Investigating the transfer of orbital angular momentum of light to micrometer-sized helical structures

Light with orbital angular momentum has a helical phase front. The number of pitches that the Poynting vector makes per wavelength determines the angular momentum number ℓ. Light with ℓ=1 has 1 helical turn (pitch) per wavelength. If the beam diameter and the wavelength are each ~1µm in size, then the pitch angle for ℓ=1 is ~45º. A µ-sized helical structure held in optical tweezers usually starts to rotate due to linear momentum transfer [1,2]. If its pitch angle is similar to the one of the tweezing light, its rotation should depend on the helicity of the light’s orbital angular momentum. When the structure and the light have the same helicity (looking toward the light source) the rotation should stop or slow down, when they have opposite helicity the rotation should speed up. To investigate this effect, we tweeze helical bacteria with light of ℓ=-1,0 and+1 and observe their rotational speed. [1] J. R. Robbins, D. A. Tierney, H. Schmitzer, Applied Physics Letters, 88, 023901, 2006 [2] H. Schmitzer, M. Henstridge, D. Engle, W. Dultz, D. Tierney, Proceedings to the 110th Spring meeting of the German Society of Applied Optics in Brescia, June 2009