Minimum detectable displacement in near-field scanning optical microscopy

The probe-to-sample separation in near-field scanning optical microscopes can be regulated by a noncontact atomic shear force sensing scheme that allows simultaneous acquisition of optical and shear force images. We have measured the minimum detectable displacement that can be achieved with a scheme based on diffracting a focused laser beam from the vibrating probe. The minimum detectable displacement determines the smallest resolvable change in force acting on the probe. The measured shot-noise-limited value is 2.8×10 ^-3 Å_rms/Hz,and the practical sensitivity is limited by thermal vibration noise to 7×10^-3 Å_rms/Hz. These values compare well with those calculated theoretically.