  A photonic force microscope comprises of an optically trapped micro-probe and
a position detection system to track the motion of the probe. Signal collection
for motion detection is often carried out using the backscattered light off the
probe - however, this mode has problems of low S/N due to the small
back-scattering cross-sections of the micro-probes typically used. The position
sensors often used in these cases are quadrant photodetectors. To ensure
maximum sensitivity of such detectors, it would help if the detector size
matched with the detection beam radius after the condenser lens (which for
backscattered detection would be the trapping objective itself). To suit this
condition, we have used a miniature displacement sensor whose dimensions makes
it ideal to work with 1:1 images of micron-sized trapped probes in the
back-scattering detection mode. The detector is based on the quadrant photo-IC
in the optical pick-up head of a compact disc player. Using this detector, we
measured absolute displacements of an optically trapped 1.1 um probe with a
resolution of ~10 nm for a bandwidth of 10 Hz at 95% significance without any
sample or laser stabilization. We characterized our optical trap for different
sized probes by measuring the power spectrum for each probe to 1% accuracy, and
found that for 1.1 um diameter probes, the noise in our position measurement
matched the thermal resolution limit for averaging times up to 10 ms. We also
achieved a linear response range of around 385 nm with crosstalk between axes
~4% for 1.1 um diameter probes. The detector has extremely high bandwidth (few
MHz) and low optical power threshold - other factors that can lead to it's
widespread use in photonic force microscopy.
