Highly Focused Conical Optical Field for Pico-Newton Scale Force Sensing

We propose to use a highly focused conical optical field generated by cone-frustum-shaped annular core fiber (ACF) tip for Pico-Newton scale force sensing. And we theoretically calculate the wave propagation in the ACF tip and its surrounding medium based on angular spectrum representation. The propagation properties and focusing effect of the highly focused conical optical field are also discussed and demonstrated experimentally. The ACF tip provides very strong focusing effect for the guided mode in the ACF. We show that there are stable three-dimensional trap-points for Rayleigh particles in the main lobe and every ring of the focused field with Bessel-like profile. And the transverse trapping forces are much greater (almost 30 times) than their corresponding longitudinal radiation forces so that a particle trapped by highly focused conical beam is only sensitive for disturbing force in longitudinal direction. Force sensitivities in longitudinal direction and the detection range are also discussed.