Optical forces are crucial in understanding light–matter interactions across various scientific fields, including atomic physics, optics, photonics, and nanotechnology. Since the advent of laser tweezers, significant advancements have been made in optical trapping, binding, sorting, and transporting particles using optical forces. Current research focuses on enhancing control over different types of optical forces to better manage the dynamics and interactions of microparticles and nanoparticles.
Laser tweezers, which use intensity-gradient forces from a strongly focused laser beam, act as point-like traps to capture particles. These conservative forces draw particles toward the minima of potential energy. In contrast, non-conservative optical forces, such as phase-gradient optical forces, are more versatile, capable of pulling, rotating, and moving particles. Phase-gradient forces arise from the phase gradients of a focused light beam and have been used in curve-shaped trapping laser beams to move particles along a curve. This mechanism relies on designing the beam's phase gradient to redirect light radiation pressure along the curve. While the potential of phase gradient forces acting in higher dimensions is recognized, achieving this remains a challenging problem that has yet to be addressed.
To address this complex problem, Professor Jose A. Rodrigo's research group at Universidad Complutense de Madrid (Spain) has recently developed an innovative optical trap in the form of a surface-shaped laser trap with a conformable phase-gradient field, which redirects part of the light radiation pressure across a surface. The novelty of this optical manipulation tool lies in its customizable phase-gradient force field, extended in two dimensions, allowing for multifunctional remote motion control of multiple particles. This enables particles to move across the surface and interact in ways that would not be possible using conventional optical tools. Relevant research results were recently published in Photonics Research, Volume 12, Issue 10, 2024. [José A. Rodrigo, Enar Franco, Óscar Martínez-Matos, "Surface laser traps with conformable phase-gradient optical force field enable multifunctional manipulation of particles," Photonics Res. 12, 2088 (2024)]
This study aims to enhance optical manipulation techniques for controlling the movement of nanoparticles facilitating their interactions within the surrounding environment. Traditional laser traps struggle with manipulating multiple particles, especially when control of their translational and spinning motion is required. The new surface-shaped laser traps can capture and autonomously transport multiple nanoparticles, controlling both their translation and spin.
The study demonstrates how this laser tool moves single and multiple plasmonic nanoparticles (gold nanospheres) and forms spinning optically bound (OB) dimers, the smallest optical matter systems. As shown in Fig. 1(a), particles move across the surface via programmed light-driven orbital transport. Multiple particles, guided by the phase-gradient force field, can interact electromagnetically to form OB dimers (Fig. 1(b)). These particles act as switchable miniature motor rotors, rotating due to optical binding forces and optical torque from a circularly polarized surface laser trap.
Fig. 2 shows triangle-shaped laser traps where particle orbital motion can be also programmed. The optical force field can be configured for transport tasks (Fig. 2(a)) and to create strong particle confinement barriers (Fig. 2(b)). These results highlight the multifunctional performance of the surface laser traps, regardless their shape, with significant implications for light-matter interactions and complex particle system control. This could advance the creation of novel light-driven nanomotors and optical matter structures.
Figure 1. (a) Numerical simulation of the light-driven orbital transport of a gold nanosphere (radius 200 nm) across a circular ribbon-like surface trap (circularly polarized, left CP). The phase distribution and intensity-gradient of the laser beam are shown along with the track of the particle. (b) In the experiment the particle travels across the surface at a mean speed of 50 mm/s, as expected (see Visualization 2). (c) Experimental results: Multiple particles simultaneously transported on the same surface laser trap. The particles are able to travel and interact among themselves forming stable OB dimers spinning at 6 Hz (see Visualization 5).
Figure 2. (a) Numerical simulation and experimental results for the light-driven orbital transport of a gold nanosphere (radius 200 nm) across a triangular surface laser trap with a rectangular hole (circularly polarized), see Visualization 9. (b) Triangular surface laser trap without a hole (circularly polarized). This example illustrates the ability to set inward and outward radial optical force fields. The phase distribution and intensity gradient of the laser beam are shown along with the particle track. Experiment: Multiple particles travel across the surface, directed by these radial optical force fields. The optical force field can also create a barrier at the surface boundary to prevent fast particles from escaping the trap if needed (see Visualization 10).
Corresponding author Professor Jose A. Rodrigo stated: "We detail the creation of surface laser traps with tailored multifunctional optical force fields, combined with optical torque from polarized light. This approach allows us to control particle dynamics and enable particle-particle interactions beyond the capabilities of conventional optical tools. By structuring a laser beam in an application-specific manner, we developed customizable surface laser traps that enhance the study and control of complex interacting particle systems, including plasmonic structures of high interest in optics and photonics. This work exemplifies the synergy between physics, mathematics, and technology in addressing challenging optical manipulation problems."
Future research will expand the application of surface laser traps to manipulate a diverse set of nano and micro-particles, including inorganic materials and biological structures such as cells. The main goal is to harness the versatility of these innovative laser traps to study and control complex systems of interacting particles, paving the way for novel technological applications in real-world scenarios.
