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Time-resolved spectroscopy of semiconductors with single-cycle THz pulses


Background. The far-IR portion of the electromagnetic spectrum spanning the 100 GHz - 10 THz range is called THz region and corresponds to wavelengths in the range 30 μm < λ < 3 mm and photon energies in the range 0.4 meV < hν < 40 meV (3 - 330 cm-1). This region of the electromagnetic spectrum is widely exploited nowadays in applications to security, life sciences, analytical sciences, molecular spectroscopy and solid-state physics.
By exploiting a nonlinear optical effect called optical rectification, single-cycle coherent pulses in the THz region can be generated in nonlinear crystals by ultrashort laser pulses in the IR. The THz pulses can then be detected in amplitude and phase through electro-optical sampling techniques exploiting again ultrashort IR laser pulses.
Motivations. Understanding the charge transport properties of a semiconductor is essential for the development of innovative optoelectronic devices. The role of different quasiparticles like free carriers, excitons or polarons in charge transport can be discriminated by THz radiation; time-resolved THz spectroscopy can also uncover out-of-equilibrium dynamics of photogenerated charge carriers through pump-probe measurements.
Thesis goal. This thesis work aims at the study of charge transport dynamics in organic and inorganic semiconductors by time-resolved IR/VIS/UV-pump + THz-probe spectroscopy.
Thesis activities and perspectives. The activity will be developed in the framework of collaborations with CNR, IIT, UniMI. Among several cases, two classes of materials are currently under investigation and will be considered in the thesis work:

a) Polarons in Lead-Halide Perovskites. These materials are intensively investigated since might be exploited in new-generation photovoltaic devices. When charge carriers are promoted to the conduction band by photon absorption, their transport properties are strongly affected by the interaction with the lattice ions. The coupling of the electron with the lattice deformation is described by a quasi-particle called polaron, whose properties affect the carrier mobility in the material.
The formation dynamics, the energy relaxation and the decay of such polarons will be studied in the THz spectral domain in order to determine the intrinsic limitations to charge transport in lead-halide perovskites.

b) Ultrafast Phonon-Assisted Absorption in Semiconductors. When a direct band-gap semiconductor absorbs photons, electrons are promoted to the conduction band. The conductivity of the photoexcited material is often modeled as that of a Drude-like free electron gas. However, if the photon energy is larger than the direct gap and the photon flux is substantial, side valleys of the conduction band are strongly populated; in this case the interaction of the electron gas with lattice vibrations is no longer negligible and leads to a more complex transient conduction behavior. A prototypical case is gallium arsenide, that shows resonances in the THz spectral region related to electron-phonon coupling. The dynamics of this coupling will be studied in the THz domain as a function of the delay from the impulsive optical excitation, in order to enlighten the nature of this electron-phonon coupling and its evolution during intervalley electron scattering.