Resumen de Interacció entre la llum i el moviment en nanoestructures desordenades
The interaction of light trapped in an optical cavity and motional degrees of freedom in cavity optomechanical systems has emerged as a mechanism enabling both fundamental research in mesoscopic quantum physics and high-performance microscale devices for applications such as sensing or optical signal processing. The dynamics of such systems are reduced to a small set of governing parameters that can be engineered by design. Nevertheless, control over these parameters becomes challenging in nanometer-scale structures like optomechanical crystal cavities due to unavoidable fabrication imperfections. This imposes severe limits in state-of-the-art systems and disorder is seen as a nuisance. In this thesis, we propose instead to harness its potential. In a disordered lattice, the interplay between order and disorder in multiple scattering offers an alternative route to confine light, i.e., Anderson localization, a phenomenon well known for electrons in solid-state physics. In principle, the same phenomenon happens for elastic waves (phonons), leading to tightly localized mechanical modes. However, direct observation of Anderson localization of phonons in the GHz range remains elusive, due to the lack of practical phonon transitions in the solid state and limited far-field radiation for read-out. Can we use disorder-induced optical cavities to locally probe Anderson localization of GHz mechanical vibrations via their optomechanical interaction? What is the likelihood to find spatially co-localized photons and phonons? Are these two waves equally sensitive to fabrication imperfection? Can we manipulate, via light, the mechanical degrees of freedom in such a system? These scientific questions articulate this thesis.
In order to answer these, two main requirements are identified. The first is exploring high quality factor (Q) optical cavities, since the transduction of mechanical motion scales with it. The second is the level of overlap between the localized fields, i.e. the statistical level of co-localization, since acoustic and optical modes appear at uncorrelated positions due to their complex interference nature. The first requirement is achieved in both standard and slotted slow-light photonic crystal waveguides, where we observe high-Q ( up to 100000) optical Anderson localization. In particular, one of the designs simultaneously operates as a phononic waveguide. We demonstrate transduction of thermally-activated motion via Anderson-localized optical modes in slotted photonic crystal waveguides at two frequency ranges: low-frequency in-plane mechanical modes spanning 100-500 MHz and high-frequency ~7 GHz guided mechanical modes. At both frequency ends, the light field is used to amplify mechanical motion up to coherent self-sustained oscillations. At the 7 GHz band, the explored system constitutes a perfect platform to observe high-frequency phonon localization phenomena. However, these two-dimensional optomechanical crystal waveguides lack any a priori mechanism that guarantees a high degree of co-localization. To circumvent this issue we propose using periodic-on-average one-dimensional GaAs/AlAs Distributed Bragg Reflectors. A statistical enhancement of the vacuum optomechanical coupling rate, g, is found, making this system a promising candidate to explore Anderson localization of even higher frequency (~20 GHz) phonons using ultra-fast pump-probe coherent phonon spectroscopy. We use this experimental technique to all-optically probe a spacer-less phononic nanocavity created by concatenating two perfectly periodic multilayers, i.e., a 0D topological state, a testbed to understand the most basic implications of bulk topology on interfaces. Last, we explore their propagating counterpart, topological interface waveguides and quantify their potential for robust backscattering-free photon transport at the nanoscale, a premise for compact and efficient circuit and cavity optomechanics based on topological edge states.
