Beam sharpener produces laser 'knife'
By Colin Barras (Image: Nanfang Yu) You may think of lasers as needle-like crisp beams. But the semiconductor lasers used for everything from CDs to fibre-optic broadband are more like the fuzzy, spreading beams from a flashlight. Intricate arrangements of curved mirrors or lenses are needed to tame these into usable beams, especially if they need to travel long distances. A new device that fits to the front of the laser source makes the process much simpler and more efficient. It was developed by an international research team led by Federico Capasso at Harvard University and his graduate student Nanfang Yu. So far they can reduce the spread of a beam in one plane by 25 times. Instead of using lenses or mirrors, the team straps a 200-micrometer tall metallic structure, dubbed a plasmonic collimator, to the front of the laser source. At the top of the device is a horizontal slit, just 2 micrometers high. That slit is placed directly over the laser source. It is so narrow that around half the beam passing through is sharply diffracted towards the surface of the metal. As it hits the metal, the high-energy laser light turns into surface plasmons – waves of energy rippling through the electrons of the collimator’s surface. The remaining light travels through the slit unimpeded. Meanwhile, the surface plasmons travel down the face of the collimator. The plasmons encounter a series of small horizontal grooves in the collimator’s surface that jolt them back into light again. Recycling the surface plasmons like this means that very little energy is lost from the system. The distances between the slit and each groove are chosen to ensure that all the beams emerging from the collimator are synchronised, coming together to form one beam tightly focused in the vertical direction. While the beam divergence from a semiconductor laser is around 60° in all directions, Capasso’s team reduce it to just 2.4° in the vertical plane. The result is like a knife blade, sharpened in one direction but extending out in another. The researchers now plan to make a version that can sharpen a laser in two dimensions, like a pencil sharpener, which would bring it to a sharp point. “This is a first important step towards full beam collimation,” says Yu. “We have preliminary results showing that this can be done by using semicircular grooves.” Journal reference: Nature Photonics (DOI: