Real Time Sensing of One Dimensional Nanostring Vibrations

Micromechanical resonators host localized vibrations which make them useful tools for sensing and exploring the physics of small-scale systems. Devices such as silicon nitride cantilevers or suspended carbon nanotubes can be used as sensing devices through measurements of how their resonant frequencies change due to local perturbations. The small size of these resonators enables extremely precise resolution of masses and forces. While reducing the size of mechanical resonators conceptually improves sensitivity, when a resonator is shrunk down to the molecular scale, a much more complex regime emerges. Polymers, for instance, are known to be highly entropic: coupling to their environment causes large thermal fluctuations among their many flexural modes. The dynamics of such systems are well understood in highly damped environments such as biopolymers inside a living cell. However, in underdamped environments the dynamics are largely unexplored.<br/>In this study, single molecules of DNA are suspended between SU-8 nanopillars with superhydrophobic surfaces that clamp the DNA nanostrings to both ends. The vibrations are measured by using a scannable photonic crystal cavity coupled to a tapered optical fiber. When the cavity is positioned close to the DNA, the near field coupling between the cavity and resonator allows for real-time measurement of the flexural dynamics of the nanostring via optical readout. These measurements are conducted at room temperature and can be used to further study the complex behavior of the nanostring's flexural modes such as thermalization, nonlinear coupling, and chaotic behavior. Looking forward, resonators made from DNA can be programmed to have arbitrary structures to tune their phononic character for added functionality and to probe the physics of nanostrings.