Plasmons put laser light on the straight and narrow
By: Shane Ennerson
A semiconductor laser that has been engineered to emit a narrow beam of light without the use of additional lenses could help to reduce the cost of optical systems.
Plasmonic collimatorResearchers in the US and Japan have devised a simple way to generate a nearly parallel beam of light from a semiconductor laser — without the need for bulky and expensive lenses. Instead, a patterned metallic film is used to absorb divergent light from the laser and reemits it in one direction. The team says that the technique — which relies on collective electronic excitations called plasmons — could make semiconductor lasers cheaper, smaller and more efficient.
Although semiconductor lasers are very small (some are micrometer-sized) and can be integrated within electronic devices, the light-emitting region of the laser is about the same size as the wavelength of the laser light. This means that the light emitted from the laser is diffracted, often by as much as several tens of degrees.
As most laser applications require a collimated beam with a much smaller divergence, the light is usually collimated by placing a high-quality lens with a large collection angle (or numerical aperture) at the laser output. Unfortunately, that makes semiconductors more expensive and relatively bulky. Now, however, Nanfang Yu, Federico Capasso and colleagues at Harvard University in the US, along with researchers at the optical-equipment maker Hamamatsu Photonics, have collimated laser light by placing a thin, patterned metal film on the output facet of a semiconductor laser.
Parallel grooves
The film has an aperture slit, which is about 2 µm wide and is adjacent to a series of parallel grooves that are 0.8 µm wide, 1.5 µm deep and separated by 8.9 µm. The collimator was fabricated on the surface of a semiconductor laser that emits infrared light at a wavelength of 9.9 µm.
Nanfang Yu and Federico CapassoSome of the laser light travelling through the aperture is absorbed, creating surface plasmons — collective excitations involving large numbers of electrons on the surface of the metal film. As the plasmons propagate across the film, they are scattered by the grooves, which results in the plasmons being converted back into light with the same wavelength as the laser.
The size of the aperture and grooves are chosen so that the reemitted light undergoes constructive interference to form a beam of light that propagates outward and perpendicular to the laser facet.
The divergence of the plasmon-collimated laser was about 2.4o, whereas the divergence of the same laser before being fitted with the collimator was about 63o.
New dimensions
Although this collimator only works in one dimension, Yu says that the team are now working on a 2D design involving a circular aperture surrounded by concentric rings of grooves. "Preliminary results in our group have shown that this scheme works very well: a divergence of a few degrees in the horizontal and vertical planes has been achieved in a quantum-cascade laser," said Yu.
Freelance writer working for Dragonlasers at http://www.dragonlasers.com
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