Rotation Detection in Light-Driven

he ability of light to exert a torque on macroscopic objects has been known since the experiments of Beth in 1936,1 who demonstrated that po-larized light transfers angular momentum to a birefringent wave plate. Since the de-velopment of optical tweezers2 for manipu-lation and control of materials at the micro-and nanoscale, the need to produce con-trolled rotations in addition to translations has led to a number of different techniques for rotating microparticles. These include methods that rely on properties such as par-ticle birefringence, as in Beth’s original ex-periment,3 form birefringence of anisotropi