Optical tweezers and optical spanners with Laguerre-Gaussian modes

We numerically model the axial trapping forces within optical tweezers arising from Laguerre-Gaussian laser modes. For an 8 mu m diameter sphere suspended in water, the higher-order modes produce an axial trapping force several times larger than that of the fundamental. Partial absorption results in a transfer of the orbital angular momentum from the Laguerre-Gaussian mode to the trapped particle. This results in the rotation of the particle by what may be called an optical spanner (wrench). For an absorption coefficient of alpha = 5700 m(-1) and a laser power of 10 mW, we find that an 8 mu m diameter sphere would acquire an angular acceleration of 10 x 10(4) rad s(-2) and a limiting angular velocity of 0.2 rad s(-1).