Spin-to-orbital angular momentum conversion in a strongly focused optical beam

As a fundamental property of light, the angular momentum of photons has been of great interest. Here, we demonstrate that optical spin-to-orbital angular momentum conversion can occur in a homogeneous and isotropic medium. This Letter presents both theoretical and experimental studies of this conversion in a tightly focused beam and shows that the orbital rotation speeds of trapped particles are altered because of this conversion as predicted by theory.