Coherent mixing of mechanical excitations in nano-optomechanical structures (0908.1128v1)

Abstract: The combination of large per-photon optical force and small motional mass attainable in nanocavity optomechanical systems results in strong dynamical back-action between mechanical motion and the cavity light field. In this work we study the optical control of mechanical motion within two different nanocavity structures, a zipper nanobeam photonic crystal cavity and a double-microdisk whispering-gallery resonator. The strong optical gradient force within these cavities is shown to introduce signifcant optical rigidity into the structure, with the dressed mechanical states renormalized into optically-bright and optically-dark modes of motion. With the addition of internal mechanical coupling between mechanical modes, a form of optically-controlled mechanical transparency is demonstrated in analogy to electromagnetically induced transparency of three-level atomic media. Based upon these measurements, a proposal for coherently transferring RF/microwave signals between the optical field and a long-lived dark mechanical state is described.

• The paper reveals that optical gradient forces can induce coherent mixing and splitting of mechanical modes in nanocavity structures.
• The experimental approach uses a zipper nanobeam cavity and a double-microdisk resonator to achieve tunable frequency control, with the flapping mode shifting from 8.3 MHz to over 15 MHz.
• The theoretical framework, based on Hamiltonian models, predicts Fano-like resonances and EIT analog effects that pave the way for advances in optomechanical applications.

Coherent Mixing of Mechanical Excitations in Nano-Optomechanical Structures: A Comprehensive Overview

The paper of coherent mixing of mechanical excitations in nano-optomechanical systems (NOMS) presented in the discussed paper offers critical insights into the dynamic interactions between optical and mechanical domains at the nanoscale, particularly emphasizing the optical control over mechanical motion. Utilizing two distinct nanocavity structures—a zipper nanobeam photonic crystal cavity and a double-microdisk whispering-gallery resonator—the research zeroes in on the optical gradient force's capacity to induce significant optical rigidity and facilitate coherent mechanical mixing.

Mechanism and Experimental Approach

At the core of the paper is the investigation of how large optical gradient forces, achieved within these nanocavities, influence mechanical motion. The research delineates the resultant phenomena into inducement of optical rigidity, renormalization of mechanical modes, and optomechanical interference resembling phenomena such as electromagnetically induced transparency (EIT) observed in atomic systems.

The zipper nanobeam optomechanical cavity and the double-microdisk resonator serve as exemplary systems to demonstrate these effects. The zipper cavity, known for its strong optomechanical coupling, showcases how its mechanical modes can split into optically bright and dark modes with tuning of laser-cavity detuning and intra-cavity photon numbers. In parallel, the examination of the double-disk resonator elucidates the intricate dynamic between 'flapping' and 'breathing' mechanical modes, characterized by frequency modulation enabled through optical spring effects.

Numerical Findings and Theoretical Implications

The experimental methodology, employing optical fiber nanoprobe coupling and analyzing radio-frequency (RF) transmission spectra, reveals significant results. Frequency tunability of mechanical modes reaches impressive ranges, directly correspondent to the modulation of intra-cavity forces. For instance, the optical spring effect in the double-disk system allows the 'flapping' mode to be adjusted from 8.3 MHz to beyond 15 MHz, a formidable numerical indicator of the system's controllability.

A theoretical framework, utilizing Hamiltonians to model mechanical interactions, underscores the mechanisms at play. These integrate internal mechanical coupling within the modes, enhancing the prospect of Fano-like and EIT analog interference. Theoretical predictions align rigorously with experimental observations, notably in the manifestation of Fano-type resonances and mechanical modes exhibiting dressed states under optical influences.

Implications and Future Direction

The findings suggest substantive implications for the manipulation and control of mechanical and optical states within such systems. The demonstrated capability of optical tuning of mechanical transparency paves the way for significant advances in classical and potentially quantum information processing, foretelling possible applications in optical buffering, signal transfer, and even photonic-phononic state conversion.

While the experiments predominantly focus on thermal excitation, the paper hints at the greater potential of controlled optical excitation to extend these dynamics. This perspective aligns with the advances in RF/microwave photonics, proposing novel methods for coherent information storage and retrieval akin to EIT systems, but within the NOMS framework.

Future work could further explore optimizing the structural parameters and material choices to enhance mechanical Q-factors and further reduce optical losses. Additionally, transitioning these findings from a classical regime to a quantum mechanical one, especially in the context of cooling to ground states and single-phonon operations, constitutes a promising trajectory.

In conclusion, the paper presents a comprehensive paper of coherent mixing induced by optical forces within nanoscale mechanical systems, enriching the understanding of optomechanical interactions and laying the groundwork for innovative applications in integrated photonics and beyond.