Can a single nanomechanical mode generate a frequency comb?

Doubly-clamped nanostring resonators excel as high Q nanomechanical systems enabling room temperature quality factors of several 100,000 in the 10 MHz eigenfrequency range. Dielectric transduction via electrically induced gradient fields provides an integrated control scheme while retaining the large mechanical quality factor [1]. Dielectrically controlled nanostrings are an ideal testbed to explore a variety of dynamical phenomena ranging from multimode coupling to coherent control [2].

Here I will focus on the nonlinear dynamics of a single, resonantly driven mode. The broken time reversal symmetry gives rise to the squeezing of the string’s fluctuations. As a result of the high mechanical Q factor, the squeezing ratio is directly accessible from a spectral measurement [3]. It is encoded in the intensities of the two spectral peaks arising from the slow dynamics of the system in the rotating frame.

For stronger driving, an onset of self-sustained oscillation is observed which leads to the generation of a nanomechanical frequency comb. The effect is a consequence of a resonantly induced negative effective friction force induced by the drive. This is the first observation of a frequency comb arising solely from a single mode and a single, resonant drive tone [4].

References
[1] Q. P. Unterreithmeier et al., Universal transduction scheme for nanomechanical systems based on dielectric forces, Nature 458, 1001 (2009)

[2] T. Faust et al., Coherent control of a classical nanomechanical two-level system, Nature Physics 9, 485 (2013)

[3] J. S. Huber et al., Spectral Evidence of Squeezing of a Weakly Damped Driven Nanomechanical Mode, Phys. Rev. X 10, 021066 (2020)

[4] J. S. Ochs et al., Frequency Comb from a Single Driven Nonlinear Nanomechanical Mode, Phys. Rev. X 12, 041019 (2022)