Nonlinear THz spectroscopy in low-dimensional semiconductors using a free-electron laser

Sources for intense THz pulses allow one to investigate interesting nonlinear effects in various solid-state systems in a time-resolved manner. Here we discuss perturbative and non-perturbative nonlinear effects in graphene-based systems and GaAs/AlGaAs quantum wells, respectively, which are excited by intense, tunable, narrowband THz pulses from a free-electron laser (FEL).
For the graphene-based samples we have pursued two strategies to resonantly enhance the linear and nonlinear response. One way is to apply a magnetic field perpendicular to the graphene layer.
This splits the linear band structure into a set of non-equidistant Landau levels. Four-wave mixing experiments on the lowest Landau levels reveal a strong nonlinearity and a rapid dephasing of the microscopic polarization on sub-ps timescales [1]. The second path for resonant enhancement is the fabrication of ribbons and discs of doped graphene that feature a strong plasmonic response [2]. The nonlinearity corresponding to the plasmonic response is based on a red-shift and broadening of the absorption line caused by carrier heating.
While the previously discussed phenomena are perturbative nonlinear effects, FEL pulses are also well suited to generate non-perturbative effects such as the intraexcitonic Autler-Townes effect and double dressing of polaritons in microcavities [3, 4]. Here we present novel results on the dressing of subbands in a single GaAs/AlGaAs quantum well by THz photons. To this end, the transition between the second and third subband was resonantly pumped with the FEL. Broadband probing by THz time-domain spectroscopy using GaP crystals for optical rectification enabled us to observe the THz-dressing of the electronic states. Namely Autler-Townes splitting occurs at the transition between the first and second subband while the transition from the second to the third subband is split into a Mollow triplet.
We are grateful to our collaborators M. M. Jadidi, T. E. Murphy, A. Belyanin, and E. Malic.
REFERENCES
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