# From Far-Field to Near-Field Micro- and Nanoparticle Optical Trapping
## 3 Plasmonic Optical Tweezers  
Recent advances in the fields of optics and nano-optics have helped to overcome the diffraction limit problem by using evanescent fields instead of propagating ones; these have the intrinsic property of confinement beyond the diffraction limit. A detailed analysis can be found in [33; 34]. The current trend is to use metallic nanostructures (see [35] for a recent review on different platforms) in which surface plasmons can be excited at resonant frequencies and that concentrate the electric field to create highly intense fields, thereby significantly increasing the trapping potential depth that a nanoparticle may experience. In 1992, [PERSON] et al. demonstrated the motion of microparticles under illumination from evanescent fields [36] and they were soon followed by [PERSON] et al. [37]. Some years later, in 1997, [PERSON] et al. theoretically proposed and calculated optical trapping at the nanoscale, using enhanced evanescent fields from a laser-illuminated metallic nanotip [8]. [PERSON] et al., at around the same time, did similar work, but used a metallic nano-aperture instead of a tip [38]. Figure 5 shows their proposed geometrical model. Analytical work on the calculation of forces acting on a dielectric sphere from an evanescent field can also be found in [39]. The advantage of using this kind of configuration comes from the fact that the incident field no longer directly creates the trapping potential, but rather excites the surface plasmons (SP) on the metal/dielectric interface. The SPs, in turn, create the strong evanescent field that provides the trapping potential. The main benefit of trapping using an evanescent field is th  
Figure 5: The geometrical model, studied by [PERSON] et al., for subwavelength particle trapping, utilising the evanescent field near a metallic nano-aperture. This is the most commonly studied and reported geometry due to its simplicity to fabricate using the focussed ion beam (FIB) milling technique. Figure reproduced with permission from [38]. a very high field gradient, thus exerting a large trapping force (see Equation (9)) with no need to increase the incident intensity, thereby leading to a reduction of radiation damage to the sample. In other words, superior trapping conditions can be achieved with much lower illumination power compared to conventional optical tweezers. It was then just a matter of time for plasmonic optical tweezers to be realised. In 2007, [PERSON] et al., using a geometry of total internal reflection similar to the one shown in Figure 6a, and a pattern consisting of 4.8 \(\upmu\)m-diameter gold discs fabricated on glass, performed multiple trapping of 4.88 \(\upmu\)m polystyrene colloids [9]. Note that the laser beam was unfocussed, with a waist of about 100 \(\upmu\)m and the intensity was more than 10 times lower than that required for conventional optical tweezers with similar characteristics. Theoretical work had been done earlier in order to study the forces arising in such a configuration [40]. Also, in earlier experimental work, the authors used a photonic force microscope to measure the plasmon radiation forces acting on polystyrene beads at the localised surface plasmon resonance. They reported forces 40 times stronger than those obtained in the absence of SP excitation [41]. It is important to mention the two distinct types of surface plasmons. Surface plasmon polaritons (SPP) are propagating electromagnetic surface waves that appear at the metal/dielectric interface due to the motion of the metal's free electrons driven by the incident electromagnetic field. They are evanescent modes and, thus, they produce localised fields with a high intensity decaying exponentially away from the metal surface. Due to the very large intensity gradient, they exert strong gradient forces on the trapped particles (see Equation (9)), thereby producing stable traps.