Momentum exchange between light and nanostructured matter

Doktorsavhandling, 2021

An object's translational and rotational motion is associated with linear and angular momenta. When multiple objects interact the exchange of momentum dictates the new system's motion. Since light, despite being massless, carries both linear and angular momentum it too can partake in this momentum exchange and mechanically affect matter in tangible ways. Due to conservation of momentum, any such exchange must be reciprocal, and the light therefore acquires an opposing momentum component. Hence, light and matter are inextricably connected and one can be manipulated to induce interesting effects to the other. Naturally, any such effect is facilitated by having strongly enhanced light-matter interaction, which for visible light is something that is obtained when nanostructured matter supports optical resonances. This thesis explores this reciprocal relationship and how nanostructured matter can be utilised to augment these phenomena.

Once focused by a strong lens, light can form optical tweezers which through optical forces and torques can confine and manipulate small particles in space. Metallic nanorods trapped in two dimensions against a cover glass can receive enough angular momentum from circularly polarised light to rotate with frequencies of several tens of kilohertz. In the first paper of this thesis, the photothermal effects associated with such optical rotations are studied to observe elevated thermal environments and morphological changes to the nanorod. Moreover, to elucidate upon the interactions between the trapped particle and the nearby glass surface, in the thesis' second paper a study is conducted to quantify the separation distance between the two under different trapping conditions. The particle is found to be confined ~30-90 nm away from the surface.

The momentum exchange from a single nanoparticle to a light beam is negligible. However, by tailoring the response of an array of nanoparticles, phase-gradient metasurfaces can be constructed that collectively and controllably alter the incoming light's momentum in a macroscopically significant way, potentially enabling a paradigm shift to flat optical components. In the thesis' third paper, a novel fabrication technique to build such metasurfaces in a patternable polymer resist is investigated. The technique is shown to produce efficient, large-scale, potentially flexible, substrate-independent flat optical devices with reduced fabricational complexity, required time, and cost.

At present, optical metasurfaces are commonly viewed as stationary objects that manipulate light just like common optical components, but do not themselves react to the light's changed momentum. In the last paper of this thesis, it is realised that this is an overlooked potential source of optical force and torque. By incorporating a beam-steering metasurface into a microparticle, a new type of nanoscopic robot – a metavehicle – is invented. Its propulsion and steering are based on metasurface-induced optical momentum transfer and the metavehicle is shown to be driven in complex shapes even while transporting microscopic cargo.