The researchers use a near-field scanning technique capable of extracting the amplitude, phase, and polarization of optical modes confined to a photonic crystal waveguide that they first used for slow-light characterization in 2005. Their current paper, published in the Journal of the Optical Society of America B, uses the technique to explore two key features of slow light in photonic crystal waveguides: coupling into the slow-light mode from a normal fast-light mode as well as experimental determination of the bandstructure of the waveguide.
Coupling of a waveguide mode into a mode whose group velocity is 10 to 1000 times lower is difficult because of the associated difference in energy density of the two modes. However, previous theoretical work by the researchers has lead to designs with coupling efficiencies exceeding 90%. These photonic crystal waveguide geometries take advantage of non-energy-carrying evanescent modes at the interface between fast- and slow-light waveguide sections to facilitate energy density matching at the interface. By fitting mathematical functions that theoretically describe the field behavior inside photonic crystal waveguides to the near-field data, the current work has experimentally demonstrated the presence of the evanescent modes at the fast-light/slow-light interface. This is an exciting result as it demonstrates and confirms designs for achieving the important task of efficient coupling to slow-light devices.
Their method is also capable of extracting the spatial wavevectors associated with the propagating modes at a given frequency with good resolution. Photonic crystal band diagrams are complicated as a result of the presence of band folding, bandgaps and band crossings, and anticrossings. The authors' measurement technique combined with the novel functional fitting procedure yields high-resolution band diagrams capable of visualizing the complicated shapes of the waveguide bands previously unseen in lower-resolution band diagram measurement methods.
This work represents an important step forward in making passive slow-light devices practical. It also enables exciting future pursuits such as determining experimentally the slowing factor of a given device and investigating devices designed for slow light over larger bandwidths.
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