Free electrons in heavily doped semiconductors operate in the hydrodynamic regime, where oscillating velocity, current and electromagnetic field terms can mix and produce relatively strong nonlinear effects in the mid-infrared and terahertz ranges, where the material behaves as a free-electron system. We have designed and realized electron-doped InGaAs nanoantennas with the aim of measuring the efficiency of Third Harmonic Generation (THG) and comparing it with the nonlinearity coefficients predicted by a hydrodynamic model. To observe THG from nanoantennas, we used a difference-frequency generation source of mid-infrared short pulses with center-wavelength tunable between 12 and 6 micrometers. Four different doping levels and several dipole antenna lengths were investigated. The volume-normalized THG efficiencies of free-electrons are much higher than those of the crystal host, as directly shown by analysis of an undoped sample. The THG efficiency is found to peak at a mid-infrared excitation wavelength that depends on the free electron concentration, mirroring the decrease of the plasma wavelength with increasing carrier concentration.
Plasmonic nanoantenna designs are quickly evolving in the direction of practical molecular sensing applications hence their wavelength range is being extended from the visible towards the mid-infrared. The problem of obtaining, in the mid-infrared, the same degree of plasmonic confinement obtained with gold in the visible range is related to the perfect conductor behavior of metals at long wavelengths. Here we fabricated bow-tie nanoantennas made of bottom-up assembled “metallic germanium” with free electron density of the order of 1020 cm-3 and therefore short plasma wavelength of 4.5 μm. We demonstrate the existence in the antenna gaps of confined hotspots with radius of the order of 100 nm, which we imaged by near-field photoexpansion microscopy at a wavelength of 5.8 μm in order to provide a clear proof of strong field confinement in the mid-infrared.
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