Nonlinear couplings between photons and electrons in new materials give rise to a wealth of interesting nonlinear phenomena [1]. This includes frequency mixing, optical rectification or nonlinear current generation, which are of particular interest for generating radiation in spectral regions that are difficult to access, such as the terahertz gap. Owing to its specific linear dispersion and high electron mobility at room temperature, graphene is particularly attractive for realizing strong nonlinear effects [2]. However, since graphene is a centrosymmetric material, second-order nonlinearities a priori cancel, which imposes to rely on less attractive third-order nonlinearities [3]. It was nevertheless recently demonstrated that dc-second-order nonlinear currents [4] as well as ultrafast ac-currents [5] can be generated in graphene under optical excitation. The asymmetry is introduced by the excitation at oblique incidence, resulting in the transfer of photon momentum to the electron system, known as the
We show that the Landau levels in epitaxial graphene in presence of localized defects are significantly modified compared to those of an ideal system. We report on magneto-spectroscopy experiments performed on high quality samples. Besides typical interband magneto-optical transitions, we clearly observe additional transitions that involve perturbed states associated to short-range impurities such as vacancies. Their intensity is found to decrease with an annealing process and a partial self-healing over time is observed. Calculations of the perturbed Landau levels by using a delta-like potential show electronic states both between and at the same energies of the Laudau levels of ideal graphene. The calculated absorption spectra involving all perturbed and unperturbed states are in very good agreement with the experiments.
We report on a magnetic-field investigation of a In0.53Ga0.47As/GaAs0.51Sb0.49 terahertz quantum cascade laser. Owing to the suppression of inter-Landau level non-radiative scattering, the device performances are strongly improved at high magnetic field. Working temperature up to 190 K and current threshold of 450 A/cm 2 are measured at 11 teslas, comparable to the state-of-the-art GaAs/AlGaAs terahertz lasers. The nominally symmetric three-well structure presents significantly better performance with negative bias polarisation because of the inverted-interface roughness impact on the laser action.
Measurements of current have been performed on a very long wave infrared quantum cascade detector under strong magnetic field applied parallel to the growth axis, both under dark and light conditions. The analysis of dark current as a function of temperature highlights three regimes of transport involving the different energy levels of the structure. For photocurrent analysis, we developed a model based on a rate equation approach taking into account all the electronic levels of the structure. This model is in agreement with the oscillatory component of the experimental magnetophotocurrent. It allows to identify the key points controlling the electronic transport such as extraction from the upper level of the optically active quantum well, location of ionized impurities and scattering mechanisms involved in the structure. This work is valuable for the future conception of highperformance quantum cascade detectors in infrared and far infrared range.
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