Magnetic force and optical force sensing with ultrathin silicon resonator

Abstract

In this article, we demonstrated magnetic and optical force measurements using an ultrathin silicon cantilever down to 20 nm or 50 nm in thickness. The cantilever was heated in an ultrahigh vacuum for enhancing the Q factor and a magnetic particle was mounted at the end of the cantilever using a manipulator. The vibration was measured by a laser Doppler vibrometer and its signal was fed to an opposed metal electrode for electrostatic self-oscillation. An application of a magnetic field with a coil exerted a force to the magnetic material, which results in the change of the resonant frequency. However, the change in the mechanical properties of the cantilever, due to mechanical instability and temperature variation, drifts the resonance peak. Force balancing between the magnetic force and an electrostatic force in the opposite phase can minimize the vibration amplitude. From the electrostatic force at the minimum point, the exerted force can be estimated. A magnetic moment of 4×10−20 J/T was measured by this method. The same technique was also applied to measure the optical force of ∼10−17 N, impinging on the cantilever by a laser diode.