Small-divergence semiconductor lasers by plasmonic collimation

Surface plasmons offer the exciting possibility of improving the functionality of optical devices through the subwavelength manipulation of light. We show that surface plasmons can be used to shape the beams of edge-emitting semiconductor lasers and greatly reduce their large intrinsic beam divergence. Using quantum cascade lasers as a model system, we show that by defining a metallic subwavelength slit and a grating on their facet, a small beam divergence in the laser polarization direction can be achieved. Divergence angles as small as 2.4° are obtained, representing a reduction in beam spread by a factor of 25 compared with the original 9.9-µm-wavelength laser used. Despite having a patterned facet, our collimated lasers do not suffer significant reductions in output power (∼100 mW at room temperature). Plasmonic collimation provides a means of efficiently coupling the output of a variety of lasers into optical fibres and waveguides, or to collimate them for applications such as free-space communications, ranging and metrology. Nanfang Yu and colleagues show that plasmonics can be used to reduce the spread of laser beams. They demonstrate their technique using a quantum cascade laser, and show that by defining a metallic subwavelength slit and grating onto the facet of the laser, a beam divergence of 2.4 degrees can be achieved. The technique can potentially be used to collimate the beams from a variety of different lasers.