Plasmonic collimators could disable the use of optical systems for semiconductor lasers

Jul 28, 2008 07:46 GMT  ·  By

We often think of lasers as sharp, highly focused beams of light, but the truth is that most of the time lasers have high divergences, spreading light into beams similar to those produced by flashlights. This is especially true for semiconductor lasers, such as those used in building CD-ROM devices and in fiber-optics, which usually require complex optic systems in order to bring the beam into a usable form.

A team of researchers led by Frederico Capasso and graduate student Nanfang Yu of the Harvard University has now created a new highly directional semiconductor laser by placing a device known as plasmonic collimator in front of the light source. The device is able to reduce the divergence of the laser beam by 25 times, thus disabling the necessity for other optical systems.

The new plasmonic collimator stands only 200-micrometers tall and is created out of a series of metallic surfaces, having on top a horizontal slit only 2 micrometers tall, placed directly over the laser source. The size of the slit is precisely calibrated so that half of the beam passing through it is diffracted to the metal surfaces.

When this happens the laser beam is forced into forming surface plasmons, meaning that part of the energy of the light is used to create a wave of energy rippling through the electrons of the collimator, while the rest passes through the slit without encountering any resistance. As the surface plasmons travel through the collimator, horizontal grooves in the surface convert them back into light. This enables the collimator to lose little energy as the beam is focused.

As a result, the beam of light exiting the collimator is highly focused vertically, which gives it a divergence of only 2.4 degrees in the vertical plane, as opposed to typical laser systems available today that have divergences as high as 60 degrees. The next step, the research team said, is to create a collimator that can produce a focused laser beam in both directions.

"This is a first important step towards full beam collimation. We have preliminary results showing that this can be done by using semicircular grooves," said Nanfang Yu.