  An important source of innovation in nanophotonics is the idea to scale down
known radio wave technologies to the optical regime. One thoroughly
investigated example of this approach are metallic nanoantennas which employ
plasmonic resonances to couple localized emitters to selected far-field modes.
While metals can be treated as perfect conductors in the microwave regime,
their response becomes Drude-like at optical frequencies. Thus, plasmonic
nanoantennas are inherently lossy. Moreover, their resonant nature requires
precise control of the antenna geometry. A promising way to circumvent these
problems is the use of broadband nanoantennas made from low-loss dielectric
materials. Here, we report on highly directional emission from active
dielectric leaky-wave nanoantennas made of Hafnium dioxide. Colloidal
semiconductor quantum dots deposited in the nanoantenna feed gap serve as a
local light source. The emission patterns of active nanoantennas with different
sizes are measured by Fourier imaging. We find for all antenna sizes a highly
directional emission, underlining the broadband operation of our design.
