# From Far-Field to Near-Field Micro- and Nanoparticle Optical Trapping
## 3 Plasmonic Optical Tweezers
However, because SPPs are pure evanescent modes, direct coupling to propagating light is not possible and, in order to excite them, a different geometrical approach is required. The most common experimental method relies on the Kretschmann configuration [42; 43], which is shown in Figure 6a. Light is coupled into SPPs under total internal reflection in order to compensate for the light momentum mismatch. The crucial parameter in this configuration is the angle of incidence, \(\theta\), which controls both the scattering and the gradient force, allowing for tuning of the total trapping force. Figure 6: (**a**) Schematic of the Kretschmann configuration [42; 43] used to excite the surface plasmon polaritons (SPP). Light is coupled into SPPs under total internal reflection in order to compensate for the light momentum mismatch. The angle of incidence, \(\theta\), controls both the scattering and the gradient forces, allowing for tuning of the total trapping force. (**b**) Excitation of the localised surface plasmons (LSP) can happen under direct illumination of the metallic nanostructures. The geometrical characteristics of the nanostructures on the metallic thin film define the resonance frequency and the forces that arise from the plasmonic field. Inset shows the exponential decay of the evanescent plasmonic field from the surface of the material. In contrast, localised surface plasmons (LSP) are related to the bound electrons that are present near to nano-apertures or nanoparticles much smaller than the wavelength of the electromagnetic field. Bound electrons are susceptible to a damping oscillation due to the nucleus attraction and, as a result, they have a characteristic resonance frequency, unlike SPPs that can be excited over a wide range of frequencies. The benefits of LSPs are that they can directly couple to propagating light and their resonance frequency can be tuned by changing the size and the shape of the nano-aperture/nanoparticle (Figure 6b). In a theoretical work done on LSPs, the dramatic dependence of the strength of the excited evanescent field on the frequency of the incident electromagnetic field was presented [40]. Detailed mathematical analysis on the excitation of surface plasmons and the forces arising can be found in Ref. [14; 44]. To date, many different configurations have been reported using SPs for efficient trapping of subwavelength particles, such as plasmonic nanodots [45], nano-antennas [46], nanocavities [47] and nano-apertures of different shapes and sizes [48; 49], relying on very low incident power. Note that, in these cases, rigorous calculations need to be done beforehand in order to determine crucial parameters such as resonance wavelength and polarisation in compliance with the plasmonic field excitation requirements. Additionally, the fabrication of such nanostructures can also be a challenging task. The most popular techniques to fabricate structures of these sizes is focussed ion beam (FIB) milling and electron beam lithography (EBL), with a resolution of about 10 nm. With these considerations, the implementation of plasmonic optical tweezers can, overall, be relatively time consuming. The main disadvantage with plasmonic structures is the conductive nature of metals, which is linked to heat induction and dissipation to the surrounding environment. The excitation of the LSP leads to a frequency-dependent absorption of light by the structures in the metallic thin film, which provides maximum absorption for maximum plasmonic field intensity. The dynamic behaviour of the nanoparticles in and around the optical potential well can be strongly affected by the resultant photothermal effects and various studies have investigated this phenomenon [50; 51]. Methods to suppress the heat dissipation have been proposed [52; 53] and these could be applied in a synergistic way to mitigate the problem. Additionally, controlled fabrication of the plasmonic structures could enable researchers to take advantage of the self-induced back action effect [54] (see below).