Optical metasurfaces for momentum exchange between light and matter

Doctoral thesis, 2024

Light, despite being massless, carries both linear and angular momentum, allowing it to interact with matter in ways that induce tangible mechanical effects, such as translation and rotation. These effects are governed by the conservation of momentum, which dictates that any change in the momentum of light must be counterbalanced by a corresponding change in the momentum of the interacting material. Consequently, an optical force or torque can lead to observable mechanical translation and rotation. This thesis explores the reciprocal interaction between light momentum and matter, focusing on the manipulation of optical momentum through engineered subwavelength structures known as metasurfaces. This mutual exchange significantly influences both light and matter, offering opportunities to control one by altering the other.

Metasurfaces offer a wide range of possibilities for manipulating light momentum by modulation of the amplitude, phase, and polarization of transmitted and reflected light. They can also serve as compact replacements of traditional bulky elements, such as lenses and spatial light modulators. An example of this is demonstrated in the first appended paper, where a cylindrical metalens combined with a beam deflector is used to optically trap and transport particles along its line focus. Other examples, including the generation of holographic patterns and beam deflectors, are also presented in this report.

In the process of shaping a light beam, optical metasurfaces are often viewed as stationary elements that do not respond to the changing momentum of light. However, the metasurfaces themselves are subject to momentum exchange and reaction optical forces. This effect becomes evident when a metasurface is incorporated into a lightweight micro-scale object, called a metaparticle, which is free to move across a surface.

This thesis studies a new type of rotary metaparticle that can spin by redirecting linear photon momentum through pairs of beam deflector metasurfaces. These metaparticles consist of a transparent SiO2 structure as the body, with embedded silicon metagratings to simulate the operational behavior of blazed gratings. Moreover, the second paper in this thesis investigates the collective behavior of spinning metaparticles, revealing unconventional orbiting patterns as they interact with each other. This study led to the discovery of a new type of optical gradient force, which acts perpendicular to the classical optical gradient force. Finally, the third paper demonstrates that the reaction optical forces generated by metagratings enable large metaparticles, up to 100 µm in diameter, to rotate by bending light at high angles. It is also shown that the torque generated by a rotor can rotate hundreds of passive microparticles in solution, suggesting potential applications for particle mixing in microfluidics.

The thesis is organized as follows: The first part is devoted to studying the fundamentals and developments of static optical metasurfaces by categorizing them based on various aspects, and the second part is dedicated to the study of optical forces and torques. The third part investigates the results that highlight the versatility of stationary and movable metasurfaces in facilitating optical manipulation of objects. Finally, the last chapter describes metasurface fabrication and characterization techniques, as well as various simulation method used in this work.