Abstract
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.
Highlights
The divergence angle u of a beam produced by an edge-emitting semiconductor laser is diffraction-limited to a value u % arcsin(l/T) in the plane normal to the waveguide layers, where l is the laser wavelength and T is the thickness of the waveguide core
A small beam divergence in the vertical direction of quantum cascade lasers (QCLs) is realized by coupling the outgoing laser radiation through a slit into the surface plasmon (SP) modes of a metallic grating on the laser facet; see Fig. 1a for a schematic of the device
The far-field of our QCLs in the vertical direction no longer corresponds to the diffraction from an aperture defined by the waveguide core in the original device, but rather to the interference pattern created by the large number of grooves that span the laser facet
Summary
The divergence angle u of a beam produced by an edge-emitting semiconductor laser is diffraction-limited to a value u % arcsin(l/T) in the plane normal to the waveguide layers, where l is the laser wavelength and T is the thickness of the waveguide core. A similar expression holds for the divergence angle parallel to the layers. In the case of mid-infrared quantum cascade lasers (QCLs), T is limited to a few micrometres and u typically ranges from 40 to 808 (full-width at half-maximum, FWHM) in the polarization direction (z-axis, Fig. 1a), which is normal to the waveguide layers. Divergent beams from semiconductor lasers are focused or collimated with lenses or curved mirrors, which usually require meticulous optical alignment. It is not practical to suppress the vertical divergence by growing thick laser active cores; such devices would require unrealistically high voltages for operation and would have heat dissipation problems
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