Mid Infrared Nonlinear Plasmonics using Germanium Nanoantennas on Silicon Substrates

TL;DR

This paper demonstrates third harmonic generation in mid-infrared plasmonic germanium nanoantennas on silicon, enabling ultrafast, tunable, coherent light sources with high spatial resolution.

Contribution

It introduces germanium nanoantennas resonant in the mid-infrared for nonlinear plasmonics, with experimental and simulation validation of their nonlinear optical properties.

Findings

Successful third harmonic generation observed in germanium nanoantennas

Spatially resolved maps of linear and nonlinear emission provided

Field enhancement inside antennas drives nonlinear frequency mixing

Abstract

We demonstrate third harmonic generation in plasmonic antennas made of highly doped germanium and designed to be resonant in the mid infrared. Owing to the near-field enhancement, the result is an ultrafast, sub-diffraction, coherent light source tunable between 3 and 5 micrometer wavelength on a silicon substrate. To observe nonlinearity in this challenging spectral region, a high-power femtosecond laser system equipped with parametric frequency conversion in combination with an all-reflective confocal microscope setup is employed. We show spatially resolved maps of the linear scattering cross section and the nonlinear emission of single isolated antenna structures. A clear third order power dependence as well as the mid-infrared emission spectra prove the nonlinear nature of the light emission. Simulations support the observed resonance length of the double rod antenna and demonstrate that the field enhancement inside the antenna material is responsible for the nonlinear frequency mixing.