Optical frequency combs have been around long enough for the Nobel prize awarded for their development to be over a decade old. Mechanical analogues of phononic frequency combs, on the other hand, were only first theorized in 2014. After experimentally demonstrating the bulk vibrational phenomenon via coupling of two modes, authors now report in Applied Physics Letters the demonstration of three-mode four-wave mixing and therefore the clear possibility of multimode phononic frequency combs.
While the photonic and phononic spectral properties of combs share similar basic principles, generating phononic modes involves different fundamental dynamics. Without the intensity-focusing optical Kerr effect to generate nonlinear excitations, parametric excitation of phonon modes results from the driving frequency’s intrinsic coupling to phonon eigenfrequencies that then, in turn, produce higher order terms. In this experiment, the two coupled mechanical modes are lower in frequency than that of the driving mode, but the theoretical description underlying this realization also allows for parametrically exciting higher frequency modes.
Measurements of a suspended aluminum nitride-on-silicon device, made with conventional micro/nano fabrication processes, enabled discovery of the three-mode four-wave mixing process. Electrodes patterned on the device facilitated sinusoidal driving in the RF regime and measurements of the resulting response. The authors also noted surprising behavior in individual mode values as drive levels increased, despite always summing to the single-valued driving frequency. Though there wasn’t a discernible function, this could have been due to nonlinear feedback.
Frequency combs, especially multimode and broadband manifestations, are useful tools for metrology and frequency mapping applications. Mechanical combs also offer a unique set of sensing applications with potential for intrinsic sensing of slow varying processes or environmental changes. The authors look to broaden their investigational spectrum to multiple higher mode frequencies in further studies.
Source: “Phononic frequency comb via three-mode parametric resonance,” by Adarsh Ganesan, Cuong Do, and Ashwin Seshia, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.5003133.
