Gain-assisted harmonic generation in near-zero permittivity metamaterials made of plasmonic nanoshells

We investigate enhanced harmonic generation processes in gain-assisted, near-zero permittivity metamaterials composed of spherical plasmonic nanoshells. We report the presence of narrow-band features in transmission, reflection and absorption induced by the presence of an active material inside the core of the nanoshells. The damping-compensation mechanism used to achieve the near-zero effective permittivity condition also induces a significant increase in field localization and strength and, consequently, enhancement of linear absorption. When only metal nonlinearities are considered, second- and third-harmonic generation efficiencies obtained by probing the structure in the vicinity of the near-zero permittivity condition approach values as high as 10 ^-7 for irradiance value as low as 10 MW cm ^-2. These results clearly demonstrate that a relatively straightforward path now exists for the development of exotic and extreme nonlinear optical phenomena in the kWcm ^-2 range.