Active Q control in tuning-fork-based atomic force microscopy

The authors present comprehensive theoretical analysis and experimental realization of active Q control for the self-oscillating quartz tuning fork (TF). It is shown that the quality factor Q can be increased (decreased) by adding the signal of any phase lag, with respect to the drive signal, in the range of theta(1) to theta(1)+pi (theta(1)+pi to theta(1)+2 pi), where theta(1) is the characteristic constant of TF. Experimentally, the nominal Q value of 4.7x10(3) is decreased to 1.8x10(3) or increased to 5.0x10(4) in ambient condition, where the minimum detectable force is estimated to be 4.9x10(-14) N at 1 Hz. The novel Q control scheme demonstrated in the widely used quartz TF is expected to contribute much to scanning probe microscopy of, in particular, soft and biological materials. (C) 2007 American Institute of Physics.