Tiny 'Bow-Tie' Microlasers Make it Big
Photo from Lay-Language Version of Paper FB13.01
Tiny Lasers Make it Big: High-Power Directional Emission From Semiconductor "Bow-tie" Micro-lasers
Scientists at Lucent Technologies' Bell Laboratories have developed a radically new resonator design that can dramatically increase the output power and directionality of micro-disk semiconductor lasers. The results were featured at a Monday afternoon session at the APS Centennial meeting in Atlanta, Georgia, in March.
"Miniaturization is a key word often cited for semiconductor devices, built upon the concept that the smaller the active volume of a device, the less electrical power it will consume and the faster it will operate," says Bell Labs, Claire Gmachl, a member of the research team. "This in turn greatly eases the limitations imposed on power supplies and cooling devices, and allows more and more devices to be packed together even more densely." A similar line of thought is equally valid for semiconductor lasers, widely used in applications such as data transfer, telecommunication, and CD-players.
However, the miniaturization of conventional semiconductor lasers faces two problems: smaller devices in general have higher losses, which degrade their laser properties, and usually provide less light output. One of the best ways to solve these problems is to improve reflectivity of the laser resonator mirrors. This was achieved intrinsically by the development of micro-disk semiconductor lasers by Richart Slusher and coworkers at Bell Labs in 1991 - then the world's smallest lasers. In these devices, "Laser action takes place on so-called 'whispering gallery' modes, named after an effect found in medieval churches, where even whispers can travel long distances along the curved inner surfaces of arches and domes," says Gmachl.
The laser operates by confining the light through total internal reflection. The light rays reflect repeatedly from the boundary with the same angle of incidence, which is greater than the allowed maximum angle for refracting out of the medium. Hence the light circulates along the inner boundary of the laser almost infinitely, making the tiny laser-resonator appear to the light like a big 'infinite' one curled up inside the disk, and enabling one to make very compact lasers. However, the devices proved unsuitable for most technological applications because such lasers produce very low power and require additional components to direct the small amount of emitted light.
The new resonator design demonstrated by Gmachl's team increased the output power and directionality of such micro-disk lasers by up to a factor of one thousand. "The design of these new devices was guided by Chaos theory, in a way that is closely related to the description of a ball moving on an oddly shaped billiard table," says Gmachl. The researchers fabricated miniature cylinder lasers - a few hundredths of a millimeter across - which were smoothly deformed from circular symmetry. The lasers have a cross-section that has been elongated in one direction and squeezed in the perpendicular direction. At weak deformations, the devices are dominated by chaotic motion of the light rays in the cavity ("the billiard table"). At higher deformations a different type of laser resonance appears and is responsible for highly directional and high power emission.
These bow tie shaped resonances are stable resonator modes, using only parts of the cylinder-laser's perimeter as resonator mirrors, resulting in strongly directional light output. The reflectivity of the boundary is very high, but not quite unity, as it was in the whispering gallery lasers. This allows the laser to have a low threshold and to reach a high output power. So the lasers emit with high power into very specific directions and even improved some of their general laser characteristics. The effect should be largely independent of the particular laser or semiconductor laser material, but the Bell Labs team achieved their results using a quantum cascade laser emitting in the mid-infrared wavelength region, as it is particularly suited for whispering-gallery type geometries.
[Adapted from a lay language paper by Claire Gmachl of Lucent Technologies' Bell Laboratories.]