# From Far-Field to Near-Field Micro- and Nanoparticle Optical Trapping
## 3 Plasmonic Optical Tweezers
### Self-Induced Back Action Effect
According to [PERSON] et al., in order to optimise \(\kappa_{SIBA}\), the cavity has to be constructed such that the _back-action parameter_, \(v=\delta\omega_{0}(x_{p})/\Gamma\), is maximised [62]. This means that, while the particle is trapped in the centre of the trap, the resonance shift is such that the photon flux from the cavity is less than the maximum possible. As a consequence, when the particle moves away from the centre of the trap, the resonance shift causes the photon flux to increase and, thus, the intensity of the transmitted light increases. From Equation (9) an increase to the intensity leads to an increase to the gradient force, which restores the particle back to the centre of the trap. Then, the photon flux and the intensity decrease and, again, the particle tends to move away from the trap centre. This kind of feedback is referred to as _\"optomechanical coupling\"_ because there is a continuous response between light and mechanical motion. The field of optomechanics in plasmonics is rather unexplored and, to our knowledge, there is only one work that reports an optomechanical coupling constant [60]. This optomechanical coupling not only relaxes the requirements for high power trapping, but also prevents the sample from overheating since most of the time the particle is trapped using a low intensity. It remains open to exploration to find ways to increase the optomechanical coupling constant and to achieve even higher particle confinement and motion transduction. In Figure 10a, the vertical dashed line represents the excitation laser wavelength, the black lineshape is the empty cavity mode resonance, and the orange is the shifted one due to particle trapping. In the first case (Figure 10(ai), blue-shifted) the cavity resonance is set to be blue-detuned from the excitation laser, such that when the particle is trapped, the resonance red-shift increases the photon flux to the maximum value and the gradient force reaches a maximum. However, when the particle moves away from the centre of the trap, the lineshape blue shifts towards the empty cavity resonance and the intensity decreases. In order to increase the gradient force, the power of the laser has to, externally, be increased, thus, it is not the most efficient scenario for trapping. In the case where the empty mode resonance is slightly blue-detuned (Figure 10(aii), resonance), the red-shifting due to a trapping event creates a symmetrical lineshape around the laser wavelength. As the particle tries to escape from the trap, the resonance moves towards the empty mode value and causes an increase in the photon flux and the light intensity of light, thus increasing the gradient force. Finally, in the case where the empty cavity resonance is designed to be red-detuned from the excitation laser (Figure 10(aiii), red-shifted), the trapped object further red-shifts the resonance, leading to significant reduction in the intensity and the gradient force. In this configuration, the trapping becomes very inefficient and an increase to the laser power is necessary to keep the particle in the trap. The intensity required to keep the particle efficiently in the trap is less in the second case where the SIBA effect contributes to an increase in the total trapping stiffness. This was also experimentally observed for the first time by [PERSON] et al. [60]. Figure 10b shows the experimental data that confirm the superior trapping efficiency of a plasmonic cavity, designed to be slightly blue-detuned from the excitation wavelength (Figure 10(aii,bii)). The remarkable effect of SIBA is now apparent and, by proper design of the plasmonic structure, we can have a larger trapping stiffness at a lower laser power, thereby reducing heat transfer to the specimen.