Parametric amplification in nanomechanical structures is demonstrated by modulating a purely intrinsic mechanical parameter of the system-the stress-via piezoelectric electromechanical coupling. Large resonance amplitude and quality factor enhancement due to parametric pumping are observed under both vacuum and ambient pressure conditions. Exploration of the region of parametric instability yields results that agree with parametric amplification theory. © 2010 American Institute of Physics. ͓doi:10.1063/1.3505500͔Recent technological advances have enabled the fabrication of mechanical resonators down to the nanometer scale.
1Resonant nanoelectromechanical systems ͑NEMS͒ possess such properties as extremely high frequencies, 2,3 high mechanical responsivity, large quality factors, and operability at low required power, making NEMS resonators promising in a variety of applications ranging from mass and force sensing 4-6 to quantum limited measurements. 7 However, as electromechanical systems are miniaturized, readout of mechanical motion becomes increasingly difficult, as most forms of electrical transduction signals scale with device size. One potential solution is amplification of the signal in the mechanical domain using parametric effect before applying electrical transduction.Parametric resonance is of interest to many areas of research ranging from quantum optics 8 to plasma physics.
9Since the initial demonstration of a resonant mechanical parametric system by Rugar and Grütter, 10 it has attracted significant attention.11-14 The simplest parametric amplification scheme involves periodic modulation of the spring constant of the system at twice its fundamental resonance frequency. In previous manifestations of mechanical parametric amplification, the nature of the spring constant modulation was usually capacitive, 11,12 although other methods exploited intrinsic residual stress, 14 Lorentz force, 15 or an external feedback loop.16 Direct parametric tuning of the tensile stress using piezoelectric electromechanical coupling was introduced earlier in large scale microelectromechanical resonator operating at 140 kHz.17 Here, we demonstrate piezoelectric parametric amplification and actuation in high frequency and very-high frequency, submicron-scale NEMS resonators.Modulation of the stress of a nanomechanical doubly clamped beam results in resonance frequency change:for length L, Young's modulus E, density , dimension in the direction of vibration t h , and stress . The displacement of the central point of the beam is modeled as harmonic oscillator of effective mass m, spring constant k 1 , and quality factor Q with Duffing 19 nonlinearity we obtain the following equation of motion:where k p = 0.3k 1 L 2 / ͑Et h 2 ͒ is the spring constant modulation amplitude, and F͑t͒ = F 0 cos͑ D t + ͒ is a driving force. The parametric modulation is performed at twice the resonance frequency, leading to a vibrational amplitude gain given by:where k t is the threshold parametric pumping force 2k 1 / Q. The second term in this...