All Optical Zepto-Newton-Meter Nanoscale Silk Sensor

We show a new class of optical silk torsion pendulum (TOP) with zepto-Newton-meter (zNm) sensitivity achieved by exploiting hitherto unknown ultrasoft twist response of tough silk fibers in ultrahigh vacuum. We demonstrate several macroscopic microgram TOPs using three different silk types showing 6 orders of magnitude wide range of sensitivity calibrated using up to zNm optical torques. Remarkably, a nanoscale diameter capture silk exhibits about 20  fNm/rad torsion constant with low damping and responds to sub-400 zNm impulses over many oscillation cycles. We have isolated sub-3.0±0.2  zNm/s torque velocity sensitivity with an integration time of the order of hours. We show systematic measurements of optical torsional damping induced by well-timed subcycle impulses as well as periodic on-off torquing down to 470 zNm amplitude, in quantitative agreement with model simulations. Our calculation suggests that a 100 nm diameter silk nanofibril can open a route to sub-yocto-Nm sensitive macroscopic TOP thereby making silk sensors attractive for real-world ultrasensitive metrology applications.