Abstract
We demonstrate third harmonic generation in plasmonic antennas consisting of highly doped germanium grown on silicon substrates and designed to be resonant in the mid-infrared frequency range that is inaccessible with conventional nonlinear plasmonic materials. Owing to the near-field enhancement, the result is an ultrafast, subdiffraction, coherent light source with a wavelength tunable between 3 and 5 µm, and ideally overlapping with the fingerprint region of molecular vibrations. To observe the nonlinearity in this challenging spectral window, a high-power femtosecond laser system equipped with parametric frequency conversion in combination with an all-reflective confocal microscope setup is employed. We demonstrate 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 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.
Highlights
Plasmonic nanoantennas[1,2], i.e., resonant metallic structures with sub-optical-wavelength sizes, are one of the key components in advanced nano-optical applications
In conclusion, we were able to demonstrate for the first time coherent nonlinear emission from plasmonic subwavelength structures in the mid-IR frequency range
This achievement is enabled by the crucial advancements in the growth of epitaxial group-IV semiconductors and of heavily doped germanium on silicon substrates in particular
Summary
Plasmonic nanoantennas[1,2], i.e., resonant metallic structures with sub-optical-wavelength sizes, are one of the key components in advanced nano-optical applications. By directing and concentrating far-field electromagnetic radiation into subdiffraction-limited near-field volumes, plasmonic nanoantennas are ideal tools for accessing single quantum systems with light. Another benefit of the light concentration capabilities of resonant antennas is the ability to access nonlinear optical phenomena[3] even with minute electromagnetic field amplitudes. This approach has been exploited successfully in the near-IR spectral range to generate second harmonic[2], third harmonic[4], or even higher order phenomena[3,5]. In the case of graphene, a high degree of 2D confinement and optical field manipulation can be reached in the mid-IR region[10]
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