|a) When a microscopic object is brought close to a tightly focused laser beam, it will be drawn to the centre of the beam and become trapped by optical gradient forces. The object is not static within the trap, but fluctuates as a result of thermal brownian motion. (Black arrows denote field lines.) |
b) In the presence of the patterned substrate, the trapping beam simultaneously excites localized surface plasmons causing a strong field enhancement. The presence of the plasmon modes creates additional optical gradient forces, which suppress the brownian motion of the trapped object, thus offering better confinement.
Image and text: Nature Photonics