Photonic nanostructures can improve the efficiency and functionality of optoelectronic devices by controlling propagation and extraction of light in waveguides. The paradigmatic example is the extraction of light by matching the grating and waveguide mode momenta. A higher functionality can be achieved, however, via strong coupling between waveguide and plasmon modes. Here, we show that such hybridized systems provide a convenient strategy to tailor the dispersion band of the waveguide. By controlling the metallic grating height, the bands evolve from a Dirac cone to a flatband and a parabolic profile. We show an application of this system to light emitting waveguides, where a flatband leads to a sharply defined emission cone, with a 1.8 fold gain in emission efficiency and a Purcell factor of 2.4. We show that 72.2% of all radiated power is contained within the flatband region and is eight times larger than the total emission by the dipole in a homogenous medium. The system thus combines efficiency gain with the opportunity of beam shaping.

• Received 18 March 2021
• Revised 26 April 2021
• Accepted 28 April 2021

DOI:https://doi.org/10.1103/PhysRevB.103.205423