Vortex look for lasers
Jul 12, 1999
Complex patterns of optical vortices have been observed in the output of semiconductor lasers by researchers at the Israel Institute of Technology, also known as the Technion, in Haifa. Jacob Scheuer and Meir Orenstein also observed that the patterns produced by their laser became more complicated as the current through the laser was increased (Science 285 230). Possible applications of the research include optical data storage, distribution and processing, and the laser cooling of particles.
Scheuer and Orenstein used so-called vertical cavity surface-emitting lasers. In these devices the light is emitted from the top of the active region of the laser, rather than from the edge as in conventional semiconductor lasers. This gives a two-dimensional output in which the patterns can form, rather than the one-dimensional output from conventional devices. The Israeli team used microcavity lasers in which the active region contained three 8 nanometre thick layers of indium gallium arsenide. The laser output had a diameter of 20 microns.
Vortex structures occur widely in science and nature - in gases, fluids, plasmas and DNA, for example. The optical vortices produced by Scheuer and Orenstein are the smallest and most complex spontaneously-produced vortices ever seen. In an optical vortex the phase of the electric field experiences a singularity at the centre of the beam, where the amplitude also goes to zero. The vortex has an order or a charge, which is the number of times the phase changes by a factor of 2p on the path around the vortex. When the current through the laser was increased above 16 milliamps, a complex figure-of-eight pattern consisting of two vortices with the same charge was created, followed by a four-vortex pattern at 17.2 mA. As the current was increased further, a radial symmetric pattern was formed, followed by arrays of three, five and seven vortices.