Optical angular-momentum transfer to trapped absorbing particles

Particle rotation resulting from the absorption of light carrying angular momentum has been measured. When absorbing CuO particles (1 - 5 mu m) were trapped in a focused ''donut'' laser beam, they rotated, due to the helical phase structure of the beam. Changing the polarization of the light from plane to circular caused the rotation frequency to increase or decrease, depending on the sense of the polarization with respect to the helicity of the beam. Rotation frequencies were obtained by Fourier analysis of amplitude fluctuations in the backscattered light from the particles.