Fabry Perot fiber interferometer for millikelvin AFM cantilever detection*

We describe the design and construction of a position detection system for pendulum geometry non-contact atomic force microscopy (nc-AFM) suitable for a millikelvin ultra-high vacuum environment. We use a single fiber-optic cable to create a Fabry-Perot interferometer to detect the oscillation frequency and amplitude of an inverted-geometry silicon cantilever. Our detector achieves sub-nanometer resolution while dissipating less than 250uW, which makes it suitable for operation in a dilution refrigerator. We use a compensating piezo to solve phase-slippage of the laser signal, enabling stable positional accuracy over the hours necessary to acquire an AFM image. Furthermore, the fiber-cantilever alignment can be remotely calibrated with our unique XYZ piezo walker that uses metal-ceramic sliding surfaces for more robust operation in ultra high vacuum and cryogenic temperatures.  Our entire detection system, including XYZ alignment walkers, occupies a minimal in-situ footprint of 24mm x 28mm x 52mm, located near the tip-sample junction.