Lighting the path: Optical trapping in active matter and beyond

From the earliest observations of deflection of comet tails by sunlight in the 17th century to the sophisticated manipulation of microscopic entities using lasers in the 21st century, the interplay between light and matter has ceaselessly captivated and challenged humanity. The unique ability of light to interact with matter has illuminated pathways into the microworld, driving innovations in numerous fields of science. This thesis aims to offer an in-depth exploration of various aspects of optical forces, merging advanced computational tools with experimental novelties, all bound together by the central theme of manipulating the microscopic world using light. In the initial chapter, we delve into the basics of optical trapping. Starting with its foundational theories, we gradually transition into the practical aspects of setting up and using optical tweezers. Additionally, I detail the wide-ranging applications of this tool, highlighting their significant contributions to fields such as biology, environmental science, and active matter. In the succeeding chapter, we confront a challenge that has complicated optical forces research: the computationally demanding nature of force calculations, which results in significantly long computations. By weaving machine learning into the tapestry of conventional approaches, we can achieve calculations that combine both speed and precision. This innovation unlocks the doors to numerically probing complex systems, from ellipsoids in double traps to red blood cells and the environmentally harmful microplastics. As we wade through this chapter, it becomes evident that the confluence of optics (not only optical forces) and artificial intelligence is not just a marriage of convenience but a symbiotic fusion, signaling the dawn of an exciting chapter in this discipline. In the third chapter, we maintain our focus on the microscale, but our subjects come from the vast reaches of space. Here, the focus is on using optical tweezers as a precise tool for examining cosmic dust. These tiny interstellar grains, spread throughout the cosmos, are more than just specks that complicate our space observations; they carry information about celestial evolution and processes. By probing and characterizing these particles, we underline the potential of optical tweezers in advancing our understanding of the universe and its mysteries. In the fourth chapter, I introduce a microengine with full orbital motion control. At its heart is a Janus particle, intriguingly trapped not at the center of the beam, but at a precise distance where thermal forces pushing outward balance with the optical forces pulling inward. This equilibrium can be adjusted by varying the beam power. Adding another layer of complexity, when the system is exposed to circularly polarized light, the particle starts to rotate. Remarkably, this rotational motion can be halted or even reversed by simply shifting to linearly polarized light or using circularly polarized light in the opposing direction. This chapter unveils the vast potential of combining both optical and thermal effects. In the concluding chapter, the emphasis is on how optical forces can move particles, not just confine them, within the domain of active matter. Shape-asymmetric particles exhibit an inherent ability to self-propel due to the momentum transfer via transverse optical forces. This unique propulsion mechanism is further enhanced when we introduce a light-absorbing coating, which instigates thermophoretic effects. These capped particles are propelled, charting their course along intricate pathways. The pathways are shaped by the interplay between deterministic optical forces and random Brownian motion, and are influenced by variables such as particle size and the distribution of light. A numerical model that closely aligns with our experimental findings offers further clarity. This chapter doesn’t merely spotlight a novel application of optical forces; the implications of this study extend beyond targeted applications, shedding light on broader phenomena such as bacterial motion and animal migration, and enriching our understanding of determinants of motion across various scientific domains. Every chapter highlights the unique powers of optical forces, and together they provide a detailed understanding of how we use light to work with and understand the tiny world around us. As you navigate this thesis, I extend an invitation: marvel at the versatility of light and envisage a future where its interplay with matter continues to unlock new scientific and technological horizons.