Dielectric forces have been known to act on polarisable materials subject to electric field gradients for a long time. This phenomenon, that is well known even from the macroscopic world, since it allows e.g. a charged object to bend a thin water jet, is technologically employed to manipulate small particles. Examples range from optical tweezers, where the trapping force is controlled via the intensity of a laser beam, to dielectrophoresis, where electric fields are employed to control the motion of particles suspended in a liquid.

With extensive advances in micropatterning over the last years, the fundamental study on mechanical motion rushed towards micro- and nanoscale objects. Moreover, as these systems are very sensitive to their environment, they are increasingly explored and employed by engineers in applications ranging from sensing to signal processing. However, despite a strong research effort, the transduction of nanomechanical systems still remains a challenge, as has been recently pointed out in a paper by Masmanidis et al. (Science 317, 780 (2007). The quest consists in finding an actuation and detection scheme that is efficient, integrable, local and tunable, yet does not introduce dissipation or constrain the resonator material. Present devices are either actuated using local mechanisms such as magnetomotive, piezo-electrical, capacitive or electrothermal schemes or schemes relying on external drives. While the former are broadband, integrable and efficient, they strongly restrict the choice of resonator material, often requiring metal electrodes or other conducting elements leading to strong dissipation. The latter, in turn, do not impose material constraints, but are neither broadband nor fully scalable.

We have successfully adopted the simple concept of dielectric forces for the surprisingly efficient transduction of nanomechanical systems: An arbitrary polarisable nanomechanical resonator, in our case a low-dissipation silicon nitride beam, is polarized and driven with voltages applied to subjacent gate electrodes. This layout has been enabled by a novel fabrication strategy involving postprocessing of freely suspended devices to define the drive electrodes below the resonator plane. However, the underlying scheme can be universally applied to nearly all materials. Both actuation and detection based on dielectric forces can be performed. Even more, the scheme exceeds the possibilities of optically induced forces, e.g. in optical tweezers, by allowing control of polarization and force via two independent parameters. The polarization induced in the beam can be controlled by a DC voltage, while the actuating force is produced with an RF modulation.

Our most prominent findings include:

  • Dielectric actuation separates the driving scheme from the driven element, thus allowing for an independent optimization. Particularly, it enables efficient acuation of ultralow-dissipation resonators since our scheme couples to the static polarisation of an arbitrary mechanical element induced by an external field.
  • The scheme allows on-chip transduction: the signal is converted locally, thereby enabling signal processing. Even more, it is highly integrable and scalable to large arrays which makes it interesting for sensing applications.
  • For the chosen geometry, our scheme allows for voltage-tuning of the resonance by more than 1000 full width at half max (FWHM). This in turn enables parametric driving which is beneficial for an efficient transduction since it allows to actuate and to detect at different frequencies.
  • The presented scheme is extremely broadband and can be applied to the highest frequency nanomechanical resonators available today.
  • With RF excitation voltages down to the range of a few ┬ÁV, the actuation is extremely efficient. This is enabled by the independently controlled polarization of the resonator via the applied DC voltage which can be employed to generate a large dipole moment, resulting in a strongly increased responsivity.

[1]    Q. Unterreithmeier et al.,
        Universal transduction scheme for nanomechanical systems
        based on dielectric forces,
        Nature, 458, 1001 (2009) [doi:10.1038/nature07932 ].