Plasmonic mid-infrared third harmonic generation in germanium nanoantennas

Area of Science:

• Optics and Photonics
• Materials Science
• Nonlinear Optics

Background:

• Conventional nonlinear plasmonic materials struggle to access the mid-infrared (mid-IR) frequency range.
• Mid-IR light is crucial for molecular fingerprinting due to its overlap with vibrational modes.

Purpose of the Study:

• To demonstrate third harmonic generation (THG) in highly doped germanium (Ge) plasmonic antennas.
• To develop an ultrafast, subdiffraction, coherent light source tunable in the mid-IR (3-5 µm).

Main Methods:

• Fabrication of highly doped Ge antennas on silicon substrates.
• Utilizing a high-power femtosecond laser system with parametric frequency conversion.
• Employing an all-reflective confocal microscope setup for spatially resolved measurements.

Main Results:

• Successful demonstration of THG in Ge antennas resonant in the mid-IR.
• Observation of a tunable, coherent light source between 3 and 5 µm.
• Spatially resolved mapping of linear scattering and nonlinear emission, showing third-order power dependence.

Conclusions:

• Highly doped Ge antennas enable efficient nonlinear frequency mixing in the mid-IR.
• Near-field enhancement within the antennas is key to the observed nonlinear process.
• This technology offers a novel ultrafast light source for mid-IR spectroscopy and sensing.