The fact that light carries both linear and angular momentum is well-known to physicists. One application of the linear momentum of light is for optical tweezers, in which the refraction of a laser beam through a particle provides a reaction force that draws the particle towards the centre of the beam. The angular momentum of light can also be transfered to particles, causing them to spin. In fact, the angular momentum of light has two components that act through different mechanisms on various types of particle. This Review covers the creation of such beams and how their unusual intensity, polarization and phase structure has been put to use in the field of optical manipulation.

Tweezers with a twist

http://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/ufk_papers/magnetic_tweezers/croquette.pdf

Magnetic Tweezers: Micromanipulation and Force Measurement at the Molecular Level

Some 16 years ago, Allen et al. [Phys. Rev. A 45, 8185 (1992)] recognised that laser beams which carried an angular momentum additional to photon spin, could be realized in the laboratory. Such beams have helical phase fronts and so have an azimuthal component to the Poynting vector, which results in angular momentum along the beam axis. This orbital angular momentum, very often combined with spin to make optical angular momentum, has given rise to many developments. These range from optical spanners for driving micro-machines to high dimensional quantum entanglement and new opportunities in quantum information processing. The concept of orbital angular momentum is now leading to new understanding of a wide range of phenomena, including fundamental processes in Bose-Einstein condensates, while the associated technologies have led to new applications in optical tweezing and microscopy.

Advances in optical angular momentum

Optical tweezers were first realized 15 years ago by Arthur Ashkin and co-workers at the Bell Telephone Laboratories. Since that time there has been a steady stream of developments and applications, particularly in the biological field. In the last 5 years the flow of work using optical tweezers has increased significantly, and it seems as if they are set to become a mainstream tool within biological and nanotechnological fields. In this article we seek to explain the underpinning mechanism behind optical tweezers, to review the main applications of optical tweezers to date, to present some recent technological advances and to speculate on future applications within both biological and non-biological fields.

http://people.physics.illinois.edu/selvin/prs/498ibr/tweezers.pdf

Lights, action: optical tweezers

Laser beams containing higher-order phase singularities can be produced with high efficiency computer generated holograms made with very simple equipment. Using such holograms in an optical tweezers experiment we have successfully trapped reflective and absorptive particles in the dark central spot of a focused charge 3 singularity beam. Angular momentum absorbed from the beam can set particles into rotation.

Optical Particle Trapping with Higher-order Doughnut Beams Produced Using High Efficiency Computer Generated Holograms

A single-beam optical trapping of microscopic particles and biological cells is demonstrated by using higher-order mode Nd : YAG laser beams at 1.064 μm for the first known time. A stronger trapping force by a TEM01* beam than that by a TEM00 beam was observed and 360° rotation of a trapped red blood cell with a TEM0n beam was achieved.

M. Ishigure

Papers

OPtical Trapping And Rotational Manipulation Of Microscopic Particles And Biological Cells Using Higher-Order Mode Nd: Yag laser beams