© 2019 Optical Society of America. The ability of both bowtie and semicircle antennas to enhance waveguide input coupling efficiency in thin trench waveguides was studied for broadband applications. The waveguide outputs of trench waveguides with and without antennas were measured using a tunable laser with wavelengths ranging from 570 nm to 960 nm under both parallel and perpendicular polarization conditions. The results indicated that the waveguides with antennas performed better than the trench waveguide alone for parallel polarization conditions with maximum enhancements of 51 and 32 from the simulations, and 14.4 and 5.2 experimentally for the bowtie and semicircle, respectively. Conversely, there was no significant enhancement for the perpendicular polarization. This was attributed to the excitation of either the gap mode or the surface plasmon mode, depending on polarization.