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A photonic force microscope comprises of an optically trapped micro-probe anda position detection system to track the motion of the probe. Signal collectionfor motion detection is often carried out using the backscattered light off theprobe - however, this mode has problems of low S/N due to the smallback-scattering cross-sections of the micro-probes typically used. The positionsensors often used in these cases are quadrant photodetectors. To ensuremaximum sensitivity of such detectors, it would help if the detector sizematched with the detection beam radius after the condenser lens (which forbackscattered detection would be the trapping objective itself). To suit thiscondition, we have used a miniature displacement sensor whose dimensions makesit ideal to work with 1:1 images of micron-sized trapped probes in theback-scattering detection mode. The detector is based on the quadrant photo-ICin the optical pick-up head of a compact disc player. Using this detector, wemeasured absolute displacements of an optically trapped 1.1 um probe with aresolution of ~10 nm for a bandwidth of 10 Hz at 95% significance without anysample or laser stabilization. We characterized our optical trap for differentsized probes by measuring the power spectrum for each probe to 1% accuracy, andfound that for 1.1 um diameter probes, the noise in our position measurementmatched the thermal resolution limit for averaging times up to 10 ms. We alsoachieved 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 (fewMHz) and low optical power threshold - other factors that can lead to it'swidespread use in photonic force microscopy.

Measurement of probe displacement to the thermal resolution limit in photonic force microscopy using a miniature quadrant photodetector