Appl. Phys. Lett. 93, 181101 (2008)
Compared with vertical-surface emitters, one of the disadvantages of edge-emitting semiconductor lasers has been the large beam divergence that made far-field focusing difficult to achieve. Nanfang Yu and colleagues from Harvard University now demonstrate how highly directional output can be achieved by placing a plasmonic collimator directly on the laser facet. Separated from the semiconductor laser through a thin insulating alumina layer, the plasmonic collimator consists of a gold layer with a number of half-circle grooves. The periodic structure directly couples the surface plasmons in the gold layer to the laser light. Beam collimation is then achieved through the interference of the actual laser beam and the light scattered back from the rings. The achieved improvements in directionality from an edge-emitting quantum-cascade laser are significant, with divergence angles being reduced from up to 80° down to a minimum of 2.4°. At the same time, the power output was boosted by up to 57%. Further improvements in design and fabrication of subwavelength structures integrated with semiconductor lasers may lead to further advances in beam engineering.
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