  We demonstrate experimentally the operation of tapered Yagi-Uda nanoantennas
for broadband unidirectional emission enhancement. The measured transmittance
spectra show that, in comparison to untapered reference structures, the tapered
nanoantennas exhibit distinct wide-band spectral resonances. The performed
full-vectorial numerical calculations are in good qualitative agreement with
the measured spectra, further revealing how the near-field profiles of the
tapered nanoantennas are directly reflecting their broadband characteristics.
