Measuring static femto-Newton optical forces by a nanofiber with white-light interferometry

By now, it is still extremely challenging to accurately measure femto-Newton (10-15 N) optical forces. Here, a simple scheme of a suspended tapered nanofiber coupled with a glass substrate is demonstrated to generate and sense the fN optical force, where the ~10 nm deformation of the nanofiber can be resolved by white light interferometry and an ultra-low stiffness of 0.540 fN/nm is obtained for the suspended nanofiber through Brownian motion. Owing to the ultra-low stiffness of the nanofiber, the scheme provides a cost-effective method to sense and measure ultra-weak (5.2 fN) optical force. Moreover, the scheme also provides a simple way to exploit fN optical forces for implementation of micro-Watt all-optical devices.