Very high frequency probes for atomic force microscopy with silicon optomechanics

Abstract

Atomic force microscopy (AFM) has been constantly supporting nanosciences and nanotechnologies for over 30 years, being present in many fields from condensed matter physics to biology. It enables measuring very weak forces at the nanoscale, thus elucidating interactions at play in fundamental processes. Here we leverage the combined benefits of micro/nanoelectromechanical systems and cavity optomechanics to fabricate a sensor for dynamic mode AFM at a frequency above 100 MHz. This is two decades above the fastest commercial AFM probes, suggesting opportunity for measuring forces at timescales unexplored so far. The fabrication is achieved using very-large scale integration technologies inherited from photonic silicon circuits. The probe's ring optomechanical cavity is coupled to a 1.55 um laser light and features a 130 MHz mechanical resonance mode with a quality factor of 900 in air. A limit of detection in displacement of 3.10-16 m/sqrt(Hz) is obtained, enabling the detection of the Brownian motion of the probe and paving the way for force sensing experiments in the dynamic mode with a working vibration amplitude in the picometer range. Inserted in a custom AFM instrument embodiment, this optomechanical sensor demonstrates the capacity to perform force-distance measurements and to maintain a constant interaction strength between tip and sample, an essential requirement for AFM applications. Experiments show indeed a stable closed-loop operation with a setpoint of 4 nN/nm for an unprecedented sub-picometer vibration amplitude, where the tip-sample interaction is mediated by a stretched water meniscus.