Optical metasurfaces for momentum exchange between light and matter

Licentiate thesis, 2023

Light carries energy and momentum that can be transferred to matter. Because of the conservation of total momentum, a change in light momentum due to interaction with an object must be compensated by an equal but opposite change in the momentum of that object. The resulting optical forces may in turn cause observable mechanical translation and rotation of the object, as a consequence of a change in linear and angular momentum of the light, respectively. These effects can be controlled by structuring the impinging light field and/or by engineering the optical properties of the object.
Metasurfaces, flat structures with engineered subwavelength building blocks, provide various possibilities for tailoring optical forces and momentum exchange. A metasurface can be used as a compact replacement for conventional bulky optical elements used to shape light, such as lenses and spatial light modulators. An example of this is provided in the appended paper, where a cylindrical metalens with a linear phase gradient is used to optically trap and translate small particles along its line focus. But in the process of shaping a light beam, a metasurface is itself subject to momentum exchange and optical forces. This effect can be observed if a metasurface is incorporated into a micro-scale object allowed to move freely across a surface. Several examples of such meta-particles propelled by light are provided in the thesis.
The thesis is organized as follows: The first part is devoted to studying the fundamentals and developments of optical metasurfaces by categorizing them based on aspects of material composition, working wavelength, and applications. Then a brief review of different actuation mechanisms used for the manipulation of micro and nano-objects is provided, with a focus on using optical fields as the external source of energy and momentum.
The second part of the thesis is dedicated to results that highlight the versatility of stationary and movable metasurfaces in facilitating optical manipulation of objects. Finally, a description of the metasurface fabrication and characterization methods used is provided.