Opto-mechanical interactions in nanowire waveguides

Abstract

Controlling light at the nanoscale is a current research frontier, as advanced nanofabrication techniques allow fabrication of complex nanophotonic structures. Such structures offer a path to integrable on-chip applications including signal processing, switching and routing. In specially designed flexible nanocircuits, the force exerted by light can significantly deform the structure, and in turn, change the optical response of the structure accordingly, resulting in opto-mechanical interactions. These interactions present new possibilities to achieve novel optical functionalities in tunable nanophotonic devices. This thesis formulates and discusses several approaches for tailoring the opto-mechanical interactions in nanowire waveguides, which can be suspended in air, enabling flexible structure tuning by optical forces. Chapter 1 introduces and reviews the field of opto-mechanics and the current state-of-the-art with a particular focus on opto-mechanical interactions in nanophotonic structures. I also introduce the key physical concepts and theoretical framework underpinning the following chapters. In Chapter 2 I develop approaches for controlling optically induced forces between side-coupled waveguide nanocavities by introducing a longitudinal shift. The analysis predicts that optical cavity enhanced transverse forces can be tuned from repulsive to attractive, or even suppressed for particular longitudinal shifts. Additionally, the shift induces longitudinal forces on the waveguides, in contrast to unshifted structures. In chapter 3 I show that a similar approach can control forces between photonic-crystal nanowires in the slow-light regime. In this case, the increased optical energy density of waveguide modes close to the photonic band edge can significantly enhance the optical forces. I also compare the relative benefits of slow-light and cavity structures for opto-mechanical applications. Chapter 4 presents a theoretical analysis of nonlinear dynamics associated with opto-mechanical self-action. Novel nonlinear opto-mechanical interactions are revealed in coupled suspended nanocavities that are driven by two detuned laser frequencies. Such driving enables simultaneous excitation of odd and even optical supermodes, which induce gradient forces of opposite signs. Competition between these forces produces opto-mechanical potentials with large barriers and narrow wells. These types of potentials were suggested for precise displacement control in the static regime. However I find that self-induced oscillations appear even at the deep global potential minima for mechanical damping rates below a certain threshold, and I identify a new regime of chaotic switching between mechanical deformations of opposite signs. Chapter 5 presents experimental results for suspended chalcogenide (Ge11.5As24Se64.5) waveguide nanocavities designed to experience an attractive optical force due to substrate coupling. This chalcogenide material features strong Kerr nonliearity and low two-photon absorption, which is attractive for fast nonlinear signal processing combined with opto-mechanical tunability. Optical cavities of quality factor {u0303}104 were fabricated and comprehensively characterized, revealing a strong thermo-optic response. Experimental measurements and theoretical simulations reveal that a Fabry-Perot cavity formed by the waveguide end facets strongly modifies the optical bistability, suggesting new possibilities for engineering the thermo-optic response of nanostructures. Theoretical modelling predicts that such structures could be suitable for opto-mechanical experiments when placed in a vacuum chamber to avoid air damping of mechanical oscillations. I present conclusions and discuss future directions in the final chapter 6.