An ultra-sensitive opto-mechanical force sensor has been built and tested in the optics laboratory at INFN Trieste. Its application to experiments in the Dark Energy sector, such as those for Chameleon-type WISPs, is particularly attractive, as it enables a search for their direct coupling to matter. We present here the main characteristics and the absolute force calibration of the KWISP (Kinetic WISP detection) sensor. It is based on a thin Si3N4 micro-membrane placed inside a Fabry-Perot optical cavity. By monitoring the cavity characteristic frequencies it is possible to detect the tiny membrane displacements caused by an applied force. Far from the mechanical resonant frequency of the membrane, the measured force sensitivity is 5.0e-14 N/sqrt(Hz), corresponding to a displacement sensitivity of 2.5e-15 m/sqrt(Hz), while near resonance the sensitivity is 1.5e-14 N/sqrt(Hz), reaching the estimated thermal limit, or, in terms of displacement, 7.5e-16 N/sqrt(Hz). These displacement sensitivities are comparable to those that can be achieved by large interferometric gravitational wave detectors.
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