. A strong field gradient along the optical axis is required in this case. To manipulate the particle at a distance from the surface, we propose an Archimedes spiral plasmonic lens with a circular groove (CG-ASPL). The optical properties and parameters influencing the trapping performance of CG-ASPL are fully analyzed and discussed. By illuminating the structure with circular polarization and structure optimization, we can reduce the required optical power down to 2.4 mW for trapping particle of 1 µm in diameter with groove width and height of 100 and 125 nm, respectively. The particle can be stably trapped with trapping potential of 4138 kBT/W in the far-field region (1.1λ) owing to constructive interference of the scattered SPP waves. Furthermore, this structure is ultra-compact with a size of about 6.7 µm in diameter. We believe the results demonstrated in this work would be very useful for lab-on-a-chip applications and many others.