# From Far-Field to Near-Field Micro- and Nanoparticle Optical Trapping
## 4 Conclusions and Future Perspectives  
It is interesting to see how a relatively basic idea can be transformed into a very useful technique for manipulating matter, finally leading to the awarding of a Nobel Prize in Physics. Optical tweezers, whether conventional or plasmonic, are now widely used in many fields of research, due to such scientific efforts. A big advantage of optical tweezers is that they are relatively easy and inexpensive to build or to modify according to research needs. Thus, they have found application in various fields such as physics [67], biosciences [68; 69; 70], and chemistry [71]. In many cases, they serve as a tool to trap or manipulate matter while simultaneously making other measurements, such as Raman spectroscopy of biological samples [72; 73; 74]. There is also an increased interest in exploring the area between particle trapping and atom cooling to study quantum phenomena at the mesoscopic scale [75; 76], indicating that optical tweezers may even impact the development of quantum-based technologies. Very recently, ground state laser cooling of a subwavelength sized dielectric particle  
Figure 10: (**a**) Three different cases of the resonance profile of a plasmonic cavity (black line). In case (i), (ii), and (iii) the cavity is designed to be blue-detuned, slightly blue-detuned and red-detuned, respectively, from the excitation laser (vertical dashed line). The orange line represents the frequency shift of the resonance of the cavity due to the existence of the particle near by it. In case (ii), the SIBA effect has a positive influence on the trap and, whenever the particle tries to escape, the photon flux (intensity) increases, which causes, increase to the gradient force. (**b**) Experimental measurements of the total trapping stiffness as a function of the incident laser intensity. (**b**)(i) corresponds to case (**a**)(i), where as the intensity decreases both the total and the empty cavity trapping stiffness decrease, making the trap inefficient. (**b**)(ii) corresponds to case (**a**)(ii), and clearly shows that, as the laser intensity decreases, the empty cavity trapping stiffness decreases, but the total stiffness increases due to the positive contribution from the SIBA effect, thus achieving a stable, self-adjustable trap. Figures reproduced from [60], under Creative Commons license: CC BY-NC-ND 4.0 [[https://creativecommons.org/licenses/by-nc-nd/4.0/](https://creativecommons.org/licenses/by-nc-nd/4.0/)]([https://creativecommons.org/licenses/by-nc-nd/4.0/](https://creativecommons.org/licenses/by-nc-nd/4.0/)).  
trapped in an optical tweezers has been reported [77], providing a route to explore topics such as quantum sensing using macrosystems, so it is certainly a very exciting time for further advances in the field.  
Of course, there are many open questions and challenges to overcome in order to optimise the techniques so that they can be used to efficiently trap not only particles in the range of less than 10 nm, but also biological samples such as proteins, viruses and DNA for which the treatment as a spherical dielectric particles is inadequate. Although not discussed here, the geometrical shape and material of the trapped object play a significant role in the trapping conditions and, in particular for biomaterials, heating and rapid temperature changes must be taken into consideration. One of the biggest challenges for progress in optical trapping is the ability to fully control and manipulate many trapped nano-objects simultaneously. While this has been achieved for micro-objects by using modulation of the light field, it has yet to be realized in the nanoscale. We can expect to see this barrier surmounted in the near future.  
The authors would like to thank Okinawa Institute of Science and Technology Graduate University for funding, and [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] for constructive comments.  
The authors declare no conflict of interest.