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.
Accurate and reliable characterization of solids and liquids is essential in medical applications as well as in pharmaceutical and industrial production sites. Vibrational microspectroscopy techniques such as quantum cascade laser based (QCL-IR), Fourier Transform IR-microscopes or near field scanning imagers have been established. Here we demonstrate how modern IR- spectroscopy and imaging from micro to nanoscale can provide a deep insight on the chemical composition. Here we use a dental application as an example for highlighting the potential of combining different techniques. The different results achieved by using complementary systems are presented.
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