Tapered Yagi-Uda Nanoantennas for Broadband Unidirectional Emission
  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. 
Isabelle Staude, Ivan S. Maksymov, Manuel Decker, Andrey E.   Miroshnichenko, Dragomir N. Neshev, Chennupati Jagadish and Yu...
physics.optics