Enhancing nonlinear processes from dielectric nanoantennas: The role of the substrate

We investigate the impact of different substrates on the radiation efficiency of nonlinear processes from dielectric nanoantennas. Several approaches have been considered to optimize the nonlinear radiation efficiency from these structures, but all the strategies have generally failed at limiting the amount of radiation at the harmonic frequencies that is lost in the substrate. It is well known that epsilon-near-zero (ENZ) materials, namely material showing a zero-crossing for the real part of the dielectric permittivity, have peculiar linear properties, such as their ability to realize perfect electromagnetic tunneling and re-direct antennas electromagnetic radiation. Here we first compare the second harmonic signal radiated from a cylindrical AlGaAs nanoantenna placed over different materials, including an ENZ substrate and found that the ability to re-direct the electromagnetic radiation is particularly favored when the ENZ condition occurs at the second harmonic frequency. These results let us foresee a novel approach to improve nonlinear processes at the nanoscale and the possibility to realize novel functionalities, such as beam steering and tailored antenna directivities thanks to the tunability of ENZ materials.