Laser beams have an intrinsic spread. Normally this spread is overcome using lenses or curved mirrors to focus or collimate the light, but this typically involves meticulous optical alignment. Researchers, reporting online this week in Nature Photonics, have shown that it is possible to reduce this beam spread using surface plasmons — fluctuations in the electron density, which can be excited by light, at the boundary of two materials.
Nanfang Yu and colleagues use surface plasmons to shape the beams produced by quantum-cascade lasers — semiconductor lasers that emit light in the mid- to far-infrared. They etch a metallic slit and grooves onto the front of the laser. These structures allow the laser light to be efficiently coupled to surface plasmon waves and then scattered into a very narrow beam of light.
To make this work, the team has to optimize several properties of the slit–groove structure, including the width of the slit and the width and depth of each groove. The resulting laser beams have 25 times less spread, with an angle of about 2.4 degrees. A variety of beams could be engineered in this way, offering a more efficient way of coupling light into optical fibres.
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