Ciarlo, Antonio (2023) Intrinsic feedback effect optical trapping. [Tesi di dottorato]

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Feedback control is used in optical trapping to improve its performance, for example to trap particles using the lowest mean power possible or for cooling their motion.Feedback effects are usually implemented using external systems that control the power of the trapping laser and, often, they are very complex and expensive. To solve these issues, intrinsic feedback effects produced by the trapping system itself can be used, thereby avoiding the need for external systems. In this thesis, the intrinsic feedback effect that arises when trapping particles inside the laser cavity, not outside it as usual, is studied by further developing a novel type of optical tweezers called intracavity optical tweezers. The novel experimental setup here proposed allows trapping a particle in single-beam configuration and in double-beam configuration, which also makes it possible to generate a double-trap configuration by slightly misaligning the two beams. Trapping in single-beam configuration in water and, more importantly, in air is characterised by the presence of the intrinsic feedback effect, which improves the trapping efficiency compared to the standard optical tweezers when trapping in water but not in air. Experiments done in double-beam configuration show a better confinement of the trapped particle and that, when the particle enters the trap, the feedback effect changes drastically the power of the two beams that are found to be anticorrelated. Moreover, trapping in double-trap configuration causes the particle to transit between the two slightly misaligned traps periodically. These transitions are characterised by the presence of the intrinsic feedback effect as the power of the two beams changes periodically, in an anticorrelated way, following the trajectory of the particle. In conclusion, this thesis demonstrated the presence of the intrinsic feedback effect when trapping particles inside a laser cavity not only in single-beam, but also in double-beam configuration and in double-trap configuration. The latter, in particular, demonstrates that the particle acts as a micro-isolator by reducing the power of the laser beam trapping the particle itself. Furthermore, trapping in air in single beam configuration is a huge step towards trapping in vacuum, which could lead to more intriguing physics such as the study of chaotic effects and the cooling of the trapped particles with the consequent possibility of observing quantum effects on mesoscale.

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