Linearly polarized light can exert a torque on a birefringent object when passing through it. This phenomena, present in Maxwells equations, was revealed by Poynting and beautifully demonstrated in the pioneer experiments of Beth and Holbourn. Modern uses of this effect lie at the heart of optomechanics with angular momentum exchange between light and matter. A milestone of controlling movable massive objects with light is the reduction of their mechanical fluctuations, namely cooling. Optomechanical cooling has been implemented through linear momentum transfer of the electromagnetic field in a variety of systems, but remains unseen for angular momentum transfer to rotating objects. We present the first observation of cooling in a rotational optomechanical system. Particularly, we reduce the thermal noise of the torsional modes of a birefringent optical nanofiber, with resonant frequencies near 200 kHz and a Q-factor above $mathbf{2times10^4}$. Nanofibers are centimeter long, sub-micrometer diameter optical fibers that confine propagating light, reaching extremely large intensities, hence enhancing optomechanical effects. The nanofiber is driven by a propagating linearly polarized laser beam. We use polarimetry of a weak optical probe propagating through the nanofiber as a proxy to measure the torsional response of the system. Depending on the polarization of the drive, we can observe both reduction and enhancement of the thermal noise of many torsional modes, with noise reductions beyond a factor of two. The observed effect opens a door to manipulate the torsional motion of suspended optical waveguides in general, expanding the field of rotational optomechanics, and possibly exploiting its quantum nature for precision measurements in mesoscopic systems.
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