Controlling nanomaterial properties with the angular momentum of light

Abstract

The properties of nanomaterials can be tailored through structural and geometrical design, chemical functionalization, strain engineering and other techniques. However,many existing methods for property control are either irreversible or depend on complexphysical set-ups, limiting their practicality and broader technological implementation. This thesis addresses that challenge by developing a non-contact, all-optical approach that leverages the orbital angular momentum carried by Laguerre-Gaussian beams. The method is investigated for two different applications. In the first instance, the focus is given to orbital angular momentum transfer from Laguerre-Gaussian beams to two dimensional (2D) materials. A theoretical framework for calculating optical forces and torques in dielectric media is presented and angular momentum beams are implemented into a numerical simulation software to predict their effects on the 2D materials. Building on this, a novel, non-contact experimental method is developed to induce wrinkling in two common examples of 2D materials, monolayer graphene and WS2, using the optical torques of Laguerre-Gaussian beams. The out-of-plane deformations and property changes are characterized using various experimental techniques, including electrical conductance measurements, Raman spectroscopy, atomic force microscopy and photoluminescence. The method is reversible and spatially-selective and only limited by sample heterogeneity and monolayer-substrate interactions. In the second instance, the application of optical angular momentum is extended to chiral sensing. The dynamic control of the optical activity of chiral shuriken meta-materials is demonstrated through numerical simulations and experimental dichroism measurements under varying beam focusing conditions. Together, this thesis highlights the potential of angular momentum beams as a versatile tool for controlling nanomaterial properties with high spatial precision